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Title: Form and Function: A Contribution to the History of Animal
Morphology

Author: E. S. Russell

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*** START OF THE PROJECT GUTENBERG EBOOK FORM AND
FUNCTION: A CONTRIBUTION TO THE HISTORY OF ANIMAL
MORPHOLOGY ***

FORM AND FUNCTION
A CONTRIBUTION TO THE

HISTORY OF ANIMAL MORPHOLOGY

By E. S. RUSSELL,

M.A., B.Sc., F.Z.S.

ILLUSTRATED

LONDON

JOHN MURRAY, ALBEMARLE STREET, W.

1916

All rights reserved

Transcriber's Note: Obvious printer errors have
been corrected, all other inconsistencies in
spelling and punctuation are as in the original.

PREFACE

This book is not intended to be a full or detailed history of animal
morphology: a complete account is given neither of morphological
discoveries nor of morphological theories. My aim has been rather to call
attention to the existence of diverse typical attitudes to the problems of
form, and to trace the interplay of the theories that have arisen out of them.
The main currents of morphological thought are to my mind three—the
functional or synthetic, the formal or transcendental, and the materialistic or
disintegrative.
The first is associated with the great names of Aristotle, Cuvier, and von
Baer, and leads easily to the more open vitalism of Lamarck and Samuel
Butler. The typical representative of the second attitude is E. Geoffroy St.
Hilaire, and this habit of thought has greatly influenced the development of
evolutionary morphology.
The main battle-ground of these two opposing tendencies is the problem of
the relation of function to form. Is function the mechanical result of form,
or is form merely the manifestation of function or activity? What is the
essence of life—organisation or activity?
The materialistic attitude is not distinctively biological, but is common to
practically all fields of thought. It dates back to the Greek atomists, and the
triumph of mechanical science in the 19th century has induced many to
accept materialism as the only possible scientific method. In biology it is
more akin to the formal than to the functional attitude.
In the course of this book I have not hidden my own sympathy with the
functional attitude. It appears to me probable that more insight will be
gained into the real nature of life and organisation by concentrating on the
active response of the animal, as manifested both in behaviour and in
morphogenesis, particularly in the post-embryonic stages, than by giving
attention exclusively to the historical aspect of structure, as is the custom of
"pure morphology." I believe we shall only make progress in this direction
if we frankly adopt the simple everyday conception of living things—which

many of us have had drilled out of us—that they are active, purposeful
agents, not mere complicated aggregations of protein and other substances.
Such an attitude is probably quite as sound philosophically as the opposing
one, but I have not in this place attempted any justification of it. I have
touched very lightly upon the controversy between vitalism and materialism
which has been revived with the early years of the present century. It hardly
lends itself as yet to historical treatment, and I could hardly hope to
maintain with regard to it that objective attitude which should characterise
the historian.
The main result I hope to have achieved with this book is the
demonstration, tentative and incomplete as it is, of the essential continuity
of animal morphology from the days of Aristotle down to our own time. It
is unfortunately true that modern biology, perhaps in consequence of the
great advances it has made in certain directions, has to a considerable extent
lost its historical consciousness, and if this book helps in any degree to
counteract this tendency so far as animal morphology is concerned, it will
have served its purpose.
I owe a debt of gratitude to my friends Dr James F. Gemmill and Prof. J.
Arthur Thomson for much kindly encouragement and helpful criticism. The
credit for the illustrations is due to my wife, Mrs Jehanne A. Russell. One is
from Nature; the others are drawn from the original figures.
E. S. R.
Chelsea, 1916.

CONTENTS

CHAP. Page
I. The Beginnings of Comparative Anatomy 1
II. Comparative Anatomy before Cuvier 17
III. Cuvier 31
IV. Goethe 45
V. Etienne Geoffroy St Hilaire 52
VI. The Followers of Etienne Geoffroy St Hilaire 79
VII. The German Transcendentalists 89
VIII. Transcendental Anatomy in England—Richard Oven 102
IX. Karl Ernst von Baer 113
X. The Embryological Criterion 133
XI. The Cell-Theory 169
XII. The Close of the Pre-evolutionary Period 190
XIII. The Relation of Lamarck and Darwin to Morphology 213
XIV. Ernst Haeckel and Carl Gegenbaur 246
XV. Early Theories on the Origin of Vertebrates 268
XVI. The Germ-layers and Evolution 288
XVII. The Organism as an Historical Being 302
XVIII. The Beginnings of Causal Morphology 314
XIX. Samuel Butler and the Memory Theories of Heredity 335
XX. The Classical Tradition in Modern Morphology 345

Index 365

ILLUSTRATIONS

FIG. Page
1. Hyoid Arch of the Conger. (Original.) 58
2. "Vertebra" of a Pleuronectid. (Geoffroy.) 61
3. Abdominal Segment of the Lobster. (Geoffroy.) 63
4. Ideal Typical Vertebra. (Owen.) 102
5. Natural Typical Vertebra. (Owen.) 103
6. The Archetype of the Vertebrate Skeleton. (Owen.) 105
7. Ideal Transverse Section of a Vertebrate Embryo. (Von
Baer.) 119
8. Gill-slits of the Pig Embryo. (Rathke.) 134
9. Meckel's Cartilage and Ear-ossicles in Embryo of Pig.
(Reichert.) 145
10. Cranial Vertebræ and Visceral Arches in Embryo of
Pig. (Reichert.) 148
11. Embryonic Cranium of the Adder. (Rathke.) 152
12. Transverse Section of Chick Embryo. (Remak.) 211
13. Development of the Ascidian Larva (Kowalevsky.) 272
14. Transverse Section of the Worm Nais. (Semper.) 280
15. The Five Primary Stages of Ontogeny. (Haeckel.) 292

FORM AND FUNCTION
CHAPTER I

THE BEGINNINGS OF COMPARATIVE ANATOMY

The first name of which the history of anatomy keeps record is that of
Alcmaeon, a contemporary of Pythagoras (6th century B.C.). His interests
appear to have been rather physiological than anatomical. He traced the
chief nerves of sense to the brain, which he considered to be the seat of the
soul, and he made some good guesses at the mechanism of the organs of
special sense. He showed that, contrary to the received opinion, the seminal
fluid did not originate in the spinal cord. Two comparisons are recorded of
his, one that puberty is the equivalent of the flowering time in plants, the
other that milk is the equivalent of white of egg.[1] Both show his bias
towards looking at the functional side of living things. The latter
comparison reappears in Aristotle.
A century later Diogenes of Apollonia gave a description of the venous
system. He too placed the seat of sensation in the brain. He assumed a vital
air in all living things, being in this influenced by Anaximenes whose
primitive matter was infinite air. In following out this thought he tried to
prove that both fishes and oysters have the power of breathing.[2]
A more strictly morphological note is struck by a curious saying of
Empedocles (4th century B.C.), that "hair and foliage and the thick plumage
of birds are one."[3]
In the collected writings of Hippocrates and his school, the Corpus
Hippocraticum, of which no part is later than the end of the 5th century,
there are recorded many anatomical facts. The author of the treatise "On the
Muscles" knew, for instance, that the spinal marrow is different from
ordinary marrow and has membranes continuous with those of the brain.
Embryos of seven days (!) have all the parts of the body plainly visible.

Work on comparative embryology is contained in the treatise "On the
Development of the Child."[4]
The author of the treatise "On the Joints," which Littré calls "the great
surgical monument of antiquity," is to be credited with the first systematic
attempt at comparative anatomy, for he compared the human skeleton with
that of other Vertebrates.

Aristotle (384-322 B.C.)[5] may fairly be said to be the founder of
comparative anatomy, not because he was specially interested in problems
of "pure morphology," but because he described the structure of many
animals and classified them in a scientific way. We shall discuss here the
morphological ideas which occur in his writings upon animals—in the
Historia Animalium, the De Partibus Animalium, and the De Generatione
Animalium.
The Historia Animalium is a most comprehensive work, in some ways the
finest text-book of Zoology ever written. Certainly few modern text-books
take such a broad and sane view of living creatures. Aristotle never forgets
that form and structure are but one of the many properties of living things;
he takes quite as much interest in their behaviour, their ecology,
distribution, comparative physiology. He takes a special interest in the
comparative physiology of reproduction. The Historia Animalium contains
a description of the form and structure of man and of as many animals as
Aristotle was acquainted with—and he was acquainted with an
astonishingly large number. The later De Partibus Animalium is a treatise
on the causes of the form and structure of animals. Owing to the importance
which Aristotle ascribed to the final cause this work became really a treatise
on the functions of the parts, a discussion of the problems of the relation of
form to function, and the adaptedness of structure.
Aristotle was quite well aware that each of the big groups of animals was
built upon one plan of structure, which showed endless variations "in excess
and defect" in the different members of the group. But he did not realise
that this fact of community of plan constituted a problem in itself. His
interest was turned towards the functional side of living things, form was
for him a secondary result of function.

Yet he was not unaware of facts of form for which he could not quite find a
place in his theory of organic form, facts of form which were not, at first
sight at least, facts of function. Thus he was aware of certain facts of
"correlation," which could not be explained off-hand as due to correlation
of the functions of the parts. He knew, for instance, that all animals without
front teeth in the upper jaw have cotyledons, while most that have front
teeth on both jaws and no horns have no cotyledons (De Gen., ii. 7).
Speaking generally, however, we find in Aristotle no purely morphological
concepts. What then does morphology owe to Aristotle? It owes to him,
first, a great mass of facts about the structure of animals; second, the first
scientific classification of animals;[6] third, a clear enunciation of the fact of
community of plan within each of the big groups; fourth, an attempt to
explain certain instances of the correlation of parts; fifth, a pregnant
distinction between homogeneous and heterogeneous parts; sixth, a
generalisation on the succession of forms in development; and seventh, the
first enunciation of the idea of the Échelle des êtres.
(1) What surprises the modern reader of the Historia Animalium perhaps
more than anything else is the extent and variety of Aristotle's knowledge of
animals. He describes more than 500 kinds.[7] Not only does he know the
ordinary beasts, birds, and fishes with which everyone is acquainted, but he
knows a great deal about cuttlefish, snails and oysters, about crabs, crawfish
(Palinurus), lobsters, shrimps, and hermit crabs, about sea-urchins and
starfish, sea-anemones and sponges, about ascidians (which seem to have
puzzled him not a little!). He has noticed even fish-lice and intestinal
worms, both flat and round. Of the smaller land animals, he knows a great
many insects and their larvæ. The extent of his anatomical knowledge is
equally surprising, and much of it is clearly the result of personal
observation. No one can read his account of the internal anatomy of the
chameleon (Hist. Anim., ii.), or his description of the structure of cuttlefish
(Hist. Anim., iv), or that touch in the description of the hermit crab (Hist.
Anim., iv.)—"Two large eyes ... not ... turned on one side like those of crabs,
but straight forward"—without being convinced that Aristotle is speaking of
what he has seen. Naturally he could not make much of the anatomy of
small insects and snails, and, to tell the truth, he does not seem to have
cared greatly about the minutiæ of structure. He was too much of a Greek
and an aristocrat to care about laborious detail.

Not only did he lay a foundation for comparative anatomy, but he made a
real start with comparative embryology. Medical men before him had
known many facts about human development; Aristotle seems to have been
the first to study in any detail the development of the chick. He describes
this as it appears to the naked eye, the position of the embryo on the yolk,
the palpitating spot at the third day, the formation of the body and of the
large sightless eyes, the veins on the yolk, the embryonic membranes, of
which he distinguished two.
(2) Aristotle had various systems of classifying animals. They could be
classified, he thought, according to their structure, their manner of
reproduction, their manner of life, their mode of locomotion, their food, and
so on. Thus you might, in addition to structural classifications, divide
animals into gregarious, solitary and social, or land animals into
troglodytes, surface-dwellers, and burrowers (Hist. Anim., i.).
He knew that dichotomous classifications were of little use for animals (De
Partibus, i. 3) and he explicitly and in so many words accepted the principle
of all "natural" classification, that affinities must be judged by comparing
not one but the sum total of characters. As everyone knows, he was the first
to distinguish the big groups of animals, many of which were already
distinguished roughly by the common usages of speech. Among his
Sanguinea he did little more than define with greater exactitude the limits of
the groups established by the popular classification. Among the
"exsanguineous" animals, however, corresponding to our Invertebrates, he
established a much more definite classification than the popular, which is
apt to call them indiscriminately "shellfish," "insects," or "creeping things."
He went beyond the superficialities of popular classification, too, in clearly
separating Cetacea from fishes. He had some notion of species and genera
in our sense. He distinguished many species of cuttlefish—Octopus
(Polypus) of which there were many kinds, Eledone (Moschites) which he
knew to have only one row of suckers while Octopus has two, Argonauta,
Nautilus, Sepia, and apparently Loligo media (= his Teuthis) and L. vulgaris
(or forbesii) which seems to be his Teuthos. He had a grasp of the principles
which should be followed in judging of the natural affinities of species. For
example, he knew that the cuckoo resembles a hawk. "But," he says, "the
hawk has crooked talons, which the cuckoo has not, nor does it resemble
the hawk in the form of its head, but in these respects is more like the

pigeon than the hawk, which it resembles in nothing but its colour; the
markings, however, upon the hawk are like lines, while the cuckoo is
spotted" (Hist. Anim., Cresswell's trans., p. 147, London, 1862).
The groups he distinguished were—man, viviparous quadrupeds, oviparous
quadrupeds, birds, fishes, Cetacea, Cephalopoda, Malacostraca (= higher
Crustacea), Insecta (= annulose animals), Testacea (= molluscs,
echinoderms, ascidians). A class of Acalephæ, including sea-anemones and
sponges, was grouped with the Testacea. The first five groups were classed
together as sanguineous, the others as exsanguineous, from the presence or
absence of red blood.
Besides these classes "there are," he says, "many other creatures in the sea
which it is not possible to arrange in any class from their scarcity"
(Creswell, loc. cit., p. 90).
(3) Aristotle's greatest service to morphology is his clear recognition of the
unity of plan holding throughout each of the great groups.
He recognises this most clearly in the case of man and the viviparous
quadrupeds, with whose structure he was best acquainted. In the Historia
Animalium he takes man as a standard, and describes his external and
internal parts in detail, then considers viviparous quadrupeds and compares
them with man. "Whatever parts a man has before, a quadruped has
beneath; those that are behind in man form the quadruped's back"
(Cresswell, loc. cit., p. 26). Apes, monkeys, and Cynocephali combine the
characteristics of man and quadrupeds. He notices that all viviparous
quadrupeds have hair. Oviparous quadrupeds resemble the viviparous, but
they lack some organs, such as ears with an external pinna, mammæ, hair.
Oviparous bipeds, or birds, also "have many parts like the animals
described above." He does not, however, seem to realise that a bird's wings
are the equivalent of a mammal's arms or fore-legs. Fishes are much more
divergent; they possess no neck, nor limbs, nor testicles (meaning a solid
ovoid body such as the testis in mammals), nor mammæ. Instead of hair
they have scales.
Speaking generally, the Sanguinea differ from man and from one another in
their parts, which may be present or absent, or exhibit differences in "excess
and defect," or in form. Unity of plan extends to all the principal systems of

organs. "All sanguineous animals have either a bony or a spinous column.
The remainder of the bones exist in some animals; but not in others, for if
they have the limbs they have the bones belonging to them" (Cresswell, loc.
cit., p. 60). "Viviparous animals with blood and feet do not differ much in
their bones, but rather by analogy, in hardness, softness, and size"
(Cresswell, loc. cit., p. 59). The venous system, too, is built upon the same
general plan throughout the Sanguinea. "In all sanguineous animals, the
nature and origin of the principal veins are the same, but the multitude of
smaller veins is not alike in all, for neither are the parts of the same nature,
nor do all possess the same parts" (Cresswell, loc. cit., p. 56). It will be
noticed in the first and last of these three quotations that Aristotle
recognises the fact of correlation between systems of organs—between
limbs and bones, and between blood-vessels and the parts to which they go.
Sanguineous animals all possess certain organs—heart, liver, spleen,
kidneys, and so on. Other organs occur in most of the classes—the
œsophagus and the lungs. "The position which these parts occupy is the
same in all animals [sc. Sanguinea]" (Cresswell, loc. cit., p. 39).
Unity of plan is observable not only in the Sanguinea, but also within each
of the other large groups. Aristotle recognises that all his cuttlefish are alike
in structure. Among his Malacostraca he compares point by point the
external parts of the carabus (Palinurus), and the astacus (Homarus), and he
compares also the general internal anatomy of the various "genera" he
distinguishes. As regards Testacea, he writes, "The nature of their internal
structure is similar in all, especially in the turbinated animals, for they differ
in size and in the relations of excess; the univalves and bivalves do not
exhibit many differences" (Cresswell, loc. cit., p. 83). There is an
interesting remark about "the creature called carcinium" (hermit-crab), that
it "resembles both the Malacostraca and the Testacea, for this in its nature is
similar to the animals that are like carabi, and it is born naked" (Cresswell,
loc. cit., p. 85). In the last phrase we may perhaps read the first recognition
of the embryological criterion.
With the recognition of unity of plan within each group necessarily goes the
recognition of what later morphology calls the homology of parts. The parts
of a horse can be compared one by one with the parts of another viviparous
quadruped; in all the animals belonging to the same class the parts are the

same, only they differ in excess or defect—these remarks are placed in the
forefront of the Historia Animalium. Generally speaking, parts which bear
the same name are for Aristotle homologous throughout the class. But he
goes further and notes the essential resemblance underlying the differences
of certain parts. He classes together nails and claws, the spines of the
hedgehog, and hair, as being homologous structures. He says that teeth are
allied to bones, whereas horns are more nearly allied to skin (Hist. Anim.,
iii.). This is an astonishingly happy guess, considering that all he had to go
upon was the observation that in black animals the horns are black but the
teeth white. One cannot but admire the way in which Aristotle fixes upon
apparently trivial and commonplace facts, and draws from them far-
reaching consequences. He often goes wrong, it is true, but he always errs
in the grand manner.
While Aristotle certainly recognised the existence of homologies, and even
had a feeling for them, he did not clearly distinguish homology from
analogy. He comes pretty near the distinction in the following passage.
After explaining that in animals belonging to the same class the parts are
the same, differing only in excess or defect, he says, "But some animals
agree with each other in their parts neither in form nor in excess and defect,
but have only an analogous likeness, such as a bone bears to a spine, a nail
to a hoof, a hand to a crab's claw, the scale of a fish to the feather of a bird,
for that which is a feather in the bird is a scale in the fish" (Cresswell, loc.
cit., p. 2). One of these comparisons is, however, a homology not an
analogy, and the last phrase throws a little doubt upon the whole question,
for it is not made clear whether it is position or function that determines
what are equivalent organs.
In the De Partibus Animalium there occurs the following passage:
—"Groups that only differ in degree, and in the more or less of an identical
element that they possess, are aggregated under a single class; groups
whose attributes are not identical but analogous are separated. For instance,
bird differs from bird by gradation, or by excess and defect; some birds
have long feathers, others short ones, but all are feathered. Bird and Fish are
more remote and only agree in having analogous organs; for what in the
bird is feather, in the fish is scale. Such analogies can scarcely, however,
serve universally as indications for the formation of groups, for almost all
animals present analogies in their corresponding parts."[8] It is thus

similarity in form and structure which determines the formation of the main
groups. Within each group the parts differ only in degree, in largeness or
smallness, softness and hardness, smoothness or roughness, and the like
(loc. cit., i., 4, 644b). These passages show that Aristotle had some
conception of homology as distinct from analogy. He did not, however,
develop the idea. What Aristotle sought in the variety of animal structure,
and what he found, were not homologies, but rather communities of
function, parts with the same attributes. His interest was all in organs, in
functioning parts, not in the mere spatial relationship of parts.
This comes out clearly in his treatise On the Parts of Animals, which is
subsequent to, and the complement of, his History of Animals. The latter is
a description of the variety of animal form, the former is a treatise on the
functions of the parts. He describes the plan of the De Partibus Animalium
as follows:—"We have, then, first to describe the common functions,
common, that is, to the whole animal kingdom, or to certain large groups, or
to members of a species. In other words, we have to describe the attributes
common to all animals, or to assemblages, like the class of Birds, of closely
allied groups differentiated by gradation, or to groups like Man not
differentiated into subordinate groups. In the first case the common
attributes may be called analogous, in the second generic, in the third
specific" (i, 5, 645b, trans. Ogle). The alimentary canal is a good example of
a part which is "analogous" throughout the animal kingdom, for "all
animals possess in common those parts by which they take in food, and into
which they receive it" (Cresswell, loc. cit., p. 6).
The De Partibus Animalium becomes in form a comparative organography,
but the emphasis is always on function and community of function. Thus he
treats of bone, "fish-spine," and cartilage together (De Partibus, ii., 9, 655a),
because they have the same function, though he says elsewhere that they are
only analogous structures (ii., 8, 653b). In the same connection he describes
also the supporting tissues of Invertebrates—the hard exoskeleton of
Crustacea and Insects, the shell of Testacea, the "bone" of Sepia (ii., 8,
654a). Aristotle took much more interest in analogies, in organs of similar
function, than in homologies. He did recognise the existence of homologies,
but rather malgré lui, because the facts forced it upon him.

His only excursion into the realm of "transcendental anatomy" is his
comparison of a Cephalopod to a doubled-up Vertebrate whose legs have
become adherent to its head, whose alimentary canal has doubled upon
itself in such a way as to bring the anus near the mouth (De Partibus, iv., 9,
684b). It is clear, however, that Aristotle did not seek to establish by this
comparison any true homologies of parts, but merely analogies, thus
avoiding the error into which Meyranx and Laurencet fell more than two
thousand years later in their paper communicated to the Académie des
Sciences, which formed the starting-point of the famous controversy
between Cuvier and E. Geoffroy St Hilaire (see Chap. V., below).
Moreover, Aristotle did not so much compare a Cephalopod with a doubled-
up Vertebrate as contrast Cephalopods (and also Testacea) with all other
animals. Other animals have their organs in a straight line; Cephalopods and
Testacea alone show this peculiar doubling up of the body.
(4) Aristotle was much struck with certain facts of correlation, of the
interdependence of two organs which are not apparently in functional
dependence on one another. Such correlation may be positive or negative;
the presence of one organ may either entail the presence of the other, or it
may entail its absence. Aristotle has various ways of explaining facts of
correlation. He observed that no animal has both tusks and horns, but this
fact could easily be explained on the principle that Nature never makes
anything superfluous or in vain. If an animal is protected by the possession
of tusks it does not require horns, and vice versa. The correlation of a
multiple stomach with deficient development of the teeth (as in Ruminants)
is accounted for by saying that the animal needs its complex stomach to
make up for the shortcomings of its teeth! (De Partibus, iii., 14, 674b.)
Other examples of correlation were not susceptible of this explanation in
terms of final causes. He lays stress on the fact, in the main true, of the
inverse development of horns and front teeth in the upper jaw, exemplified
in Ruminants. He explains the fact in this way. Teeth and horns are formed
from earthy matter in the body and there is not enough to form both teeth
and horns, so "Nature by subtracting from the teeth adds to the horns; the
nutriment which in most animals goes to the former being here spent on the
augmentation of the latter" (De Partibus, iii., 2, 664a, trans. Ogle). A similar
kind of explanation is offered of the fact that Selachia have cartilage instead

of bone, "in these Selachia Nature has used all the earthy matter on the skin
[i.e., on the placoid scales]; and she is unable to allot to many different parts
one and the same superfluity of material" (De Partibus, ii., 9, 655a, trans.
Ogle). Speaking generally, "Nature invariably gives to one part what she
subtracts from another" (loc. cit., ii., 14, 658a).
This thought reappears again in the 19th century in E. Geoffroy St Hilaire's
loi de balancement and also in Goethe's writings on morphology. For
Aristotle it meant that Nature was limited by the nature of her means, that
finality was limited by necessity. Thus in the larger animals there is an
excess of earthy matter, as a necessary result of the material nature of the
animal; this excess is turned by Nature to good account, but there is not
enough to serve both for teeth and for horns (loc. cit., iii., 2, 663b).
But there are other instances of correlation which seem to have taxed even
Aristotle's ingenuity beyond its powers. Thus he knew that all animals
(meaning viviparous quadrupeds) with no front teeth in the upper jaw have
cotyledons on their fœtal membranes, and that most animals which have
front teeth in both jaws and no horns have no cotyledons (De Generatione,
ii., 7). He offers no explanation of this, but accepts it as a fact.
We may conveniently refer here to one or two other ideas of Aristotle
regarding the causes of form. He makes the profound remark that the
possible range of form of an organ is limited to some extent by its existing
differentiation. Thus he explains the absence of external (projecting) ears in
birds and reptiles by the fact that their skin is hard and does not easily take
on the form of an external ear (De Partibus, ii, 12). The fact of the inverse
correlation is certain; the explanation is, though very vague, probably
correct.
In one passage of the De Partibus Aristotle clearly enunciates the principle
of the division of labour, afterwards emphasised by H. Milne-Edwards. In
some insects, he says, the proboscis combines the functions of a tongue and
a sting, in others the tongue and the sting are quite separate. "Now it is
better," he goes on, "that one and the same instrument shall not be made to
serve several dissimilar ends; but that there shall be one organ to serve as a
weapon, which can then be very sharp, and a distinct one to serve as a
tongue, which can then be of spongy texture and fit to absorb nutriment.

Whenever, therefore, Nature is able to provide two separate instruments for
two separate uses, without the one hampering the other, she does so, instead
of acting like a coppersmith who for cheapness makes a spit and
lampholder in one" (iv., 6, 683a).
(5) The first sentence of the Historia Animalium formulates, with that
simplicity and directness which is so characteristic of Aristotle, the
distinction between homogeneous and heterogeneous parts, in the mass the
distinction between tissues and organs. "Some parts of animals are simple,
and these can be divided into like parts, as flesh into pieces of flesh; others
are compound, and cannot be divided into like parts, as the hand cannot be
divided into hands, nor the face into faces. All the compound parts also are
made up of simple parts—the hand, for example, of flesh and sinew and
bone" (Cresswell, loc. cit., p. 1).
In the De Partibus Animalium he broadens the conception by adding
another form of composition. "Now there are," he says, "three degrees of
composition; and of these the first in order, as all will allow, is composition
out of what some call the elements, such as earth, air, water, fire.... The
second degree of composition is that by which the homogeneous parts of
animals, such as bone, flesh, and the like, are constituted out of the primary
substances. The third and last stage is the composition which forms the
heterogeneous parts, such as face, hand, and the rest" (ii., 1, 646a, trans.
Ogle).
In the Historia Animalium the homogeneous parts are divided into (1) the
soft and moist (or fluid), such as blood, serum, flesh, fat, suet, marrow,
semen, gall, milk, phlegm, fæces and urine, and (2) the hard and dry (or
solid), such as sinew, vein, hair, bone, cartilage, nail, and horn. It would
appear from this enumeration that Aristotle's distinction of simple and
complex parts does not altogether coincide with our distinction of tissues
and organs. We should not call vein a tissue, nor do we include under this
heading non-living secretions. But in the De Partibus Animalium Aristotle,
while still holding to the distinction set forth above, is alive to the fact that
his simple parts include several different sorts of substances. He
distinguishes among the homogeneous parts three sets. The first of these
comprises the tissues out of which the heterogeneous parts are constructed,
e.g., flesh and bone; the second set form the nutriment of the parts, and are

invariably fluid; while the third set are the residue of the second and
constitute the residual excretions of the body (ii., 2, 647b). He sees clearly
the difficulty of calling vein or blood-vessel a simple part, for while a
blood-vessel and a part of it are both blood-vessel, as we should say
vascular tissue, yet a part of a blood-vessel is not a blood-vessel. There is
form superadded to homogeneity of structure (ii., 2, 647b). Similarly for the
heart and the other viscera. "The heart, like the other viscera, is one of the
homogeneous parts; for, if cut up, its pieces are homogeneous in substance
with each other. But it is at the same time heterogeneous in virtue of its
definite configuration" (ii., 1, 647a, trans. Ogle).
Aristotle, therefore, came very near our conception of tissue. He was of
course not a histologist; he describes not the structure of tissues, which he
could not know, but rather their distribution within the organism; his section
on the homogeneous parts of Sanguinea (Historia Animalium, iii., second
half) is largely a comparative topographical anatomy; in it, for instance, he
describes the venous and skeletal systems.
This distinction which Aristotle drew plays an important part in all his
writings on animals, particularly in his theory of development. It was a
distinction of immense value, and is full of meaning even at the present day.
No one has ever given a better definition of organ than is implied in
Aristotle's description of the heterogeneous parts—"The capacity of action
resides in the compound parts" (Cresswell, loc. cit., p. 7). The
heterogeneous parts were distinguished by the faculty of doing something,
they were the active or executive parts. The homogeneous parts were
distinguished mainly by physical characters (De Generatione, i., 18), but
certain of them had other than purely physical properties, they were the
organs of touch (De Partibus, ii., 1, 647a).
(6) In a passage in the De Generatione (ii, 3) Aristotle says that the embryo
is an animal before it is a particular animal, that the general characters
appear before the special. This is a foreshadowing of the essential point in
von Baer's law (see Chap. IX. below).
He considers also that tissues arise before organs. The homogeneous parts
are anterior genetically to the heterogeneous parts and posterior to the
elementary material (De Partibus, ii., 1, 646b).

(7) We meet in Aristotle an idea which later acquired considerable vogue,
that of the Échelle des êtres(or "scale of beings"), that organisms, or even
all objects organic or inorganic, can be arranged in a single ascending
series. The idea is a common one; its first literary expression is found
perhaps in primitive creation-myths, in which inorganic things are created
before organic, and plants before animals. It may be recognised also in
Anaximander's theory that land animals arose from aquatic animals, more
clearly still in Anaxagoras' theory that life took its origin on this globe from
vegetable germs which fell to earth with the rain. Anaxagoras considered
animals higher in the scale than plants, for while the latter participated in
pleasure (when they grew) and pain (when they lost their leaves), animals
had in addition "Nous." In Empedocles' theory of evolution, the vegetable
world preceded the animal. Plato, in the Timaeus, describes the whole
organic world as being formed by degradation from man, who is created
first. Man sinks first into woman, then into brute form, traversing all the
stages from the higher to the lower animals, and becoming finally a plant.
This is a reversal of the more usual notion, but the idea of gradation is
equally present.
Aristotle seems not to have believed in any transformation of species, but
he saw that Nature passes gradually from inanimate to animate things
without a clear dividing line. "The race of plants succeeds immediately that
of inanimate objects" (Cresswell, loc. cit., p. 94). Within the organic realm
the passage from plants to animals is gradual. Some creatures, for example,
the sea-anemones and sponges, might belong to either class.
Aristotle recognised also a natural series among the groups of animals, a
series of increasing complexity of structure. He begins his study of structure
with man, who is the most intricate, and then takes up in turn viviparous
and oviparous quadrupeds, then birds, then fishes. After the Sanguinea he
considers the Exsanguinea, and of the latter first the most highly organised,
the Cephalopods, and last the simplest, the lower members of his class of
the Testacea. In treating of generation (in Hist. Animalium, v.) he reverses
this order. In the De Generatione (Book ii., 1) there is given another serial
arrangement of animals, this time in relation to their manner of
reproduction. There is a gradation, he says, of the following kind:—

1. Internally viviparous Sanguinea producing a perfect
2. Externally viviparous Sanguinea animal
3. Oviparous Sanguinea—producing a perfect egg.
4. Animals producing an imperfect egg (one which increases
in size after being laid).
5. Insects, producing a scolex (or grub).
In Aristotle's view the gradation of organic forms is the consequence, not
the cause, of the gradation observable in their activities. Plants have no
work to do beside nutrition, growth, and reproduction; they possess only the
nutritive soul. Animals possess in addition sensation and the sensitive or
perceptive soul—"their manner of life differs in their having pleasure in
sexual intercourse, in their mode of parturition and rearing their young"
(Hist. Anim., viii., trans. Cresswell, p. 195). Man alone has the rational soul
in addition to the two lower kinds.

As it is put in the De Partibus (ii., 10, 656a, trans. Ogle), "Plants, again,
inasmuch as they are without locomotion, present no great variety in their
heterogeneous parts. For, where the functions are but few, few also are the
organs required to effect them.... Animals, however, that not only live but
feel, present a greater multiformity of parts, and this diversity is greater in
some animals than in others, being most varied in those to whose share has
fallen not mere life but life of high degree. Now such an animal is man."
With the great exception of Aristotle, the philosophers of Greece and Rome
made little contribution to morphological theory. Passing mention may be
made of the Atomists—Leucippus, Democritus, and their great disciple
Lucretius, who in his magnificent poem "De Natura Rerum" gave
impassioned expression to the materialistic conception of the universe. But
the full effect of materialism upon morphology does not become apparent
till the rise of physiology in the 17th and 18th centuries, and reaches its
culmination in the 19th century. The evolutionary ideas of Lucretius
exercised no immediate influence upon the development of morphology.
[1] E. Zeller, Greek Philosophy, Eng. trans., i., 522 f.n., London 1881. Other
particulars as to Alcmaeon in T. Gomperz, Greek Thinkers, Eng. trans., i.,
London, 1901.
[2] Zeller, loc. cit., i., p. 297.

[3] Gomperz, loc. cit., i., p. 244.
[4] R. Burckhardt, Biologie u. Humanismus, p. 85, Jena, 1907.
[5] See the interesting account of Aristotle's biological work in Prof. D'Arcy W.
Thompson's Herbert Spencer lecture (1913) and his translation of the Historia
Animalium in the Oxford series.
[6] On Aristotle's forerunners, see R. Burckhardt, "Das koïsche Tiersystem, eine
Vorstufe des zoologischen Systematik des Aristoteles." Verh. Naturf. Ges. Basel,
xx., 1904.
[7] T. E. Lones, Aristotle's Researches in Natural Science, pp. 82-3, London,
1912.

[8] De Partibus Animalium, i., 4, 644a trans. W. Ogle, Oxford, 1911.

CHAPTER II

COMPARATIVE ANATOMY BEFORE CUVIER

For two thousand years after Aristotle little advance was made upon his
comparative anatomy. Knowledge of the human body was increased not
long after his death by Herophilus and Erasistratus, but not even Galen
more than four centuries later made any essential additions to Aristotle's
anatomy.
During the Middle Ages, particularly after the introduction to Europe in the
13th century of the Arab texts and commentaries, Aristotle dominated men's
thoughts of Nature. The commentary of Albertus Magnus, based upon that
of Avicenna, did much to impose Aristotle upon the learned world. Albertus
seems to have contented himself with following closely in the footsteps of
his master. There are noted, however, by Bonnier certain improvements
made by Albertus on Aristotle's view of the seriation of living things. "He is
the first," writes Bonnier, "to take the correct view that fungi are lower
plants allied to the most lowly organised animals. From this point there
start, for Albertus Magnus, two series of living creatures, and he regards the
plant series as culminating in the trees which have well-developed flowers."
[9]

Aristotle's influence is predominant also in the work of Edward Wotton
(1492-1555), who in his book De differentiis animalium adopted a
classification similar to that proposed by Aristotle. He too laid stress upon
the gradation shown from the lower to the higher forms.
In the 16th century, two groups of men helped to lay foundations for a
future science of comparative anatomy—the great Italian anatomists
Vesalius, Fallopius and Fabricius, and the first systematists (though their
"systems" were little more than catalogues) Rondeletius, Aldrovandus and
Gesner.
The anatomists, however, took little interest in problems of pure
morphology; the anatomy of the human body was for them simply the
necessary preliminary of the discovery of the functions of the parts—they
were quite as much physiologists as anatomists.
One of them, Fabricius, made observations on the development of the chick
(1615). Harvey, who was a pupil of Fabricius, likewise published an
account of the embryology of the chick.[10] In his philosophy and habit of
thought Harvey was a follower of Aristotle. It is worth noting that in his
Exercitationes anatomicae de motu cordis (1628) there is a passage which
dimly foreshadows the law of recapitulation in development which later had
so much vogue.[11]
A stimulating contribution to comparative anatomy was made by Belon,[12]
who published in 1555 a Histoire de la nature des Oyseaux, in which he
showed opposite one another a skeleton of a bird and of a mammal, giving
the same names to homologous bones. The anatomy of animals other than
man was indeed not altogether neglected at this time. Coiter (1535-1600)
studied the anatomy of Vertebrates, discovering among other things the
fibrous structure of the brain. Carlo Ruini of Bologna wrote in 1598 a book
on the anatomy of the horse.[13] Somewhat later Severino, professor at
Naples, dissected many animals and came to the conclusion that they were
built upon the same plan as man.[14] Willis, of Oxford and London, in his
Cerebri Anatome (1659) recognised the necessity for comparative study of
the structure of the brain. He found out that the brain of man is very like
that of other mammals, the brain of birds, on the contrary, like that of

fishes![15] He described the anatomy of the oyster and the crayfish. He had,
however, not much feeling for morphology.
The foundation of the Jardin des Plantes at Paris in 1626 and the subsequent
addition to it of a Museum of Natural History and a menagerie gave a great
impulse to the study of comparative anatomy by supplying a rich material
for dissection. Advantage was taken of these facilities, particularly by
Claude Perrault and Duverney.[16] In a volume entitled De la Mécanique
des Animaux, Perrault recognises clearly the idea of unity of type, and even
pushes it too far, seeking to prove that in plants there exists an arterial
system and veins provided with valves.[17]
The beginning of the 17th century saw the invention of the microscope,
which was to have such an enormous influence upon the development of
biological studies. It did not come into scientific use until well on in the
middle of the century. Just before it came into use Francis Glisson (1597-
1677), an Englishman, gave in the introduction to his treatise on the liver an
account of the notions then current on the structure of organic bodies. He
classifies the parts as "similar" and "organic," the former determined by
their material, the latter by the form which they assume. The similar parts
are divided into the sanguineous or rich in blood and the spermatic. Both
sets are further subdivided according to their physical characters,[18] the
latter, for instance, into the hard, soft, and tensile tissues. The classification
resembles greatly that propounded by Aristotle, though it is notably inferior
in the details of its working out.
For Aristotle, as for all anatomists before the days of the microscope, the
tissues were not much more than inorganic substances, differing from one
another in texture, in hardness, and other physical properties. They
possessed indeed properties, such as contractility, which were not inorganic,
but as far as their visible structure was concerned there was little to raise
them above the inorganic level. The application of the microscope changed
all that, for it revealed in the tissues an organic structure as complex in its
grade as the gross and visible structure of the whole organism. Of the four
men who first made adequate use of the new aid, Malpighi, Hooke,
Leeuenhoek, and Swammerdam, the first-named contributed the most to
make current the new conceptions of organic structure. He studied in some
detail the development of the chick. He described the minute structure of

the lungs (1661), demonstrating for the first time, by his discovery of the
capillaries, the connection of the arteries with the veins. In his work, De
viscerum structura (1666), he describes the histology of the spleen, the
kidney, the liver, and the cortex of the brain, establishing among other
things the fact that the liver was really a conglomerate gland, and
discovering the Malpighian bodies in the kidney. This work was done on a
broad comparative basis. "Since in the higher, more perfect, red-blooded
animals, the simplicity of their structure is wont to be involved by many
obscurities, it is necessary that we should approach the subject by the
observation of the lower, imperfect animals."[19] So he wrote in the De
viscerum structura, and accordingly he studied the liver first in the snail,
then in fishes, reptiles, mammals, and finally man. In the introduction to his
Anatome plantarum (1675), in which he laid the foundations of plant
histology, he vindicates the comparative method in the following words:
—"In the enthusiasm of youth I applied myself to Anatomy, and although I
was interested in particular problems, yet I dared to pry into them in the
higher animals. But since these matters enveloped in peculiar mystery still
lie in obscurity, they require the comparison of simpler conditions, and so
the investigation of insects[20] at once attracted me; finally, since this also
has its own difficulties I applied my mind to the study of plants, intending
after prolonged occupation with this domain, to retrace my steps by way of
the vegetable kingdom, and get back to my former studies. But perhaps not
even this will be sufficient; since the simpler world of minerals and the
elements should have been taken first. In this case, however, the
undertaking becomes enormous and far beyond my powers."[21] There is
something fine in this life of broad outlines, devoted whole-heartedly to an
idea, to a plan of research, which required a lifetime to carry out.
An important histological discovery dating from this time is that of the finer
structure of muscle, made by Stensen (or Steno) in 1664. He described the
structure of muscle-fibres, resolving them into their constituent fibrils.
To the microscope we owe not only histology but the comparative anatomy
of the lower animals. Throughout the 17th and 18th centuries the discovery
of structure in the lower animals went on continuously, as may be read in
any history of Zoology.[22] We content ourselves here with mentioning only
some representative names.

In the 17th century Leeuenhoek, applying the microscope almost at random,
discovered fact after fact, his most famous discovery being that of the
"spermatic animalcules."
Swammerdam studied the metamorphoses of insects and made wonderfully
minute dissections of all sorts of animals, snails and insects particularly. He
described also the development of the frog. It is curious to see what a grip
his conception of metamorphosis had upon him when he homologises the
stages of the frog's development with the Egg, the Worm, and the Nymph of
insects (Book of Nature, p. 104, Eng. trans., 1785). He even speaks of the
human embryo as being at a certain stage a Man-Vermicle.
In the 18th century, Réaumur and Bonnet continued the minute study of
insects, laying more stress, however, on their habits and physiology than
upon their anatomy. Lyonnet made a most laborious investigation of the
anatomy of the willow-caterpillar (1762). John Hunter (1728-93) dissected
all kinds of animals, from holothurians to whales. His interest was,
however, that of the physiologist, and he was not specially interested in
problems of form. It is interesting to note a formulation in somewhat
confused language of the recapitulation theory. The passage occurs in his
description of the drawings he made to illustrate the development of the
chick. It is quoted in full by Owen (J. Hunter, Observations on certain Parts
of the Animal Œconomy, with Notes by Richard Owen. London, 1837.
Preface, p. xxvi). We give here the last and clearest sentence—"If we were
to take a series of animals from the more imperfect to the perfect, we should
probably find an imperfect animal corresponding with some stage of the
most perfect."
The tendency of the time was not towards morphology, but rather to general
natural history and to systematics, the latter under the powerful influence of
Linnæus (1707-1778). The former tendency is well represented by Réaumur
(1683-1757) with his observations on insects, the digestion of birds, the
regeneration of the crayfish's legs, and a hundred other matters. To this
tendency belong also Trembley's famous experiments on Hydra (1744), and
Rösel von Rosenhof's Insektenbelustigungen (1746-1761).
Bonnet (1720-1793) deserves special mention here, since in his Traité
d'Insectologie (1745), and more fully in his Contemplation de la Nature

(1764), he gives the most complete expression to the idea of the Échelle des
êtres.
This idea seems to have taken complete possession of his imagination. He
extends it to the universe. Every world has its own scale of beings, and all
the scales when joined together form but one, which then contains all the
possible orders of perfection. At the end of the Preface to his Traité
d'Insectologie (Œuvres, i., 1779) he gives a long table, headed "Idée d'une
Échelle des êtres naturels," and rather resembling a ladder, on the rungs of
which the following names appear:—
Man. Shell Fish. Stones.
Orang-utan. Tube-worms. Figured stones.
Ape. Clothes-worms. Crystals.

Quadrupeds. Insectes. Salts.
Flying squirrel. Gall insectes. Vitriols.
Bat. Taenia.
Ostrich. Polyps. Metals.
Sea Nettles.
Birds. Sensitive plant. Half-metals.
Aquatic birds.
Amphibious birds. Plants. Sulphurs.
Flying Fish. Lichens. Bitumens.
Moulds.
Fish. Fungi, Agarics. Earths.
Creeping fish. Truffles. Pure earth.
Eels. Corals, and Coralloids.
Water sepents. Lithophytes. Water.
Asbestos.
Serpents. Talc, Gypsums. Air.
Slugs. Selenites, Slates.
Snails. Fire.

More subtile matter.

The nature of the transitional forms which he inserts between his principal
classes show very clearly his entire lack of morphological insight—the
transitions are functional. The positions assigned to clothes-moths and
corals are very curious! The whole scheme, so fantastic in its details, was
largely influenced by Leibniz's continuity philosophy, and is in no way an
improvement on the older and saner Aristotelian scheme.
Robinet, in the fifth volume of his book De la nature (1761-6), foreshadows
the somewhat similar views of the German transcendentalists. "All beings,"
he writes, "have been conceived and formed on one single plan, of which
they are the endlessly graduated variations: this prototype is the human
form, the metamorphoses of which are to be considered as so many steps
towards the most excellent form of being."[23]
The idea of a gradation of beings appears also in Buffon (1707-1788), but
here it takes more definitely its true character as a functional gradation.[24]
"Since everything in Nature shades into everything else," he says, "it is
possible to establish a scale for judging of the degrees of the intrinsic
qualities of every animal."[25]
He is quite well aware that the groups of Invertebrates are different in
structural plan from the Vertebrates—"The animal kingdom includes
various animated beings, whose organisation is very different from our own
and from that of the animals whose body is similarly constructed to ours."
[26]

He limits himself to a consideration of the Vertebrates, deeming that the
economy of an oyster ought not to form part of his subject matter! He has a
clear perception of the unity of plan which reigns throughout the vertebrate
series.[27] What is new in Buffon is his interpretation of the unity of plan.
For the first time we find clearly expressed the thought that unity of plan is
to be explained by community of origin.
Buffon's utterances on this point are, as is well known, somewhat
vacillating. The famous passage, however, which occurs in his account of
the Ass shows pretty clearly that Buffon saw no theoretical objection to the
descent of all the varied species of animals from one single form. Once

admit, he argues, that within the bounds of a single family one species may
originate from the type species by "degeneration," then one might
reasonably suppose that from a single being Nature could in time produce
all the other organised beings.[28] Elsewhere, e.g., in the discourse De la
Dégéneration des Animaux,[29] Buffon expresses himself with more
caution. He finds that it is possible to reduce the two hundred species of
quadrupeds which he has described to quite a small number of families
"from which it is not impossible that all the rest are derived."[30] Within
each of the families the species branch off from a parent or type species.
This we may note is a great advance on the linear arrangement implied in
the idea of an Échelle des êtres.[31]
It is a mistake to suppose that Buffon was par excellence a maker of
hypotheses. On the contrary he saw things very sanely and with a very open
mind. He expressly mentions the great difficulties which one encounters in
supposing that one species may arise from another by "degeneration." How
does it happen that two individuals "degenerate" just in the right direction
and to the right stage so as to be capable of breeding together? How is it
that one does not find intermediate links between species? One is reminded
of the objections, not altogether without validity, which were made to the
Darwinian theory in its early days. I cannot agree with those who think that
Buffon was an out-and-out evolutionist, who concealed his opinions for fear
of the Church. No doubt he did trim his sails—the palpably insincere "Mais
non, il est certain, par la révélation, que tous les animaux ont également
participé à la grace de la création,"[32] following hard upon the too bold
hypothesis of the origin of all species from a single one, is proof of it. But
he was too sane and matter-of-fact a thinker to go much beyond his facts,
and his evolution doctrine remained always tentative. One thing, however,
he was sure of, that evolution would give a rational foundation to the
classification which, almost in spite of himself, he recognised in Nature. If,
and only if, the species of one family originated from a single type species,
could families, be founded rationally, avec raison.
Buffon was, curiously enough, rather unwilling to recognise any systematic
unit higher than the species. Strictly speaking there are only individuals in
Nature; but there are also groups of individuals which resemble one another
from generation to generation and are able to breed together. These are

species—Buffon adheres to the genetic definition of species—and the
species is a much more definite unit than the genus, the order, the class,
which are not divisions imposed by us upon Nature. Species are definitely
discontinuous,[33] and this is the only discontinuity which Nature shows us.
Buffon put his views into practice in his Histoire Naturelle, where he
describes species after species, never uniting them into larger groups. We
have seen, however, how the facts forced upon him the conception of the
"family."
Buffon was no morphologist. He left to Daubenton what one might call the
"dirty work" of his book, the dissection and minute description of the
animals treated.
But Buffon was a man of genius, and accordingly his ideas on morphology
are fresh and illuminating. Few naturalists have been so free from the
prejudices and traditions of their trade. He makes in the Discours sur la
Nature des Animaux[34] a distinction, which Bichat and Cuvier later
developed with much profit, between the "animal" and the "vegetative" part
of animals.[35] The vegetative or organic functions go on continuously, even
in sleep, and are performed by the internal organs, of which the heart is the
central one. The active waking life of the animal, that part of its life which
distinguishes it from the plant, involves the external parts—the sense-
organs and the extremities. An animal is, as it were, made up of a complex
of organs performing the vegetative functions, assimilation, growth, and
reproduction, surrounded by an envelope formed by the limbs, the sense-
organs, the nerves and the brain, which is the centre of this "envelope."[36]
Animals may differ from one another enormously in the external parts,
particularly in the appendicular skeleton, without showing any great
difference in the plan and arrangement of their internal organs. Quadrupeds,
Cetacea, birds, amphibians and fish are as unlike as possible in external
form and in the shape of their limbs; but they all resemble one another in
their internal organs. Let the internal organs change, however—the external
parts will change infinitely more, and you will get another animal, an
animal of a totally different nature. Thus an insect has a most singular
internal economy, and, in consequence, you find it is in every point
different from any vertebrate animal.

In this contrast, on the whole justified, between the importance of variations
in the "vegetative" and variations in the "animal" parts, one may see
without doing violence to Buffon's thought, an indication of the difference
between homology and analogy. It is usually in the external parts, in the
organs by which the animal adapts itself to its environment, that one meets
with the greatest number of analogical resemblances. This contrast of
vegetative and animal parts and their relative importance for the discovery
of affinities was at any rate a considerable step towards an analysis of the
concept of unity of plan.
To Xavier Bichat (1771-1802) belongs the credit of working out in detail
the distinction drawn by Aristotle and Buffon between the animal and the
vegetative functions. Bichat was not a comparative anatomist; his interest
lay in human anatomy, normal and pathological. So his views are drawn
chiefly from the consideration of human structure.
He classifies functions into those relating to the individual and those
relating to the species. The functions pertaining to the individual may be
divided into those of the animal and those of the organic life.[37] "I call
animal life that order of functions which connects us with surrounding
bodies; signifying thereby that this order belongs only to animals"
(p. lxxviii.). Its organs are the afferent and efferent nerves, the brain, the
sense-organs and the voluntary muscles; the brain is its central organ.
"Digestion, circulation, respiration, exhalation, absorption, secretion,
nutrition, calorification, or production of animal heat, compose organic life,
whose principal and central organ is the heart" (p. lxxix.).
The contrast of the animal and the organic life runs through all Bichat's
work; it receives classical expression in his Recherches Physiologiques sur
la Vie et la Mort (1800). The plant and the animal stand for two different
modes of living. The plant lives within itself, and has with the external
world only relations of nutrition; the animal adds to this organic life a life of
active relation with surrounding things (3rd ed., 1805, p. 2). "One might
almost say that the plant is the framework, the foundation of the animal, and
that to form the animal it sufficed to cover this foundation with a system of
organs fitted to establish relations with the world outside. It follows that the
functions of the animal form two quite distinct classes. One class consists in
a continual succession of assimilation and excretion; through these

functions the animal incessantly transforms into its own substance the
molecules of surrounding bodies, later to reject these molecules when they
have become heterogeneous to it. Through this first class of functions the
animal exists only within itself; through the other class it exists outside; it is
an inhabitant of the world, and not, like the plant, of the place which saw its
birth. The animal feels and perceives its surroundings, reflects its
sensations, moves of its own will under their influence, and, as a rule, can
communicate by its voice its desires and its fears, its pleasures or its pains. I
call organic life the sum of the functions of the former class, for all
organised creatures, plants or animals, possess them to a more or less
marked degree, and organised structure is the sole condition necessary to
their exercise. The combined functions of the second class form the 'animal'
life, so named because it is the exclusive attribute of the animal kingdom"
(pp. 2-3).
In both lives there is a double movement, in the animal life from the
periphery to the centre and from the centre to the periphery, in the organic
life also from the exterior to the interior and back again, but here a
movement of composition and decomposition. As the brain mediates
between sensation and motion, so the vascular system is the go-between of
the organs of assimilation and the organs of dissimilation.
The most essential structural difference between the organs of animal life
and the organs of organic life is, in man and the higher animals at least, the
symmetry of the one set and the irregularity of the other—compare the
symmetry of the nerves and muscles of the animal life with the
asymmetrical disposition of the visceral muscles and the sympathetic
nerves, which belong to the organic life.
Noteworthy differences exist between the two lives with respect to the
influence of habit. Everything in the animal life is under the dominion of
habit. Habit dulls sensation, habit strengthens the judgment. In the organic
life, on the contrary, habit exercises no influence. The difference comes out
clearly in the development of the individual. The organs of the organic life
attain their full perfection independently of use; the organs of the animal
life require an education, and without education they do not reach
perfection (loc. cit., p. 127).

Bichat was the founder of what was known for a time as General Anatomy
—the study of the constituent tissues of the body in health and disease. His
classification of tissues was macroscopical and physiological; he relied
upon texture and function in distinguishing them rather than upon
microscopical structure. The tissues he distinguished are as follows:—[38]
1. The cellular membrane. 12. Fibro-cartilage.
2. Nerves of animal life. 13. Muscles of organic life.
3. Nerves of organic life. 14. Muscles of animal life.
4. Arteries. 15. Mucous membrane.
5. Veins. 16. Serous membrane.
6. Exhalants. 17. Synovial membrane.
7. Absorbents and glands. 18. The Glands.
8. Bones. 19. The Dermis.
9. Medulla. 20. Epidermis.
10. Cartilage. 21. Cutis.
11. Fibrous tissue.
The "cellular membrane" seems to mean undifferentiated connective tissue;
"exhalants" are imperceptible tubes arising from the capillaries and
secreting fat, serum, marrow, etc.; the "absorbents and glands" are the
lymphatics and the lymphatic glands.
In Bichat's eyes this resolution of the organism into tissues had a deeper
significance than any separation into organs, for to each tissue must be
attributed a vie propre, an individual and peculiar life. "When we study a
function we must consider the complicated organ which performs it in a
general way; but if we would be instructed in the properties and life of that
organ we must absolutely resolve it into its constituent parts."[39] The
tissues have, too, a great importance for pathology, for diseases are often
diseases of tissues rather than of organs.[40]
[9] Le Monde végétal, p. 41, Paris, 1907.
[10] Exercitationes de generatione animalium, 1651. For an account of Harvey's
work on generation and development, see Em. Rádl's masterly Geschichte der
biologischen Theorien, i., pp. 31-8, Leipzig, 1905.
[11] The passage runs:—"Sic natura perfecta et divina nihil faciens frustra, nec
quipiam animali cor addidit, ubi non erat opus, neque priusquam esset ejus usus,

fecit; sed iisdem gradibus in formatione cujuscumque animalis, transiens per
omnium animalium constitutiones (ut ita dicam) ovum, vermem, fœtum,
perfectionem in singulis acquirit."
[12] See I. Geoffroy St Hilaire, Essais de Zoologie générale, p. 71, Paris, 1841.
[13] M. Foster, Lectures on the History of Physiology, Cambridge, p. 53, 1901.
[14] Zootomia democritea, Nuremberg, 1645; Antiperipatias, seu de respiratione
piscium, Amsterdam, 1661.
[15] Rádl, loc. cit., i., p. 50.
[16] Perrault et Duverney, Mémoires pour servir à l'histoire des Animaux, Paris,
1699.
[17] F. Houssay, Nature et Sciences naturelles, Paris, p. 76, n.d.
[18] Foster, loc. cit., p. 85.
[19] Trans. by Foster, loc. cit., p. 113.
[20] He made a careful study of the silkworm.
[21] "Etenim, ferventi actatis calore, Anatomica aggressus, licet circa peculiaria
fuerim solicitus, in perfectioribus tamen haec rimari sum ausus. Verum, cum
haec propriis tenebris obscura jaceant, simplicium analogismo egent; inde
insectorum indago illico arrisit; quae cum et ipsa suas habeat difficultates ad
Plantarum perquisitionem animum postremo adjeci, ut diu hoc lustrato mundo
gressu retroacto Vegetantis Naturae gradu, ad prima studia iter mihi aperirem.
Sed nec forte hoc ipsum sufficiet cum simplicior Mineralium Elementorumque
mundus praeire debeat. At in immensum excrescit opus, et meis viribus omnino
impar," Opera Omnia, i., p. 1, London, 1686.
[22] See particularly E. Rádl, loc. cit.. 1 Teil. J. V.. Carus, Geschichte der
Zoologie, München, 1872.
[23] For a good historical account of the gradation theories see Thienemann's
paper in the Zoologische Annalen (Würzburg) iii., pp. 185-274, 1910, from
which the quotation from Robinet is taken.
[24] Histoire naturelle, i., p. 13; ii, p. 9; iv., p. 101; and xiv., pp. 28-9, 1749 and
later.
[25] No translation can render the beauty of the original—"Comme tout se fait et
que tout est par nuance dans la Nature ..." (iv., p. 101).
[26] Hist. nat., iv., p. 5.
[27] See particularly his comparison of the skeleton of the horse with that of
man. Hist. Nat., iv., p. 381, also p. 13.
[28] Loc. cit., p. 382.
[29] Tome xiv., pp. 311-374.
[30] Tome xiv., p. 358.

[31] See also "Oiseaux," Tome i., pp. 394, 395. Pallas in 1766 adopted for the
whole animal kingdom this branching arrangement.
[32] "But this cannot be, for it is certain by revelation that all animals have
equally participated in the grace of creation."
[33] iv., p.385.
[34] iv., pp. 3-110.
[35] It has been revived in our own days by Bergson, Matière et Mémoire, p. 57.
[36] iv., pp. 7-15.
[37] Anatomie Générale, Paris, 1801, Eng. trans. 1824.
[38] Anatomie Générale, Eng. trans., i., p. lii.
[39] Anatomie Générale, Eng. trans., i., p. lviii.
[40] Loc cit., i., sect. vii.

CHAPTER III

CUVIER

Cuvier was perhaps the greatest of comparative anatomists; his work is, in
the best sense of the word, classical.
Like all his predecessors, like Aristotle, like the Italian anatomists, Cuvier
studied structure and function together, even gave function the primacy.

Some functions, he says,[41] are common to all organised bodies—origin by
generation, growth by nutrition, end by death. There are also secondary
functions. Of these the most important, in animals at least, are the faculties
of feeling and moving. These two faculties are necessarily bound up
together; if Nature has given animals sensation she must also have given
them the power of movement, the power to flee from what is harmful and
draw near to what is good. These two faculties determine all the others. A
creature that feels and moves requires a stomach to carry food in. Food
requires instruments to divide it, liquids to digest it. Plants, which do not
feel and do not move, have no need of a stomach, but have roots instead.

Thus the "Animal Functions" of feeling and moving determine the character
of the organs of the second order, the organs of digestion. These in their
turn are prior to the organs of circulation, which are a means to the end of
distributing the nutrient fluid or blood to all parts of the body. These organs
of the third order are not only dependent on those of the second order, but
are also not even necessary, for many animals are without them. Only
animals with a circulatory system can have definite breathing organs—
lungs or gills. Plants, and animals without a circulation, breathe by their
whole surface.
There is accordingly a rational order of functions, and therefore of the
systems of organs which perform them. The most important are the Animal
Functions, with their great organ-system, the neuro-muscular mechanism.
Then come the digestive functions, and after them, and in a sense accessory
to them, the functions and organs of circulation and respiration. The last
three may be grouped as the Vital Functions.
The Animal Functions not only determine the character of the Vital
Functions, but influence also the primary faculty of generation, for animals'
power of movement has rendered their mode of fecundation more simple,
has therefore had an effect on their organs of generation.
This division into "Animal" and "Vital" functions recalls Buffon's and
Bichat's distinction of the "animal" and the "vegetative" lives. Cuvier
apparently took this idea from Buffon, for he says that a plant is an animal
that sleeps.[42] But the idea is as old as Aristotle, who discusses the "sleep"
of embryos and of plants in the last book of the De Generatione animalium.
The distinction between animal and vegetative life is, of course, based for
Aristotle in the difference between the ψυχή ἀισθητική and the ψυχή
θρέπτική. Cuvier, like Aristotle, Buffon, and Bichat, makes the heart the
centre of the "vegetative" organs.
It is important to note that Cuvier puts function before structure, and infers
from function what the organ will be. "Plants," he writes, "having few
faculties, have a very simple organisation."[43] It is only after having
discussed and classified functions that Cuvier goes on to examine organs.
First his views on the composition of the animal body. Aristotle
distinguished three degrees of composition—the "elements," the

homogeneous parts, and the heterogeneous parts or organs. Cuvier does the
same. Some small advance has been made in the two thousand years'
interval, due in the first place to the progress of chemistry, and in the
second to the invention of the microscope. To the first circumstance Cuvier
owes his knowledge that the inorganic substances forming the first degree
of composition are principally C, N, H, O, and P, combined to form
albumen, fibrine, and the like, which are in their turn combined to form the
solids and fluids of the body. To the latter circumstance Cuvier owes the
statement that the finest fragments into which mechanical division can
resolve the organism are little flakes and filaments, which, joined up loosely
together, form a "cellulosity." The discovery of the true cellular nature of
animal tissues did not come till much later, till some years after Cuvier's
death in 1832. Knowledge of histological detail was, however, considerable
by the beginning of the 19th century. Cuvier knew, for example, that each
muscle fibre has its own nerve fibre. But he gives no elaborate account of
the homogeneous parts, no detailed histology. On the other hand his
treatment of the heterogeneous parts or organs is detailed and masterly.[44]
The main systems of organs are, in order of importance, the nervous and
muscular, the digestive, the circulatory, and the respiratory. Each organ or
system of organs may have many forms. If any form of any organ could
exist in combination with any form of all the others there would be an
enormous number of combinations theoretically possible. But these
combinations do not all exist in Nature, for organs are not merely
assembled (rapprochés), but act upon one another, and act all together for a
common end. Accordingly only the combinations that fulfil these conditions
exist in Nature. Cuvier thus dismisses the question of a science of possible
organic forms and considers only the forms or combinations actually
existing. This question of the possibility of a "theoretical" morphology of
living things, after the fashion of the morphology of crystals with their
sixteen possible types, was raised in later years by K. G. Carus, Bronn, and
Haeckel.
Organisms, then, are harmonious combinations of organs, and the harmony
is primarily a harmony of functions. Every function depends upon every
other, and all are necessary. The harmony of organs and their mutual
dependence are the results of the interdependence of function. This thought,
the recognition of the functional unity of the organism, is the fundamental

one at the base of all Cuvier's work. Before him men had recognised more
or less clearly the harmony of structure and function, and had based much
of their work upon this unanalysed assumption. Cuvier was the first
naturalist to raise this thought to the level of a principle peculiar to natural
history. "It is on this mutual dependence of the functions and the assistance
which they lend one to another that are founded the laws that determine the
relations of their organs; these laws are as inevitable as the laws of
metaphysics and mathematics, for it is evident that a proper harmony
between organs that act one upon another is a necessary condition of the
existence of the being to which they belong."[45]
This rational principle, peculiar to natural history, Cuvier calls the principle
of the conditions of existence, for the following reason:—"Since nothing
can exist that does not fulfil the conditions which render its existence
possible, the different parts of each being must be co-ordinated in such a
way as to render possible the existence of the being as a whole, not only in
itself, but also in its relations with other beings, and the analysis of these
conditions often leads to general laws which are as certain as those which
are derived from calculation or from experiment."[46]
By "conditions of existence" he means something quite different from what
is now commonly understood. The idea of the external conditions of
existence, the environment, enters very little into his thought. He is intent
on the adaptations of function and organ within the living creature—a point
of view rather neglected nowadays, but essential for the understanding of
living things. The very condition of existence of a living thing, and part of
the essential definition of it, is that its parts work together for the good of
the whole.
The principle of the adaptedness of parts may be used as an explanatory
principle, enabling the naturalist to trace out in detail the interdependence
of functions and their organs. When you have discovered how one organ is
adapted to another and to the whole, you have gone a certain way towards
understanding it. That is using teleology as a regulative principle, in Kant's
sense of the word. Cuvier was indeed a teleologist after the fashion of Kant,
and there can be no doubt that he was influenced, at least in the exposition
of his ideas, by Kant's Kritik der Urtheilskraft, which appeared ten years
before the publication of the Leçons d'Anatomie Comparée. Teleology in

Kant's sense is and will always be a necessary postulate of biology. It does
not supply an explanation of organic forms and activities, but without it one
cannot even begin to understand living things. Adaptedness is the most
general fact of life, and innumerable lesser facts can be grouped as
particular cases of it, can be, so far, understood.
Cuvier's famous principle of correlation, the corner-stone of his work, is
simply the practical application to the facts of structure of the principle of
functional adaptedness. By the principle of correlation, from one part of an
animal, given sufficient knowledge of the structure of its like, you can in a
general way construct the whole. "This must necessarily be so: for all the
organs of an animal form a single system, the parts of which hang together,
and act and re-act upon one another; and no modifications can appear in one
part without bringing about corresponding modifications in all the rest."[47]
The logical basis of the principle is sound. The functions of the parts are all
intimately bound up with one another, and one function cannot vary without
bringing in its train corresponding modifications in the others. Structure and
function are bound up together; every modification of a function entails
therefore the modification of an organ. Hence from the shape of one organ
you can infer the shape of the other organs—if you have sufficiently
extensive empirical knowledge of functions, and of the relation of structure
to function in each kind of organ. Given an alimentary canal capable of
digesting only flesh, and possessing therefore a certain form, you know that
the other functions must be adapted to this particular function of the
alimentary canal. The animal must have keen sight, fine smell, speed,
agility, and strength in paws and jaws. These particular functions must have
correspondingly modified organs, well-developed eyes and ears, claws and
teeth. Further, you know from experience that such and such definitely
modified organs are invariably found with the carnivorous habit, carnassial
teeth, for example, and reduced clavicles. From a "carnivorous" alimentary
canal, then, you can infer with certainty that the animal possessed carnassial
teeth and the other structural peculiarities of carnivorous animals, e.g., the
peculiar coronoid process of the mandible. From the carnassial tooth you
can infer the reduced clavicle, and so on. "In a word, the form of the tooth
implies the form of the condyle; that of the shoulder blade that of the claws,
just as the equation of a curve implies all its properties."[48]

Similarly the great respiratory power of birds is correlated with their great
muscular strength, and renders necessary great digestive powers. Hence the
correlated structure of lungs, muscles and their attachments, and alimentary
canal, in birds.
Not only do systems of organs, by being adjusted to special modifications
of function, influence one another, but so also do parts of the same organ.
This is noticeably the case with the skeleton, where hardly a facet can vary
without the others varying proportionately, so that from one bone you can
up to a certain point deduce all the rest.
We deduce the necessity, the constancy, of these co-existences of organs
from the observed reciprocal influence of their functions. That being
established, we can argue from observed constancy of relation between two
organs an action of one upon the other, and so be led to a discovery of their
functions. But even if we do not discover the functional interdependencies
of the parts, we can use the established fact of the constant co-existence of
two parts as proof of a functional correlation between them.
Correlation is either a rational or an empirical principle, according as we
know or do not know the interdependence of function of which it is the
expression. Even when we apply the rational principle of correlation it
would be useless in our hands if we had not extensive empirical knowledge;
when we use an empirical rule of correlation we depend entirely upon
observation. "There are a great many cases," writes Cuvier,[49] "where our
theoretical knowledge of the relations of forms would not suffice, if it were
not filled out by observation," that is to say, there are many cases of
correlation not yet explicable in terms of function. From a hoof you can
deduce the main characters of herbivores (with a certain amount of
assistance from your empirical knowledge of herbivores), but could you
from a cloven hoof deduce that the animal is a ruminant, unless you had
observed the constancy of relation, not directly explicable in terms of
function, between cloven hoofs and chewing the cud? Or could you deduce
from the existence of frontal horns that the animal ruminates?
"Nevertheless, since these relations are constant, they must necessarily have
a sufficient cause; but as we are ignorant of this cause, observation must
supplement theory; observation establishes empirical laws which become
almost as certain as the rational laws, when they are based upon a sufficient

number of observations.... But that there exist all the same hidden reasons
for all these relations is partly revealed by observation itself, independently
of general philosophy."[50] That is to say, even correlations for which no
explanation in terms of function can be supplied are probably in reality
functional correlations. This may, in some cases, be inferred from the
graded correspondence of two sets of organs. For example, ungulates which
do not ruminate, and have not a cloven hoof, have a more perfect dentition
and more bones in the foot than the true cloven-hoofed ruminants. There is
a correlation between the state of development of the teeth and of the foot.
This correlation is a graded one, for camels, which have a more perfect
dentition than other ruminants, have also a bone more in their tarsus. It
seems probable, therefore, that there is some reason, that is, some
explanation in terms of function, for this case of correlation.
Nevertheless, the fact remains that many correlations are not explicable in
terms of function, and the substitution of correlation as an empirical
principle for correlation as a rational principle marks for Cuvier a step away
from his functional comparative anatomy towards a pure morphology. It is
significant that in later times the term correlation has come to be applied
more especially to the purely empirical constancies of relation, and has lost
most of its functional significance. But the correlation of the parts of an
organism is no mere mathematical concept, to be expressed by a coefficient,
but something deeper and more vital.
Cuvier interpreted the functional dependence of the parts in terms of what
we now call the general metabolism. He had a clear vision of the constant
movement of molecules in the living tissue, combining and recombining, of
the organism taking in and intercalating molecules from outside from the
food and rejecting molecules in the excretions, a ceaseless tourbillon vital.
"This general movement, universal in every part, is so unmistakably the
very essence of life that parts separated from a living body straightway die."
[51]
The organisation of the body, the arrangement of its solids and liquids,
is adapted to further the tourbillon vital. "Each part contributes to this
general movement its own particular action and is affected by it in
particular ways, with the result that, in every being, life is a unity which
results from the mutual action and reaction of all its parts."[52]

Cuvier, however, did not resolve life into metabolism, nor reduce vital
happenings to the chemical level. The form of organised bodies is more
essential than the matter of which they are composed, for the matter
changes ceaselessly while the form remains unchanged. It is in form that we
must seek the differences between species, and not in the combinations of
matter, which are almost the same in all.[53] The differences are to be sought
at the level of the second and third degrees of composition.
The existence of differences of form introduces a new problem, the problem
of diversity. There are only a few possible combinations of the principal
organs, but as you get down to less important parts the possible scope of
variation is greatly increased, and most of the possible variations do exist.
Nature seems prodigal of form, of form which needs not to be useful in
order to exist. "It needs only to be possible, i.e., of such a character that it
does not destroy the harmony of the whole."[54] We seize here the relation
of the principle of the adaptedness of parts to the problem of the variety of
form. The former is in a sense a regulative and conservative principle which
lays down limits beyond which variation may not stray. In itself it is not a
fountain of change; there must be another cause of change. This thought is
of great importance for theories of descent.
Cuvier has no theory to account for the variety of form: he contents himself
with a classification. There are two main ways of classifying forms; you
may classify according to single organs or according to the totality of
organs. By the first method you can have as many classifications as you
have organs, and the classifications will not necessarily coincide. Thus you
can divide animals according to their organs of digestion into two classes,
those in which the alimentary canal is a sac with one opening (zoophytes)
and those in which the canal has two openings,[55] a curious forestalment, in
the rough, of the modern division of Metazoa into Cœlentera and
Cœlomata.
It is only by taking single organs that you can arrange animals into long
series, and you will have as many series as you take organs. Only in this
way can you form any Échelle des êtres or graded series; and you can get
even this kind of gradation only within each of the big groups formed on a
common plan of structure; you can never grade, for example, from

Invertebrates to Vertebrates through intermediate forms[56] (which is
perfectly true, in spite of Amphioxus and Balanoglossus!).
In the Règne Animal Cuvier restricts the application of the idea of the
Échelle within even narrower limits, refusing to admit its validity within the
bounds of the vertebrate phylum, or even within the vertebrate classes. This
seems, however, to refer to a seriation of whole organisms and not of
organs, so that the possibility of a seriation of organs within a class is not
denied. Cuvier was, above all, a positive spirit, and he looked askance at all
speculation which went beyond the facts. "The pretended scale of beings,"
he wrote, "is only an erroneous application to the totality of creation of
partial observations, which have validity only when confined to the sphere
within which they were made."[57] This remark, which is after all only just,
perfectly expresses Cuvier's attitude to the transcendental theories, and was
probably a protest against the sweeping generalisations of his colleague,
Etienne Geoffroy St Hilaire.
A true classification should be based upon the comparison of all organs, but
all organs are not of equal value for classification, nor are all the variations
of each organ equally important. In estimating the value of variations more
stress should be laid on function than on form, for only those variations are
important which affect the mode of functioning. These are the principles on
which Cuvier bases the classification of animals given in the Leçons,
Article V., "Division des animaux d'après l'ensemble de leur organisation."
The scheme of classification actually given in the Leçons recalls curiously
that of Aristotle, for there is the same broad division into Vertebrates, with
red blood, and Invertebrates, almost all with white blood. Nine classes
altogether are distinguished—Mammals, Birds, Reptiles, Fishes, Molluscs,
Crustacea, Insects, Worms, Zoophytes (including Echinoderms and
Cœlenterates).
A maturer theory and practice of classification is given in the Règne Animal
of seventeen years later. Here the principle of the subordination of
characters (which seems to have been first explicitly stated by the younger
de Jussieu in his Genera Plantarum, 1789,[58]) is more clearly recognised.
The properties or peculiarities of structure which have the greatest number
of relations of incompatibility and coexistence, and therefore influence the
whole in the greatest degree, are the important or dominating characters, to

which the others must be subordinated in classification. These dominant
characters are also the most constant.[59] In deciding which characters are
the most important Cuvier makes use of his fundamental classification of
functions and organs into two main sets. "The heart and the organs of
circulation are a kind of centre for the vegetative functions, as the brain and
the spinal cord are for the animal functions."[60] These two organ-systems
vary in harmony, and their characters must form the basis for the
delimitation of the great groups. Judged by this standard there are four
principal types of form,[61] of which all the others are but modifications.
These four types are Vertebrates, Molluscs, Articulates, and Radiates. The
first three have bilateral, the last has radial symmetry. Vertebrates and
Molluscs have blood-vessels, but Articulates show a functional transition
from the blood-vessel to the tracheal system. Radiates approach the
homogeneity of plants; they appear to lack a distinct nervous system and
sense organs, and the lowest of them show only a homogeneous pulp which
is mobile and sensitive. All four classes are principally distinguished from
one another by the broad structural relations of their neuromuscular system,
of the organs of the animal functions. Vertebrates have a spinal cord and
brain, an internal skeleton built on a definite plan, with an axis and
appendages; in Molluscs the muscles are attached to the skin and the shell,
and the nervous system consists of separate masses; Articulates have a hard
external skeleton and jointed limbs, and their nervous system consists of
two long ventral cords; Radiates have ill-defined nervous and muscular
systems, and in their lowest forms possess the animal functions without the
animal organs.
This well-rounded classification of animal forms is in a sense the crown of
Cuvier's work, for the principle of the subordination of characters, in the
interpretation which he gives to it, is a direct application of his principle of
functional correlation. Each of the great groups is built upon one plan. The
idea of the unity of plan has become for Cuvier a commonplace of his
thought, and it is tacitly recognised in all his anatomical work. But he never
takes it as a hard-and-fast principle which must at all costs be imposed upon
the facts.
Cuvier has become known as the greatest champion of the fixity of species,
but it is not often recognised that his attitude to this problem is at least as

scientific as that of the evolutionists of his own and later times. No doubt he
became dogmatic in his rejection of evolution-theory, but he was on sure
ground in maintaining that the evolutionists of his day went beyond their
facts. He considered that certain forms (species) have reproduced
themselves from the origin of things without exceeding the limits of
variation. His definition of a species was, "the individuals descended from
one another or from common parents, together with those that resemble
them as much as they resemble one another."[62] "These forms are neither
produced nor do they change of themselves; life presupposes their
existence, for it cannot arise save in organisations ready prepared for it."[63]
He based his rejection of all theories of descent upon the absence of definite
evidence for evolution. If species have gradually changed, he argued, one
ought to find traces of these gradual modifications.[64] Palæontology does
not furnish such traces. Again, the limits of variation, even under
domestication, are narrow, and the most extreme variation does not
fundamentally alter the specific type. Thus the dog has varied perhaps most
of all, in size, in shape, in colour. "But throughout all these variations the
relations of the bones remain the same, and the form of the teeth never
changes to an appreciable extent; at most there are some individuals in
which an additional false molar develops on one side or the other."[65] This
second objection is the objection of the morphologist. It would be an
interesting study to compare Cuvier's views on variation with those of
Darwin, who was essentially a systematist.
Cuvier's first objection was of course determined to some extent by the
imperfection of the palæontological knowledge of his time. But even at the
present day the objection has a certain force, for although we have definite
evidence of many serial transformations of one species into another along a
single line, for example, Neumayr's Paludina series, yet at any one
geological level the species, the lines of descent, are all distinct from one
another.[66]
Cuvier recognised very clearly that there is a succession of forms in time,
and that on the whole the most primitive forms are the earliest to appear.
Mammals are later than reptiles, and fishes appear earlier than either. As
Depéret puts it, "Cuvier not only demonstrated the presence in the
sedimentary strata of a series of terrestrial faunas superimposed and

distinct, but he was the first to express, and that very clearly, the idea of the
gradual increase in complexity of these faunas from the oldest to the most
recent" (p. 10).
He did not believe that the fauna of one epoch was transformed into the
fauna of the next. He explained the disappearance of the one by the
hypothesis of sudden catastrophes, and the appearance of the next by the
hypothesis of immigration. He nowhere advanced the hypothesis of
successive new creations. "For the rest, when I maintain that the stony
layers contain the bones of several genera and the earthy layers those of
several species which no longer exist, I do not mean that a new creation has
been necessary to produce the existing species, I merely say that they did
not exist in the same localities and must have come thither from elsewhere."
[67]
It was left to d'Orbigny to teach the doctrine of successive creations, of
which he distinguished twenty-seven (Cours élémentaire de palaeontologie
stratigraphique, 1849).
Cuvier, however, can hardly have believed that all species were present at
the beginning, since he does admit a progression of forms. Probably he had
no theory on the subject, for theories without facts had little interest for
him. At any rate it is a mistake to think that Cuvier was a supporter of the
theological doctrine of special creation. His philosophy of Nature was
mechanistic, and he dedicated his Recherches sur les Ossemens Fossiles to
his friend Laplace. He admitted the idea of evolution at least so far as to
conceive of a development of man from a savage to a civilised state.[68] He
refused to accept the extravagant evolutionary theory of Demaillet and the
somewhat confused theory of Lamarck (whom he joins with Demaillet),[69]
just as he rejected the transcendental theories of Geoffroy St Hilaire,
because they seemed to him not based upon facts.
[41] Leçons d'Anatomie Comparée, tome i., pp. 10 et scq., 1800.
[42] Leçons d'Anatomie Comparée, i., p. 18.
[43] Loc. cit., i., p. 13.
[44] Leçons d'Anatomie Comparée, tome i., Articles iii.-iv., 1800.
[45] Leçons d'Anatomie Comparée, i., p. 47.
[46] Le Règne Animal, i., p. 6, 1817.
[47] Histoire des Progrès des Sciences naturelles depuis 1789, i., p. 310, 1826.

[48] Recherches sur les Ossemens Fossiles, i., p. 60, 1812.
[49] Ossemens fossiles, i., p. 60.
[50] Loc. cit., i., p. 63.
[51] Leçons d'Anatomie Comparée, i., p. 6.
[52] Le Règne Animal, i., p. 16.
[53] Hist. Prog. Sci. Nat., i., p. 187, 1826.
[54] Leçons, i., p. 58.
[55] Loc. cit., i., Article iii.
[56] Loc. cit., i., p. 60.
[57] Règne Animal, i., p. xx.
[58] Cuvier, Hist. Prog. Sci. Nat., i., p. 288, 1826.
[59] Règne Animal, i., p. 10.
[60] Règne Animal, p. 55.
[61] First propounded by Cuvier in 1812, Ann. Mus. d'Hist. Nat., xix.

[62] Règne Animal, i., p. 19.
[63] Loc. cit., p. 20.
[64] Recherches sur les Ossemens Fossiles, i., p. 74, 1812.
[65] Loc. cit., p. 79.
[66] See C. Depéret, Les transformations du Monde animal, Paris, 1907, and G.
Steinmann, Die geologischen Grundlagen der Abstammungslehre, Leipzig,
1908.
[67] Recherches, i., p. 81.
[68] Règne Animal, i., p. 91.
[69] Ossemens Fossiles, i., p. 26.

CHAPTER IV

GOETHE

Science, in so far as it rises above the mere accumulation of facts, is a
product of the mind's creative activity. Scientific theories are not so much
formulæ extracted from experience as intuitions imposed upon experience.
So it was that Goethe, who was little more than a dilettante,[70] seized upon
the essential principles of a morphology some years before that morphology
was accepted by the workers.
Goethe is important in the history of morphological method because he was
the first to bring to clear consciousness and to express in definite terms the
idea on which comparative anatomy before him was based, the idea of the
unity of plan. We have seen that this idea was familiar to Aristotle and that
it was recognised implicitly by all who after him studied structure
comparatively. In Goethe's time the idea had become ripe for expression. It
was used as a guiding principle in Goethe's youth particularly by Vicq
d'Azyr and by Camper. The former (1748-1794), who discovered[71] in the
same year as Goethe (1784) the intermaxillary bone in man, pointed out the
homology in structure between the fore limb and the hind limb, and
interpreted certain rudimentary bones, the intermaxillaries and rudimentary

clavicles, in the light of the theory that Vertebrates are built upon one single
plan of structure.
"Nature seems to operate always according to an original and general plan,
from which she departs with regret and whose traces we come across
everywhere" (Vicq d'Azyr, quoted by Flourens, Mém. Acad. Sci., XXIII.,
p. xxxvi.).
Peter Camper (1722-1789), we are told by Goethe himself in his
Ostéologie, was convinced of the unity of plan holding throughout
Vertebrates; he compared in particular the brain of fishes with the brain of
man.
The idea of the unity of plan had not yet become limited and defined as a
strictly scientific theory; it was an idea common to philosophy, to ordinary
thought, and to anatomical science. We find it expressed by Herder (who
perhaps got it from Kant) in his Ideen zur Philosophie der Geschichte der
Menschheit (1784), and it is possible that Goethe became impressed with
the importance of the idea through his conversations with Herder. Be that as
it may, it is certain that Goethe sought for the intermaxillaries in man only
because he was firmly convinced that the skeleton in all the higher animals
was built upon one common plan and that accordingly bones such as the
intermaxillaries, found well developed in some animals, must also be found
in man. The idea was not drawn from the facts, but the facts were
interpreted and even sought for in the light of the idea. "I eagerly worked
upon a general osteological scheme, and had accordingly to assume that all
the separate parts of the structure, in detail as in the whole, must be
discoverable in all animals, because on this supposition is built the already
long begun science of comparative anatomy."[72]
The principle comes to clear expression in his Erster Entwurf einer
allgemeinen Einleitung in die vergleichende Anatomie (1795).[73] He writes:
—"On this account an attempt is here made to arrive at an anatomical type,
a general picture in which the forms of all animals are contained in potentia,
and by means of which we can describe each animal in an invariable order."
[74]
His aim is to discover a general scheme of the constant in organic parts,
a scheme into which all animals will fit equally well, and no animal better
than the rest. When we remember that the type to which anatomists before

him had, consciously or unconsciously, referred all other structure was man
himself, we see that in seeking after an abstract generalised type Goethe
was reaching out to a new conception. The fact that only the structure of
man and the higher animals was at all well-known in his time led Goethe to
think that his general Typus would hold for the lower animals as well,
though it was to be arrived at primarily from a study of the higher animals.
All he could assert of the entire animal kingdom was that all animals agreed
in having a head, a middle part, and an end part, with their characteristic
organs, and that accordingly they might, in this respect at least, be reduced
to one common Typus. Goethe's knowledge of the lower animals was not
extensive.
Though Goethe did not work out a criterion of the homology of parts with
any great clearness, he had an inkling of the principle later developed by E.
Geoffroy St Hilaire, and called by him the "Principle of Connections."
According to this principle, the homology of a part is determined by its
position relative to other parts. Goethe expresses it thus:—"On the other
hand the most constant factor is the position in which the bone is invariably
found, and the function to which it is adapted in the organic edifice."[75] But
from this sentence it is not clear that Goethe understood the principle as one
of form independent of function, for he seems to consider that the
homology of an organ is partly determined by the function which it
performs for the whole. He wavers between the purely formal or
morphological interpretation of the principle of connections and the
functional. We find him in the additions to the Entwurf (1796), saying:
—"We must take into consideration not merely the spatial relations of the
parts, but also their living reciprocal influence, their dependence upon and
action on one another."[76] But in seeking for the intermaxillary bone in man
he was guided by its position relative to the maxillaries—it must be the
bone between the anterior ends of the maxillaries, a bone whose limits are
indicated in the adult only by surface grooves.
As a matter of fact Goethe's morphological views are neither very clearly
expressed nor very consistent. This comes out in his treatment of the
relation between structure and function. Sometimes he takes the view that
structure determines function. "The parts of the animal," he writes, "their
reciprocal forms, their relations, their particular properties determine the

life and habits of the creature."[77] We are not to explain, he says, the tusks
of the Babirussa by their possible use, but we must ask how it comes to
have tusks. In the same way we must not suppose that a bull has horns in
order to gore, but we must investigate the process by which it comes to
have horns to gore with. This is the rigorous morphological view. On the
other hand he admits elsewhere that function may influence form.
Apparently he did not work out his ideas on this point to logical clearness,
and Rádl[78] is probably correct in saying that the following quotation with
its double assertion represents most nearly Goethe's position:—
"Also bestimmt die Gestalt die Lebensweise des Thieres, Und
die Weise zu leben, sie wirkt auf alle Gestalten Mächtig zurück."
[79]

His best piece of purely morphological work was his theory of the
metamorphosis of plants. Stripped of its vaguer elements, and of the crude
attempt to explain differences in the character of plant organs by differences
in the degree of "refinement" of the sap supplied to them, the theory is that
stem-leaves, sepals, petals, and stamens are all identical members or
appendages. These appendages differ from one another only in shape and in
degree of expansion, stem-leaves being expanded, sepals contracted, petals
expanded, and so on alternately. It is equally correct to call a stamen a
contracted petal, and a petal an expanded stamen, for no one of the organs is
the type of the others, but all equally are varieties of a single abstract plant-
appendage.
What Goethe considered he had proved for the appendages of plants he
extended to all living things. Every living thing is a complex of living
independent beings, which "der Idee, der Anlage nach," are the same, but in
appearance may be the same or similar, different or unlike.[80] Not only is
there a primordial animal and a primordial plant, schematic forms to which
all separate species are referable, but the parts of each are themselves units,
which "der Idee nach," are identical inter se. This fantasy can hardly be
taken seriously as a scientific theory; it seems, however, to have been what
guided Goethe in his "discovery" of the vertebral nature of the skull. Just as
the fore limb can be homologised with the hind limb, so, reasoning by
analogy, the skull should be capable of being homologised with the
vertebræ. To what ludicrous extremes this doctrine of the repetition of parts

within the organism was pushed we shall see when we consider the theories
of the German transcendentalists of the early nineteenth century.
Though Goethe's morphological views were lacking in definiteness he hit
upon one or two ideas which proved useful. Thus he enunciated the "law of
balance" long before Etienne Geoffroy St Hilaire, the law "that to no part
can anything be added, without something being taken away from another
part, and vice versa."[81] He saw, too, what a help to the interpretation of
adult structure the study of the embryo would be, for many bones which are
fused in the adult are separate in the embryo.[82] This also was a point to
which the later transcendentalists gave considerable attention.
So far we have spoken of Goethe as if he were merely the prophet of formal
morphology; we have pointed out how he brought to clear expression the
morphological principle implicit in the idea of unity of type, and how he
seized upon some important guiding ideas, such as the principle of
connections. But Goethe was not a formalist, and he was very far from the
static conception of life which is at the base of pure morphology. His
interest was not in Gestalt or fixed form, Bildung or form change. He saw
that Gestalt was but a momentary phase of Bildung, and could be
considered apart and in itself only by an abstraction fatal to all
understanding of the living thing. Mephistopheles scoffs at the scholars who
would explain a living creature by anatomising it:

"Dann hat er die Theile in seiner Hand,
Fehlt leider! nur das geistige Band."[83]

Goethe kept clear of this mistake; he knew that the artist comes nearer to
the truth than the analyst.
In the fragment entitled Bildung und Umbildung organischer Naturen
(1807), introductory to a reprint of his paper on the "Metamorphosis of
Plants," we get an exposition of his general views on living things. He
points out there how we try to understand things by separating them into
their parts. We can, it is true, resolve the organism into its structural
elements, but we cannot recompose it or endow it with life by joining up the
parts. Hence we require some other means of understanding it. "In all ages
even among scientific men there can be discerned a yearning to apprehend

the living form as such, to grasp the connection of their external visible
parts, to interpret them as indications of the inner activity, and so, in a
certain measure, to master the whole conceptually." This science which
should discover the inner meaning of organic Bildung is called Morphology.
[84]
In Morphology we should not speak of Gestalt or fixed form, or if we
do we should understand by it only a momentary phase of Bildung. Form is
of interest not in itself but only as the manifestation of the inner activity of
the living being. Over development, he says elsewhere, there presides a
formative force, a bildende Kraft or Bildungstrieb, which works out the idea
of the organism. Living things, in his view of them, strive to manifest an
idea. They are Nature's works of art—and so, incidentally, they require an
artist to interpret them.
This profound conception of the nature of life is applied not only to the
growing changing individual but also to the whole changing world of
organisms. They are all manifestations of a living shaping power which
moulds them. This shaping power, immanent in all life, is conceived to
work according to a general plan, and so we get an explanation of the fact
that living things seem simply varieties of one common type.
"If we once recognise," says Goethe, "that the creative spirit brings into
being and shapes the evolution of the more perfect organic creatures
according to a general scheme, is it altogether impossible to represent this
original plan if not to the senses at least to the mind...?"[85]
Such an interpretation of the unity of plan reaches perhaps beyond the
bounds of science.
[70] See Kohlbrugge, "Hist. krit. Studien über Goethe als Naturforscher," Zool.
Annalen. v., 1913, pp. 83-231.
[71] Or re-discovered, according to Kohlbrugge.
[72] Cotta ed., vol. ix., p. 448.
[73] "First Draft of a General Introduction to Comparative Anatomy."
[74] Cotta ed., ix., p. 463.
[75] Cotta ed., p. 478.
[76] Loc. cit., p. 491.
[77] Entwurf, Cotta ed., ix., p. 465.

[78] Geschichte der biologischen Theorien, i., p. 266.
[79] "So the form determines the manner of life of the animal, and the manner of
life in its turn reacts powerfully upon all forms."
[80] Bildung und Umbildung organischer Naturen, 1807.
[81] Cotta ed., ix., p. 466.
[82] Loc. cit., pp. 474-5.
[83] Then he has all the parts within his hand, excepting only, sad to say, the
living bond.
[84] Goethe was the inventor of the word.
[85] Cotta ed., ix., p. 490.

CHAPTER V

ETIENNE GEOFFROY SAINT-HILAIRE

E. Geoffrey made an experiment, unsuccessful but instructive. He tried to
found a science of pure morphology; he failed: his failure showed, once and
for all, that a pure morphology of organic forms is impracticable.

Already, in 1796, in one of his earliest memoirs,[86] Geoffroy was guided by
the idea that Nature has formed all living things upon one plan. Organs
which seem anomalous are merely modifications of the normal; the trunk of
an elephant is formed by the excessively prolonged nostrils, the horn of a
rhinoceros is simply a mass of adhering hairs. In general, however varied
their form, all organs are simply variations of a common scheme; Nature
employs no new organs. Organs which are rudimentary, such as the
clavicles in the ostrich and the nictitating membrane in man, bear witness to
the unity of plan. In this Geoffroy goes no further than his predecessors.
They too had recognised homologies of organs; they too had interpreted
rudimentary organs as vestiges of an original plan.
In a series of papers published in 1807, Geoffroy took a further step, and
sought to establish homologies which were not obvious—homologies, too,
not so much of organs as of parts.

These memoirs (published in the Annales du Muséum d'Histoire naturelle,
vols. ix. and x., 1807) dealt with the homology between the bones of the
pectoral fin and girdle in fish and the bones of the arm and shoulder-girdle
in higher Vertebrates, with the homologies of the bones of the sternum, and
with the determination of the pieces of the skull, particularly in the
crocodile. All Geoffroy's morphological doctrine is found in them, but for
the full expression of his views we must take his chief work, the
Philosophie anatomique, particularly the first volume (1818). This volume
contains, beside the important "Discours préliminaire" and "Introduction"
which we shall presently consider in detail, five memoirs, which deal with
the various bones connected with the respiratory organs in fishes (the bones
of the operculum, of the hyoid, of the branchial arches, of the pectoral
girdle), and seek to discover their homologies with corresponding bones in
air-breathing Vertebrates.
"Can the organisation of vertebrated animals be referred to one uniform
type?" This is the question with which the Philosophie anatomique opens,
the question to which the whole book is an answer. But is it not generally
acknowledged by naturalists that Vertebrates are built upon one uniform
plan, that, for instance, the fore limb may be modified for running,
climbing, swimming, or flying, yet the arrangement of the bones remain the
same? How else could there be a "natural method" of classification?[87]
But the homologies so drawn repose upon a vague and confused feeling for
likenesses; they are not based upon an explicit principle. What general
principle can be applied? "Now it is evident that the sole general principle
one can apply is given by the position, the relations, and the dependencies
of the parts, that is to say, by what I name and include under the term of
connections." For instance, the part known as the hand in man and generally
as the fore foot in other Vertebrates, is the fourth part in order in the anterior
member, and its homologue can always be recognised by this fact of its
connections (p. xxvi.). The principle of connections serves as a guide in
tracing an organ through all its functional transformations, for "an organ can
be deteriorated, atrophied, annihilated, but not transposed" (p. xxx.).
It is this principle which enables one to follow out in detail the further
fundamental conception that in every Vertebrate there are found the same
"organic materials," or units of construction. This conception, which

Geoffroy calls the Théorie des analogues (p. xxxii.), is clearly one part of
the old idea of the unity of type; it teaches the unity of composition of
organic beings, while the Principe des connexions adds the unity of plan.
Both conceptions are logically implicit in the vague notion of unity of type;
Geoffroy disengaged them, and pushed each to its logical extreme.
Most of the ordinary homologies of structure in air-breathing Vertebrates
have already been seized, he continues, for they are more or less obvious,
and many intermediate states exist (p. xxxiv.). But ordinary methods of
comparison fail when the attempt is made to homologise the structure of
fishes with that of air-breathing Vertebrates, for the homologies are
anything but obvious and no intermediate organs are found.
Most air-breathing Vertebrates have a larynx, a trachea, and bronchi, which
are absent in fish; and fish have many parts which seem to be absent in
higher Vertebrates. But apply the "Theory of Analogues"; it teaches that
there can be no organ peculiar to fish and not found in other Vertebrates;
apply the "Principle of Connections," it will show which organs are
homologous in the two types (p. xxxv.).
Comparative anatomists, with few exceptions, had hitherto taken man as the
type, and referred all structure to his; Geoffroy's principles led him to give
preference to no one animal in particular, but to seize upon each part in the
species in which it reaches the maximum of its development (p. xxxvi.). He
is thus led to refer all structures to a generalised abstract type. In this
abstract type each organ exists at the maximum of its development, each
organ shows all its potentialities realised. In a way, therefore, this type, this
abstraction, gives the scheme of the possible transformations of each organ.
It is true Geoffroy does not refer to this "Archetype" in so many words, but
it must always have been vaguely present in his mind. He has this idea in
his head when he says in one of his later works, "There is, philosophically
speaking, only a single animal."[88] The "single animal" is simply the
generalised type.
Having laid down his two principles Geoffroy goes on to apply them to the
difficult case of the comparison of the skeleton of fish with the skeleton of
the higher Vertebrates. "My present task is to demonstrate that there is no

part of the bony framework of fishes that cannot find its analogue in the
other vertebrated animals."[89] It seems at first sight that many bones are
peculiar to fish, formed expressly for performing the functions which fish
do not share with higher animals. These are the bones connected with
respiration—the operculum, the branchiostegal rays, the branchial arches,
and others. That the peculiar bones should be connected with the respiratory
functions is only natural, for the contrast between fish and higher
Vertebrates is essentially a contrast between water-breathing and air-
breathing animals. Considering first the general form of the skeleton in fish,
we are met at once with a difficulty; there is no obvious homologue in
fishes of the neck, the trunk, and the abdomen of higher animals. What
apparently corresponds to the trunk is in fishes crowded close up under the
head. But, after all, it is not of the essence of the vertebrate type to have the
trunk and the abdomen attached at definite and invariable distances along
the vertebral column—that is a notion surviving from the anatomy which
made man its type. The "trunk" differs in position according to the class, in
quadrupeds, birds, and fishes (p. 9). Now, says Geoffroy, allow me this one
hypothesis, that the trunk with its organs can, as it were, move bodily along
the vertebral column, so as to be found in one class near the front end of the
vertebral column, in another about the middle, and in a third near the end,
then I can show you in detail that the constituent parts of this trunk are
found in all classes to be invariably in the same positions relatively to one
another (p. 10). It is important to note this hypothesis of a "metastasis"
which Geoffroy makes, for it is the key to the understanding of many of the
far-fetched homologies which he tries to establish. It is, of course, clear that
this hypothesis is in formal contradiction with his principal hypothesis of
the invariability of connections, and that he, so to speak, gets a hold on his
fish to apply his principle of connections only by admitting at the very
outset an exception to his primary principle. A further application of the
hypothesis of metastasis will be noticed below in connection with the
determination of the sternum of fishes. We note here an interpretation of the
first metastasis in terms of functional adaptation. "The constant and violent
action of the tail, if it does not go so far as actually to displace and move
forward the internal organs, at least fits in well with an arrangement in
which the organs are so disposed" (p. 99).

The first memoir deals with the homologies of the opercular bones.
Geoffroy considers that the external opening of the ear corresponds to the
external opening of the gill-chamber, which lies between the operculum and
the pectoral girdle. The ear communicates with the buccal cavity by the
Eustachian tube, so does the branchial chamber by means of the gill-slits.
The auditory chamber of higher Vertebrates is, therefore, the homologue of
the branchial chamber in fish; the opercular bones in fish and the ossicles of
the ear in other Vertebrates stand in close relation to this chamber; therefore
the opercular bones are the homologues of the ossicles of the ear, the
interoperculum corresponding to the malleus, the suboperculum to the
lenticular, the minute lower part of the suboperculum to the incus, the
operculum to the stapes, and the pre-operculum to the tympanic ring. In
making these particular determinations Geoffroy professes to be led by his
principle of connections. The pre-operculum has, he says, the same
connections with neighbouring bones as the tympanic bone in other
Vertebrates, and the other pieces of the gill-cover are homologised with
particular ear-ossicles according to the order in which they stand to one
another. The second memoir in the book deals with the sternum, and affords
a very good example of Geoffroy's method of dealing with the facts of
structure. We shall omit here any detailed reference to the other three
memoirs, which deal with the hyoid, with the branchial arches and the
structures which correspond in air-breathing Vertebrates, and with the bones
of the shoulder-girdle.
In the memoir on the sternum Geoffroy's first care is to arrive at a definition
of what a sternum is. He defines it partly by its functions, partly by its
connections, as the system of bones which covers and protects the thorax,
and gives attachment to certain groups of muscles.
The most highly developed sternum (according to this definition) is the
plastron of the tortoise, whose structure it dominates (p. 103). It is
important, therefore, to determine of how many bones the plastron is
composed, since the full number of elementary parts of which an organ is
composed is best seen when the organ is at the maximum of its
development. There are nine bones in the plastron of the tortoise. "The
conclusion to be drawn from this is that every sternum, provided that it is
not inhibited in its development by some obstacle, is composed of nine
elementary parts" (p. 105). These nine bones are in Geoffroy's

nomenclature, the episternals, the hyosternals, the hyposternals, the
xiphisternals, which are all paired bones, and the entosternal, which is
unpaired. The arrangement of them is in the tortoise:—

The articulations in the tortoise are indicated by the connecting lines.
Geoffroy tries to show that the sternum in other animals is composed of
these nine bones, or at least of a certain number of them, always in the same
invariable relative positions. Thus in birds the sternum consists of five
pieces, of a huge keeled entosternal, and of two "annexes" on either side,
which are the hyo-and hyposternals. These are separate only in young birds.
Occasionally, especially in young birds, rudiments of episternals and
xiphisternals also occur. The minuteness of the episternals and the
xiphisternals

Fig 1.—Hyoid Arch of the Conger.(Original)
may be attributed to the gigantic size of the entosternal, in accordance with
the Loi de balancement. In the other air-breathing Vertebrates the nine
sternal elements can according to Geoffroy be discovered without great
difficulty. But when we come to the determination of the sternum in fishes,
difficulties abound, which Geoffroy solves in the following way. He points
out that between the clavicles (cleithra) and the hyoid bone (basihyal) in
fishes there is a long median bone (urohyal) which is attached in front by
two strong tendons to the horns of the hyoid and is free behind (see Fig. 1).
Gouan (1720) had seen in this bone the homologue of the sternum.
Geoffroy adopts this view, but considers that this bone alone cannot
represent the whole sternum. He finds the representatives of other bones of
the sternum in the large bones (epihyal and ceratohyal, or the two pieces of
the ceratohyal) which are comprised in the hyoid arch. But he is
immediately met by the difficulty that this complex of bones is situated in
front of the pectoral girdle, whereas the sternum in higher Vertebrates lies
behind the pectoral girdle. He reflects, however, that the gills of fish,
situated in front of the clavicles, are merely the lungs under another name.
The gills have become shifted forward by a metastasis similar to that which
brought the whole thoracic organs far forward in fish. This being so, their
supporting elements, the sternum and the ribs, must have moved with them,
and are hence to be found in front of the pectoral girdle.
Geoffroy's next step is to point out that the only possible homologues of
sternal ribs are the branchiostegal rays, which arise from the large bones of
the hyoid arch. If these are sternal ribs, the bones to which they are attached

must be the hyo- and hyposternals or "annexes," the bones from which in
birds the ribs take their origin.
The unpaired sternal bone (urohyal) cannot be homologous with the
entosternal, for it has no connections with the annexes. He decides that it
must represent the episternals, for in some young birds there is a two-
headed episternal to which two strong tendons are attached, just in the same
way as the unpaired piece in fish is bound to the bones of the hyoid by two
tendons. "Thus it is not the sternum as a whole that has shifted in front of
the clavicles and covered with its side pieces the gills placed there; it is a
piece exclusively piscine, in the sense that it is only in the class of fishes
that it reaches the maximum of its development" (p. 83).
To sum up, the sternum in all four vertebrate classes is composed of the
same elements, arranged always in the same way. "One is ... led to the
conception of an ideal type of sternum for all Vertebrates, which then,
considered from a lower standpoint, resolves itself into several secondary
forms according as the whole or the majority of the constituent materials
are employed, or even as these elements come to change their respective
dimensions or proportions" (p. 134). As to the elementary constituents,
"they give proof of individuality, and sometimes even, in certain
abnormalities, of independence, and rise to the level of primary
organisatory materials" (p. 132). What holds good for the sternum holds
good for other organs—and accordingly the unity of plan and composition
can be demonstrated for all the organs of Vertebrates.
Soon after the publication of the Philosophie anatomique (1818) Geoffroy
went further in his search for unity, and maintained that the structure of
insects and Crustacea could be reduced to the vertebrate type.
He proposed to replace Cuvier's classification of the animal kingdom into
the four large groups, Vertebrata, Mollusca, Articulata, and Radiata by the
following classification:—[90]

Hauts-Vertébrés (Vertebrata, Cuv.).
Vertébrés
Dermo-Vertébrés (Articulata, Cuv.).

Mollusques (Mollusca, Cuv.)
.Invertébrés.
Rayonnés (Radiata, Cuv.).

The idea upon which is based the comparison of Articulates with
Vertebrates is that each skeletal segment of Articulates is a vertebra. In the
Hauts-vertébrés the vertebræ are internal; in the Dermo-vertébrés they are
external. "Every animal lives either outside or inside its vertebral column."
[91]
The essence of a vertebra is not its form, nor its function, but its
composition from four elementary pieces which unite round a central space
(Isis, loc. cit., p. 532). Serres had shown that in the higher animals every
vertebra is formed from four centres of ossification, that the body of the
vertebra is at first tubular, and that afterwards it becomes filled up. In
lobsters and crabs each segment is composed of four elementary pieces, as
may be seen most easily in young ones. "Accordingly each segment
corresponds to a true vertebra in composition: there is the same number of
'materials,' the same order in the course of ossification, the same kind of
articulation, the same annular arrangement, the same empty space in the
middle" (p. 534). The only difference is that in Articulates the central space
is very great and contains all the organs of the body, whereas in the higher
Vertebrates the body of the vertebra becomes completely filled up. In the
thoracic region of Crustacea it is not the whole segment with part of the
carapace which corresponds to a vertebra, but merely the part round the
ventral nerve-cord (endophragmal skeleton).
If the skeleton of the segment in Articulates corresponds to the body of a
vertebra and is here external, then the appendages of the Articulate must
correspond to ribs (p. 538). The full development of this thought is found in
a Memoir of 1822, "Sur la vertèbre."[92] He takes as the typical vertebra that
of a Pleuronectid, probably the turbot. His original figure is reproduced
(Fig. 2).
He includes as part of the vertebra not only the neural (e′, e″) and hæmal
(o′, o″) arches, but also, above and below these, the radialia (a″, u′) and the
fin-rays (a′, u″). (Neither the radialia nor the fin-rays are, by the way, in the
same transverse plane as the body of the vertebra). Every vertebra, he
considers, contains these nine pieces—the cycleal (or body), the two perials
(e′, e″) and the two epials (a′, a″) above, the two paraals (o′, o″) and the two

cataals (u′, u″) below. The epials and the
cataals are in reality paired bones which in
fish mount one on top of the other to support
the median fins. In the cranial region—the
skull is formed of modified vertebræ—the
epials and perials open out so as to form the
walls and roof of the brain; in the thoracic
region the paraals and cataals reach their
maximum of development and perform the
same service for the thoracic organs, the
paraals becoming vertebral, and the cataals
sternal, ribs.
We have seen that in Arthropods the body of
the vertebra (cycleal) forms the open ring of
the segment, which lies immediately under
the skin, the vertebral tube coinciding with
the epidermal tube. The homologues of the
other eight pieces of the vertebra must Fig. 2.
accordingly be sought in the external "Vertebra" of a Pleuronectid.
appendages. At first sight there seems here a (After Geoffroy.)
contradiction of the principle of connections, for the appendages in
Arthropods are lateral, whereas the paired bones of the vertebra are dorsal
and ventral. But there is in reality no contradiction, for "what our law of
connections absolutely requires is that all organs, whether internal or
external, should stand to one another in the same relations; but it is all one
whether the box (coffre) that encloses them lies with this or that side on the
ground. What similarities in the organisation of man and the digitate
mammals, and yet what differences between their attitudes when standing!
The same holds true as regards the normal attitudes of the pleuronectids and
the other fishes" (p. 107).
The exact way in which Geoffroy homologised the parts of the appendages
in Arthropods with the paired pieces of the typical vertebra is best shown by
the reproduction of his figure of an abdominal segment of the lobster
(Fig. 3), in which the parts homologous with those represented in the figure
of the typical vertebra (Fig. 2) are indicated by the same letters. The
ingenuity of the comparison is astonishing.

The comparison of the Arthropod with the Vertebrate is extended also to the
internal organs. The internal organs of the Arthropod are shown to stand in
the same order to one another as in the Vertebrate, only the organs are
inverted. Thus the nervous system is dorsal in the Vertebrate, ventral in the
Arthropod. Turn the Arthropod on its back and the relative positions of the
systems of organs are the same as in the Vertebrate. The relation of the
organs to the external tube is of course different in Arthropods and
Vertebrates, but this is no contradiction of the principle of connections.
"Such a tube, although it is the organs essential to life that it contains, can
yet behave in different ways with regard to the mass of these organs: the
principle of connections demands only that all the organs maintain with one
another fixed and definite relations; but the principle would be in no way
invalidated if the whole mass had rotated inside the tube" (p. 112).
Geoffroy pushed the analogy between
Arthropods and Vertebrates very far,
for he asserted that every piece in the
skeleton of an insect was homologous
with some bone in Vertebrates, that it
stood always in its proper place, and
remained faithful to at least one of its
connections.[93] It does not appear
that he attempted to prove in detail
this very big assumption, but the
beginnings of a detailed comparison
are found in the paper of 1820, Sur
l'organisation des insectes. Six
segments are distinguished in an
insect—the head, the three divisions
of the thorax, the abdomen, and the
terminal segment of the abdomen
(p. 455).
Fig. 3.
The skeleton of the insect's head is Abdominal Segment of the Lobster.
said to correspond to the bones of the (After Geoffroy.)
face, to the bones of the cerebrum and to the hyoid of higher Vertebrates,
the skeleton of the prothorax to the bones of the cerebellum, of the palate,
and the pieces of the larynx, the skeleton of the mesothorax to the parietals,

interparietals, and opercular bones, and that of the metathorax to the
skeleton of the thorax of Vertebrates. The pieces of the abdomen and of the
terminal segment correspond to the bones of the abdomen and coccyx
(p. 458). It does not need the subsequent likening of the hind wings of
insects to the air bladder of fish, and of the stigmata to the pores of the
lateral line, to convince one finally of the fancifulness of the whole
comparison.
In 1830 two young naturalists, Meyranx and Laurencet, presented to the
Académie des Sciences a memoir in which they likened a Cephalopod to a
Vertebrate bent back at the level of the umbilicus, saying that the Vertebrate
in this position had all its organs in the same order as in the Cephalopod.
Geoffroy took up this idea with enthusiasm, seeing in it a further
application of his master-idea of the unity of plan and composition. By
means of this comparison Mollusca definitely took their place in the Échelle
des êtres, after the Articulata, just as Geoffroy had maintained in 1820,
saying that crabs formed a link between the other Crustacea and the
molluscs.[94] The comparison brought him nearer to the end he had in view,
the reference of all animal structure to one single type.
But in championing the memoir of Meyranx and Laurencet, Geoffroy found
himself in direct antagonism with Cuvier, who held that his four
"Embranchements" had each a separate and distinct plan of structure. In a
paper read to the Academy in February 1830,[95] Cuvier easily demolished
the crude comparison of the Cephalopod to the Vertebrate. He gave
diagrams of the internal organs of a Cephalopod and of a Vertebrate bent
back in the manner indicated by Meyranx and Laurencet, and he showed in
detail that the arrangement of the main organs was quite different, that the
likeness would have been much greater if the Cephalopod had been likened
to a Vertebrate doubled up the other way,[96] but that even then the
arrangement of the organs would not be the same. The organs, too, of the
Cephalopod are differently constructed. He sums up his criticism by saying:
—"I give true and summary expression to all these facts when I say that
Cephalopods have several organs in common with Vertebrates, which fulfil
in either case similar functions, but that these organs are differently
arranged with respect to one another, and often constructed in a different
way; that they are in Cephalopods accompanied by several other organs

which Vertebrates do not possess, whilst the latter on their side have many
organs which Cephalopods lack" (p. 257). Geoffroy could not accept this
commonsense view of the matter, but made a fight for his transcendental
theories. This was the beginning of the famous controversy between
Geoffroy and Cuvier which so excited the interest of Goethe. It was a
struggle between "comparative anatomy" and "morphology," between the
commonsense teleological view of structure and the abstract,
transcendental. Geoffroy brought forward all his theories on the homology
of the skeleton of fish with the skeleton of higher Vertebrates, and tried to
prove by them his great principle of the unity of plan and composition;
Cuvier took Geoffroy's homologies one by one, and showed how very slight
was their foundation. Cuvier was on sure ground in insisting upon the
observable diversities of structural type, and his vast knowledge enabled
him to score a decisive victory.[97]
The controversy was not, as we are sometimes told, a controversy between
a believer in evolution and an upholder of the fixity of species, although it
raised a question upon which evolution theory was to throw some light.
In these Darwinian days Geoffroy has reaped a little posthumous glory as
an early believer in evolution. That he did believe in evolution to a limited
extent is certain; that his theory of evolution was, as it were, a by-product of
his life-work, is also certain. Geoffroy was primarily a morphologist and a
seeker after the unity hidden under the diversity of organic form. His theory
of evolution had as good as no influence upon his morphology, for he did
not to any extent interpret unity of plan as being due to community of
descent. His morphological, non-evolutionary standpoint comes out quite
clearly in several places in the Philosophie anatomique. He does not derive
the structure of the higher Vertebrates from the simpler structure of the
lower, but when he finds in fish a part at the maximum of its development,
he speaks of the same part, rudimentary in the higher forms, as being, as it
were, held in reserve for use in the fish. Thus, speaking of the episternal in
fish which forms the central piece of its sternum, he says, "it is a bone that
is rudimentary in birds (one might almost add a bone that is held in reserve
in birds for this fate) which is destined to form in the centre the principal
keel of this new machine" (p. 84). Again, with reference to the homology of
the ossicles of the ear with the opercular bones in fish, "employing other
resources equally hidden and rudimentary, Nature makes profitable use of

the four tiny ossicles lodged in the auditory passage, and, raising them in
fish to the greatest possible dimensions, forms from them these broad
opercula...." (p. 85). Or you may take it the other way about, and start from
the organisation of fishes; opercular bones are of no use to air-breathing
animals, so they dwindle away, and are pressed into the service of the ear,
although they are of little use in hearing (p. 46).
There is here no thought of evolution; in later years, however, his
researches upon fossil crocodilians led him to consider the possibility that
the living species were descended from the antediluvian. For the factors of
the transformation he refers to Lamarck's hypotheses.[98] In a memoir of
1828,[99] dealing with the possible genetic relation of living to fossil
species, he still regards the question as more or less open. Although fossil
species are mostly different from living species are we therefore to
conclude, he asks, that they are not the ancestors of the present day forms?
"The contrary idea arises more naturally in the mind; for otherwise the six-
days' creation would have had to be repeated and new beings produced by a
fresh creation. Now this proposition, contrary as it is to the most ancient
historical traditions, is inadmissible" (p. 210). It is sufficiently clear from
this quotation that Geoffroy was thinking only of a transformation of the
antediluvian species created by God, and by no means of an evolution of all
species from one primitive type. In matters of religion Geoffroy was
orthodox. He goes on to point out how great a resemblance there is in
essential structure between fossil and living species. All find their place in
one scheme of classification; does it not seem that all are modifications "of
one single being, of that abstract being or common type, which it is always
possible to denote by the same name?" (p. 211). This type is abstract, not
actual, and it is certainly not conceived as an original ancestor of all
animals.
The fullest development of Geoffroy's views on evolution is found in his
memoir "Le degré d'influence du monde ambiant pour modifier les formes
animales."[100] Here the relation of his evolution-theory to his morphology
is pointed out. The principle of unity of plan and composition cannot be the
final goal of zoology; there must follow on it a philosophical study of the
differences between organic forms. The causes of these differences are to be
found in the environment (pp. 66-7). Geoffroy seems here to be moving

from a pure to a causal morphology. It is probable, he continues, that living
species have descended by uninterrupted generation from the antediluvian
species (p. 74), and that they have in the process become modified through
external influences.
Now of all functions respiration is the most important, and upon respiration
everything is regulated. "If it be admitted that the slow progression of the
centuries has brought in its train successive changes in the proportion of the
different elements of the atmosphere, it follows as a rigorously necessary
consequence that the organisation has been proportionately influenced by
them" (p. 76). The respiratory milieu changes, the species change with it, or
are eliminated (p. 79). We may see, perhaps, in the stress which Geoffroy
lays upon respiration and the respiratory milieu a result of his constant
obsession with the comparison of fish with air-breathing Vertebrates.
In the first geological period, we read in another Memoir of the same year,
[101]
when ammonites and Gryphæa flourished, hot-blooded animals with
lungs could not exist. "A lung constructed like that of mammals and birds
would not have been adapted to the essence of the respiratory element such
as in my conception of it the system of the environing air used to be"[102]
(p. 58).
Geoffroy does not tell us exactly how the milieu is to act upon the
organism; the whole theory is little more than a sketch and a pointing out of
the way for future research—and in this prophetic enough. The action of
external agents was apparently considered as physical, and no power of
active adaptation was ascribed to the organism.
From a passage in the memoir "Sur la Vertèbre" we may perhaps infer that
he believed increasing complexity of structure to be due to a realisation of
potentialities, to the development of parts present in the lower animals only
in potency—"the organisation ... only awaits favourable conditions to rise,
by addition of parts, from the simplicity of the first formations to the
complication of the creatures at the head of the scale" (p. 112). Evolution
takes place as the environment allows, and in a sense in opposition to the
environment.
He believed in saltatory evolution, for he considered that the lower
oviparous Vertebrates could not be transformed into birds by slow

modification, but only by a sudden transformation of their lungs, which
would bring about the other characteristics of birds (p. 80). He considered,
too, that transformations could arise by means of monstrous development
(p. 86). In this connection the experiments which he made on the hen's
egg[103] in order to produce artificial monstrosities are significant, though
his purpose was rather to obtain proof of the inadequacy of the
preformation hypothesis.[104]
It seems probable enough that if Geoffroy had developed his views on
evolution he would finally have been led to interpret unity of plan in terms
of genetic relationship. But as it was he remained at his morphological
standpoint. He did not interpret rudimentary organs as useless heritages of
the past; he preferred to think that Nature had prepared double means for
the same function, one or other being predominant according as the animal
lived in the water or on the land. "To the animal that lives exclusively in the
air Nature has granted an organisation suited to this mode of respiration,
without however suppressing the other corresponding means, that is to say,
without depriving it of a second system which is applicable only to the
mode of respiration by the intermediary of water, and vice versa."[105]
He seems, in one instance at least, to have hit upon the root-idea of the
biogenetic law, but he was far from appreciating its significance. He
recognised that an amphibian in its development passed through a stage
when it was in all essentials similar to a fish, and he saw in this visible
transformation a picture of the evolutionary transformation. "An
amphibian," he writes,[106] "is at first a fish under the name of tadpole, and
then a reptile [sic] under that of frog.... In this observed fact is realised what
we have above represented as an hypothesis, the transformation of one
organic stage into the stage immediately superior." But it is not clear that he
considered the development of the amphibian to be a repetition of its
ancestral history.
He went, however, a certain length towards recognising the main principle
of a law which was a commonplace of German transcendental thought, and
was developed later by his disciple E. Serres, the law that the higher
animals repeat during their development the main features of the adult
organisation of animals lower in the scale. Thus he compared fish as
regards certain parts of their structure with the fœtus of mammals. He

compared also Articulates with embryonic Vertebrates in respect of their
vertebræ, for in the higher Vertebrates the body of the vertebra is tubular at
an early stage of development, and in Articulates the body of the vertebra
remains tubular permanently (supra, p. 61). As regards their vertebræ,
"insects occupy a place in the series of the ages and developments of the
vertebrate animals, that is to say, they realise one of the states of their
embryo, as fishes do one of the states of their fœtal condition."[107]
This idea was destined to exercise a great influence upon the development
of morphology. A further development of the thought is that certain
abnormalities in the higher animals, resulting from arrest of development,
represent states of organisation which are permanent in the lower animals.
[108]

So far we have considered Geoffroy's theories in their application to the
facts. We go on to discuss the theories themselves, and the general
conception of living things which underlies them.
The principle of unity of plan and composition is the keynote of Geoffroy's
work. It states that the same materials of organisation are to be found in all
animals, and that these materials stand always in the same general spatial
relations to one another. The "materials of organisation" are not necessarily
organs in the physiological sense, and indeed the principle of the unity of
plan cannot be upheld if the unity has reference to organs only. This became
clear to Geoffroy, especially in his later years. In 1835 he wrote, speaking
of the principle of the unity of plan, "I have, moreover, regenerated this
principle, and obtained for it universality of application, by showing that it
is not always the organs as a whole, but merely the materials composing
each organ, that can be reduced to unity."[109] Even in the Philosophie
anatomique he deals rather with parts than with organs; he deals, for
instance, with the elementary parts of the sternum, not with the organ
"sternum" in its totality. The functions of the sternum vary, and the primary
protective function of the sternum may be assumed by quite other parts,
e.g., by the clavicles in fish, which protect the heart.[110]
True homologies can be established between materials of organisation but
not always between organs, which may be composed of different
"materials."

Almost as a corollary to this comes the further view that form is of little
importance in determining homologies. An organ is essentially an
instrument for doing a particular kind of work, and its form is determined
by its function. Organs which perform the same function are usually similar
in form though the elementary materials composing them may be different.
This is seen in many cases of convergence. Organs, therefore, which
perform the same function and are similar in external form are not
necessary homologous. Conversely, the same complex of materials, say a
fore limb, may take on the most varied shapes according as the function of
the organ changes—but homology remains though form changes.
Accordingly, form is one of the least important elements to be considered in
determining a homology. "Nature," he wrote in one of his early papers,
"tends to repeat the same organs in the same number and in the same
relations, and varies to infinity only their form. In accordance with this
principle I shall have to draw my conclusions, in the determining the bones
of the fish's skull, not from a consideration of their form, but from a
consideration of their connections."[111]
Again, after comparing a vertebra of the Aurochs with an abdominal
segment of the crab, he says, "I have insisted upon an identity which has
extended to the least important relation of all, that of form."[112]
Geoffroy's morphological units or materials of organisation were in the case
of the skeleton—with which his researches principally deal—the single
bones. But the interesting point is that he sought his skeleton-units in the
embryo, and considered each separate centre of ossification as a separate
bone. Coalescence of bones originally separate is one of the most usual
events in development, and it is an occurrence which, more than any other,
tends to obscure homologies. Because of its coalescence with the
maxillaries, the intermaxillary in man was not discovered until Vicq d'Azyr
and Goethe found it separate in the embryo. Apparently quite independently
of Goethe, Geoffroy hit upon this plan of seeking in the embryo the primary
elements or materials of organisation. In an early paper on the skull of
Vertebrates,[113] where he is concerned with showing that each bone of the
fish's skull has its homologue in the skull of higher Vertebrates, he is faced
with the difficulty that the skull of the fish has more bones than the skull of
higher Vertebrates. "Having had the inspiration," he writes, "to reckon as

many bones as there are distinct centres of ossification, and having made a
consistent trial of this method, I have been able to appreciate the correctness
of the idea: fish, in their earliest stages, are in the same conditions relatively
to their development as the fœtuses of mammals, and hence bear out the
theory" (p. 344). So, too, in dealing with the homologies of the sternal
elements (supra, p. 57) he treats as separate bones the "annexes" of the
sternum in birds, though these are separate only in the young.
If the same materials of organisation are present in all animals, and if they
are arranged always in the same positions relatively to one another, how
does it come about that animal forms are so varied, what explanation can be
offered of the diversities of organic structure? Geoffroy's main answer to
this question is his Loi de balancement. The law was enunciated by him
already in 1807.[114] We take the following quotation, which represents his
thought most nearly, from the Cours de l'histoire naturelle des Mammifères
(1829). "According to our manner of regarding the organisation of
mammals, there is only a single animal modified by the inverse reciprocal
variation of all or some of its parts. Now, from the fact that there is only one
single general animal, it follows that for each section of its components or
for each of its organs there is available only a given quantity of formative
materials. Now suppose that the distribution of these materials has not been
made in such a way as to ensure an exact equilibrium between all the parts
concerned, one organ will get more than its share, another less. My law of
the compensation of organs is founded on these principles" (i., Leçon 16,
p. 12). "The atrophy of one organ turns to the profit of another; and the
reason why this cannot be otherwise is simple, it is because there is not an
unlimited supply of the substance required for each special purpose."[115]
The nutritive material available is limited for each species; if one part gets
more than its share the other parts must get less—that is all the law means.
As an example, take the minuteness of the episternals and xiphisternals in
birds, as contrasted with the huge size of the entosternal. "The minuteness
of the episternals and xiphisternals might be imputed to this gigantic piece
diverting to its own profit the nutritive fluid, since the bigger it is the
smaller these are."[116]
One has constantly to remember in dealing with Geoffroy's theories that he
was not an evolutionist, but purely a morphologist. It is therefore, perhaps,

to ask too much to require of him an explanation of the causes of diversity.
The morphologist describes, classifies, generalises; he does not seek for
causes. But we must leave this question aside in order to discuss how far
Geoffroy's theory of the unity of plan and composition fits the facts. As
Geoffroy himself admitted on several occasions, his theory was an à priori
one, a theory hit upon by hasty induction, then erected into a principle and
imposed upon the facts. No more than Goethe did he extract his principle
from a sufficient mass of data.
Now he found his theory to be in its pure form unworkable; he found, for
example, that the skeleton of fishes could not be compared directly, bone
for bone, with the skeleton of higher Vertebrates; he had to admit
differences of position of whole sets of organs in the two groups, he had to
admit various metastases, before he could bring the skeleton of fish into
line. And these metastases are due to functional requirements—for example,
the forward position of sternum and thoracic organs in fish is an adaptation
to swimming.
So he does not so much demonstrate the unity of plan of whole organisms
as the unity of plan of particular corresponding parts of them. Thus he does
not prove or attempt to prove that Articulates are in all points like
Vertebrates, but simply that their skeleton is built upon the same plan as that
of Vertebrates. The rest of the organs, while still comparable with the organs
of Vertebrates, stand in different relations to the skeleton. An Articulate
therefore, on his own showing, is not, as a whole, built upon the same
general structural plan as a Vertebrate.
Further, he does not always remain true to his principles, for he does not
establish homologies of parts entirely by their connections but sometimes
by their functions as well. Thus the sternum, or rather the complex of
sternal elements, is defined and discovered in particular cases not by its
connections only but also by its functions. The framework of the gills is
homologised part by part with the framework of the lungs, not because the
relations of the framework to the rest of the skeleton are the same in fish
and air-breathing Vertebrates, but simply because gills are considered the
equivalents of lungs—a comparison which is purely physiological.

Even with these concessions to the functional view of living things,
Geoffroy was unable to make good his contention that all animals are built
upon the same plan. His arguments failed to carry conviction to his
contemporaries, and Cuvier in particular subjected them to destructive, and
indeed final, criticism.
The paper, already referred to, in which Cuvier disposed of the
transcendentalists' comparison of Cephalopods and Vertebrates is of great
significance, for it states in the clearest way the radical opposition between
the functional and the formal attitudes to living things.
Cuvier points out that if by unity of composition is meant identity, then the
statement that all animals show the same composition is simply not true—
compare a polyp with a man!—on the other hand, if by unity is meant
simply resemblance or homology, the statement is true within certain limits,
but it has been employed as a principle since the days of Aristotle, and the
theory of unity of composition is original only in so far as it is false. He
admits, however, that Geoffroy has seized upon many hidden homologies,
especially by his valuable discovery of the importance of fœtal structure. In
all this Cuvier is undoubtedly right. Unity of plan and composition, as
Geoffroy conceived it, simply does not exist. Cuvier goes on to say that this
principle of Geoffroy's, in the greatly modified form in which it can be
accepted, and has been accepted from the dawn of zoology, is not the sole
and unique principle of the science. On the contrary, it is merely a
subordinate principle, subordinate to a higher and more fruitful principle,
that, namely, of the conditions of existence, of the adaptation (convenance)
of the parts, of the co-ordination of the parts for the rôle which the animal is
to play in Nature. "That is the true philosophical principle," he says,
"whence may be deduced the possibility of certain resemblances, the
impossibility of certain others; it is the rational principle from which
follows the principle of the unity of plan and composition, and in which at
the same time it finds those limits, which some would like to disregard"
(p. 248).
Geoffroy's position is the direct contrary. He holds that the principle of the
unity of plan and composition is the true base of natural history,[117]and that
this unity limits the possible transformations of the organism. Thus,
speaking of the influence of the respiratory medium, he says, "All the same

this influence of the external world, if it has ever become a cause which
disturbed organisation, must necessarily have been confined within fairly
narrow limits; animals must have opposed to it certain conditions inherent
to their nature, the existence of the same materials composing them, and a
manifest tendency to resemble one another, and to reproduce invariably the
same primordial type."[118] Unity of plan and composition is, on this view,
prior to adaptation and limits adaptation. Cuvier's view, on the contrary, is
that the necessity of functional and ecological adaptation accounts for the
repetition of the same types of structure. There are, of all the possible
combinations of organs, only a few viable types—those whose structure is
adapted to their life. Therefore it is reasonable that these few types should
be repeated in innumerable exemplars. One must remember, in order to
appreciate Cuvier's view, that he was not obsessed, as we are, by the idea of
evolution.
Cuvier thought in terms of organs, not in terms of "materials of
organisation." He held that the resemblances between the organs of one
class of animals and the organs of another were due to the similarity of their
functions. "Let us conclude, then, that if there are resemblances between the
organs of fish and those of other classes, it is only in the measure that there
is a resemblance between their functions."[119] There are only a few kinds of
organs, each adapted for a particular function, and these organs are
necessarily repeated from class to class.—"As the animal kingdom has
received only a limited number of organs, it is inevitable that some at least
of these organs should be common to several classes."[120]
Geoffroy thought in terms of "materials," of parts of indefinite function,
parts which might take on any function. He insists upon the necessity of
disregarding function when tracing out the unity of composition. He
considers, in direct opposition to Cuvier's interpretation of structural
resemblance as due to similarity of function, that unity of composition is the
primary fact, and similarity of function subsidiary. In his reply in the
Mammifères (1829) to Cuvier's criticisms in the Histoire naturelle des
Poissons (1828), he insists on the necessity of excluding function from
consideration in any truly philosophical treatment of comparative anatomy
(Discours prél., p. 25). Cuvier held that function determined structure, or at
least that the necessity of adaptation ruled the transformations of form.

Geoffroy considered that structure determined function, that changes of
structure, however they might arise, caused changes of function. "Animals,"
he writes, "have no habits but those that result from the structure of their
organs; if the latter varies, there vary in the same manner all their springs of
action, all their faculties and all their actions."[121]
Again, "a vegetarian régime is imposed upon the Quadrumana by their
possession of a somewhat ample stomach, and intestines of moderate
length."[122] The hand of the bat has become so modified as to constrain the
bat to live in the air.[123]
The best example of Geoffroy's insistence upon the priority of structure to
function, and so of his purely morphological attitude, is perhaps his
interpretation, already alluded to, of the appendages of Articulates. The
segments of the Articulate are, he says, the equivalents of the bodies of the
vertebræ of higher forms. Now "from the circumstance that the vertebra is
external, it results that the ribs must be so too; and, as it is impossible that
organs of such a size can remain passive and absolutely functionless, these
great arms, hanging there continually at the disposition of the animal, are
pressed into the service of progression, and become its efficient
instruments."[124] The ribs become locomotory appendages.
We may compare the similar thought that the ear ossicles are simply
opercular bones reduced and turned to other uses.
Geoffroy could not but recognise the correlation of structure to function, for
this is a fact which imposes itself upon every observer. He recognised also
correlation between functions, as when he pointed out the connection
between increased respiration and enhanced muscular activity in birds.[125]
He interpreted structure at times in terms of function, the short, strong
clavicle of the mole as an adaptation to digging, the keeled sternum of birds
as an adaptation to flying, and so on. But we may say that his whole
tendency was to disregard function, to look upon it as subsidiary. He
protests against arguing from function and habits to structure, as an "abuse
of final causes."[126] He was not so convinced as Cuvier was of the all-
importance of functional correlation; in this view he was probably
confirmed by his work on teratology. It did not surprise him that Insects, in
which lungs, heart and circulation have disappeared(!), should yet have a

skeleton built upon the same plan as the skeleton of Vertebrates, which
possess these organs; the correlation of organ-systems is not so close as to
prevent this.[127] So too, although the other organs of the insect are all
inside the body of the vertebræ, they are yet comparable with the organs of
Vertebrates.[128] The existence of rudimentary organs also seemed to him an
argument against too strict a correlation of parts.
The contrast between the teleological attitude, with its insistence upon the
priority of function to structure, and the morphological attitude, with its
conviction of the priority of structure to function, is one of the most
fundamental in biology.
Cuvier and Geoffroy are the greatest representatives of these opposing
views. Which of them is right? Is there nothing more in the unity and
diversity of organic forms than the results of functional adaptation, or is
Geoffroy right in insisting upon an element of unity which cannot be
explained in terms of adaptation? If there be an irreducible element of unity,
is there any truth in Geoffroy's suggestion that this unity results from a
power which is exercised in the world of atoms where are elements of
inalterable character?[129]
The problem as Geoffroy and Cuvier understood it was not an evolutionary
one. But the problem exists unchanged for the evolutionist, and evolution-
theory is essentially an attempt to solve it in the one direction or the other.
Theories such as Darwin's, which assume a random variation which is not
primarily a response to environmental changes, answer the problem in
Geoffroy's sense. Theories such as Lamarck's, which postulate an active
responsive self-adaptation of the organism, are essentially a continuation
and completing of Cuvier's thought.
[86] "Mémoire sur les rapports naturels des makis," Magasin Encyclopèdique,
vii.
[87] Discours préliminaire, pp. xv.-xxiv.
[88] Études progressives d'un Naturaliste, p. 50, Paris, 1835.
[89] Philosophie Anatomique., i., Introduction, p. 1.
[90] "Sur une colonne vertébrale et ses côtes dans les insectes apiropodes,"
(Acad. Sci., Feb. 12, 1820). Printed in Isis, pp. 527-52, 1820 (2).
[91] "Sur l'organisation des insectes," p. 458. Isis, pp. 452-62, 1820 (2).

[92] Mém. Mus. d'Hist. nat., ix., pp. 89-119, Pls. v-vii.
[93] Sur l'organisation des insectes, p. 459.
[94] Isis, p. 549.
[95] Published in Ann. Sci. Nat., xix., pp. 241-59, 1830.
[96] Cf. Aristotle (supra, p. 10).
[97] For an account of the controversy reference may be made to I. Geoffroy St
Hilaire, Vie Travaux et Doctrine scientifique d'Etienne Geoffroy St Hilaire, Paris,
1847; also Semper, Arb. zool. zoot. Instit. Würzburg, iii., 1876-7, K. E. von Baer,
Lebensgeschichte Cuviers, ed. L. Stieda, 1897, and J. Kohlbrugge, in Zoolog.
Annalen, v., pp. 143-95. 1913.
[98] "Recherches sur l'organisation des Gavials," Mém. Mus. d'Hist. nat., xii.,
1825.
[99] Mém. Mus. d'Hist. nat., xvii., pp. 209-29.
[100] Mém. Acad. Sci., xii., pp. 63-92, 1833.
[101] Mém. Acad. Sci., xii., pp. 43-61, 1833.
[102] Geoffroy's French style is at times incredibly bad, and more or less literal
translations of his sentences are apt to read queerly!
[103] Mém. Mus. d'Hist. nat., xiii., p. 289, 1826.
[104] Mém. Mus. d'Hist. nat., xviii., p. 221, 1828. His teratological work is
important, and is chiefly contained in the second volume of the Philosophie
anatomique.
[105] Phil. anat., i., p. 449.
[106] Mém. Acad. Sci., xii., p. 82, 1833.
[107] Mém. Mus. d'Hist. nat., ix., p. 101, 1822.
[108] Cours de l'histoire naturelle des Mammifères, i., Leçon 3, p. 13, 1829.
[109] Études progressives d'un Naturaliste, p. 59, f.n., Paris, 1835.
[110] Phil. Anat., i., p. 444.
[111] Ann. Mus. d'Hist. nat., x., p. 344, 1807.
[112] Isis, p. 534, 1820 (2).
[113] Ann. Mus. d'Hist. nat., x., pp. 342-65, 1807.
[114] loc. cit., x., p. 343.
[115] Phil. anat., i., 450, f.n. Cf. Aristotle (supra, p. 11).
[116] Loc. cit., p. 136.
[117] Mammifères, i., Discours prél., p. 18.

[118] Phil. anat., i., p. 208.
[119] Cuvier and Valenciennes, Hist. nat. Poissons, i., p. 550, 1828.
[120] Cuvier and Valenciennes, loc. cit., p. 544.
[121] Mammifères, i., Leçon 4, p. 17.
[122] Loc. cit., Leçon 5, p. 8.
[123] Loc. cit., Leçon 13, p. 6.
[124] Isis, p. 539, 1820 (2).
[125] Mammifères, i., Leçon 4, p. 6.
[126] Mammifères, Discours prél., p. 7.
[127] Isis, p. 460, 1820 (2).
[128] Mém. Mus. d'Hist. nat., ix., p. 102, 1822.
[129] Mém. Acad. Sci.., xii., p. 76, 1833.

CHAPTER VI

THE FOLLOWERS OF ETIENNE GEOFFROY SAINT-HILAIRE

Geoffroy's theories were not generally accepted by his contemporaries, but
his methods had considerable influence, especially in France, where many
made essays in pure morphology.
His chief follower was Serres, who is mentioned indeed in the Philosophie
anatomique as a fellow-worker. Serres was primarily a medical anatomist;
his interest lay in human anatomy and embryology, normal and
pathological.
His best early work was an Anatomie comparée du cerveau (1824-26),
which met with a flattering reception from Cuvier.[130] He laid great stress
upon the development of the brain and spinal cord in the different classes,
and was quick to point out analogies not only between adult but also
between embryonic structures. He paid much attention to cases of
correlation, and noted a great many; he observed, for instance, a constant

relation between the development of the spinal cord and of the corpora
quadrigemina, and between the size of the corpora quadrigemina and the
volume of the optic nerves and eyes. In this the influence of Cuvier is
unmistakable.
Serres' early theoretical views are to be found in a series of papers in the
Annales des Sciences naturelles,[131] under the general title Recherches
d'Anatomie transcendante, sur les Lois de l'Organogénie appliquées à
l'anatomie pathologique, also published separately. We follow these papers
in our exposé of Serres' doctrine, reserving for a future chapter (Chap. XII.)
the consideration of his matured views of thirty years later.
In the first of them he points out how neither position nor function has
proved altogether sufficient to establish homologies. In the early days
anatomists were guided by form; when form failed them, they traced an
organ in its changes throughout the series of animals by considering its
function. This method was satisfactory enough as regards the organs of the
nutritive life. But in the organs of the life of relation, in the nervous system,
the functions of the parts were difficult to discover, and their form very
changeful. Hence a new principle was required, and Serres found it in the
thought which he probably owed to the German transcendentalists (see
Chap. VII.), that the permanent structure of the lower animals could be
compared with phases in the development of the higher, and particularly of
man, or, as he put it, that comparative anatomy was often only a fixed and
permanent anthropogeny, and anthropogeny a fugitive and transitory
comparative anatomy (xi., p. 106).
"In rising towards the first formations," he writes, "transcendental anatomy
recognised that one and the same organ, however complicated its definitive
form might be, repeated in its transitory states the organic simplicities of the
lower classes. Thus the primitive heart of birds was first of all a canal, then
a pocket or single cavity, then finally the complex organ of the class.
Comparative anatomy was thus seen to be repeated and reproduced by
embryogeny" (xii., p. 85).
His explanation of the fact of repetition is that, "in animals belonging to the
lower classes the formative force, whatever it may be, has a less energetic
impulsion than in the higher animals, and hence the organs pass through

only a part of the transformations which those of the higher forms undergo;
and it is for this reason that they show permanently the organic dispositions
which are only transitory in the embryo of man and the higher Vertebrates.
Hence these double aortas, these double venæ cavæ which one observes
more or less constantly among reptiles" (xxi., p. 48).
The number of stages in embryogeny is proportionate to the complexity of
the adult; the younger the embryo the simpler its organs—such is the
general formula of the relation between the embryo and the adult. But here
in Serres' doctrine of parallelism a complication enters. He observed that
embryonic organs did not always develop in a piece, by simple growth, but
often were formed by the union of separately formed parts or layers. Thus
the kidney in man is formed by the fusion of a number of "little kidneys,"
and the spinal cord reaches its full development by the laying down of
successive layers within it. He was greatly impressed with this fact, which,
as a convinced believer in epigenesis, he used with great effect against the
preformistic theories. "This method of isolated formation," he wrote, "is
noticed in early stages in the thyroid, the liver, the heart, the aorta, the
intestinal canal, the womb, the prostate, the clitoris, and the penis" (xi.,
p. 69). So, too, in the development of the skeleton, ossification proceeds
from separate centres, foramina are formed by the fusion of separate bones
round them. In his memoir, Lois d'Osteogénie (1819), Serres established
several laws of ossification based upon this principle of separate formation.
[132]

How is the fact of multiple formation to be reconciled with the principle of
repetition, according to which organs are simplest in the early embryo and
in the lower animals? But observation shows that, as a rule, the further
down the scale you go the more divided organs become—the more
numerous the bones of the skull, for example. There is thus a parallel
between multiple formation of organs in the embryos of the higher
Vertebrates and their subdivided state in the lower. Take, for example, the
kidney. In the genus Felis, and in birds, each kidney has two lobes, in the
elephant four, in the otter ten, in the ox twelve to fourteen. The human
kidney in its development starts with about a dozen lobes, and the number
diminishes as the kidney grows. Thus the permanent state of the kidney in
the animals mentioned is reproduced by the stages of its development in
man (xii., p. 126).

So, too, at the second or third month the uterus of the human embryo is
bicornuate, and afterwards passes through stages comparable to the adult
and permanent uterus of rodents, ruminants, and carnivores. There is indeed
a time in the development of the human embryo when it resembles in many
of its organs the adult stage of various lower animals. It is about this time
that it possesses a tail.
We note that Serres' theory of parallelism applies, strictly speaking, only to
organs, not to organisms, although he, too, readily fell into the error of
supposing that the organisation of an embryo could be compared as a whole
with the adult organisation of an animal lower in the scale. Thus he wrote in
one of his later papers[133]—"As our researches have made clear, an animal
high in the organic scale only reaches this rank by passing through all the
intermediate states which separate it from the animals placed below it. Man
only becomes man after traversing transitional organisatory states which
assimilate him first to fish, then to reptiles, then to birds and mammals."
Serres was not altogether free from the besetting sin of the
transcendentalists—hasty generalisation.
The law of parallelism applied not only to Vertebrates but also to
Invertebrates. In a short paper[134] of 1824 Serres attempted an explanation
of the nervous system of Invertebrates. Invertebrates, he considered, lacked
the cerebrospinal axis of Vertebrates, and their nervous system was the
homologue of the sympathetic system of Vertebrates. The relation of the
invertebrate to the vertebrate nervous system being thus fixed, can the
nervous system of Invertebrates be reduced to one plan? It does not seem
possible to establish a common plan for the adult nervous systems. But
apply the principle of parallelism, which has proved so valuable within the
limits of the vertebrate series. Taking insects as the highest class, we find
that there are three stages in the development of their nervous system; in the
first the nervous system is composed of two separate strands, in the second
the strands unite round the œsophagus, in the third they unite also behind.
Now in Bulla aperta, stage (1) is permanent; in Clio, Doris, Aplysia,
Tritonia, Sepia, Helix, stage (2) is permanent, and in Unio stage (3). In fact,
all the varieties of the nervous system of molluscs fall into one or other of
these three classes. "It follows, then, that as regards their nervous system,
the Mollusca are more or less advanced larvæ of insects" (p. 380). The law
of parallelism is here applied to single organ-systems, but in later years

Serres applied it to whole organisations also, saying that the lower
Invertebrates were permanent embryos of the higher.
In the paper of 1834, already referred to, Serres pushed his speculations
further and attempted to establish the unity of type of all animals,
Vertebrates and Invertebrates alike—a favourite pastime of the
transcendentalists. It is incontestable, he admits, that adult Invertebrates are
quite different in structure from adult Vertebrates, "but if one regards them
as what I take them to be, namely, permanent embryos, and if one compares
their organisation with the embryogeny of Vertebrates, one sees the
differences disappear, and from their analogies arise a crowd of unsuspected
resemblances" (loc. cit., p. 247).
The last point of Serres' doctrine which calls for remark is his interpretation
of abnormalities as being often comparable to grades of structure permanent
in the lower animals. Thus the double aorta which may occur as an
abnormality in man is the normal and permanent state in reptiles. This idea,
of course, he got from Etienne Geoffroy St Hilaire. It is further developed
in his "Théorie des formations et des déformations organiques appliquée à
l'anatomie comparée des monstruosités (1832), and in his final large
memoir of 1860 (see below, p. 205).
In 1816 appeared a fine piece of work by J. C. Savigny on the homologies
of the appendages in Articulates. The standpoint was that of pure
morphology. "I am convinced," he wrote, "that when a more complete
examination has been made of the mouth of insects, properly so called, that
is to say, having six legs and two antennæ, it will be found that whatever
form it affects it is always essentially composed of the same elements....
The organ remains the same, only the function is modified or changed—
such is Nature's constant plan."[135] In this the influence of Geoffroy can be
traced; but the work was very free from the exaggerations of the
transcendentalists, and many of Savigny's homologies are accepted even to-
day. The first memoir dealt with the mouth-parts of insects; the second with
the anterior appendages of Articulates generally. Savigny shows that the
mouth-parts of insects can be reduced to the type shown in Orthoptera,
where there are clearly two mandibles, two maxillæ, and a lower lip formed
by the fusion of two second maxillæ. All other insects have these same
mouth-parts, disposed in the same order, however much their form may

have been modified in response to new functions. He goes on to compare
the anterior set of appendages in a long series of Articulates, in Julus,
Scolopendra, Cancer, Gammarus, Cyamus, Nymphon, Phalangium, Apus,
Caligus, Limulus, and a few others. For Crustacea he established the
homologies now accepted, of the mandibles with the mandibles of insects,
of the first and second pairs of maxillæ with the parts so named in insects,
and so on. He is quite clear that the maxillipedes of Crustacea are the
homologues of the feet of Hexapoda. "Their disposition must lead one to
think that the six anterior feet of Julus, that is to say, all the feet of the
Hexapoda, are here transformed into jaws" (loc. cit., p. 48). In Scolopendra
also there is a similar transformation of two pairs of legs into auxiliary
jaws. In Gammarus, where there is only the first pair of maxillipedes, the
other two pairs have become "retransformed" into feet. We find him
supporting his comparison of the three anterior pairs of legs in Julus to the
three pairs of legs in insects by an argument drawn from embryology; for
only the first three pairs of feet are present in Julus at birth (Degeer), "an
observation, which, together with their position, should cause them to be
considered as the representatives of the six thoracic feet of Hexapoda"
(p. 44).
His comparison of the Arachnid appendages with those of insects and
Crustacea is very curious. As his starting-point he takes Cyamus, which has
antennæ (two pairs) and mouth parts (four pairs) as in many Crustacea, and
then seven pairs of legs; he compares with it Nymphon, which has in all
seven pairs of appendages. These appendages he homologises with the
seven pairs of legs of Cyamus, so that the first appendage in Nymphon
corresponds to the seventh appendage of Cyamus. This homology is
extended to all Arachnids; their first two pairs of appendages, however they
may be modified as "false" mandibles and "false" maxillæ, really
correspond to the second and third maxillipedes in Crustacea, and to the
second and third pairs of feet in insects. It is interesting to note that he treats
Limulus as an Arachnid, pointing out that there is as much difference
between Apus and Limulus as between Cancer and Phalangium. He
describes the "gnathobases" in Phalangium and Limulus. We may note that
he had just an inkling of the modern doctrine that all the appendages of
Articulates consist of a basal joint bearing an inner and an outer terminal
piece, for he observes that the "cirri" of the maxillipedes of Crustacea give

the appendage the same bifid appearance as the appendages of the abdomen
and the thoracic legs of Mysis (p. 50).
V. Audouin, in his memoir, Recherches anatomiques sur le thorax des
animaux articulés,[136] applied the principle of the unity of plan and
composition to the exoskeleton of insects, Crustaceans, and Arachnids. His
guiding ideas were, "(1) that the skeleton of articulated animals is formed of
a definite number of pieces, which are either distinct or intimately fused
with one another; (2) that in many cases, some pieces diminish or altogether
disappear, while others reach an excessive development; (3) that the
increase of one piece seems to exert on the neighbouring pieces a kind of
influence which explains all the differences one finds between the
individuals of each order, family and genus" (Sep. copy, p. 16p. Geoffroy
had already stated, without proof, that the parts of the Arthropod's skeleton,
however they might change in shape and size, remained faithful to the
principle of connections, at least at their points of insertion.[137] Audouin
gave the detailed demonstration of this by his accurate and minute
determination of the pieces of the arthropod skeleton. He recognised that
the body of Arthropods was made up of a series of similar rings, and that
even the compact head of insects consisted of fused segments. In each
segment Audouin distinguished a fixed number of hard chitinous parts, the
dorsal tergum, the ventral sternum, the lateral "flanc" of three pieces, all to
be recognised by their positions relative to one another. Many of the names
which he proposed are still in use; it was he who introduced the terms
prothorax, mesothorax, and metathorax, for the three segments of the
insect's thorax. He used Geoffroy's Loi de balancement to explain cases of
correlative development, such as the relation between the size of the front
wings and the development of the mesothorax. In another paper Audouin
compared the three pieces of the dorsal skeleton of Trilobites to the tergum
and the upper part of the "flanc."[138] In a third paper of about the same time
he tried to establish the homologies of the segments throughout the
Articulate series—with less success than Savigny.
Later on, in conjunction with Milne-Edwards, he demonstrated the unity of
composition of the nervous system in Crustacea, showing how the
concentrated system of the crab was formed by the same series of ganglia as
in the Macrura.

The entomologist Latreille also tackled the problem of the homologies of
the segments in the different classes of Arthropods (Cuvier, loc. cit.,
p. cclxxii.). He thought he could find fifteen segments in all Arthropods. He
made the retrograde step of likening the head of insects to a single segment.
But some of his homologies showed morphological insight, e.g., his
comparison of the "first jaws" of Arachnids to antennæ, because they were
placed above the upper lip. It was he who first pointed out the resemblance
of the leaf-like gills of Ephemerid larvæ to wings, and suggested that wings
were "a sort of tracheal feet."
He made also a rather hazy and speculative contribution on Okenian lines to
the problem of the relation of Arthropods to Vertebrates, likening the
carapace of Crustacea to an enormously developed hyoid, the appendages
of the tail to the ventral and anal fins of fish. The masticatory organs of
Arthropods were jaws disjointed at their symphysis; antennæ, nostrils
turned outside in.

Dugès also made a comparison of Articulates with Vertebrates.[139] He did
not accept Geoffroy's vertebral theory of the Arthropod skeleton, though he
admitted that in Arthropods the dorsal surface was turned towards the
ground, basing this assumption on the position of the nervous system, and
also, curiously enough, on the inverted position of the embryo on the lower
surface of the yolk. He considered that the mandibles and first maxillæ of
Arthropods were the homologues of the upper and lower jaws of
Vertebrates, adducing as confirmatory evidence the fact that in snakes the
rami are separate. The labium was the equivalent of the hyoid, the labial
palps and maxillipedes the equivalent of the "hyoid" elements which form
the branchial arches.
But Dugès' main contribution to morphological method was his conception
of the living organism as a colony of lesser units, which were themselves
real "organisms." "By organism the author means a complex of organs
which taken together suffice to constitute, ideally or actually, a complete
animal. An 'organism' is, as it were, an elementary or simple animal; several
organisms combined form a complex animal" (p. 255). Dugès hit upon this
principle, which was first suggested to him by A. Moquin-Tandon's work on
the leech (1827), as a great aid in demonstrating the unity of plan and
composition throughout the animal kingdom.[140] According to his view

there are three main types of animals—(1) Biserials, including bilaterally
symmetrical animals, composed of two parallel series of "organisms"; (2)
Radiates, composed of "organisms" arranged like the spokes of a wheel;
and (3) Raceme-animals, in which the separate "organisms" were disposed
more or less irregularly, in bunches (p. 257). The unitary "organism" is
supposed to be the same in all, only the arrangement differing. Dugès of
course admitted that the centralisation of the complete organism became
greater the higher it stood in the scale, and that this held good also in
individual development. The appendages of Articulates and Vertebrates
were thought of as the members of as many separate organisms. He went so
far as to suggest that the fingers of a man's hand were the free extremities of
as many thoracic members.
Dugès' conception of the organism has often been revived since in a saner
form, e.g., by E. Perrier, and it has a certain validity. It has much affinity
with the similar conceptions of Goethe and the German transcendentalists.
[130] Mem. Acad. Sci., iv., pp. cclxxxiv.-ccci., 1824.
[131] Ann. Sci. Nat., xi., xii., 1827; xvi., 1829; xxi., 1830.
[132] See Rádl, loc. cit., i., pp. 225-6.
[133] Ann. Sci. nat. (2), ii., p. 248, 1834.
[134] Ann. Sci. nat., iii., pp. 377-80, 1824.
[135] Mémoires sur les Animaux sans Vertèbres, Part I., p. 10, Paris, 1816.
[136] Ann. Sci. Nat., (1), i., pp. 97-135, 416-432, 1824.
[137] Isis, p. 456, 1820 (2).
[138] Cuvier, Mém. Acad. Sci., iv., p. cclxx., 1824.
[139] Acad. Sci. 18th Oct. 1831. Extract in Ann. Sci. Nat., xxiv., pp. 254-60,
1831.
[140] His views were more fully elaborated in his Mémoire sur la conformité
organique dans l'échelle animale, Montpellier, 1832.

CHAPTER VII

THE GERMAN TRANSCENDENTALISTS

To complete our historical survey of the morphology of the early 19th
century we have now to turn back some way and consider the curious
development of morphological thought in Germany under the influence of
the Philosophy of Nature. We have already seen many of these notions
foreshadowed by Goethe, who had considerable affinity with the
transcendentalists, but the full development of transcendental habits of
thought comes a little later than the bulk of Goethe's scientific work, and
owes more to Kielmeyer and Oken than to Goethe himself.
A great wave of transcendentalism seems to have passed over biological
thought in the early 19th century, arising mainly in Germany, but
powerfully affecting, as we have seen, the thought of Geoffroy and his
followers. Many ideas were common to the French and German schools of
transcendental anatomy, the fundamental conception that there exists a
unique plan of structure, the idea of the scale of beings, the notion of the
parallelism between the development of the individual and the evolution of
the race. It is difficult to disentangle the part played by each school and to
determine which should have the credit for particular theories and
discoveries. The philosophy seems to have come chiefly from Germany, the
science from France. It must be borne in mind that German comparative
anatomy was largely derivative from French, that the Paris Museum was the
acknowledged anatomical centre, and that Cuvier was its acknowledged
head.
It is probably correct to say that the credit mainly belongs to the German
transcendental school for the law of the parallelism between the stages of
individual development and the stages of the scale of beings, and the theory
of the repetition or multiplication of parts within the individual. The
vertebral theory of the skull is a particular application of the second of these
generalisations.

The law of parallelism[141] seems to have been expressed first by Kielmeyer
(1793),[142] who gave to it a physiological form, saying that the human
embryo shows at first a purely vegetative life, then becomes like the lower
animals, which move but have no sensation, and finally reaches the level of
the animals that both feel and move.

The idea was next taught by Autenrieth in 1797.[143]
Oken (1779-1851) in his early tract Die Zeugung (1805), and in his
Lehrbuch der Naturphilosophie (1809-11) elaborated the thought, and
taught that every animal in its development passes through the classes
immediately below it. "During its development the animal passes through
all stages of the animal kingdom. The fœtus is a representation of all animal
classes in time."[144] The Insect, for example, is at first Worm, next Crab,
then a perfect volant animal with limbs, a Fly (ibid., p. 542).
As Nature is "the representation of the individual activities of the spirit," so
the animal kingdom is the representation of the activities or organs of man.
The animal kingdom is therefore "a dismemberment of the highest animal,
i.e., of Man" (p. 494). Now "animals are gradually perfected, entirely like
the single animal body, by adding organ unto organ"—the way of evolution
is the way of development. Hence "animals are only the persistent fœtal
stages or conditions of Man," who is the microcosm, and contains within
himself all the animal kingdom.

Oken was himself a careful student of embryology; von Baer[145] speaks of
his work (published in Oken and Kieser, Beiträge zur vergleichenden
Zoologie, Anatomie und Physiologie, 2 pts., 1806-7) as forming the turning-
point in our understanding of the mammalian ovum. He had accordingly
actually observed a resemblance in certain details of structure between the
human fœtus and the lower animals; but the peculiar form which the law
took in his hands was a consequence of his hazy philosophy. He saw the
relation of teratological to fœtal structure, for he affirmed that
"malformations are only persistent fœtal conditions" (p. 492).
The idea of comparing the embryo of higher animals with the adult of lower
was widely spread at this time among German zoologists. We find, for
example, in Tiedemann's brilliant little textbook[146] the statement that
"Every animal, before reaching its full development, passes through the
stage of organisation of one or more classes lower in the scale, or, every
animal begins its metamorphosis with the simplest organisation" (p. 57).
Thus the higher animals begin life as a kind of fluid animal jelly which
resembles the substance of a polyp; the young mammal, like the lower

Vertebrates, has only a simple circulation, and, like them, lives in water (the
amniotic fluid); the frog is first like a worm, then develops gills and
becomes like a fish (p. 57). In his work on the anatomy of the brain,[147]
Tiedemann established the homology of the optic lobes in birds by
comparing them with fœtal corpora quadrigemina in man (see Serres, Ann.
Sci. nat., xii., p. 112).
J. F. Meckel, in 1811, devoted a long essay to a detailed proof of the
parallelism between the embryonic states of the higher animals and the
permanent states of the lower animals. In a previous memoir in the same
collection[148] (i., 1, 1808) he had made some comparisons of this kind in
dealing with the development of the human fœtus; in this memoir (ii., 1,
1811) he brings together all the facts which seem to prove the parallelism.
His collection of facts is a very heterogeneous one; he mingles
morphological with physiological analogies, and makes the most far-
fetched comparisons between organs belonging to animals of the most
diverse groups. He compares, for instance, the placenta with the gills of
fish, of molluscs and of worms, homologising the cotyledons with the
separate tufts of gills in Tethys, Scyllæa and Arenicola(p. 26). This is purely
a physiological analogy. He compares the closed anus of the early human
embryo with the permanent absence of an anus in Cœlentera, and the
embryo's lack of teeth with the absence of teeth in many reptiles and fish, in
birds, and in many Cetacea (p. 46).[149] These are merely chance
resemblances of no morphological importance. He considers bladderworms
as animals which have never escaped from their amnion, and Volvox as not
having developed beyond the level of an egg (p. 7). He lays much stress
upon likeness of shape and of relative size, comparing, for instance, the
large multilobate liver of the human fœtus with the many-lobed liver of
lower Vertebrates and of Invertebrates. In general he shows himself, in his
comparisons, lacking in morphological insight.
His treatment of the vascular system affords perhaps the best example of his
method (pp. 8-25). The simplest form of heart is the simple tubular organ in
insects, and it is under this form that the heart first appears in the
developing chick. The bent form of the embryonic heart recalls the heart of
spiders; it lies at first free, as in the mollusc Anomia. The heart consists at
first of one chamber only, recalling the one-chambered heart of Crustacea.

A little later three chambers are developed, the auricle, ventricle, and aortic
bulb; at this stage there is a resemblance to the heart of fish and amphibia.
At the end of the fourth day the auricle becomes divided into two, affording
a parallel with the adult heart of many reptiles.
In his large text-book of a somewhat later date, the System der
vergleichenden Anatomie (i., 1821), he works out the idea again and gives
to it a much wider theoretic sweep, hinting that the development of the
individual is a repetition of the evolutionary history of the race. Meckel was
a timid believer in evolution. He thought it quite possible that much of the
variety of animal form was due to a process of evolution caused by forces
inherent in the organism. "The transformations," he writes, "which have
determined the most remarkable changes in the number and development of
the instruments of organisation are incontestably much more the
consequence of the tendency, inherent in organic matter, which leads it
insensibly to rise to higher states of organisation, passing through a series of
intermediate states."[150]
His final enunciation of the law of parallelism in this same volume shows
that he considered the development of the individual to be due to the same
forces that rule evolution. "The development of the individual organism
obeys the same laws as the development of the whole animal series; that is
to say, the higher animal, in its gradual evolution, essentially passes through
the permanent organic stages which lie below it; a circumstance which
allows us to assume a close analogy between the differences which exist
between the diverse stages of development, and between each of the animal
classes" (p. 514).
He was not, of course, able fully to prove his contention that the lower
animals are the embryos of the higher, and we gather from the following
passage that he could maintain it only in a somewhat modified form. "It is
certain," he writes, "that if a given organ shows in the embryo of a higher
animal a given form, identical with that shown throughout life by an animal
belonging to a lower class, the embryo, in respect of this portion of its
economy, belongs to the class in question" (p. 535). The embryo of a
Vertebrate might at a certain stage of development, be called a mollusc, if
for instance, it had the heart of a mollusc.

He admits, too, that the highest animal of all does not pass through in his
development the entire animal series. But the embryo of man always and
necessarily passes through many animal stages, at least as regards its single
organs and organ-systems, and this is enough in Meckel's eyes to justify the
law of parallelism (p. 535).
In his excellent discussion of teratology Meckel points out how the idea of
parallelism throws light upon certain abnormalities which are found to be
normal in other (lower) forms (p. 556).[151]
We may refer to one other statement of the law of parallelism—by K. G.
Carus in his Lehrbuch der vergleichenden Anatomie (Leipzig, 1834). The
standpoint is again that of Naturphilosophie. It is a general law of Nature,
Carus thinks, that the higher formations include the lower; thus the animal
includes the vegetable, for it possesses the "vegetative" as well as the
"animal" organs. So it is, too, by a rational necessity that the development
of a perfect animal repeats the series of antecedent formations.
As we have said, the main credit for the enunciation of the law of
parallelism belongs to the German transcendental school; but the law owes
much also to Serres, who, with Meckel, worked out its implications. It
might for convenience, and in order to distinguish it from the laws later
enunciated by von Baer and Haeckel, be called the law of Meckel-Serres.
Under the "theory of the repetition or multiplication of parts within the
organism" may be included, first, generalisations on the serial homology of
parts, and second, more or less confused attempts to demonstrate that the
whole organisation is repeated in certain of the parts. The recognition of
serial homologies constituted a real advance in morphology; the
"philosophical" idea of the repetition of the whole in the parts led to many
absurdities. It led Oken to assert that in the head the whole trunk is
repeated, that the upper jaw corresponds to the arms, the lower to the legs,
that in each jaw the same bony divisions exist as in the limbs, the teeth, for
instance, corresponding to the claws (loc. cit., p. 408). It led him to
distinguish "two animals" in every body—the cephalic and the sexual
animal. Each of these has its own organs; thus "in the perfect animal there
are two intestinal systems thoroughly distinct from each other, two
intestines which belong to two different animals, the sexual and cephalic

animal, or the plant and the animal" (p. 382). The intestine of the sexual
animal is the large intestine; the lungs of the sexual animal are the kidneys,
its glottis is the urethra, its mouth the anus. So, too, the mouth is the
stomach of the head. On another line of thought the sternum is a ventral
vertebral column. Limbs are connate ribs, the digits indicating the number
of ribs included (cf. Dugès, supra, p. 88).

J. F. Meckel[152] discusses "homologies" of this kind in the thorough and
pedestrian way so characteristic of him. Not only, he says, are the right and
left halves of the body comparable with one another, but also the upper and
the lower, the dividing line being drawn at the level of the diaphragm. The
lumbar complex corresponds to the skull, the anus to the mouth, the urino-
genital opening to the nasal opening; in general, the urino-genital system
corresponds to the respiratory, the kidneys to the lungs, the ureters to
bronchi, the testes and ovaries to the thymus (he had observed the
physiological relation between the development of the thymus and the state
of the genital organs), the prostate and the uterus to the thyroid gland, and
the penis and clitoris to the tongue. The fore-limbs and girdle correspond in
detail with the hind limbs and the pelvis—a point already worked out by
Vicq d'Azyr; the dorsal and ventral halves of the body are likewise
comparable in some respects, the sternum, for example, answering in the
arrangement of its bones, muscles and arteries to the vertebral column. The
skeleton of each member is in some respects a repetition of the vertebral
column.

His brother, D. A. Meckel,[153] worked out an elaborate comparison
between the alimentary canal and the genital organs, basing the legitimacy
of the comparison upon early embryological relations and upon the state of
things in Cœlentera, where genital and digestive organs occupy the same
cavity. In his view the uterus corresponded to the stomach, the vagina to the
œsophagus, the fallopian tubes to the intestine, and so on.
The vertebral theory of the skull took its origin from the same habit of
thought. As part of the wider idea of the metameric repetition of parts it had
some scientific worth, but the theory was pushed too far, and the facts were
twisted to suit it. Among annulate animals the theory of repetition found
ample scope; Oken was able to compare with justice the jaws of crabs and
insects with their other limbs, as Savigny did later in a more scientific way.

Among Vertebrates the application of the theory of serial repetition was not
so obvious, except in the case of the vertebræ. Goethe seems to have been
the first to hit upon the idea that the skull is composed of a number of
vertebræ, serially homologous with those of the vertebral column. He tells
us that the idea flashed into his mind when contemplating in the Jewish
cemetery at Venice a dried sheep's skull. The discovery was made in 1790,
but not published till 1820.[154]
The idea seems to have been taught by Kielmeyer, one of the earliest of the
"philosophers of nature," but it was not published by him.
In a book (Cours d'Études médicales), published in 1803, Burdin
assimilated the skull to the vertebral column.
Oken, in an inaugural dissertation (Programm) Ueber die Bedeutung der
Schädelknochen,[155] published in 1807, gave to the theory its necessary
development. Autenrieth, also in 1807,[156] distinguishing separate ganglia
in the brain, was not far from the hypothesis that each of these ganglia must
have its separate vertebra.
In 1808 Duméril read a paper to the Académie des Sciences in which he
compared the skull to a gigantic vertebra, basing his hypothesis on the
similarity existing between the crests and depressions on the hinder part of
the skull and those on the posterior surfaces of the vertebræ.
After Oken's work the vertebral theory was taken up generally by both the
German and the French anatomists. Spix published in 1815 a large volume
on the skull, entitled Cephalogenesis, distinguishing (as Oken did at first)
three cranial vertebræ. Bojanus in his Anatome testudinis europæae (1819),
and in a series of papers in Isis (1817-1819, and 1821) established the
existence of a fourth cranial vertebra, and this was accepted by Oken in the
later editions of his Lehrbuch. Meckel and Carus among the Germans, de
Blainville and E. Geoffroy among the French, contributed to the
development of the theory. In England the theory was championed
particularly by Richard Owen.
It was one thing to assert in a moment of inspiration that the skull was
composed of modified vertebræ; it was quite another to demonstrate the
relation of the separate bones of the skull to the supposed vertebræ. Upon

this much uncertainty reigned; there was not even unanimity as to the
number of vertebræ to be distinguished. Goethe found six vertebræ in the
skull; Spix, and at first Oken, three only, Geoffroy seven; the accepted
orthodox number seems to have been four (Bojanus, Oken, Owen).
As an example of the method of treatment adopted we may take Oken's
matured account of the composition of the cranial vertebræ, as given in the
English translation of his Lehrbuch. "To a perfect vertebra," he says,
"belong at least five pieces, namely, the body, in front the two ribs, behind
the two arches or spinous processes" (p. 370). In the cervical vertebræ the
transverse processes represent the ribs. The skull consists of four vertebræ,
the occipital, the parietal, the frontal and the nasal, or, named after the sense
with which each is associated, the auditory, the lingual, the ocular and the
olfactory. The "bodies" of these vertebræ are the body of the occipital
(basioccipital), the two bodies of the sphenoid (basi- and pre-sphenoid), and
the vomer. The transverse processes of each are the condyles of the
occipitals (exoccipitals), the alæ of the two sphenoids (alisphenoids and
orbitosphenoids) and the lateral surfaces of the vomer. The arches or
spinous processes are the occipital crest, the parietals, the frontals, and the
nasals.
The cranium is thus composed of four rings of bone, each composed of the
typical elements of a vertebra.
The arbitrary nature of the comparison is obvious enough. As Cuvier
pointed out in the posthumous edition of his Leçons, it is only the occipital
segment that shows any real analogy with a vertebra—an analogy which
Cuvier ascribed to similarity of function. He admitted a faint resemblance
of the parietal segment to a vertebra:—"The body of the sphenoid does
indeed look like a repetition of the basioccipital, but having a different
function it takes on another form, especially above, by reason of its
posterior clinoid apophyses."[157] He denied the resemblance of the frontal
and nasal "vertebræ" to true vertebræ, pointing out that both parietals and
frontals are bones specially developed for the purpose of roofing over and
protecting the cerebrum.
A very curious development was given to the vertebral theory by K. G.
Carus, who seems to have taken as his text a saying of Oken's, that the

whole skeleton is only a repeated vertebra.[158] His system is worthy of
some consideration, for he tries to work out a geometry of the skeleton.[159]
His method of deduction is a good example of pure Naturphilosophie. Life,
he says, is the development of something determinate from something
indeterminate. A finite indeterminate thing, that is, a liquid, must take a
spherical form if it is to exist as an individual. Hence the sphere is the
prototype of every organic body. Development takes place by antagonism,
by polarity, typically by the division and multiplication of the sphere. In the
course of development the sphere may change, by expansion into an egg-
shaped body, or by contraction into a crystalline form, the changes due to
expansion being typical of living things, those due to contraction being
typical of dead. At the surface of the primitive living sphere is developed
the protective dermatoskeleton, which naturally takes the shape of a hollow
sphere; round the digestive cavity which is formed in the living sphere is
developed the splanchnoskeleton; round the nervous system (which is, as it
were, the animal within the animal) is developed the neuroskeleton. All
skeletal formations belong to one or other of these systems.
Carus defines his aim to be the discovery of the inner law which presides
over the formation of the skeleton throughout the animal kingdom; he
desires to know "how such and such a formation is realised in virtue of the
eternal laws of reason" (iii., p. 93). Here we touch the kernel of
Naturphilosophie—the search for rational laws which are active in Nature;
the discontent with merely empirical laws.
The thesis which Carus sustains is that all forms of skeleton, whether of
dermatoskeleton, splanchnoskeleton, or neuroskeleton, can be deduced
from the hollow sphere, which is the primary form of any skeleton
whatsoever (p. 95). That means, put empirically, that every skeleton can be
represented schematically by a number of hollow spheres, suitably modified
in shape, and suitably arranged. The chief modification in shape exhibited
by bones is one which is intermediate between the organic and the
crystalline series of modifications of the sphere. The organic modifications
are bounded by curved lines, the crystalline by straight; the intermediate
partly by curved and partly by straight lines. They are the dicone (the shape
of a diabolo) and the cylinder. These forms must necessarily be of
importance for the skeleton, which is intermediate between the organic and

the inorganic. "The dicone embodies the real significance of the bone,"
writes Carus. Each dicone and cylinder composing the skeleton is called by
Carus a vertebra.
We may expect then all skeletons to be composed of spheres, cylinders and
dicones in diverse arrangements. Nature being infinite, all the possible
types of arrangement of these elements must exist in the test or skeleton of
some animal, living, fossil, or to come (p. 127). One conceives easily what
the main types of skeleton must be. In some animals, e.g., sea-urchins, the
skeleton is a simple sphere; in others, e.g., starfish, secondary rows of
spheres radiate out from a central sphere or ring; in annulate animals the
skeleton consists of a row of partially fused spheres.
In Vertebrates the arrangement is more complex. There are first the
protovertebral rings of the dermatoskeleton, these being principally the ribs,
limb-girdles, and jaws. Round the central nervous system are developed the
deutovertebral rings of the neuroskeleton (vertebræ in the ordinary sense).
The apophyses and bodies of the vertebræ, and the bones of the
members[160] are composed of columns of tritovertebræ, or vertebræ of the
third order. Thus the whole vertebrate skeleton is a particular arrangement
of vertebræ, which in their turn are modifications of the primary hollow
sphere.
The German transcendentalists were more or less contemporary with E.
Geoffroy, and no doubt influenced him, especially in his later years, as they
certainly did his follower Serres. Oken indeed wrote, in a note[161]
appended to Geoffroy's paper on the vertebral column of insects, that "Mr
Geoffroy [sic] is without a doubt the first to introduce in France
Naturphilosophie into comparative anatomy, that is to say, that philosophy
one of whose doctrines it is to seek after the signification of organs in the
scale of organised beings." This is, however, an exaggeration, for Geoffroy
was primarily a morphologist, whereas the morphology of the German
transcendentalists was only a side-issue of their Naturphilosophie.
Geoffroy, on his part, exercised some influence on the transcendentalists.
He asserts[162] indeed that Spix got some of the ideas published in the
Cephalogenesis (1815) from attending his course of lectures in 1809. It is
certainly the case that Spix published before Geoffroy the view that the

opercular bones are homologous with the ear-ossicles, adopting, however, a
different homology for the separate bones.[163]
Some speculations seem to have been common to both schools—for
instance, the law of Meckel-Serres, the vertebral theory of the skull, and the
recognition of serial homology in the appendages of Arthropods (Savigny,
Oken). Latreille and Dugès, as well as Serres, clearly show in their
theoretical views the influence of Oken and the other transcendentalists.
Geoffroy's principle of connections and law of compensation were
recognised by some at least of the Germans.
But whatever his actual historical relations may have been with the German
school, Geoffroy was vastly their superior in the matter of pure
morphology. He alone brought to clear consciousness the principles on
which a pure morphology could be based: the Germans were transcendental
philosophers first, and morphologists after.
One understands from this how J. F. Meckel, who was in some ways the
leading comparative anatomist in Germany at this time, could be at once a
transcendentalist and an opponent of Geoffroy. Meckel had a curiously
eclectic mind. A disciple of Cuvier, having studied in 1804-6 the rich
collections at the Museum in Paris, the translator of Cuvier's Leçons
d'anatomie comparée, he earned for himself the title of the "German
Cuvier," partly through the publication of his comprehensive textbook
(System der vergl. Anatomie, 5 vols.), partly by his extensive and many-
sided research work, partly by his authoritative teaching. His System shows
in almost every page of its theoretical part the influence of Cuvier; and it is
through having assimilated Cuvier's teaching as to the importance of
function that Meckel combats Geoffroy's law of connections, at least in its
rigorous form. He submits that the connections of bones and muscles must
change in relation to functional requirements. He rejects Geoffroy's theory
of the vertebrate nature of Articulates. Generally throughout his work the
functional point of view is well to the fore.
Yet at heart Meckel was a transcendentalist of the German school. His
vagaries on the subject of "homologues" leave no doubt about that, and, in
spite of Cuvier, he believed, though not very firmly, in the existence of one
single type of structure.

A Cuverian by training, his lack of morphological sense threw him into the
ranks of the transcendentalists, to whom perhaps he belonged by nature.

[141] For a full account, see Kohlbrugge, Zool. Annalen, xxxviii., 1911.
[142] Rede über das Verhältnis der organischen Kräfte, Stuttgart u. Tübingen,
1793 (1814). See Rádl, loc. cit., i., p. 261; ii., p. 57.
[143] Supplem. ad historiam embryonis, Tübingen, 1797.
[144] Lehrbuch der Naturphilosophie, Eng. trans., p. 491, 1847.
[145] Ueber Entwickelungsgeschichte der Thiere, i., p. xvii., 1828.
[146] Zoologie, Landshut, i., 1808.
[147] Anatomie u. Bildungsgeschichte des Gehirns im Fötus des Menschen,
Nürnberg, 1816.
[148] Beyträge zur vergleichende Anatomie, Leipzig, i., 1808-9, ii., 1811-2.
[149] Cetacea were generally considered at this time to be mammals of low
organisation.
[150] From the French trans., which appeared under the title Traité gén. d'Anat.
comparée, i., p. 449, 1828.
[151] Cf. Geoffroy (supra, p. 70).
[152] Beyträge, ii., 2, 1812. Also in his System d. vergl. Anat., i., 1821.
[153] In J. F. Meckel's Beyträge, ii.
[154] Zur Morphologie, i., 2, p. 250, 1820; and ii., 2, pp. 122-4, 1824.
[155] See translation, giving the gist of this paper, in Huxley's Lectures on the
Elements of Comparative Anatomy, pp. 282-6, London, 1864.
[156] Reil's Archiv. f. Physiol., vii., 1807.
[157] Leçons d'anatomie comparée, 3rd ed., Brussels reprint, i., p. 414, 1836.
[158] In his Programm, U. d. Bedeut. d. Schädelknochen, 1807.
[159] Traité élémentaire d'anatomie comparée (French trans.), vol. iii., Paris,
1835. First developed in his volume Von den Ur-Theilen des Knochen und
Schalen-Gerustes, Leipzig, 1828.
[160] Dutrochet in 1821 had tried to prove that the bones of the members belong
to the type of the vertebra—the dicone.
[161] Isis, pp. 552-9, 1820 (2).
[162] Mém. Mus. d'Hist. nat., ix., 1822.
[163] Cuvier and Valenciennes, Hist. nat. Poissons, i., p. 311, f.n.

CHAPTER VIII

TRANSCENDENTAL ANATOMY IN ENGLAND—RICHARD OWEN

Richard Owen is the epigonos of transcendental morphology; in him its
guiding ideas find clear expression, and in his writings are no half-truths
struggling for utterance.

Fig. 4.—Ideal Typical Vertebra. (After Owen.)
But he was, though a staunch transcendentalist, an eclectic of the older
ideas current in his time; for he picked out what was best in the older
systems—Cuvier's teleology, Geoffroy's principle of connections, Oken's
idea of the serial repetition of parts. In particular, he assimilated the
teaching of Cuvier, the great opponent of the transcendentalists, and
reconciled it in part with his own transcendentalism. His main theoretical

views are to be found in his volume On the Archetype and Homologies of
the Vertebrate Skeleton (London, 1848). The master-idea of the book is that
the vertebrate skeleton consists of a series of comparable segments, each of
which Owen calls a vertebra.

Fig 5.—Natural Typical Vertebra; Thorax of a Bird. (After
Owen.)
His definition of a vertebra is, "one of those segments of the endo-skeleton
which constitute the axis of the body, and the protecting canals of the
nervous and vascular trunks" (p. 81). The parts of a typical vertebra are
shown in Fig. 4, which is copied from Owen's Fig. 14.
In Fig. 5 (page 103) is shown an actual vertebra, as Owen conceives it, the
"vertebra" being that of a bird.
A segment of sternum is included as the "hæmal spine" of the vertebra (hs);
the vertebral rib is the "pleurapophysis" (pl); the sternal rib the
"hæmapophysis" (h); the uncinate process of the vertebral rib is known as
the "diverging appendage" (a). The whole vertebrate skeleton is composed
of a series of vertebræ which show these typical parts. We arrive thus at the

conception of an "Archetype" of the vertebrate skeleton, such as is
represented in Fig. 6.
The archetype is only a scheme of what is usually constant in the vertebrate
skeleton, and both the number and the arrangement of the bones in any real
Vertebrate are subject to variation. "It has been abundantly proved," Owen
writes, towards the end of his volume, "that the idea of a natural segment
(vertebra) of the endoskeleton does not necessarily involve the presence of
a particular number of pieces, or even a determinate and unchangeable
arrangement of them. The great object of my present labour has been to
deduce ... the relative value and constancy of the different vertebral
elements, and to trace the kind and extent of their variations within the
limits of a plain and obvious maintenance of a typical character" (p. 146).
It goes without saying that Owen considered the skull to be formed of
vertebræ—the vertebral theory of the skull was, in his system, a deduction
from the vertebral theory of the skeleton. He recognised four cranial
vertebræ; the arrangement of them, and the relation of their constituent
bones to the parts of the typical vertebra are shown in the table appearing
on page 106. So far as their first three elements are concerned, these
vertebræ are practically identical with the vertebræ distinguished in the
classical vertebral theory of the skull, as enunciated by Oken. A divergence
appears with the determination of the other elements of the vertebræ. The
upper and lower jaws are associated with the nasal and frontal vertebræ
respectively, not however as limbs of the head, but as constituent elements
of these vertebræ. In the same way the hyoid apparatus is part and parcel of
the parietal vertebra, and the pectoral girdle and fore-limbs part of the
occipital vertebra.

Fig. 6.—The Archetype of the Vertebrate Skeleton. (After
Owen.)

Cranial Vertebræ.[164] (After Owen, 1848, p. 165.)
Vertebræ. Occipital. Parietal. Frontal. Nasal.
Centra. Basioccipital. Basisphenoid. Presphenoid. Vomer.
Neurapopbyses. Exoccipital. Alisphenoid. Orbitosphenoid. Prefrontal.
Neural Spines. Supraoccipital. Parietal. Frontal. Nasal.
Parapopbyses. Paroccipital. Mastoid. Postfrontal. None.
Pleurapophyses. Scapular. Stylohyal. Tympanic. Palatal.
Hæmapophyses. Coracoid. Ceratohyal. Articular. Maxillary.
Hæmal Spines. Episternum. Basihyal. Dentary. Premaxillary.
Diverging Fore-limb or Fin. Branchio-stegals. Operculum. Pterygoid and
Appendage. Zygoma.

Owen's reasons for considering the pectoral girdle and the fore-limb part of
the occipital vertebra are as follows. In fish the pectoral girdle is slung to
the skull by means of the post-temporal bone (supra-scapula, according to
Owen) which abuts on the occipital arch. In Lepidosiren, whose skeleton

resembles the archetype in many ways, the pectoral girdle is likewise
attached to the occipital segment.
In most other Vertebrates the pectoral girdle has shifted backwards along
the vertebral column, by a "metastasis" (Geoffroy) similar to that by which
the pelvic fins in many fish have shifted up close to the pectoral girdle. The
scapula (with supra-scapula) is the pleurapophysis, the coracoid the
hæmapophysis, of the occipital vertebra. The clavicle is homologised with
the slender bone in fish now known as the post-clavicle, which shows a
connection with the first or atlas vertebra of the vertebral column, forming,
according to Owen, the hæmapophysis of the atlas. Owen considers it no
objection to this view that in other Vertebrates the clavicle is anterior to the
coracoid—"its anterior position to the coracoid in the air-breathing
Vertebrata is no valid argument against the determination, since in these we
have shown that the true scapular arch is displaced backwards" (On the
Nature of Limbs, p. 63, London, 1849). In the pelvic girdle the ilium
corresponds to the scapula, the ischium to the coracoid, the pubis to the
clavicle. Hence the ilium is a pleurapophysis, the ischium and pubis are
both hæmapophyses. The fore-limb is the developed "appendage" of the
occipital vertebra, the hind-limb the developed "appendage" of the pelvic
vertebra. They are serially homologous with, for example, the uncinate
processes of the ribs in birds (see Figs. 5 and 6). The fore-limb is a simple
filament in Lepidosiren, and presents few joints in Proteus and Amphiuma;
in other air-breathing Vertebrates it shows a more complete development,
the humerus, radius and ulna, and the bones of the wrist and hand becoming
differentiated out.
As the fore-limb is equivalent to a single bone of the archetype, it is said to
be, in its developed state, "teleologically compound" (p. 103).
Since in the archetype every vertebra has its appendage, more than two
pairs of locomotory limbs might have been developed. "Any given
appendage might have been the seat of such developments as convert that
of the pelvic arch into a locomotive limb; and the true insight into the
general homology of limbs leads us to recognise many potential pairs in the
typical endoskeleton. The possible and conceivable modifications of the
vertebrate archetype are far from having been exhausted in the forms which
have hitherto been recognised, from the primæval fishes of the palæozoic

ocean of this planet up to the present time" (p. 102). It is not of the essence
of the vertebrate type to be tetrapodal.
In determining homologies Owen remained true to Geoffroy's principle of
connections. Speaking of an attempt which had been made to determine
homologies by the mode of development, he writes, "There exists doubtless
a close general resemblance in the mode of development of homologous
parts; but this is subject to modification, like the forms, proportions,
functions, and very substance of such parts, without their essential
homological relationships being thereby obliterated. These relationships are
mainly, if not wholly, determined by the relative position and connection of
the parts, and may exist independently of form, proportions, substance,
function and similarity of development. But the connections must be sought
for at every period of development, and the changes of relative position, if
any, during growth, must be compared with the connections which the part
presents in the classes where vegetative repetition is greatest and adaptive
modification least" (p. 6). It is interesting to note that in Owen's opinion
comparative anatomy explains embryology. Thus the scapula, which is the
pleurapophysis of the occipital vertebra, is vertical on its first appearance in
the embryo of tetrapoda, and lies close up to the head (On the Nature of
Limbs, p. 49)—the embryo shows a greater resemblance to the archetype
than the adult. "We perceive a return to it, as it were, in the early phases of
development of the highest organised of the actually existing species, or we
ought rather to say that development starts from the old point; and thus, in
regard to the scapula, we can explain the constancy of its first appearance
close to the head, whether in the human embryo or in that of the swan, also
its vertical position to the axis of the spinal column, by its general
homology as the rib or 'pleurapophysis' of the occipital vertebra" (Limbs,
p. 56).
We owe to Owen the first clear distinction between "homologous" and
"analogous" organs; it was he who first proposed the terms "homologue"
and "analogue," which he defined as follows:—"Analogue. A part or organ
in one animal which has the same function as another part or organ in a
different animal." "Homologue. The same organ in different animals under
every variety of form and function."[165]

He introduced also useful distinctions between Special, General, and Serial
Homology. "The relations of homology," he writes, "are of three kinds: the
first is that above defined, viz., the correspondency of a part or organ,
determined by its relative position and connections, with a part or organ in a
different animal; the determination of which homology indicates that such
animals are constructed on a common type; when, for example, the
correspondence of the basilar process of the human occipital bone with the
distinct bone called 'basi-occipital' in a fish or crocodile is shown, the
special homology of that process is determined. A higher relation of
homology is that in which a part or series of parts stands to the fundamental
or general type, and its enunciation involves and implies a knowledge of the
type on which a natural group of animals, the Vertebrate, for example, is
constructed. Thus when the basilar process of the human occipital bone is
determined to be the 'centrum' or 'body' of the last cranial vertebra, its
general homology is enunciated.
"If it be admitted that the general type of the vertebrate endoskeleton is
rightly represented by the idea of a series of essentially similar segments
succeeding each other longitudinally from one end of the body to the other,
such segments being for the most part composed of pieces similar in
number and arrangement, and though sometimes extremely modified for
special functions, yet never so as to wholly mask their typical character—
then any given part of one segment may be repeated in the rest of the series,
just as one bone may be reproduced in the skeletons of different species,
and this kind of repetition or representative relation in the segments of the
same skeleton I call 'serial homology'" (p. 7). As an example of serial
homology we might take the centra of the vertebræ—the vomer, the
presphenoid, the basisphenoid, the basioccipital and the series of centra in
the spinal column. Such serially repeated parts are called homotypes (p. 8).
Not all the bones of the vertebrate skeleton are included in the archetype as
constituents of the vertebræ. Thus the branchial and pharyngeal arches are
accounted part of the splanchnoskeleton, as belonging to the same category
as the heart bone of some ruminants, and the ossicles of the stomach in the
lobster (p. 70). The ossicles of the ear in mammals are "peculiar
mammalian productions in relation to the exalted functions of a special
organ of sense" (p. 140, f.n.). This recognition of a possible development of
new organs to meet new functions shows unmistakably the influence of

Cuvier. Owen was indeed well aware of the importance of the functional
aspect of living things, and he often adopted the teleological point of view.
As a true morphologist, however, he held that the principle of adaptation
does not suffice to explain the existence of special homologies. The
ossification of the bones of the skull from separate centres may be
purposive in Eutheria, in that it prevents injury to the skull at birth; but how
explain on teleological principles the similar ossification from separate
centres in marsupials, birds and reptiles? How explain above all the fact that
the centres are the same in number and relative position in all these groups?
Surely we must accept the idea of an archetype "on which it has pleased the
divine Architect to build up certain of his diversified living works" (p. 73).
In his study of centres of ossification, Owen made in point of theory a
distinct advance on his predecessors. We saw that Geoffroy recognised the
importance of studying the ossification of the skeleton, and that Cuvier
accepted such embryological evidence as an aid in determining homologies.
Owen pointed out that it was necessary to distinguish between centres of
ossification which were teleological in import and such as were purely
indicative of homological relationships. Many bones, single in the adult,
arise from separate centres of ossification, but we must distinguish between
"those centres of ossification that have homological relations, and those that
have only teleological ones; i.e., between the separate points of ossification
of a human bone which typify vertebral elements, often permanently
distinct bones in the lower animals; and the separate points which, without
such signification, facilitate the progress of osteogeny, and have for their
obvious final cause the well-being of the growing animal" (p. 105). There
is, for example, a teleological reason why in mammals and leaping
Amphibia (e.g., frogs), the long bones should ossify first at their ends, for
the brain is thus protected from concussion; in reptiles that creep there is
less danger of concussion, and the long bones ossify in the middle (p. 105).
But there is no teleological reason why the coracoid process of the scapula
should in all mammals develop from a separate centre. The coracoid is
however a real vertebral element (hæmapophysis), and in monotremes,
birds and reptiles it is in the adult a large and separate bone. Its ossification
from a separate centre in mammals has therefore a homological
significance. The scapula in mammals is an example of what Owen calls a
"homologically compound" bone. All those bones which are formed by a

coalescence of parts answering to distinct elements of the typical vertebra
are "homologically compound" (p. 105). On the other hand, "All those
bones which represent single vertebral elements are 'teleologically
compound' when developed from more than one centre, whether such
centres subsequently coalesce, or remain distinct, or even become the
subject of individual adaptive modifications, with special joints, muscles,
etc., for particular offices" (p. 106). The limb-skeleton, corresponding as it
does to a single bone of the archetype, is the typical example of a
teleologically compound bone. Owen in his definition of teleological
compoundness has combined two kinds of adaptation—(1) temporary
adaptation of bones to the exigencies of development, birth and growth
(e.g., development of long bones from separate centres); (2) definitive
adaptation of a skeletal part to the functions which it has to perform (e.g.,
teleological structure of limbs). Such adaptations are, so to speak, grafted
on the archetype.
Owen's general views on the nature of living things merit some attention.
Organic forms, according to Owen, result from the antagonistic working of
two principles, of which one brings about a vegetative repetition of
structure, while the other, a teleological principle, shapes the living thing to
its functions. The former principle is illustrated in the archetype of the
vertebrate skeleton, in the segmentation of the Articulates, in the almost
mathematical symmetry of Echinoderms, and the actually crystalline
spicules of sponges. It is the same principle which causes repetition of the
forms of crystals in the inorganic world. "The repetition of similar segments
in a vertebral column, and of similar elements in a vertebral segment, is
analogous to the repetition of similar crystals as the result of polarising
force in the growth of an inorganic body" (p. 171). This "general polarising
force" it is which mainly produces the similarity of forms, the repetition of
parts, and generally the signs of the unity of organisation. The adaptive or
"special organising force" or ἰδέα, on the other hand, produces the diversity
of organic beings. In every species these two forces are at work, and the
extent to which the general polarising or "vegetative-repetition-force" is
subdued by the teleological is an index of the grade of the species.
This view is analogous to the Geoffroyan conception that the diversity of
form is limited by the unity of plan. Owen thus ranges himself with

Geoffroy against Cuvier, who considered that diversity of form is limited
only by the principle of the adaptation of parts.
[164] Owen introduced most of the names of bones now current.
[165] Lectures on Invertebrate Animals, pp. 374, 379, 1843.

CHAPTER IX

KARL ERNST VON BAER

Von Baer was recognised as the founder of embryology even by his
contemporaries. His predecessors, Aristotle,[166] Fabricius,[167] Harvey,[168]
Malpighi,[169] Haller,[170] Wolff,[171] had made a beginning with the study
of development; von Baer, by the thoroughness of his observation and the
strength of his analysis, made embryology a science.
It was to one of the German transcendentalists that von Baer owed the
impulse to study development. Ignatius Döllinger, Professor in Würzburg,
induced three of his pupils, Pander, d'Alton and von Baer, to devote
themselves to embryological research. The development of animals was at
this time little known, in spite of recent work by Meckel (1815 and 1817),
Tiedemann (Anatomie u. Bildungsgeschichte des Gehirns, 1816), by Oken
(loc. cit., supra, p. 90), and some others.
Pander, with whom apparently Döllinger and d'Alton collaborated, was the
first to publish his results;[172] von Baer, who through absence from
Würzburg had for a time dropped his embryological studies, started to work
in 1819, after the publication of Pander's treatise, and produced in 1828 the
first volume of his master-work, Ueber Entwickelungsgeschichte der
Thiere. Beobachtung und Reflexion (Königsberg, 1828). The second volume
followed in 1837, but dates really from 1834, and was published in an
incomplete form. This second volume is intended as an introduction to
embryology for the use of doctors and science students. In it von Baer
describes in full detail the development of many vertebrate types—chick,
tortoise, snake, lizard, frog, fish, several mammals and man, basing his

remarks largely upon his personal observations, but taking account also of
all contemporary work. A separate account of the development of a fish
(Cyprinus blicca) appeared in 1835.[173]
We shall concentrate attention on the first volume. This volume contains the
first full and adequate account of the development of the chick, followed by
a masterly discussion of the laws of development in general.
When we consider that von Baer worked chiefly with a simple microscope
and dissecting needles, the minuteness and accuracy of his observations are
astonishing. He described the main facts respecting the development of all
the principal organs, and if, through lack of the proper means of
observation, he erred in detail, he made up for it by his masterly
understanding and profound analysis of the essential nature of development.
His account of the development of the chick is a model of what a scientific
memoir ought to be; the series of "Scholia" which follow contain the
deductions he made from the data, and, in so far as they are direct
generalisations from experience, they are valid for all time.
The first Scholion is directed against the theory of preformation, and
succeeds in refuting it on the ground of simple observation. The theme of
the second Scholion is that the essential nature (die Wesenheit) of the
animal determines its differentiation, that no stage of development is solely
determined by the antecedent stage, but that throughout all stages the
Wesenheit or idea of the definitive whole exercises guidance. This guidance
is shown most clearly in the regulatory processes of the germ, whereby the
large individual variations commonly presented by the early embryo are
compensated for or neutralised in the course of further development. Baer
in this shows himself a vitalist.
It is, however, the third and subsequent Scholia which must here
particularly occupy our attention, for it is in these that von Baer comes to
grips with morphological problems. Already in the second Scholion he had
definitely enunciated the law which runs as a theme throughout the volume,
the observational and the theoretical part alike, the law that development is
essentially a process of differentiation by which the germ becomes ever
more and more individualised. "The essential result of development," he
writes, "when we consider it as a whole, is the increasing independence

(Selbständigkeit) of the developing animal" (p. 148). In the third Scholion
he elaborates this thought and shows that differentiation takes place in triple
wise. The three processes of differentiation are "primary differentiation" or
layer-formation, "histological differentiation" within the layers, and the
"morphological differentiation" of primitive organs.
The first of these differentiations in time is the formation of the germ-
layers, which takes place by a splitting or separation of the blastoderm into
a series of superimposed lamellæ. Baer's account of the process in the chick
is as follows:—
"First of all, the germ separates out into heterogeneous layers, which with
advancing development acquire ever greater individuality, but even on their
first appearance show rudiments of the structures which will characterise
them later. Thus in the germ of the bird, so soon as it acquires consistency
at the beginning of incubation, we can distinguish an upper smooth
continuous surface and a lower more granular surface. The blastoderm
separates thereupon into two distinct layers, of which the lower develops
into the plastic body-parts of the embryo, the upper into the animal parts;
the lower shows clearly a further division into two closely connected
subsidiary layers—the mucous layer and the vessel-layer; the original upper
layer also shows a division into two, which form respectively the skin and
the parts which I have called the true ventral and dorsal plates—parts which
contain in an undifferentiated state the skeletal and muscular systems, the
connective tissues, and the nerves belonging to these. In order to have a
convenient term for future use, I have named this layer the muscle-layer"
(p. 153).
The process of delamination results then in the formation of four layers, of
which the upper two (composing the "animal" or "serous" layer) will give
origin to the animal (neuromuscular) part of the body, the lower pair to the
plastic or vegetative organs. The uppermost layer will form the external
covering of the embryo, and also the amniotic folds; from it there
differentiates out at a very early stage the rudiment of the central nervous
system, forming a more or less independent layer. Below the outermost
layer lies the layer from which are formed the muscular and skeletal
systems, and beneath this "muscle-layer" comes the "vessel-layer," which
gives origin to the main blood-vessels. The innermost layer of the four will

form the mucous membrane of the alimentary canal and its dependencies; at
the present stage, however, it is, like the other layers, a flat plate.
From all these layers tubes are developed by the simple bending round of
their edges. The outermost layer becomes the investing skin-tube of the
embryo; the layer for the nervous system forms the tubular rudiment of the
brain and spinal cord; the mucous layer curls round to form the alimentary
tube; the muscle layer grows upwards and downwards to form the fleshy
and osseous tube of the body wall; even the vessel layer forms a tube
investing the alimentary canal, but a part of it goes to form the medial
"Gekröse," or mesenterial complex, which departs considerably from the
tubular form.
When these tubes or "fundamental organs" are formed the process of
primary differentiation is complete. The fundamental organs, however, have
at no time actually the form of tubes; they exist as tubes only ideally, for
morphological and histological differentiation go on concurrently with the
process of primary differentiation.
Through morphological differentiation the various parts of the fundamental
organs become specialised, through unequal growth, first into the primitive
organs and then into the functional organs of the body. "Single sections of
the tubes originally formed from the layers develop individual forms, which
later acquire special functions: these functions are in the most general way
subordinate elements of the function of the whole tube, but yet differ from
the functions of other sections. Thus the nerve-tube differentiates into
sense-organs, brain and spinal cord, the alimentary tube into mouth cavity,
œsophagus, stomach, intestine, respiratory apparatus, liver, bladder, etc.
This specialisation in development is bound up with increased or
diminished growth" (p. 155). Rapid growth concentrated at one point brings
about an evagination; in this manner are formed the sense-organs from the
nerve-tube, the liver and lungs from the alimentary tube. Or increased
growth over a section of a tube causes it to swell out; in this wise the brain
develops from the nerve-tube, the stomach from the alimentary tube. The
segmentation which soon becomes so marked, particularly in the muscle
layer, is also due to a process of morphological differentiation.

At the same time that the organs of the body are being thus roughly blocked
out and moulded from the germ-layers the third process of differentiation is
actively going on. "In addition to the differentiation of the layers, there
follows later another differentiation in the substance of the layers, whereby
cartilage, muscle and nerve separate out, a part also of the mass becoming
fluid and entering the bloodstream" (p. 154). Through histological
differentiation the texture of the layers and incipient organs becomes
individualised. In its earliest appearance the germ consists of an almost
homogeneous mass, containing clear or dark globules suspended in its
substance (ii., p. 92). This homogeneity gives place to heterogeneity; the
structureless mass becomes fibrous to form muscles, hardens to form
cartilage or bone, becomes liquid to form the blood, differentiates in a
hundred other ways—into absorbing and secreting tissues, into nerves and
ganglia, and so forth. It will be noticed that the concept of histological
differentiation is independent of the cell-theory; it signifies that textural
differentiation which leads to the formation of tissues in Bichat's sense. The
tissues and the germ-layers stand in fairly close relation with one another,
for while certain tissues are formed chiefly but not exclusively in one layer,
others are formed only in one layer and never elsewhere. For example,
peripheral nerves are for the most part formed in the muscle layer, though
the bulk of the nervous tissue is formed in the walls of the nerve tube;
similarly blood and blood-vessels may arise from almost any layer, though
their chief seat of origin is the vessel-layer; on the other hand, bone is
formed only in the muscle-layer (i., p. 155, ii., pp. 92-3).
This relation of tissue to germ-layer was more fully discussed and brought
into greater prominence by Remak, from the standpoint of the cell-theory,
and it will occupy us in a later chapter (Chap. XII.).
The fourth Scholion elaborates the analysis of developmental processes still
further, and discusses in particular the scheme of development which is
shown by the Vertebrata. The characteristic structure of the vertebrate body
is brought about by a "double symmetrical" rolling together of the germ-
layers, whereby two main tubes are formed, one above and one below the
axis of the body, which is the chorda. The dorsal tube is formed by the two
animal layers, the ventral tube by all the layers combined (see Fig. 7).

The process is indicated with sufficient clearness in the diagram. It will be
seen that the real foundation and framework of the arrangement is the
muscle-layer, with its two tubes, one surrounding the central nervous
system and forming the "dorsal plates," the other surrounding the body
cavity and forming the "ventral plates." In the dorsal plates, which early
show metameric segmentation, the investing skeleton of the neural axis
develops; in the ventral plates are formed the ribs, the ventral arches of the
vertebræ, the hyoid, the lower jaw and other skeletal structures.
The alimentary or "mucous" tube and the part of the vessel layer which
invests it become so closely bound up with one another as to form a single
primitive organ—the alimentary canal. The muscles of the alimentary canal
are accordingly in all probability developed in the investing part of the
vessel layer. From the "Gekröse," or remaining part of the vessel layer
develop the Wolffian bodies (Urnieren, Pronephros), the kidneys, the sex
glands, and the series of "blood-glands"—suprarenals, thyroid, thymus and
spleen. Baer did not attach any special morphological significance to the
peritoneal lining of the body cavity, as is done in more modern forms of the
germ-layer theory. The gill-slits were largely formed by outgrowths from
the alimentary canal.

Fig. 7.—Ideal Transverse Section of a Vertebrate Embryo.
(After von Baer.)
a. Chorda. e. Vessel-layer. i. Amnion.
b. Dorsal plates. f. Alimentary tube. k. Serous membrane.
c. Ventral plates. g. Pronephros. l. Tolk sac.
d. Spinal cord. h. Skin.

In his germ-layer theory von Baer was influenced a good deal by Pander, to
whom the actual discovery of the process of layer-formation is due. Pander,
however, had distinguished only three germ-layers, an upper "serous" layer,
a lower "mucous" layer and a middle "vessel-layer." He it was who
introduced the terms "Keimhaut" (blastoderm) and "Keimblatt" (germ-
layer).
The honour of being the founder of the germ-layer theory is sometimes
attributed to C. F. Wolff, notably by Kölliker and O. Hertwig. Wolff, it is
true, in his memoir De formatione intestinorum (1768-9) showed that the
alimentary canal was first formed as a flat plate which folded round to form

a tube, and in a somewhat vaguely worded passage he hinted that a similar
mode of origin might be found to hold good for the other organ-systems.
But it seems clear that Wolff had no definite conception of the process of
layer-formation as the first and necessary step in all differentiation. This, at
any rate, was von Baer's opinion, who assigns to Pander the glory of the
discovery of the germ-layers. "You," he writes, "through your clearer
recognition of the splitting of the germ—a process which remained dark to
Wolff—have shed a light upon all forms of development" (p. xxi.).
We have now seen, following von Baer's exposition, how development is
essentially a process of differentiation, a progress from the general to the
special, from the homogeneous to the heterogeneous; we have analysed the
process into its three subordinate processes—primary, histological and
morphological differentiation. So far we have considered development in
general and the laws which govern it; we have now to consider the varieties
of development which the animal kingdom offers in such profusion, in
order to discover what relations exist between them. This is the problem set
in the fifth Scholion. Baer at once brings us face to face with the solution of
the problem attempted in the Meckel-Serres law. It is a generally received
opinion, he writes, that the higher animals repeat in their development the
adult stages of the lower, and this is held to be the essential law governing
the relation of the variety of development to the variety of adult form. This
opinion arose when there was little real knowledge of embryology; it threw
light indeed upon certain cases of monstrous development, but it was
pushed altogether too far. It complicated itself with a belief in a historical
evolution;—"People gradually learnt to think of the different animal forms
as developed one from another—and seemed, in some circles at least,
determined to forget that this metamorphosis could only be conceptual"
(p. 200). At the same time the theory of parallelism led men to rehabilitate
the outworn conception of the scale of beings, to maintain that animals form
one single series of increasing complexity, a scale which the higher
members must mount step by step in their development—from which it
followed that evolution, whether conceived as an ideal or as an historical
process, could take place only along one line, could be only progressive or
regressive. Not all the supporters of the theory of parallelism held these
extreme views, but conclusions of this kind were natural and logical
enough.

Von Baer had soon found in the course of his embryological studies that the
facts did not at all fit in with the doctrine of parallelism; the developing
chick, for example, was at a very early stage demonstrably a Vertebrate, and
did not recapitulate in its early stages the organisation of a polyp, a worm or
a mollusc. He had published his doubts in 1823, but his final confutation of
the theory of parallelism is found in this Scholion.
If it were true, he says, that the essential thing in the development of an
animal is this repetition of lower organisations, then certain deductions
could be drawn, which one would expect to find confirmed in Nature. The
first deduction would be that no structures should appear in the embryo of
the higher animals that are not found in the lower animals. But this is not
confirmed by the facts—no adult among the lower animals, for instance,
has a yolk-sac like that of the chick embryo. Again, if the law of parallelism
were true, the mammalian embryo would have to repeat the organisation of,
among other groups, insects and birds. But the embryo in utero is
surrounded by fluid and cannot possibly breathe free air, so it cannot
possibly repeat the structure of either insects or birds, which are pre-
eminently air-organisms. Generally speaking, indeed, we find in all the
higher embryos special structures which adapt them to the very special
conditions of their development, and these we never find as permanent
structures in the lower animals. The supporters of the theory of parallelism
might, however, admit the existence of such special embryonic organs
without greatly prejudicing their case, for these temporary organs stand to
some extent outside the scope of the theory.
But they would have to face a second and more important deduction from
their views, namely, that the higher animals should repeat at every stage of
their development the whole organisation of some lower animal, and not
merely agree with them in isolated details of structure. The deduction is,
however, not borne out by the facts. The embryo of a mammal resembles in
many points, at different stages of its development, the adult state of a fish;
it has gill-slits and complete aortic arches, a two-chambered heart, and so
on. But at no time does it combine all the essential characters of a fish; nor
has it ever the tail of a fish, nor the fins, nor the shape. Any recapitulation
there may be is a recapitulation of single organs, there is never a repetition
of the complete organisation of a fish. This is indeed the fundamental
criticism of the theory of parallelism; and if it applies even within the limits

of the vertebrate phylum, so much the more does it apply to comparisons
between embryonic Vertebrates and adult Invertebrates.
There are also some lesser arguments which might be urged against the
theory of parallelism. If the theory were strictly true, no state which is
permanent in a higher animal could be passed through by an animal lower
in the scale. But birds, which are lower in the scale than mammals, pass
through a stage in which they resemble mammals in certain respects much
more than they do when adult, for in an embryonic condition they agree
with mammals in having no feathers, no air sacs, no pneumatic sacs in the
bones, no beak. Their brain also resembles that of mammals more in an
earlier stage than it does later. So, too, myriapods and hydrachnids have at
birth three pairs of feet, and resemble at this stage adult insects, which form
a higher class.
Again, were the analogy between the development of the individual and the
evolution of the Échelle des êtres complete, organs and organ-systems
ought to develop in the individual in the order in which they appear in the
scale of beings. But this is not always the case. In fish the hinder extremity
develops only its terminal joint, while in the embryos of higher animals the
basal joint is the first to appear.
Another consequence one would expect to find realised, were the theory of
parallelism correct, is the late appearance in development of parts which are
confined to the higher animals. In the development of a Vertebrate
accordingly one would not expect the vertebræ to appear before the embryo
had passed through many Invertebrate stages. But experience shows the
direct contrary, for in the chick the rudiments of the vertebral axis appear
sooner than any other part.
The theory of parallelism or recapitulation then is not borne out by the
facts, and clearly cannot be the law which we are seeking. But what then is
the true relation between the variety of development and the variety of adult
structure? Before answering this question we must review the varied forms
of adult organisation and consider in what relations they stand to one
another. In particular we must enquire whether they belong to one type or to
many. One point is here cardinal—we must distinguish between the type of
organisation and the grade of differentiation. By "type" von Baer means the

structural plan of the organism. "I call the type the spatial relationship of the
organic elements and organs" (p. 208). Each type of organisation
characterises one of the big groups of animals; the lesser groups represent
"grade" modifications of the type. "The product of the degree of
differentiation and the type gives the several great groups of animals which
are called classes" (p. 208). Ausbildung (differentiation) takes place in one
or other of several directions, in adaptation, for instance, to life in the water
or to life in the air.
There are, von Baer considers, four main types—(1) the peripheral or
radiate type, (2) the longitudinal type, (3) the massive or molluscan type,
(4) the vertebrate type. The radiate type is shown by discoid infusoria, by
medusæ, by starfish and their allies. The longitudinal type characterises
such genera as Vibrio, Filaria, Gordius, and all the annulate animals.
Mollusca, rotifers, polyzoa, and such infusoria as are not included in types
(1) and (2) belong to the massive type, in which the body and its parts form
rounded masses. The longitudinal type is predominantly "animal," the
massive type predominantly "plastic" (vegetative). The vertebrate type has
both the "animal" and the "plastic" organs highly developed. In the
symmetrical arrangement of the animal parts it resembles the longitudinal
type; its plastic parts with their asymmetrical arrangement and rounded
shape belong to the massive type.
These types of von Baer inevitably recall the "Embranchements" of Cuvier,
with which they more or less coincide. It seems that von Baer arrived at his
types (from the study of adult structure) independently of Cuvier, though
the priority of publication rests with Cuvier.[174]
Now it is clear that the development of the individual, which is essentially
an Ausbildung, a differentiation, is directly comparable with the grade-
differentiation of forms within the type. And just as the type rules all its
varied modifications, so does the development of the individual take place
always within the bounds imposed by type. This is von Baer's chief
contribution to the theory of embryonic relationships—the law that "the
type of organisation determines the manner of development" (p. xxii.).
Development is not merely from the general to the special—there are at
least four distinct "general" types, from which the special is developed. The
type is fixed in the very earliest stages of development—the embryo of a

Vertebrate is from the very beginning a Vertebrate (p. 220), and it shows at
no time any agreement in total organisation with any Invertebrate. The
types are independent of one another; differentiation and development
follow a different course in each of them. Not but what some analogies can
be found between the very earliest stages of embryos of different type. Thus
vertebrate and annulate embryos agree in certain points at the time of the
formation of the primitive streak. And in the earliest stage of all, the egg-
stage, there is probably agreement between all the types. In eggs with yolk,
whether vertebrate or annulate, there is always a separation into an animal
and a plastic layer. It seems, too, as if a hollow sphere were a constant stage
in the development of all animals (pp. 224, 258). Apart from these
analogies, development takes an entirely independent course in each of the
four main types, and no embryo of one of the higher types repeats in its
development the peculiar organisation of any adult of the lower types.
If we consider now development within the type, which is the only
legitimate thing to do, we arrive at certain laws governing the relation of
embryos to one another. For instance, at a certain stage vertebrate embryos
are uncommonly alike. Von Baer had two in spirit which he was unable to
assign to their class among amniotes; they might have been lizard, bird, or
mammal, he could not say definitely which.[175] Generally the farther back
we go in the development of Vertebrates the more alike we find the
embryos. The type-characters are first to appear, then the class characters,
then the characters distinguishing the lesser classificatory groups. "From a
more general type the special gradually emerges" (p. 221). The chick is first
a Vertebrate, then a land-vertebrate, then a bird, then a land-bird, then a
gallinaceous bird, and finally Gallus domesticus. Development within the
type is a progress from the general to the special, a real evolution. The more
divergent two adults are, the farther back we must go in their development
to find an agreement between their embryos. We can sum up the case in the
following laws:—
"(1) That the general characters of the big group to which the embryo
belongs appear in development earlier than the special characters. In
agreement with this is the fact that the vesicular form is the most general
form of all; for what is common in a greater degree to all animals than the
opposition of an internal and an external surface?

"(2) The less general structural relations are formed after the more general,
and so on until the most special appear.
"(3) The embryo of any given form, instead of passing through the state of
other definite forms, on the contrary separates itself from them.
"(4) Fundamentally the embryo of a higher animal form never resembles
the adult of another animal form, but only its embryo" (p. 224).
These laws relating to development within the limits of type are destructive
of even a limited application of the theory of parallelism, for not even
within the limits of the type is there a real scale which the higher forms
must mount; each embryo develops for itself, and diverges sooner or later
from the embryos of other species, the divergence coming earlier the greater
the difference between the adult forms. It is only because the lower less-
differentiated adult forms happen to be little divergent from the generalised
or embryonic type, that they show a certain similarity with the embryos of
the higher more differentiated members of the group. Such similarity,
however, is due to no necessary law governing the development of the
higher animals; it is, on the contrary, merely a consequence of the
organisation of these lower animals (p. 224).
Von Baer goes on to show what are the distinguishing embryological
characters of the types and classes, working out a dichotomous schema of
development, which each embryo must follow, branching off early or late to
its terminal point, according to the lower or higher goal it has to reach.
One important consequence for morphology results from von Baer's laws of
differentiation within the type. If the embryo develops from the general to
the special, then the state in which each organ or organ-system first appears
must represent the general or typical state of that organ within the group.
Embryology will therefore be of great assistance to comparative anatomy,
whose chief aim it is to discover the generalised type, the common plan of
structure, upon which the animals of each big group are built. And the
surest way to determine the true homologies of parts will be to study their
early development. "For since each organ becomes what it is only through
the manner of its development, its true value can be recognised only from
its method of formation. At present, we form our judgments by an
undefined intuition, instead of regarding each organ merely as an isolated

product of its fundamental organ, and discerning from this standpoint the
correspondences and dissimilarities in the different types" (p. 233). Parts,
therefore, which develop from the same "fundamental organ," and in the
last resort from the same germ-layer, have a certain kinship, which may
even reach the degree of exact homology.
Now since the mode of development in each type is peculiar to that type,
organs of the same name in different types must not necessarily be
accounted homologous, even if they correspond exactly with one another in
their general functional relations to the rest of the organs. Thus the central
nervous system of Arthropods must not be homologised with the central
nervous system of Vertebrates, for it develops in a different manner. So, too,
the brain of Arthropods or of Mollusca is not strictly comparable with the
brain of Vertebrates. Again, the air-tubes or tracheæ of insects are, like the
trachea and bronchi of many Vertebrates, air-breathing organs. But the two
organs are not homologous, for the air-tubes of Vertebrates are developed
from the alimentary tube ("fundamental organ" of the alimentary system,
developed from the vegetative layer), while the air-tubes of insects arise
either by histological differentiation, or by invagination of the skin (p. 236).
Organs can be homologous only within the limits of the big groups; there
can be no question of homology between members of different types.
The development of plants, like the development of animals, is essentially a
progress from the general to the special (p. 242). Botanists have not been
troubled by any recapitulation theory, and in founding their big groups,
Acotyledons, Monocotyledons, and Dicotyledons, upon embryological
characters, they were guided by true principles, which ought indeed to be
followed in zoology. If we knew the development of all kinds of animals
sufficiently well, then the best way to classify them would be according to
the characters they show in their early development, for it is in early
development that they show the characters of the type in their most
generalised form. As it is, we have in our ignorance to establish the big
groups by the study of adult structure, but we find, on putting together all
we know of comparative embryology, that a classification of animals
according to the mode of their development gives, as is only natural, the
same four groups as does the study of adult structure. The four types of
development are thus:—

(1) The double-symmetrical, which is found in Vertebrates. It is called the
double-symmetrical, because in Vertebrates development takes place from a
central axis (notochord) in two directions, upwards and downwards, in such
a way that two tubes are formed, one above and one below the axis. (2) The
second type is the symmetrical, which is shown by Annulates. A primitive
streak is formed on the ventral surface of the yolk; development proceeds
symmetrically on both sides of the streak. (3) Radiate development is
probably typical of the radiate structural type. (4) In the massive type, the
development seems to be a spiral one.
Common to most modes is a separation of the germ into animal and plastic
layers, a separation which seems to be conditioned largely by the presence
of yolk. A classification based upon embryological characters ought to be
applied even to the lesser groups and would here prove itself of service.
Embryology, for instance, fully supports de Blainville's separation of
Batrachia from true reptiles,[176] for reptiles develop an amnion and
Batrachia do not.
We come now to the sixth and last Scholion. Development is a true
evolution of the special from the general, so runs von Baer's most general
law of all. This can be expressed in a slightly different way, and the words
which he chooses in the sixth Scholion to express this final and most
general result are these:—"The developmental history of the individual is
the history of the growing individuality in every respect" (p. 263). The
greatest modern treatise on embryology ends on a splendid note. One
creative thought rules all the forms of life. And more—"It is this same
thought that in cosmic space gathered the scattered masses into spheres and
bound them together in the solar system, the same that from the weathered
dust on the surface of the metallic planets brought forth the forms of life.
And this thought is nought else but life itself, and the words and syllables in
which life expresses itself are the varied forms of the living" (p. 264).
Von Baer reminds one greatly of Cuvier. There is the same sheer intellectual
power, the same sanity of mind, the same synthetic grip. Von Baer, like
Cuvier, never forgot that he was working with living things; he was
saturated, like Cuvier, with the sense of their functional adaptedness. In his
paper on the external and internal skeleton[177] he gives a masterly analysis
of the functional modifications of the limbs in Vertebrates, and the whole

paper indeed, with its sober attack on transcendentalism, is a vindication as
much of the functional point of view as of the importance of embryology.
Both Cuvier and von Baer, by the very sanity of their views, found
themselves in partial opposition to the theories current in their time. Cuvier
was the critic of Geoffroy and the transcendentalists, of Lamarck and the
believers in the Échelle des êtres, evolutionary or ideal. Von Baer also,
though influenced greatly by Naturphilosophie, turned against the
exaggerations of the transcendental school, and by his unanswerable
criticism of the theory of parallelism took away the ground from those who
too easily believed in an historical evolution.[178]
We have seen what were von Baer's criticisms of the theory of parallelism.
If we turn to the later writings of Cuvier we find the essential criticism
expressed in similar terms. Speaking of an attempt which had been made to
show that fish were molluscs developed to a higher degree, he wrote in
1828,[179] "Let us draw the conclusion that even if these animals can be
spoken of as ennobled molluscs, as molluscs raised to a higher power, or if
they are embryos of reptiles, the beginnings of reptiles, this can be true of
them only in an abstract and metaphysical sense, and that even this abstract
statement would be very far from giving an accurate idea of their
organisation." From the fact that the respiratory and circulatory organs of
fish greatly resemble those of tadpoles the conclusion has been drawn that
fish are in a sense embryos of Amphibia (p. 547). But this manner of
viewing things is none the less vicious, "for this reason ... that it considers
only one or two points and neglects all the others" (p. 548), and is directly
contrary to common sense. There is never a recapitulation of total
organisations, only at the most of single organs.
It will be remembered that Cuvier opposed and demolished the theory of
the Échelle des êtres, not only by showing that there were in Nature four
entirely different plans of animal structure, but also by demonstrating that
even the animals of each single Embranchement could not readily be
arranged in one series, that a serial arrangement was really valid only for
their separate organs. Von Baer also held that there are four distinct types of
structure; he, too, combated the idea of gradation within the limits of the
type. In so far as species represent successive stages in the development, the
Ausbildung, of the type, so far can the idea of a scale of beings be applied.

But the members of a type follow not one line of evolution but several
diverging lines, in direct adaptation to different environmental conditions,
so that a serial arrangement of them is not as a rule possible. It may be
possible to establish a serial arrangement of single organs from the simplest
to the most complex. But each organ or organ-system will require a
different serial arrangement, for the different systems vary on different lines
and an animal may be highly developed in respect of one system and little
developed in respect of all the others. Man, for instance, is the highest
animal only in respect of his nervous system. The idea of the scale of beings
has therefore only a very limited application even within the limits of the
type. Applied to the whole animal kingdom it becomes merely absurd.
Another point of resemblance between Cuvier and von Baer was that
Cuvier, though essentially a student of adult structure, did recognise the
importance of embryology; following up some observations of Dutrochet he
studied the fœtal membrane of mammals and tried to establish their
homologies.[180] And in his criticism of the vertebral theory of the skull he
advanced as an argument against the basisphenoid being a vertebral
centrum the fact (established by Kerkring, 1670), that it develops from two
centres.[181] Von Baer's relation to transcendental anatomy was in some
ways a close one, though he was a trenchant critic of the extreme views of
the school.[182] He took from Oken the idea that a simple fundamental plan
rules the organisation of all Vertebrates; "That jaws and limbs are
modifications of one fundamental form is readily apparent, and, after Oken,
the fact ought to be accepted by the majority of those naturalists who do not
refuse to admit the existence of a general type from which the diversity of
structure is developed" (i., p. 192). He accepted the vertebral theory of the
skull in its main lines, and used his embryological knowledge to support the
idea that jaws correspond to limbs—the latter point as part of the
transcendental idea that the hind end of the body repeats the organisation of
the anterior part (i., p. 192). The particular form which his theory of the
relation of jaws to limbs took is shown in the following passage:—"The
maxillary bone has ... the significance of an extremity and at the same time
that of a rib or lower arch of a vertebra, just as the pelvic bones unite in
themselves the signification of ribs and proximal members of the hinder
extremity" (Meckel's Archiv, p. 367, 1826).

He appreciated the morphological idea of the serial repetition of parts, and
gave it accurate formulation. The whole vertebrate body, he considered, was
composed of a longitudinal series of morphological elements, each of which
was made up a section from each of the fundamental organs—a vertebra, a
section of the nerve-cord, and so on (Entwickelungsgeschichte, ii., p. 53).
Groups of these morphological elements formed morphological divisions,
such as the vertebral segments of the head with their highly developed
neural arches, or the segments of the neck with their undeveloped hæmal
arches. The morphological elements are clearly shown only in the animal
parts, but there are indications in the embryo of a segmentation also of the
vegetative parts,—the gill-slits, for instance, and the vascular arches. The
vegetative parts, however, develop on the whole unsymmetrically (cf.
Bichat). These elements which von Baer distinguishes are morphological
units, as he himself points out, contrasting them with organs which are not
usually units in a morphological sense. "We call organ," he writes, "each
part that has by reason of its form or its function a certain distinctiveness,
but this concept is very indefinite, and possesses, from a morphological
point of view, little value. For this reason it seems necessary to introduce
into scientific morphology the concepts of morphological elements and
divisions" (ii., p. 84).
Von Baer exercised a very considerable influence upon the subsequent trend
of morphological theory. By his criticism of the Meckel-Serres theory, he
rid morphology for a time of an idea which was leading it astray; by his
substitution of the law that development is always from the general to the
special, he set morphologists looking for the archetype in the embryo, not in
the adult alone, and made them realise that homologies could often best be
sought in the earliest stages of development; by formulating the germ-layer
theory he supplied morphologists with a new criterion of homology, based
upon the special relations of the parts (germ-layers) which are first
differentiated in all development. He made the study of development an
essential part of morphology.
[166] De generatione Animalium.
[167] De formato fœtu, ? 1600; De formatione fœtus, 1604.
[168] Exercitationes de generatione animalium, 1651.
[169] De formatione pulli in ovo, 1673; De ovo incubato, 1686.

[170] De formatione pulli in ovo, 1757-8; Sur la formation du cœur dans le
poulet, 1758.
[171] Theoria generatioinis, 1759; De formatione intestinorum, 1768-9.
[172] Beiträge zur Entwickelung des Hühnchens im Ei. Würzburg, 1818. Also in
Latin in shorter form, 1817.
[173] Untersuchungen ü. die Entwickelungsgeschichte der Fische; Leipzig,
1835.
[174] Cuvier, in 1812, Ann. Mus. d'Hist. Nat., xix.; von Baer in 1816, Nova Acta
Acad. Nat. Cur. See Entwickelungsgeschichte der Thiere, i., p. vii., f.n.
[175] Compare a parallel passage in Prévost et Dumas:—"At the very first sight
one will be struck with the resemblance between the forms of the very early
embryos of these two classes, a resemblance so extraordinary that one cannot
refuse to admit the conclusions resulting from it. The resemblance is so striking
that one can defy the most experienced observer to distinguish in any way the
embryos of dog or rabbit ... from those of fowls or ducks of a corresponding
age."—Ann. Sci. nat., iii., p. 132, 1824.
[176] De l'organisation des Animaux, i., p. 140, 1822.
[177] "Ueber das äussere und innere Skelet," Meckel's Archiv für Anat. u.
Physiol., pp. 327-76, 1826. See, too, his Entwickelungsgeschichte, i., pp. 181, ff.
[178] Von Baer wrote an appreciative biography of Cuvier, published
posthumously in 1897, Lebensgeschichte Cuviers, ed. L. Stieda. French trans. in
Ann. Sci. Nat. (Zool.), ix., 1907.
[179] Cuvier et Valenciennes, Histoire naturelle des Poissons, i., p. 550.
[180] Mém. Mus. d'Hist. Nat., iii., pp. 98-119, 1817.
[181] Leçons d'Anatomie comparée, 3rd ed., vol. i., p. 414, Bruxelles, 1836.
[182] In the aforementioned paper in Müller's Archiv he criticises Carus
vigorously and is sarcastic on Geoffroy.

CHAPTER X

THE EMBRYOLOGICAL CRITERION

Pander's work of 1817 was the forerunner of an embryological period in
which men's hopes and interest centred round the study of development.
"With bewilderment we saw ourselves transported to the strange soil of a

new world," wrote Pander, and many shared his hopeful enthusiasm. K. E.
von Baer's Entwickelungsgeschichte was by far the greatest product of this
time, but it stands in a measure apart; we have in this chapter to consider
the lesser men who were Baer's contemporaries, friends, followers or
critics.
It was largely a German science, this new embryology, and its leaders were
all personally acquainted. Pander, von Baer and Rathke were on friendly
terms with one another; von Baer dedicated his master-work to Pander;
Rathke dedicated the second volume of his Abhandlungen to von Baer.
Interest in the new science was, however, not confined to Germany. In Italy,
Rusconi commenced in 1817 his pioneer researches on the development of
the Amphibia with a Descrizione anatomica degli organi della circolazione
delle larve delle Salamandre aquatiche (Pavia), in which he traced the
metamorphoses of the aortic arches. This was followed in 1822 by his
Amours des Salamandres aquatiques (Milan), and in 1826 by his memoir
Du développement de la grenouille (Milan). In this last paper he described
how the dark upper hemisphere of the frog's egg grows down over the lower
white hemisphere and leaves free only the yolk plug; he observed the
segmentation cavity and the archenteron, but thought that the former
became the alimentary canal; he observed and interpreted rightly the
formation of the medullary folds. The circular blastopore in the frog in later
years often went by the name of the anus of Rusconi.

In France Dutrochet[183] investigated the fœtal membranes in various
vertebrate classes; Prévost and Dumas studied the very earliest stages of
development in birds, mammals and amphibia (Ann. Sci. nat., ii., iii., 1824,
xii., 1827).
A little later came Dugès' studies of the osteology and myology of
developing amphibia (1834),[184] and Coste's careful researches into the
early developmental history of mammals.[185]
It was in 1825 that Heinrich Rathke (1793-1860), published his famous
discovery of gill-slits in the embryo of a mammal,[186] a discovery which
aroused considerable interest, and greatly stimulated embryological
research. He describes how in a young embryo of a pig he saw four slits in
the region of the neck, going right through into the œsophagus. They were

separated by partitions which he
called Kiemenbogen (gill-arches),
and immediately in front of the
first gill-slit lay the developing
lower jaw. He compared these gill-
slits with those of a dogfish. We
reproduce his drawing of the pig-
embryo (Isis, Pl. IV., fig. 1).
Later in the same year Rathke
discovered gill-slits in the chick,
[187]
in this case finding only three.
He described growing out from in
front of the first slit a structure
Fig. 8. which he compared to the
Gill-slits of the Pig Embryo. operculum or gill-cover of a fish.
(After Rathke.) These discoveries were confirmed
[188]
and extended for the chick by the embryologist Huschke, a pupil of
Oken. Like Rathke, he found only three indubitable gill-slits, but he noticed
that the body-wall in front of the first gill-slit was really composed of two
arches, which were on the whole similar to the gill-arches. The hinder of
these two seemed to him to be a horn of the hyoid, the front one, which was
bent at an angle, to be the rudiment of the upper and lower jaws (p. 401).
Between these two arches he found an opening, just as between two gill-
arches a gill-slit. This opening led into the mouth-cavity, and according to
Huschke it became the external ear-passage. He discovered also three pairs
of aortic arches in close relation with the gill-arches, so close indeed, that
he did not hesitate to call them gill-arteries, and to recognise their
resemblance with the aortic arches of fish. He traced, in part at least, the
metamorphosis which these aortic arches undergo. This part of his
discovery he developed in fuller detail in a paper of 1828,[189] in which he
gave some excellent figures.
Shortly after Huschke's first paper, von Baer published his views and
observations on this subject in a short memoir in Meckel's Archiv.[190] In
this paper he confirmed Rathke's discovery, and described the slits and
arches in the dog and the chick. Both Rathke and he found gill-slits in the

human embryo about this time (p. 557). There were generally present, he
found, four gill-slits, and, as Rathke had suggested, the first gill-arch
became the lower jaw. Von Baer also confirmed Rathke's discovery of the
operculum, assigning it, however, to the second gill-arch. He refused to
accept Huschke's derivation of the auditory meatus from the first gill-slit.
Von Baer saw what had escaped Rathke and Huschke, that there were, not
three nor four, but as many as five aortic arches.
In his view of the metamorphosis of the aortic arches in the chick the first
two pairs disappeared completely, the third pair gave rise to the arteries of
the head and the fore-limbs, the right side of the fourth arch became the
aorta, the left half of the fourth and the right half of the fifth arch became
the pulmonary arteries, while the left half of the fifth arch disappeared. This
schema, which for the last three arches was the same as Huschke's, von
Baer upheld for the chick even in the second volume of his
Entwickelungsgeschichte (p. 116); he rectified it, however, for mammals in
the same volume (p. 212), deriving both pulmonary arteries from the fifth
arch, and the aorta from the fourth left. He fully recognised the great
analogy of the embryonic arrangement of gill-arches and gill-arteries in
Tetrapoda with their arrangement in fish (i., pp. 53, 73).

Huschke, in a paper of 1832,[191] chiefly devoted to the development of the
eye, figured and described the developing upper and lower jaws, and
maintained against von Baer that the first slit turns into the auditory meatus
and the Eustachian tube.
These were the first papers of the embryological period. Before going on to
discuss the principles which guided embryological research during the next
ten or twenty years it is convenient to note what were the main lines of
work characterising the period.
The typical figure of the period is Rathke, who produced a great deal of
first-class embryological work. He was, even more than von Baer, a
comparative embryologist, and there were few groups of animals that he did
not study. His first large publication, the Beiträge zur Geschichte der
Thierwelt (i.-iv., Halle, 1820-27), contained much anatomical work in
addition to the purely embryological; he commenced here his series of
papers on the development of the genital and urinary organs, continued in

the Abhandlungen zur Bildungsund Entwickelungs-Geschichte des
Menschen und der Thiere (i., ii., Leipzig, 1832-3). A fellow-worker in this
line was Johannes Müller, whose Bildungsgeschichte der Genitalien
(Düsseldorf) appeared in 1830.
In a memoir on the development of the crayfish which appeared in 1829,
[192]
Rathke found in an Invertebrate confirmation of the germ-layer theory
propounded by Pander and von Baer. He was greatly struck by the inverted
position of the embryo with respect to the yolk. In following out the
development of the appendages he noticed how much alike were jaws and
legs in their earliest stage, and how this supported Savigny's contention that
the limbs of Arthropods belonged to one single type of structure. In his
paper (1832) on the development of the fresh-water Isopod, Asellus,[193]
Rathke returns to this point. Commenting on the original similarity in
development of antennæ, jaws and legs, he writes, "Whatever the doubts
one may have reserved as to the intimate relation existing between the jaws
and feet of articulate animals after the researches of Savigny on this subject
and mine on developing crayfish, they must all fall to the ground when one
examines with care the development of the fresh-water Asellus" (p. 147 of
French translation).
Further comparative work by Rathke is found in the two volumes of
Abhandlungen and in a book, Zur Morphologie, Reisebemerkungen aus
Taurien (1837), which contains embryological studies of many different
types, including a study of the uniform plan of arthropod limbs. Later on
Rathke devoted himself more to vertebrate embryology, producing among
other works his classical papers on the development of the adder (1839), of
the tortoise (1848), and of the crocodile (1866). He laid the foundations of
all subsequent knowledge of the development of the blood-vascular system
in a series of papers of various dates from 1838 to 1856. The diagrams in
his paper on the aortic arches of reptiles (1856) were for long copied in
every text-book.
Rathke was a foremost worker in another important line of embryological
work, the study of the development of the skeleton and particularly of the
skull. We shall discuss the history of the embryological study of the skull in
some detail below; meantime, we note the two other important lines of
research which characterise this period. One is the intensive study of the

development of the human embryo, a study pursued by, among others,
Pockels, Seiler, Breschet, Velpeau, Bischoff, Weber, Müller, and Wharton
Jones.[194] The other important line—the early development of the
Mammalia—was worked chiefly by Valentin,[195] Coste,[196] and, above all,
by Bischoff, whose series of papers[197] was justly recognised as classical.
What interests us chiefly in the work of this embryological period is, of
course, the relation of embryology to comparative anatomy and to pure
morphology. The embryologists were not slow to see that their work threw
much light upon questions of homology, and upon the problem of the unity
of plan. Von Baer, we have seen, recognised this clearly in 1828; Rathke, in
one of his most brilliant papers, the Anatomische-philosophische
Untersuchungen über den Kiemenapparat und das Zungenbein (Riga and
Dorpat, 1832), used the facts of development with great effect to show the
homology of the gill-arches and hyoid throughout the vertebrate series;
Johannes Müller made great use of embryology in his classical
Vergleichende Anatomie der Myxinoiden (i. Theil, 1836), and, according to
his pupil Reichert, firmly held the opinion that embryology was the final
court of appeal in disputed points of comparative anatomy;[198] Reichert
himself in a book of 1838 (Vergleichende Entwickelungsgeschichte des
Kopfes der nackten Amphibien) discussed the two different methods of
arriving at the "Type"—the anatomical method of comparing adults, and the
embryological method of comparing embryogenies. Of the embryological
method, he says, "Its aim is to distinguish during the formation of the
organism the originally given, the essence of the type, and to classify and
interpret what is added or altered in the further course of development.
Embryologists watch the gradual building up of the organism from its
foundations, and distinguish the fundament, the primordial form, the type,
from the individual developments; they reach thus, following Nature in a
certain measure, the essential structure of the organism, and demonstrate the
laws that manifest themselves during embryogeny" (p. vi.). The
embryologists, influenced in this greatly by von Baer, gradually felt their
way to substituting for the "Archetype" of pure morphology what one may
perhaps best call the embryological archetype. How the transition was made
we can best see by following out the course of discovery in one particular
line. We choose for this purpose the development of the skull, a subject

which excited much interest at this time and upon which much quite
fundamental work was done, particularly by Rathke and Reichert.
Following up his discovery of gill-slits and arches in the embryos of birds
and mammals, Rathke in two papers of 1832[199] and 1833[200] worked out
the detailed homologies of the gill-arches in the higher Vertebrates. He
describes how in the embryo of the Blenny there is a short, thick arch
between the first gill-slit and the mouth. A furrow appears down the middle
of the arch dividing it incompletely into two. In the anterior halves a
cartilaginous rod is developed which is connected with the skull; these rods
become on either side the lower jaw and "quadrate." In the posterior halves
two similar rods are formed which develop into the hyoid. The hyoid is at
first connected with the skull, but afterwards frees itself and becomes slung
to the "quadrate." From the hinder edge of the hyoid arch grows out the
membranous operculum, in which develop later the opercular bones and
branchiostegal rays. The upper jaw is an independent outgrowth of the
serous layer.
The serial homology of the lower jaw and quadrate with the hyoid and with
the true gill-arches was thus established in fish, and Rathke had little
difficulty in demonstrating a similar origin of lower jaw and hyoid in the
embryos of higher Vertebrates. He could even, as we have noted before,
find the homologue of the operculum in a flap which grows out from the
hyoid arch in the embryo of birds.
But Rathke could not altogether shake himself free from the transcendental
notion of the homology of jaws with ribs, and this led him to draw a certain
distinction between the first two and the remaining gill-arches, by which the
homology of the former with the ribs was asserted and the homology of the
latter denied. He thought he could show that the skeletal structures (lower
jaw, "quadrate," and hyoid) of the first two arches were formed in the
serous layer, just like true ribs, and like them in close connection with the
vertebral skeletal axis. The other, "true," gill-arches appeared to him to be
formed in the mucous layer, in the lining of the alimentary canal. They had
no direct connection with the vertebral column, and seemed therefore to
belong to what Carus[201] had called the visceral or splanchno-skeleton. He
did not, however, let this distinction hinder him from asserting the
substantial homology of all the gill-arches inter se, the first two included.

Rathke's discoveries relative to the development of the jaws, the hyoid and
the operculum, enabled him to make short work of the homologies
proposed for them by the transcendentalists. He could prove from
embryology that the jaws were not the equivalent of limbs, as so many
Okenians believed. He could reject, with a mere reference to the facts of
development, Geoffroy's comparison of the hyoid and the branchiostegal
rays in fish with sternum and ribs. He could show the emptiness of the
attempts made by Carus, Treviranus, de Blainville and Geoffroy, to
establish by anatomical comparison the homologies of the opercular bones,
for he could show that these bones were peculiar to fish, and were scarcely
indicated, and that only temporarily, in the development of other
Vertebrates.[202] He did not, however, himself realise the relation of the ear-
ossicles to the gill-arches, though he knew that Spix and Geoffroy were
quite wrong in homologising them with the opercular bones in fish. He
described, it is true, the development of the external meatus of the ear and
the Eustachian tube from the slit which appears between the first and the
second arch, as Huschke had done before him; he described, in
confirmation of Meckel, the "Meckelian process" of the hammer running
down inside the lower jaw; but the discovery of the true homologies of the
ear-ossicles was not made until a year or two later by Reichert.
In his further study of the development of Blennius viviparus, Rathke
observed some important facts about the development of the vertebral
column and skull. He found that the vertebral centra were first formed as
rings in the chorda-sheath, which give off neural and hæmal processes. The
vertebra later ossifies from four centres. The chorda (notochord) is
prolonged some little way into the head, and the base of the cranium is
formed by the expanded sheath, which reaches forward in front of the end
of the notochord. This cranial basis shows a division into three segments, in
which Rathke was inclined to see an indication of three cranial vertebræ. (It
turned out that this division into three segments did not really exist, and
Rathke later acknowledged that he had made an error of observation.) The
side walls of the skull grow out from this base and form a fibrous capsule
for the brain. The cranial section of the chorda itself shows no sign of
segmentation; but later on the cranial portion of the chorda-sheath ossifies,
like the vertebræ, from several centres. The vomer, which, in the classical
form of the vertebral theory of the skull, was the centrum of the fourth, or

foremost, cranial vertebra, does not, according to Rathke, develop in
continuity with the cranial basis and the chorda sheath, but develops
separately in the facial region.
Von Baer, like Rathke at this time, was also to some extent a believer in the
vertebral theory of the skull. In his second volume (1834, pub. 1837) he
holds that the development of the skull, as the sum of the anterior vertebral
arches, is in general the same as that of the other neural arches, and is
modified only by the great bulk of the brain (Entwickelungsgeschichte, ii.,
p. 99). He had, however, some doubts as to the entire correctness of the
vertebral theory, doubts suggested by a study of the developing skull. "In
the course of the formation of the head in the higher animals, something
additional is introduced which does not originally belong to the cranial
vertebræ. At first we see the vertebration in the hinder region of the skull
very clearly. Afterwards it becomes suddenly indistinct, as if some new
formation overlaid it" (i., p. 194).
Even more clearly is his doubt expressed in his paper on Cyprinus. "Upon
the formation of the vertebral column only this need be said, that at this
stage the notochord is very clearly seen, and the upper and lower arches and
spinous processes are visible right to the end of the tail, but the separation
into vertebræ ceases abruptly where the back passes into the head. I do not
hesitate to assert that bony fish, too, have at this stage an unsegmented
cartilaginous cranium (as cartilaginous fish have all their life), the
prominences and hollows of which constitute its only resemblance with the
vertebral type" (1835, p. 19).
A convinced supporter of the vertebral theory was Johannes Müller, who, in
his classical memoir on the Myxinoids,[203] discussed at some length the
relation between the development of the vertebræ and the development of
the skull. His memoir is principally devoted to comparative anatomy, but in
treating of the skeleton he pays much attention to development. He
describes the formation of the vertebræ in elasmobranch embryos; for the
facts regarding other Vertebrates he relies largely on work by Rathke
(Blennius, 1833) and Dugès (1834). He recognises as the basis of his
comparisons the homology of the notochord in all vertebrate embryos with
the persistent notochord which forms the chief part or the whole of the
vertebral column in the Cyclostomes. The notochord possesses an inner and

an outer sheath and the outer sheath is continuous with the basis cranii
(p. 92). It is in the outer sheath that the vertebræ develop—from four
separate pieces, in fish at least, plus an additional element which helps to
form the centrum. The skull of Vertebrates consists, according to Müller, of
three vertebræ, whose centra are the basioccipital, the basisphenoid and the
presphenoid. Other bones besides those belonging to the vertebræ are
present, but this formation out of three vertebræ gives the essential schema
for the skull. Now the brain capsule, like the sheath of the spinal cord, is a
development from the outer sheath of the notochord. If the skull consists of
vertebræ we should expect the centra of the skull-vertebræ to develop in the
outer sheath at the sides of the cranial section of the notochord as two
separate halves, just as do the bodies of the vertebræ; we should expect
further the cartilaginous side-walls of the cranium to develop in the
membranous brain-sheath just as the neural arches develop in the
membranous sheath of the spinal column. In Rathke's discovery (!) of a
segmentation of the basis cranii into three parts, and of the isolated
formation of the vomer, Müller sees a confirmation of his view that the
skull is composed of three and not four vertebræ. But there is nothing in
Rathke's observations to support the idea that the centra of the cranial
vertebræ are formed from separate halves. Müller has to be content with a
reference to the state of things in Ammocoetes (which, by the way, he did
not know to be the young of Petromyzon). In the simple skull of
Ammocoetes the base is formed chiefly by two cartilaginous bars lying
more or less parallel with the longitudinal axis of the skull and embracing
with their hinder ends the cranial portion of the notochord.
These bars, declares Müller, are clearly the still separate halves of the pars
basilaris cranii, and represent the divided centra of the two hinder cranial
vertebræ. To complete the parallel between the development of the skull
and of the vertebræ, it would have been necessary to show that the side
walls of the cranium developed in a similar manner from separate pieces.
Müller could not prove this point from the available embryological data,
and indeed the facts which he did use had to be twisted to suit his theory. A
curious apparent confirmation of his idea that the centra of the cranial
vertebræ are formed from separate halves was supplied in 1839 by Rathke's
discovery of the trabeculæ in the embryonic skull of the adder.

The next big step in the study of the development of the skull was taken by
a pupil of Müller, C. B. Reichert, who showed in his work very distinct
traces of his master's influence. Reichert's first and most important
contribution to the subject was his paper on the metamorphosis of the gill,
or, as he called them, the visceral arches in Vertebrates,[204] particularly in
the two higher classes. Reichert describes the similar origin in embryo of
bird and mammal (pig) of three "visceral" arches. These arches stand in
close relation to the three cranial vertebræ which Reichert, like Müller,
distinguishes. He makes the retrograde step of admitting only three aortic
arches, and he is not inclined to consider the three visceral arches as
equivalent to the gill-arches of fish—in his opinion they have more analogy
with ribs, though differing somewhat from ribs in their later modifications.
The visceral arches are processes of the visceral plates (von Baer), which
grow downwards and meet in the middle line, leaving between one another
and the undivided body wall three visceral slits opening into the pharynx.
The first visceral process is different in shape from the others, for it sends
forward, parallel with the head and at right angles to its downward portion,
an upper portion in which later the upper jaw is formed. The other two
processes are straight. From the hinder edge of the second visceral arch
there develops, as Rathke had seen, a fold which is comparable with the
operculum of fish. The first slit develops externally into the ear-passage,
internally into the Eustachian tube, and in the middle a partition forms the
tympanic ring and tympanum. Inside each of the visceral processes on
either side a cartilaginous rod develops. In the first process this rod shows
three segments, of which the first lies inside that portion of the process
which is parallel with the head. This upper segment forms the foundation
for the bones of the upper jaw. The lowest segment of the cartilaginous rod
becomes Meckel's cartilage, and on the outer side of this the bones of the
lower jaw are formed. The middle segment becomes in mammals the incus
(one of the ear-ossicles), and in birds the quadrate. Meckel's cartilage,
which was discovered by Meckel[205] in fish, amphibians and birds, is a
long strip of cartilage which runs from the ear-ossicle known as the hammer
in mammals,[206] to the inside of the mandible. Reichert shows how this
relation comes about.

Fig. 9.—Meckel's Cartilage and Ear-ossicles in Embryo of
Pig. (After Reichert.)
a. Mandible. h. Hammer. k. Incus.
g. Meckel's cartilage. i. Handle of Hammer. n. Stapes.

The hammer, according to his observations on the embryo of the pig, is
simply the proximal end of Meckel's cartilage, which later becomes
separated off from the long distal portion (see Fig. 9). The third ear-ossicle
of mammals, the stapes, comes not from the first arch but from the second.
The cartilaginous rod of the second arch segments like the first into three
pieces. Of these the uppermost disappears, the middle one, which lies close
up to the labyrinth of the ear, becomes the stapes, and the lowest becomes
the anterior horn of the hyoid. The stapes forms a close connection with the
hammer and the incus. In birds, where there is a single ear-ossicle, the
columella, the middle piece of arch I forms, as we have seen, the quadrate,
by means of which the lower jaw is joined to the skull. The proximal end of
Meckel's cartilage, which in mammals forms the hammer, here gives the
articular surface between the lower jaw and the quadrate. The columella is
formed from the middle piece of the three into which the cartilage of the
second arch segments. It is, therefore, the homologue of the stapes in
mammals. The third arch takes a varying share, together with the second, in
the formation of the hyoid apparatus.
In this paper Reichert made a distinct advance on the previous workers in
the same field—Rathke, Huschke, von Baer, Martin St Ange, Dugès.
Huschke was indeed the first to suggest that both upper and lower jaws
were formed in the first gill-arch. But both von Baer and Rathke[207] held
that the upper jaw developed as a special process independent of the lower

jaw rudiment, and the actual proof that the upper jaw is a derivative of the
first visceral arch seems to have been first supplied by Reichert. His
brilliant work on the development of the ear-ossicles founded what we may
justly call the classical theory of their homologies. His views were attacked
and in some points rectified, but the main homologies he established are
even now accepted by many, perhaps the majority of morphologists.
In a paper of 1838 on the comparative embryology of the skull in
Amphibia,[208] Reichert added to his results for mammals and birds an
account of the fate of the first and second visceral arches in Anura and
Urodela.
The first visceral arch, he found, gave in Amphibia practically the same
structures as in the higher Vertebrates. Its skeleton segmented, as in
mammals and birds, into three parts; the upper part gave rise to the palatine
and pterygoid in Anura, but seemed to disappear in Urodeles, where the so-
called palatine and pterygoid developed in the mucous membrane of the
mouth; the middle part gave, as in birds, the quadrate, which formed a
suspensorium for both arches; the lower part, as Meckel's cartilage, formed
a foundation for the bones of the lower jaw. Of arch II., the lower part
became the horn of the hyoid, the upper part had a varying fate. In some
Anura it formed the ossicle of the ear (homologue of the columella of birds
and the stapes of mammals), in others it disappeared. In reptiles the upper
segment of the second arch formed, as in birds, the columella.
The account of the metamorphoses of the visceral arches in Amphibia forms
only a small part of Reichert's memoir of 1838, the chief object of which
was to discover the general "typus" of the vertebrate skull, and to follow out
its modifications in the different classes. Von Baer had shown that the
generalised type appeared most clearly in the early embryo; Reichert
therefore sought the archetype of the skull in the developing embryo. He
brought to his task the preconceived notion that the skull could be reduced
to an assemblage of vertebræ, but he saw that comparative anatomy alone
could not effect this reduction; he had recourse, therefore, to embryology,
hoping to find in the simplified structure of the embryo clear indications of
three primitive cranial vertebræ (p. 121, 1837).

In the head he distinguished two
tubes, the upper formed by the
dorsal plates, the lower by the
ventral or visceral plates. Both of
these tubes were derived from the
serous or animal layer (cf. von
Baer, supra, p. 118). The walls of
the lower tube were formed by the
visceral processes, within which
later the skeleton of the visceral
arches developed. The walls of the
upper tube formed the bones and
muscles of the cranium proper. The
facial part of the head was formed
by elements from both upper and
lower tubes. The dorsal tube
showed signs of a division into
three cranial vertebræ (Urwirbeln,
Fig. 10. primitive vertebræ). In mammals
Cranial Vertebræ and Visceral Arches and birds, as Reichert had shown in
in Embryo of Pig. Ventral Aspect. his 1837 paper, the three cranial
(After Reichert.) vertebræ were indicated by
transverse furrows on the ventral surface of the still membranous skull (see
Fig. 10, p. 148).
Even in mammals and birds, however, the positions of the eye, the ear-
labyrinth, and the three visceral arches were the safest guides to the
delimitation of the cranial vertebræ (pp. 134-138, 1837). In Amphibia
generally there were no definite lines of separation on the skull itself. "At
this stage," he writes of the cartilaginous cranium of the frog, "we find no
trace of a veritable division into vertebræ in the cartilaginous trough formed
by the basis cranii and the side parts. On the contrary, it is quite continuous,
as it is also in the higher Vertebrates during the process of chondrification"
(p. 44, 1838). The vertebræ in the membranous or cartilaginous skull could
be delimited in Amphibia by the help of the eye and the ear-labyrinth,
which lie more or less between the first and second, and the second and
third vertebræ, but, above all, by the vesicles of the brain.

As in the higher Vertebrates, the visceral arches are associated with the
cranial vertebræ as their ventral extensions, being equivalent to the visceral
plates which form the ventral portion of the "primitive vertebræ" or
primitive segments of the trunk.
If the three cranial vertebræ are not very distinct in the early stages of
development when the skull is still membranous or cartilaginous, they
become clearly delimited when ossification sets in. Three rings of bone
forming three more or less complete vertebræ are the final result of
ossification. The composition of these rings is as follows:—

Base. Sides. Top.
First vertebra Presphenoid Orbitosphenoids Frontals
Second vertebra Basisphenoid Alisphenoids Parietals
Third vertebra Basioccipital Exoccipitals Supraoccipital

The other bones of the skull are not included in the vertebræ, and this is in
large part due to the fact that the sense capsules are formed separately from
the cranium (p. 29, 1838). The ear-labyrinth, it is true, fuses indissolubly
with the cranium at a later period, but the bones which develop in its
capsule are not for all that integral parts of the primitive cranial vertebræ.
This point, it is interesting to note, had already been made by Oken in his
Programm (1807). But many of the bones developed in relation to the sense
organs can find their place in the generalised embryonic schema or
archetype of the vertebrate skull, for they are of very constant occurrence
during early development.
Having arrived at a generalised embryonic type for the vertebrate skull, of
which the fundamental elements are the three cranial vertebræ and their
arches, Reichert goes on to discuss the particular forms under which the
skull appears in adult Vertebrates. He accepts in general von Baer's law that
the characters of the large groups appear earlier in embryogeny than the
characters of the lesser classificatory divisions. "When we observe new and
not originally present rudiments in very early embryonic stages, as, for
instance, that for the lacrymals, the probability is that they belong to the
distinctive development of one of the larger vertebrate groups. From these
are to be carefully distinguished such rudiments as arise later during
ossification, mostly as ossa intercalaria, in order to give greater strength to

the skull in view of the greater development of the brain, etc.; the latter give
their individual character to the smaller vertebrate groups, and comprise
such bones as the vomer, the Wormian bones, the lowermost turbinal, etc."
(p. 63, 1838).
He did not accept the Meckel-Serres law of parallelism. He recognised the
great similarity between the unsegmented cartilaginous cranium of
Elasmobranchs, and the primordial cranium of the embryos of the higher
Vertebrates, but he did not think that the cranium of Elasmobranchs was
simply an undeveloped or embryonic stage of the skulls of the higher forms.
Rather "do the Holocephala, Plagiostomata, and Cyclostomata appear to us
to be lower developmental stages individually differentiated, so that the
other fully differentiated Vertebrates cannot easily be referred directly to
their type" (p. 152, 1838). The skull of these lower fishes is itself a
specialised one; it is an individualised modification of a simple type of
skull. And this holds good in general of the skulls of the lower Vertebrates
—they are individualised exemplars of a simple general type, not merely
unmodified embryonic stages of the greatly differentiated skulls of the
higher Vertebrates (p. 250, 1838). Differentiation within the vertebrate
phylum is therefore not uniserial, but takes place in several directions.
Reichert describes two sorts of modifications of the typical skull—class
modifications and functional modifications. The causes of the modifications
which characterise classificatory groups are unknown; the second class of
modifications occur in response to adaptational requirements.
Reichert's two papers are of considerable importance, and Müller's remark
in his review[209] of them is on the whole justified. "These praiseworthy
investigations supply from the realm of embryology new and welcome
foundations for comparative anatomy" (p. clxxxvii.).
The development of the skull was, however, more thoroughly worked out
by Rathke, and with less theoretical bias, in his classical paper on the adder.
[210]
This memoir of Rathke's is an exhaustive one and deals with the
development of all the principal organ-systems, but particularly of the
skeletal and vascular. He confirmed in its essentials Reichert's account of
the metamorphoses of the first two visceral arches, describing how the
rudiment of the skeleton of the first arch appears as a forked process of the
cranial basis, the upper prong developing into the palatine and pterygoid,

the lower forming Meckel's cartilage, while the quadrate develops from the
angle of the fork. The actual bone of the upper jaw (maxillary) develops
outside and separate from the palato-pterygoid bar. The cartilaginous rod
supporting the second visceral arch divides into three pieces on each side,
of which the lower two form the hyoid, the uppermost the columella. Like
Reichert he held the visceral arches to be parts of the visceral plates,
containing, however, elements from all three germ-layers—the serous,
mucous, and vessel layers.
The first gill-slit, or, as Rathke here prefers to call it, pharyngeal slit, closes
completely in snakes and in Urodeles. It forms the Eustachian tube in all
other Tetrapoda. As regards the vertebræ, Rathke describes them as being
formed in the sheath of the chorda from paired rudiments, each of which
sends two branches upwards, and two branches downwards. The two inner
pairs of processes coalesce round the chorda, and later form the centrum;
the upper outer pair meet above the spinal column; the lower outer pair
form ribs. The odontoid process of the axis vertebra is the centrum of the
atlas (p. 120). The formation of vertebral rudiments begins close behind the
ear-labyrinth, but in front of this the chorda-sheath gives origin to a flat
membranous plate which afterwards becomes cartilaginous. This plate
reaches forward below the third cerebral vesicle as far as the infundibulum.
The notochord ends in this plate, which is the basis cranii, just at the level
of the ear-labyrinth. In no Vertebrate does the notochord extend farther
forward (p. 122). The basis cranii gives off three trabeculæ. The middle one
is small and sticks up behind the infundibulum; it is absent in fish and
Amphibia, and soon disappears during the development of the higher forms.
The lateral trabeculæ are long bars which curve round the infundibulum and
reach nearly to the front end of the head. Together they are lyre-shaped. The
cranial basis and the trabeculæ are formed, like the vertebræ, in the sheath
of the notochord, and the only differences between the two in the early
stage of their development are that the formative mass for the cranial basis
is much greater in amount than that for the vertebræ, and that the cranial
basis by means of its processes, the trabeculæ, reaches well in front of the
terminal portion of the notochord (p. 36). The capsule for the ear-labyrinth
develops quite independently of the cranial basis and the notochord. It
resembles on its first appearance, in form, position, composition, and
connections, the ear-capsule of Cyclostomes, and so do the ear-capsules of

all embryonic Vertebrates (p. 39). It manifests clearly the embryonic
archetype, ... "there exists one single and original plan of formation, as we
may suppose, upon which is built the labyrinth of Vertebrates in general"
(p. 40). When ossification sets in, the ear-capsule forms three bones, of
which two fuse with the supraoccipital and exoccipitals.
During the formation of the ear-capsule the cranial basis develops from a
plate to a trench, for in its hinder section the side parts grow up to form the
side walls of the brain, in exactly the same way as the processes of the
vertebral rudiments grow up to enclose the spinal column (pp. 122, 192).
The foundations of the skull are now complete, and ossification gradually
sets in.

Fig. 11.—Embrionic Cranium of the Adder. Ventral Aspect.
(After Rathke.)
a. Basioccipital. d. Basisphenoid. g. Trabeculæ.
b. Exoccipital. c. Alisphenoid. h. Foramen.
c. Ear capsule. f. Orbitosphenoid. i. Ptuitary space.

The basioccipital is formed in the posterior part of the basis cranii, and the
exoccipitals in the side walls of the trench in continuity with the fundament
of the basioccipital (see Fig. 11). The supraoccipital is formed in cartilage
above the exoccipitals. The basisphenoid develops, like the basioccipital, in
the flat basis cranii, but towards its anterior edge, between the large
foramen (h) and the pituitary space (i). It is formed from two centres, each
of which is originally a ring round the carotid foramen. The presphenoid
develops in isolation between the lateral trabeculæ, just behind the point
where they fuse. The side parts of the basisphenoid and presphenoid
(forming the alisphenoids and the orbitosphenoids respectively) develop in
cartilage separately from the cranial basis, not like the exoccipitals in
continuity with it. The hinder parts of the trabeculæ become enclosed by
two processes of the basisphenoid; their front parts remain in a vestigial and
cartilaginous state alongside the presphenoid. The frontals and parietals
show a peculiar mode of origin in the adder, differing from their origin in
other Vertebrates. The frontals develop in continuity with the
orbitosphenoids, the parietals in continuity with the alisphenoids, and so
have much resemblance with the vertebral neural arches which surround the
spinal column (p. 195).
Through Rathke's work the real embryonic archetype of the vertebrate skull
was for the first time disclosed. Rathke discussed this archetype and its
relation to the vertebral theory of the skull in another paper of the same year
(1839), but before going on to this paper, we shall quote from the paper on
the adder the following passage, remarkable for the clear way in which the
idea of the embryological archetype is expressed. "Whatever differences
may appear in the development of Vertebrates, there yet exists for the
different classes and orders a universally valid idea (plan, schema, or type)
ruling the first formation of their separate parts. This idea must first be
worked out, though possibly with modifications, before more special ideas
can find play. The result of the latter process, however, is that what was
formed by the first idea is not so much hidden as partially or wholly
destroyed" (p. 135).

Rathke's general paper on the development of the skull in Vertebrates[211]
treats the matter on a broader comparative basis than his paper on the adder,
and takes into account all the vertebrate classes, in so far as their
development was then known. He here makes the interesting suggestion,

later entirely confirmed, that the basis cranii or basilar plate is first laid
down as two strips, one on each side of the chorda—the structures now
known as parachordals (pp. 6, 27). For this supposition, he thinks, speaks
the structure of the skull in Ammocoetes, which in this respect is the
simplest of all Vertebrates (pp. 6, 22). In Ammocoetes, as Johannes Müller
had shown, the foundation of the skull is formed by two long cartilaginous
bars, between the hinder portions of which the notochord ends. In these
Rathke was inclined to see the homologues of his trabeculæ, and of the
parachordals which he was ready to assume from his embryological
observations.
Müller was, of course, very ready to accept Rathke's opinions on this
subject, for he considered that they supported his own theory of the
vertebral nature of the skull. After describing in his Handbuch der
Physiologie the cartilaginous bands in Ammocoetes and their highly
differentiated homologues in the Myxinoids, he writes in the later editions,
"Hence we see that in the cranium, as in the spinal column, there are at first
developed at the sides of the chorda dorsalis two symmetrical elements,
which subsequently coalesce, and may wholly enclose the chorda. Rathke
has recently observed, in the embryos of serpents and other animals, before
the formation of the proper cranial vertebræ, two symmetrical bands of
cartilage, similar to those which I discovered as a persistent structure in
Ammocoetes.... At a later period the basis cranii of vertebrate animals
contains three parts analogous to the bodies of vertebræ, the most anterior
of which, in the majority of animals, is generally small, and its development
frequently abortive, whilst in man and mammiferous animals the three are
very distinct. These parts are developed by the formation of three distinct
points of ossification, one behind the other, in the basilar cartilage."[212]
Rathke was very cautious about accepting the vertebral theory of the skull;
he saw that the facts of development were not altogether favourable to the
theory, and he gave his adherence with many reservations and saving
clauses. His general attitude may be summed up as follows.[213]
The chorda sheath is the common matrix of the vertebræ and of a large part
of the skull. The basilar plate and the trabeculæ, which are developed from
the chorda sheath, give origin to three bones, which might possibly be
considered equivalent to vertebral centra—the basioccipital, the

basisphenoid, and the Riechbein (ethmoid). The Riechbein develops from
the fused ends of the trabeculæ. The presphenoid might also be considered
as a vertebral body, but it develops independently of the basilar plate and
trabeculæ.
Now of these bones, the basioccipital is in every way equivalent to a
vertebral centrum, for it develops in the basilar plate round the notochord.
With the exoccipitals, which arise just like neural arches, it forms a true
vertebra. The supraoccipital is an accessory bone developed in relation to
bigger brains. The basisphenoid appears in the basilar plate, but in front of
the notochord, nor does it arise in exactly the same way as the centrum of a
vertebra. The basisphenoid with the alisphenoids, which develop
independently in the side walls of the brain, may, however, still be
considered as forming a vertebra, though the resemblance is not so great as
in the case of the occipital ring. The presphenoid, being long and pointed, is
very unlike a vertebral body. The orbitosphenoids develop separately from
it. The ethmoid also differs from a vertebra, for it surrounds not the whole
nervous axis as the two hinder "vertebræ" do, but only two prolongations of
it, the olfactory lobes. In its development and final form it shows no
particular resemblance to a vertebra. Its body, the pars perpendicularis
(mesethmoid) shows no similarity with a vertebral centrum. Completing the
three hinder cranial "vertebræ" and roofing in the brain are the
supraoccipital, the parietals and the frontals. The premaxillaries, vomer, and
nasals do not belong to the cranial scheme; they are covering bones
connected with the ethmoid. So, too, the ear-capsule is not part of the
cranial vertebræ, but is rather to be compared to the intercalary bones in the
vertebral column of certain fish. Summing up as regards the cranial
vertebræ Rathke writes, "We find that the four different groups of bones,
consisting of the basioccipital with its intercalary (the supraoccipital), the
basisphenoid with its intercalaries (parietals), the presphenoid with its
intercalaries (frontals), and the ethmoid with its outgrowths (turbinals and
cribriform plate), taking them in order from behind forwards, show an
increasing divergence from the plan according to which vertebræ as
commonly understood develop, so that the basioccipital shows the greatest
resemblance to a vertebra, the ethmoid the least" (p. 30).
In a posthumous volume published in 1861 the same opinion is put forward.
"In the head, too," he writes, "some vertebræ can be recognised, although in

a more or less modified form. Yet at most only four cranial vertebræ can be
assumed, and these differ from ordinary well-developed vertebræ in their
manner of formation the more the farther forward they lie."[214]
Rathke was an able and careful critic of the vertebral theory of the skull, but
he accepted it in the main. Actual attack on the theory upon embryological
grounds was begun by C. Vogt, in his work on the development of
Coregonus,[215] and in his paper on the development of Alytes.[216] He
described for Coregonus an origin of the skull in the main similar to that
established by Rathke for the adder. There was a "nuchal plate" in which the
front end of the notochord was imbedded; the notochord ended at the level
of the labyrinth; there were two lateral bands, comparable to Rathke's
lateral trabeculæ; a "facial plate" was also formed, which seems on the
whole equivalent to the plate formed by the fused anterior ends of the
trabeculæ. A little later the cranium formed a complete cartilaginous box
surrounding the brain, very similar to the adult cranium of a shark.
In his criticism of the vertebral theory of the skull, Vogt started by defining
the vertebra as a ring formed round the chorda. Now since only the occipital
segment of the skull is formed actually round the notochord, the parts of the
skull lying in front of this cannot themselves be vertebræ, though they may
be considered as prolongations of the occipital or nuchal vertebra. "We
must regard the nuchal plate as a true vertebra, modified, it is true, in its
formation and development by its particular functions. Now, since the
notochord ends with the nuchal plate we can no longer regard as vertebræ
the parts of the skull that lie beyond, such as the lateral processes of the
cranium and the facial plate, for they have no relation with the notochord"
(p. 123).
To support this view he adduced the fact that the vertebral divisions
(primitive vertebræ) visible in the trunk do not extend into the head. He
used precisely the same arguments in his paper on Alytes to destroy the
vertebral theory of the skull. We quote the following passage translated by
Huxley (1864, p. 295) from this paper. "It has therefore become my distinct
persuasion that the occipital vertebra is indeed a true vertebra, but that
everything which lies before it is not fashioned upon the vertebrate type at
all, and that efforts to interpret it in such a way are vain; that, therefore, if

we except that vertebra (occipital) which ends the spinal column anteriorly,
there are no cranial vertebræ at all."
L. Agassiz, himself a pupil of Döllinger, in the general part (1844) of his
Recherches sur les Poissons fossiles (Neuchâtel, 1833-43), repeats in the
main his pupil Vogt's criticism of the vertebral theory (vol. i., pp. 125-9).
These arguments of Vogt and Agassiz were not considered by Müller to
dispose of the theory,[217] which maintained a firm hold even upon
embryologists. It was still upheld by Reichert, and Kölliker in 1849 showed
himself convinced of its general validity.
A useful step in the analysis of the concept "vertebra" was taken by Remak,
[218]
who showed what a complex affair the formation of vertebræ really is,
involving as it does a complete resegmentation (Neugliederung) of the
vertebral column, whereby the original vertebral bodies were replaced by
the secondary definitive bodies (p. 143). Remak showed, as he thought, that
the protovertebral segmentation of the dorsal muscle-plates did not extend
into the head, and he denied Reichert's assertion (1837) that the cranial
basis in mammals showed transverse grooves delimiting three cranial
vertebræ (p. 36). The gill-slits, he considered, could not possibly be
regarded as marking the limits of head vertebræ.
In 1858 appeared Huxley's well-known Croonian Lecture, On the Theory of
the Vertebrate Skull,[219] in which he stated with great clearness and force
the case for the embryological method of determining homologies, and
criticised with vigour the vertebral theory of the skull. By this time the two
rival methods in morphology had become clearly differentiated, and Huxley
was able to contrast them, or at least to show how necessary the new
embryological method was as a corrective and a supplement to the older
anatomical, or, as he calls it, "gradation" method. Applied to the "Theory of
the Skull," the gradation method consists in comparing the parts of the skull
and vertebral column in adult animals with respect to their form and
connections. "Using the other method, the investigator traces back skull and
vertebral column to their earliest embryonic states and determines the
identity of parts by their developmental relations" (p. 541). This second
method is the final and ultimate. "The study of the gradations of structure
presented by a series of living beings may have the utmost value in

suggesting homologies, but the study of development alone can finally
demonstrate them" (p. 541). As an example of the utility and, indeed, the
necessity of applying the embryological method Huxley takes the case of
the quadrate bone in birds. This bone had been generally regarded by
anatomists as the equivalent of the tympanic of mammals, on account of its
connection with the tympanum; but Reichert showed (1837) that the same
segment of the first visceral arch developed into the incus in mammals, and
into the quadrate in birds, and that therefore the quadrate was homologous
with the incus. Similarly, on developmental grounds, the malleus or
hammer of mammals is the homologue of the articular of birds, since both
are developed from a portion of Meckel's cartilage identical in form and
connections in the two groups. The homologies of the bones connected with
the jaws in bony fishes had long been a subject of contention among
comparative anatomists; Huxley shows from his personal observations how
the development of the visceral arches throws light upon these difficulties.
The mandibular arch in the developing fish is abruptly angled, as in the
embryo of Tetrapoda; the upper prong of it ossifies into the palatine and
pterygoid; at the angle is formed the quadrate (jugal, Cuvier), and to the
quadrate is articulated the lower jaw, which ossifies round the lower prong
or Meckel's cartilage. The scheme of development of the jaws is
accordingly similar in fish to what it is in other Vertebrates, and this
similarity of development enables Huxley to recognise what are the true
homologues of the quadrate, the palatine and the pterygoid in adult bony
fish, and to prove that the symplectic and the metapterygoid (tympanal,
Cuvier) are bones peculiar to fish. In developing Amphibia Huxley found a
suspensorium of hyoid and mandibular arches similar to the hyomandibular
of fish.
Tackling his main problem of the unity of plan of the vertebrate skull,
Huxley shows, by a careful discussion of the anatomical relationships of the
chief bones in typical examples of all vertebrate classes, that there is on the
whole unity of plan as regards the osseous skull. This unity of composition
can be established, on the gradation method, by considering the connections
of the bones of the skull with one another, their relations to the parts of the
brain and to the foramina of the principal cranial nerves. The assistance of
the embryological method is, however, necessary in determining many
points with regard to the bones developed in relation to the visceral arches.

But there is a further step to be taken. "Admitting ... that a general unity of
plan pervades the organisation of the ossified skull, the important fact
remains that many vertebrated animals—all those fishes, in fact, which are
known as Elasmobranchii, Marsipobranchii, Pharyngobranchii and Dipnoi
have no bony skull at all, at least in the sense in which the words have
hitherto been used" (p. 571). The membranous or cartilaginous skull of
these fishes shows a general resemblance in its main features to the ossified
skull of other Vertebrates; the relations of the ear to the vagus and
trigeminal nerves are, for instance, the same in both; the main regions of the
cartilaginous skull can be homologised with definite bones or groups of
bones in the bony skull; but discrepancies occur. It is again to development
that we must turn to discover the true relationship of the cartilaginous to the
ossified skull. "The study of the development of the ossified vertebrate skull
... satisfactorily proves that the adult crania of the lower Vertebrata are but
special developments[220] of conditions through which the embryonic crania
of the highest members of the sub-kingdom pass" (p. 573). It is with the
embryonic cranium of higher Vertebrates that the adult skull of the lower
fishes must be compared, and the comparison will show a substantial
though not a complete agreement between them. Thus, speaking of the
development of the frog's skull, Huxley writes:—"If, bearing in mind the
changes which are undergone by the palatosuspensorial apparatus, ... we
now compare the stages of development of the frog's skull with the
persistent conditions of the skull in the Amphioxus, the lamprey, and the
shark, we shall discover the model and type of the latter in the former. The
skull of the Amphioxus presents a modification of that plan which is
exhibited by the frog's skull when its walls are still membranous and the
notochord is not yet embedded in cartilage. The skull of the lamprey is
readily reducible to the same plan of structure as that which is exhibited by
the tadpole when its gills are still external and its blood colourless. And
finally, the skull of the shark is at once intelligible when we have studied
the cranium in further advanced larvæ, or its cartilaginous basis in the adult
frog" (p. 577). Development, therefore, proves what comparative anatomy
could only foreshadow—the unity of plan of all vertebrate skulls, ossified
and unossified alike. "We have thus attained to a theory or general
expression of the laws of structure of the skull. All vertebrate skulls are
originally alike; in all (save Amphioxus?) the base of the primitive cranium
undergoes the mesocephalic flexure, behind which the notochord

terminates, while immediately in front of it the pituitary body is developed;
[221]
in all, the cartilaginous cranium has primarily the same structure—a
basal plate enveloping the end of the notochord and sending forth three
processes, of which one is short and median, while the other two, the lateral
trabeculæ, pass on each side of the space on which the pituitary body rests,
and unite in front of it; in all, the mandibular arch is primarily attached
behind the level of the pituitary space, and the auditory capsules are
enveloped by a cartilaginous mass, continuous with the basal plate between
them. The amount of further development to which the primary skull may
attain varies, and no distinct ossifications at all may take place in it; but
when such ossification does occur, the same bones are developed in similar
relations to the primitive cartilaginous skull" (p. 578).
In a word, there is a general plan or primordial type which is manifested in
the higher forms most clearly in their earliest development—an
embryological archetype therefore.
Huxley now goes on to consider the relation of this general plan or type of
the skull to the structure and development of the vertebral column. Does the
skull in its development show any signs of a composition out of several
vertebræ? The vertebral column develops as a segmented structure round
the notochord; the skull develops first as an unsegmented plate extending
far beyond the notochord. The processes of this basilar plate, the trabeculæ,
are quite unlike anything in the vertebral column. It is true that when the
process of ossification begins, separate bones are differentiated in the
basilar plate one in front of the other, giving an appearance of segmentation.
The hindmost of these bones, the basioccipital, ossifies round the
notochord, quite like a vertebral centrum, and its side parts which form the
occipital arch develop in a "remotely similar" way to the neural arches of
the vertebræ. The next bone, however, the basisphenoid, develops in front
of the notochord, and shows very little analogy with a vertebral body. The
analogy is even more far-fetched when applied to the axial bones in front of
the basisphenoid. The cranium might indeed be divided upon ossification
into a series of segments bearing a more or less remote analogy with
vertebræ. "In the process of ossification there is a certain analogy between
the spinal column and the cranium, but that analogy becomes weaker and
weaker as we proceed towards the anterior end of the skull" (p. 585). The
best way to state the facts is to say that both skull and vertebral column start

in their development from the same point, but immediately begin to
diverge. The clear indications of segmentation which fully ossified adult
skulls undoubtedly show are, therefore, secondary, and the vertebral theory
of the skull, which was originally based upon the appearance of such fully
ossified crania, is on the whole negatived by embryology.
We have now to turn back a few years in order to follow up another line of
discovery which had an important bearing upon the theory of the vertebrate
skull—the working out of the distinction between membrane and cartilage
bones.

As early as 1731, R. Nesbitt,[222] in two lectures delivered to the Royal
College of Surgeons, demonstrated that in the human fœtus some bones
were formed not in cartilage but directly in fibrous tissue, and this
observation was confirmed by other human anatomists, particularly by
Sharpey at a considerably later date. In 1822 Arendt[223] focussed attention
upon the remarkable structure of the skull of the Pike, with its cartilaginous
brain-box studded all over with bony plaques, an arrangement which had
already attracted the interest of Cuvier and Meckel. K. E. von Baer[224] in
1826 discussed at some length the relation between the bony and the
cartilaginous skull in fishes, with particular reference to the sturgeon,
coming to the following just conclusion:—"If we consider the fibrous
skeleton of Ammocoetes as the first foundation of the skeleton of
Vertebrates, we can form a series among the cartilaginous fishes, according
as a cartilaginous skeleton penetrates more and more into this fibrous
foundation. In the same way the process of ossification supplants the
cartilaginous skeleton. So long as the ossifications lie in the skin, as in the
sturgeon, they form corneous bones (Hornknochen), but when they lie
under the skin, they form true bones, e.g., the bones of the skull in the pike"
(p. 374).
Embryologists soon become aware that a similar distinction between a
primitive cartilaginous foundation and a secondary overlying ossification of
the skull showed itself in the development of all Vertebrates. Dugès, in his
Recherches sur l'ostéologie et la myologie des Batraciens (1834),
distinguished between such bones as are formed by direct ossification of the
cartilaginous groundwork of the skull, and such as are developed in the
periosteal fibrous tissue.

Reichert in 1838[225] noted that several of the skull bones in Amphibia are
formed without the intermediary of cartilage, such as the nasals, the
maxillaries and the lacrymals. So, too, the frontals and parietals of Teleosts
developed independently of the cartilaginous skull, and belonged to the
skeletal system of the skin, not to the true vertebral axial skeleton (pp. 215-
6). Even more interesting was his discovery, afterwards confirmed by
Hertwig,[226] that in the newt several bones connected with the palate were
formed in the mucous membrane of the mouth by the fusion of a number of
little conical teeth (p. 97). Certain of these bones he considered to be the
substitutes, not the equivalents, of the palatine and pterygoid of other
Vertebrates, which are formed from the upper part of the first visceral arch,
a part missing in the newt (p. 100). Owing to the difference of development
he would not homologise these bones in the newt with the palatine and
pterygoid of other Vertebrates. He recognised also that the bone now known
as the parasphenoid was developed in the frog in the mucous membrane of
the mouth, and had originally no connection with the cranial basis (p. 34).
Rathke in 1839 also allowed the distinction between cartilage and
membrane bone, but laid no stress upon it (Entw. d. Natter., p. 197).

Jacobson in 1842[227] introduced the useful term, "primordial cranium," for
the primitive cartilaginous foundation of the skull, and drew a sharp
distinction between cartilage bones and membrane bones.

In his Recherches sur les Poissons fossiles,[228] L. Agassiz used Vogt's work
on the development of Coregonus to establish a classification of the bones
of the skull in fish, a classification which had the merit of drawing a sharp
distinction between the cartilaginous groundwork and the "protective
plates" of the fish's skull. He recognised that the protective plates developed
in a different way from the other bones of the skull. "We must distinguish,"
he writes, "two kinds of ossification; one which tends to transform the
primitive parts of the embryonic cranium directly into bone, and another
which leads to the deposition of protective plates round this core, which
develop not only upon the upper surface, as has hitherto been supposed, but
also on the lateral walls and on the lower surface of the cranium" (p. 112).
In the skull of all fish there are three elements—(1) the cartilaginous base,
including the nuchal plate, the trabeculæ and the facial plate, together with
the auditory capsules; (2) the cartilaginous cerebral envelope; (3) the bony

protective plates (absent in Elasmobranchs). The bones developed in
relation to these cranial elements can be classified as follows:—(1) the
basioccipital, exoccipitals (paroccipitals?), supraoccipital and "petrous"
(rocher), developed from the nuchal plate; the ali- and orbito-sphenoids
developed from the trabeculæ; the "cranial ethmoid"[229] developed from
the facial plate; (2) the parietals, frontals and nasals formed from the
"superior" protective plate; the "anterior" and "posterior" frontals and the
temporal, from the "lateral" plates; the body of the sphenoid and the vomer
from the "inferior" plates. The other element, the cartilaginous brain-box,
does not ossify, and tends to become absorbed (p. 124).

In 1849 Kölliker published a paper[230] dealing with the morphological
significance of the distinction between membrane and cartilage bones, and
in 1850[231] he defended his views against the criticisms of Reichert[232] in a
further note entitled Die Theorie des Primordialschädels festgehalten. It is
convenient to consider these papers together. Kölliker held that there was
(1) a histological and (2) a morphological difference between the two
categories of bones. The histological development of the two kinds was
different, but this difference was not sufficient to establish a morphological
distinction between them, a distinction in their anatomical Bedeutung. The
true morphological distinction between them was their development in
different skeleton-forming layers. Membrane bones were developed in
fibrous tissue lying between the skin and the deep layer which formed the
primordial cranium, and it was this formation in a separate layer that gave
them a different morphological significance from the bones formed directly
in the deep layer. Kölliker's distinction, therefore, was between the bones
formed in the primordial cartilaginous cranium on the one hand, and the
superficial ossifications in fibrous tissue on the other hand. The
cartilaginous cranium in Kölliker's opinion was formed upon the vertebral
type, and the membrane bones were accessory. This, at least, was his
opinion in 1849. In 1850, after Stannius had shown that membrane bones
occurred as integral parts of the vertebræ in certain fish, he modified his
view of the membrane bones, and admitted them, at least in some cases, as
constituents of the cranial vertebræ.
On this morphological distinction of membrane and cartilage bones future
comparative osteology was to be based:—

"My sole aim is to state again the principle upon which comparative
osteology is to be based and extended, and this is that first place should be
assigned to anatomical considerations, and among these to the manner of
origin of the whole bone in relation to the skeleton-forming layers" (1850,
p. 290).
The homologies established by this new principle might run counter to the
homologies indicated by the study of adult structure. "Thus, for instance,
although the lower jaw in position, function, form and shape, appears to be
the same bone throughout, yet it must be admitted that it shows a difference
in the different classes. In Mammals and Man it is an entirely secondary
bone (an extremity according to Reichert), in Birds, Amphibia and Fishes
only partially so, for its articular belongs to Meckel's cartilage and is
accordingly analogous to a rib; indeed, in the Plagiostomes, etc., the whole
lower jaw along with the articular is a persistent Meckel's cartilage" (p. 290,
1850).
So, too, the supraoccipital in man cannot be fully homologised with the
supraoccipital of many mammals, for its upper half arises at first in
isolation as a secondary bone (p. 290).
Reichert objected to the distinction drawn by Kölliker, and denied that there
was either a histological or a morphological difference between membrane
and cartilage bones. It was shown a few years later by H. Müller[233] that
there was in truth no essential difference in histological development
between the two categories of bone, that the cartilage cells were replaced by
bone cells identical with those taking part in the formation of membrane
bones. The morphological distinction continued however to be recognised,
particularly by the embryologists. Rathke in his volume of 1861[234]
classified the bones of the skull according to their origin from the
primordial cranium or from the overlying fibrous layer, distinguishing as
membrane bones, the parietals, frontals, nasals, lachrymals, maxillaries and
premaxillaries, jugals, tympanic, parts of the "temporal," vomer, part of the
supraoccipitals in some mammals, and the mandible (with the exception of
the articular in such as have a quadrate bone). Huxley was also inclined in
1864[235] to recognise the distinction, but he writes with some reserve:—"Is
there a clear line of demarcation between membrane bones and cartilage
bones? Are certain bones always developed primarily from cartilage, while

certain others as constantly originate in membrane? And further, if a
membrane bone is found in the position ordinarily occupied by a cartilage
bone, is it to be regarded merely as the analogue and not as the homologue
of the latter?" (p. 296).
We may note here that many comparative anatomists of the period were
quite ready to decide Huxley's last question in a sense favourable to the
older, purely anatomical, view of homology. Owen, for instance, held that
difference of development did not disturb homologies established by form
and connections. "Parts are homologous," he writes, "in the sense in which
the term is used in this work, which are not always similarly developed:
thus the 'pars occipitalis stricte dicta,' etc., of Soemmering is the special
homologue of the supraoccipital bone of the cod, although it is developed
out of pre-existing cartilage in the fish and out of aponeurotic membrane in
the human subject."[236] Similarly he pointed to the diversities of
development of the vertebral centrum in the different vertebrate classes as
proof that development could not always be relied upon in deciding
homologies (p. 89). But he could not deny that the archetype was better
shown in the embryo than in the adult (supra, p. 108).

J. V. Carus[237] likewise stood firm for the older method of determining
homologies by comparison of adult structure. "We can regard as
homologous," he writes, "only those parts which in the fully formed animal
possess a like position and show the same topographical relations to the
neighbouring parts" (p. 389). Parts homologous in this sense might develop
in different ways, but no great importance was to be attached to such a
circumstance. Membrane and cartilage bones developed in practically the
same way, from the same skeleton-forming layer, and no morphological
significance attached to their distinction (pp. 227, 457). Embryology was of
considerable value in helping to determine homologies, but the evidence
that it supplied was contributory, not conclusive. Perhaps the greatest
service which the study of development rendered was to disentangle, by a
comparison of the earliest embryos, the generalised type (p. 389).
We have now traced, by our historical study of the theory of the skull, the
gradual evolution of the tendency to find in development the surest guide to
determining homologies. We have seen how the embryological "type" came
to be substituted, in whole or in part, for the anatomical "type" derived from

the study of adult structure. But we have had to do only with a
modification, not with a transformation, of the criterion of homology
recognised by the anatomists. Homology is still determined by position, by
connections, in the embryo as in the adult. "Similarity of development" has
become the criterion of homology in the eyes of the embryologist, but
"similarity of development" means, not identity of histological
differentiation, but similarity of connections throughout the course of
development. For the purposes of morphology, development has to be
considered as an orderly sequence of successive forms, not in its real nature
as a process essentially continuous. Morphology has to replace the living
continuity by a kinematographic succession of stages. Since it is the earliest
of these stages that manifest the simplest and most generalised structural
relations of the parts, it is in the earlier stages that homologies can be most
easily determined. But these homologies are still determined solely by the
relative positions and connections of the parts, just as homologies are
determined in the last of all the stages of development, the adult state. And
since the generalised type is shown most clearly in the earliest stages and
tends to become obscured by later differentiation, homologies observed in
embryonic life are to be upheld even if the relations in adult life seem to
indicate different interpretations.

[183] See review by Cuvier, Mém. Mus. Hist, nat., iii., pp. 82-97, 1817.
[184] Mém. Savans étrangers, vi. Extract in Ann. Sci. nat. (2) i. (Zool.), pp. 366-
72, 1834.
[185] Recherches sur la génération des Mammifères, 1834. Embryogénie
comparée, 1837.
[186] "Kiemen bey Säugthieren," Isis, pp. 747-9, 1825.
[187] "Kiemen bey Vögeln," Isis, pp. 1100-1, 1825.
[188] "Ueber die Kiemenbogen und Kiemengefässe beym bebrüteten
Hühnchen," Isis, xx., pp. 401-3, 1827. (Read in Sept. 1826 to the Versammlung
der deutschen Naturforscher und Aerzte, then recently founded by Oken).
[189] Isis, pp. 160-4, Pl. II., 1828.
[190] "Ueber die Kiemen und Kiemengefässe in den Embryonen der
Wirbelthiere," Meckel's Archiv for 1827, pp. 556-68. Also in Ann. Sci. nat., xv.,
pp. 266-80, 280-4, 1828.
[191] Meckel's Archiv, vi., pp. 1-47, 1832.
[192] Untersuchungen über die Bildung und Entwickelung der Fluss-Krebses,
Leipzig, folio, 1829. Preliminary notice in Isis, pp. 1093-1100, 1825.
[193] "Untersuchungen über die Bildung und Entwickelung der Wasser-Assel.,"
Abh. z. Bild. u. Entwick.-Gesch., i., pp. 1-20, 1832. Translated in Ann. Sci. nat.
(2), ii., (Zool.), pp. 139-57, 1834.
[194] Kölliker, Entwickelungsgeschichte, 2nd ed., p. 17, Leipzig, 1879.
[195] Handbuch der Entwickelungsgeschichte des Menschen und ... der
Säugethiere und Vögel, Berlin, 1835.
[196] Embryogénie comparée, 1837; Histoire générale du développement des
corps organisés, 1847-49.
[197] Entwickelungsgeschichte des Kaninchen-Eies, Braunschweig, 1842;
Entwickelungsgeschichte des Hunde-Eies, Braunschweig, 1845;
Entwickelungsgeschichte des Meerschweinchens, Giessen, 1852;
Entwickelungsgeschichte des Rehes, Giessen, 1854.
[198] "It is the rôle of embryology, as my great teacher says, to form the court of
appeal for comparative anatomy, and it is from embryology particularly, which
has in the last decades provided such signal instances of the unravelling of
obscure problems, that we have to expect a definite clearing up of the problems
relating to the development of the head."—Müller's Archiv, p. 121, 1837.
[199] Anat.-phil. Unters. ü. d. Kiemenapparat u. d. Zungenbein, Riga and
Dorpat, 1832.
[200] "Bildungs- und Entwickelungs-geschichte des Blennius viviparus,"
Abhandl. z. Bild. u. Entwick.-Gesch. des Menschen u. der Thiere, ii., pp. 1-68,

Leipzig, 1833.
[201] Von den Ur-Theilen des Knochen und Schalen-Gerustes, Leipzig, 1828.
[202] Kiemenapparat, pp. 107-118.
[203] Vergleichende Anatomie der Myxinoiden. Part I. (Osteology and Myology).
(Abh. königl. Akad. Wiss. Berlin, for 1834, pp. 65-340, 9 pls., 1836.) Also
separately.
[204] "Ueber die Visceralbogen der Wirbelthiere in Allgemeinen und deren
Metamorphosen bei den Vögeln und Säugethiere," Müller's Archiv, pp. 120-222,
1837.
[205] Handbuch d. menschl. Anatomie, iv., p. 47.
[206] This was shown by Serres (Ann. Sci. nat., xi., p. 54 f.n., 1827), who found
in a human embryo a long cartilaginous piece extending from the ear-ossicles to
the inside of the lower jaw, and suggested that it was the foundation of the
permanent mandible.
[207] Abhandl., i., p. 102, 1832; ii., p. 25, 1833. (Blennius paper).
[208] Vergleichende Entwickelungsgeschichte des Kopfes der nackten
Amphibien, Königsberg, quarto, 276 pp., 1838.
[209] Müller's Archiv for 1838.
[210] Entwickelungsgeschichte der Natter, Königsberg, 1839.
[211] Bemerkungen über die Entwickelung des Schädels der Wirbelthiere,
Königsberg, 1839.
[212] Handbuch der Physiologie des Menschen, Koblenz, 1835; Eng. trans. by
W. Baly, ii., p. 1615, 1838.
[213] For a full statement of Rathke's conclusions, see the translation given by
Huxley in Lectures on the Elements of Comparative Anatomy, London, 1864.
[214] Entwickelungsgeschichte der Wirbelthiere, p.142, 1861.
[215] Embryologie des Salmones. A separate volume of L. Agassiz's Histoire
naturelle des Poissons d'Eau douce de l'Europe centrale, Neuchâtel, 1842.
[216] Untersuchungen über die Entwickelungsgeschichte der
Gebürtshelferkröte, Solothurn, 1842.
[217] Müller's Archiv for 1843, p. ccxlviii.
[218] Untersuchtingen über die Entwickelung der Wirbelthiere, Berlin, 1850-55.
[219] Delivered 17th June 1858. Reprinted in The Scientific Memoirs of T. H.
Huxley, edited by M. Foster and E. Ray Lankester, vol. i., pp. 538-606 (1898).
[220] Cf. Reichert, supra, p. 149.

[221] The origin of the pituitary body from the roof of the mouth was first
described by Rathke (1839).
[222] Human Osteogeny explained in two Lectures, London, 1736.
[223] De capitis ossei Esocis lucii structura singulari. Dissert. inaug.
Regiomonti, 1822.
[224] "Ueber das äussere und innere Skelet," Meckel's Archiv, pp. 327-76, 1826.
[225] Vergl. Entwick. d. Kopfes d. nackten Amphibien (p. 186).
[226] Arch. f. mikr. Anat., xi., Suppl., 1874.
[227] "Om Primordial-Craniet," Förhandlingar Skand. Naturf. Möle, Stockholm,
1842.
[228] Vol. I., General part, pub. 1844.
[229] Entosphenoid, Owen.
[230] Zweiter Bericht zootom. Anstalt zu Würzburg, 1849.
[231] Zeits. f. wiss. Zool., ii., pp. 281-91.
[232] Müller's Archiv for 1849, pp. 443-515.
[233] Zeits. f. wiss Zool., ix., 1858.
[234] Entw. d. Wirbelthiere, pp. 139-40, 1861.
[235] Lectures on the Elements of Comparative Anatomy.
[236] On the Archetype of the Vertebrate Skeleton, p. 5, 1848.
[237] System der thierischen Morphologie, Leipzig, 1853.

CHAPTER XI

THE CELL-THEORY.

With the founding of the cell-theory by Schwann in 1839 an important step
was taken in the analysis of the degrees of composition of the animal body.
Aristotle had distinguished three—the unorganised material, itself
compounded of the four primitive elements, earth and water, air and fire,
the homogeneous parts or tissues and the heterogeneous parts or organs, and
this conception was retained with little change even to the days of Cuvier

and von Baer. Those of the old anatomists who speculated on the relations
of organic elements to one another were dominated by Aristotle's simple
and profound classification, and proposed schemes which differed from his
only in detail. Bichat enlarged and deepened the concept of tissue, but the
degree of composition below this was for him, as for all anatomists of his
time, a fibrous or pulpy "cellulosity," living, indeed, but showing no
uniform and elemental structure. It was Schwann's merit to interpose
between the tissue and the mere unorganised material a new element of
structure, the cell. And, as it happened, a few years before Schwann
published his cell-theory, Dujardin hinted at another degree of composition
which was later to take its place between the cell and the chemical elements
—sarcode or protoplasm.
As is well known, the concept of the cell arose first in botany. Robert
Hooke discovered cells in cork and pith in 1667, and his discovery was
followed up by Grew and Malpighi in 1671, and by Leeuenhoek in 1695.
But they did not conceive the cell as a living, independent, structural unit.
They were interested in the physiology of the plant as a whole, how it lived
and nourished itself, and they studied cells and sieve-tubes, wood fibres and
tracheæ with a view rather to finding out their functions and their
significance for the life of the plant than to discovering the minutiæ of their
structure. The same attitude was taken up by the few botanists who in the
18th century paid any heed to the microscopical anatomy of plants. For C.
F. Wolff,[238] the formation of cells was a result of the secretion of drops of
sap in the fundamental substance of the plant, this substance remaining as
cell-walls when cell-formation was completed—no idea here of cells as
units of structure.
In the early 19th century, interest in plant anatomy revived somewhat, and
much work was done by Treviranus, Mirbel, Moldenhawer, Meyen and von
Mohl.[239] As a result of their work the fact was established that the tissues
of plants are composed of elements which can, with few exceptions, be
reduced to one simple fundamental form—the spherical closed cell. Thus
the vessels of plants are formed by coalescence of cells, fibres by the
elongation of cells and the thickening and toughening of their walls. At this
time, interest was concentrated on the cell-wall, to the almost total neglect
of the cell-contents; the "matured framework" of plant cells, to use Sach's
convenient phrase, was the chief, almost the sole, object of study. And it

was natural enough that the mere architecture of the plant should
monopolise interest, that the composition of the tissues out of the cells, and
the fitting together of the tissues to form the plant should awaken and hold
the curiosity of the investigator; even the modifications of the cell-walls
themselves, their rings and spiral thickenings and pits, offered a fascinating
field of enquiry.
The idea that the cell-contents might show a characteristic and individual
structure had hardly dawned upon botanists when Schleiden published his
famous paper, Beiträge zur Phytogenesis.[240] Schleiden's theme in this
paper is the origin and development of the plant cell, a subject then very
obscure, in spite of pioneer work by Mirbel. A few years before, Robert
Brown had called attention to the presence in the epidermal cells of orchids
and other plants of a characteristic spot which he called the areola or
nucleus.[241] Schleiden saw the importance of this discovery, confirmed the
constant presence of the nucleus in young cells, and held it to be an
elementary organ of the cell. He named it the cytoblast because, in his
opinion, it formed the cell. It was embedded in a peculiar gummy
substance, the cytoblastem, which formed a lining to the cellulose cell-wall.
Within the nucleus there was often a small dark spot or sphere—the
nucleolus. The nucleus, Schleiden thought, originated as a minute granule
in the cytoblastem which gradually increased in size, becoming first a
nucleolus (Kernchen), and then, by further condensation of matter round it,
a nucleus. Several nuclei might be formed in this way in a single cell. New
cells took their origin directly from a full-grown nucleus, in a peculiar way
which Schleiden describes as follows:—"As soon as the cytoblasts have
reached their full size a delicate transparent vesicle arises on their surface;
this is the young cell, which at first takes the shape of a very flat segment of
a sphere, of which the plane surface is formed by the cytoblast, the convex
side by the young cell itself, which lies upon the cytoblast like a watch-
glass on a watch" (p. 145). The young cells increase in size and fill up the
cavity of the old cell, which is in time resorbed. Cell-development always
takes place within existing cells, and either one or many new cells may be
formed within the mother-cell. Schleiden's views on cell-formation were
drawn from some rather imperfect observations on the embryo-sac and
pollen-tube, but he extended his theory to cell-formation in general. Though
wrong in almost all respects the theory had at least the merit of fixing

attention upon the really important constituents of the cell, the nucleus and
the cell-plasma. To Schleiden, too, we owe the conception of the cell as a
more or less independent living unity, whose life is not entirely identified
with the life of the plant as a whole. "Each cell," he writes, "carries on a
double life; one a quite independent and self-contained life, the other a
dependent life in so far as the cell has become an integral part of the plant"
(p. 138).
So long as the definition of the plant cell embraced little more than the
hardened cell-wall it was little wonder that "cells" in this sense were not
recognised in animal tissues, except in a few exceptional cases—as in the
notochord by Johannes Müller.[242] Careful observation of animal tissues
discovered in some cases the existence of discontinuous units of structure,
but these were not, as a rule, recognised before 1838 as analogous to plant
cells. Von Baer, for example, observed that the young chick embryo was
composed partly of an albuminous mass and partly of Kügelchen or little
globules suspended in it (Entwickelungsgeschichte, i., pp. 19, 144). Since
such Kügelchen disposed in a row formed the notochord (i., p. 145) it seems
probable that his Kügelchen were really cells. Similarly A. de
Quatrefages[243] in 1834 saw and figured segmentation spheres in the
developing egg of Limnæa, but he called them globules and did not
recognise their analogy with the cells of plants. According to M'Kendrick,
[244]
Fontana, so far back as 1781,[245] described cells with nuclei in various
tissues, and used acids and alkalis to bring out their structure more clearly.
But it was not till 1836-7-8 that a fairly widespread occurrence of cells in
animal tissues was recognised. The pioneer in this seems to have been
Purkinje, who described cells in the choroidal plexus in 1836,[246] and
compared gland cells with the cells of plants in 1837.[247] Henle in 1837[248]
and 1838[249] described various kinds of epithelial tissue, distinguishing
them according to the kind of cell composing them; he also discovered the
mode of growth of stratified epithelium. Valentin[250] appears to have seen
cells in cartilage and epithelium even before Henle, and to have observed
cells in the blastoderm of the chick. In his report on the progress of anatomy
during 1838 Johannes Müller was able to refer to quite a number of papers
dealing with the occurrence of cells in animal tissues. In addition to those
already noted, he mentions work by Breschet and Gluge on the cells of the
umbilical cord, by Dumortier on the cells in the liver of molluscs, by

Remak and by Purkinje on nerve cells, by Donné on the cells of the
conjuctiva, cornea and lens. He reports, too, that Turpin had compared the
epithelial cells of the vagina with the cell-tissue of plants. Müller himself
had not only recognised the cellular nature of the notochord, but had
observed the cells of the vitreous humour, fat cells and pigment cells, and
even the nuclei of cartilage cells. From Schwann (1839) we learn that C. H.
Schults had followed back the corpuscles of the blood to their original state
of nucleated cells, and that Werneck had recognised cells in the embryonic
lens. A preliminary notice of Schwann's own work appeared in 1838
(Froriep's Notizen, No. 91, 1838), the full memoir in 1839, under the title
Mikroskopische Untersuchungen über die Uebereinstimmung in der
Struktur und dem Wachstume der Tiere und Pflanzen.[251]
Theodor Schwann was a pupil of Johannes Müller, and we know that
Müller took much interest in the new histology. It is probably to his
influence that we owe Schwann's brilliant work on the cell, which appeared
just after Schwann left Berlin for Löwen. Schwann was himself, as his later
work showed, more a physiologist than a morphologist; he did quite
fundamental work on enzymes, discovering and isolating the pepsin of the
gastric juice; he proved that yeast was not an inorganic precipitate but a
mass of living cells; he carried out experiments directed to show that
spontaneous generation does not occur. We shall see in his treatment of the
cell-theory clear indications of his physiological turn of mind. Schwann was
only twenty-nine when his master-work appeared, and the book is clearly
the work of a young man. It has the clear structure, the logical finish, which
the energy of youth imparts to its chosen work. So the work of Rathke's
prime, the Anatomische-philosophische Untersuchungen of 1832 shows
more vigour and a more reasoned structure than his later papers. Schwann's
book is indeed a model of construction and cumulative argument, and even
for this reason alone justly deserves to rank as a classic.
The first section of his book is devoted to a detailed study of the structure
and development of cartilage cells and of the cells of the notochord, and to
a comparison of these with plant cells. He accepts Schleiden's account of
the origin and development of nuclei and cells as a standard of comparison;
and he seeks to show that nucleus and nucleolus, cell-wall and cell-
contents, show the same relations and behave in the same manner in these
two types of animal cells as in the plant-cells studied by Schleiden. The

types of cell which he chose for this comparison are the most plant-like of
all animal cells, and he was even able to point to a thickening of the cell-
wall in certain cartilage cells, analogous to the thickening which plays so
important a part in the outward modification of plant-cells. The analogy
indeed in structure and development between chorda and cartilage cells and
the cells of plants seemed to him complete. The substance of the notochord
consisted of polyhedral cells having attached to their wall an oval disc
similar in all respects to the nucleus of the plant-cell, and like it containing
one or more nucleoli. Inside the mother-cell were to be found young
developing cells of spherical shape, lacking however a nucleus. Cartilage
was even more like plant tissue. It was composed of cells, each with its cell
membrane. The cells lay close to one another, separated only by their
thickened cell-wall and the intercellular matrix, showing thus even the
general appearance of the cellular tissue of plants. They contained a nucleus
with one or two nucleoli, and the nucleus was often resorbed, as in plants,
when the cell reached its full development. Other nuclei were in many cases
present in the cell, round which young cells could be seen to develop, in
exactly the same manner as in plants. These nuclei had accordingly the
same significance as the nuclei of plants, and deserved the same name of
cytoblasts or cell-generators. The true nucleus of the cartilage cell was
probably in the same way the original generator of the mother-cell.
Having proved the identity in structure and function of the cells of these
selected tissues with the cells of plants, as conceived by Schleiden,
Schwann had still to show that the generality of animal tissues consisted
either in their adult or in their embryonic state of similar cells. This
demonstration occupies the second and longest section of his book.
His method is throughout genetic; he seeks to show, not so much that all
animal tissues are actually in their finished state composed of cells and
modifications of cells, as that all tissues, even the most complex, are
developed from cells analogous in structure and growth with the cells of
plants.
All animals develop from an ovum; it was his first task to discover whether
the ovum was or was not a cell. It happened that, some years before
Schwann wrote, a good deal of work had been done on the minute structure
of the ovum, particularly by Purkinje and von Baer. Purkinje in 1825[252]

discovered and described in the unfertilised egg of the fowl a small vesicle
containing granular matter, which he named the Keimbläschen or germinal
vesicle. It disappeared in the fertilised egg. As early as 1791 Poli had seen
the germinal vesicle in the eggs of molluscs, but the first adequate account
was given by Purkinje. In 1827[253] von Baer discovered the true ova of
mammals and cleared up a point which had been a stumbling block ever
since the days of von Graaf, who had described as the ova the follicles now
bearing his name.[254] Even von Graaf had noticed that the early uterine
eggs were smaller than the supposed ovarian eggs; Prévost and Dumas[255]
had observed the presence in the Graafian follicle of a minute spherical
body, which, however, they hesitated to call the ovum; it was left to von
Baer to elucidate the structure of the follicle and to prove that this small
sphere was indeed the mammalian ovum. His discovery was confirmed by
Sharpey and by Allen Thomson. Von Baer found the germinal vesicle in the
eggs of frogs, snakes, molluscs, and worms, but not in the mammalian
ovum; he considered the whole mammalian ovum to be the equivalent of
the germinal vesicle of birds—a comparison rightly questioned by Purkinje
(1834). In 1834 Coste[256] discovered in the ovum of the rabbit a vesicle
which he considered to be the germinal vesicle of Purkinje; he observed
that it disappeared after fertilisation. Independently of Coste, and very little
time after him, Wharton Jones[257] found the germinal vesicle in the
mammalian ovum. Valentin in 1835,[258] Wagner in 1836,[259] and Krause in
1837,[260] added considerably to the existing knowledge of the structure of
the ovum. Wagner in his Prodromus called attention to the widespread
occurrence, within the germinal vesicle of a darker speck which he called
the Keimfleck or germinal spot, known sometimes as Wagner's spot. He
recognised the Keimfleck in the ova of many classes of animals from
mammals to polyps. Frequently more than one Keimfleck occurred.
Schwann had therefore a good deal of exact knowledge to go upon in
discussing the significance of the ovum for the cell-theory. There were two
possible interpretations. Either the ovum was a cell and the germinal vesicle
its nucleus, or else the germinal vesicle was itself a cell within the larger
cell of the ovum and the germinal spot was its nucleus. Schwann had some
difficulty in deciding which of these views to adopt, but he finally inclined
to the view that the ovum is a cell and the germinal vesicle its nucleus,
basing his opinion largely upon observations by Wagner which tended to

prove that the germinal vesicle was formed first and the ovum subsequently
formed round it. But the ovum was not, in Schwann's view, a simple cell,
for within it were contained yolk-granules, one set apparently containing a
nucleus, the others not. Even the second set, those composing the yellow
yolk, were considered by Schwann to deserve the name of cells, because,
although a nucleus could not be observed in them, they had a definite
membrane, distinct from their contents—a conception of the cell obviously
dating from the earliest botanical notions of cells as little sacs. The yolk
cells were not mere dead food material but living units which took part in
the subsequent development of the egg. The relation between the
unfertilised egg and the blastoderm which arises from it is not made
altogether clear by Schwann. According to his account the cells of the
blastoderm are formed actually in the ovum. Round the nucleus of the egg
appears a Niederschlag or precipitate which is the rudiment of the
blastoderm (p. 68). When the egg leaves the ovary the nucleus disappears,
leaving behind it this rudiment of the blastoderm, which rapidly grows and
increases in size. The blastoderm of the chick before incubation is found to
be composed of spherical anucleate bodies which Schwann considers to be
cells, because they almost certainly develop into the cells of the incubated
blastoderm, which are clearly recognisable as such after eight hours'
incubation. The serous and mucous layers can be distinguished after sixteen
hours' incubation, and it is found that the cells of the serous layer contain
definite nuclei, though such seem to be absent in the cells of the mucous
layer. Between the two layers other cells are formed belonging to the vessel
layer, which is, however, in Schwann's opinion not a very definitely
individualised layer.
Schwann's next step is a detailed demonstration of the origin of each tissue
from simple cells such as those composing the incubated blastoderm.
"The foregoing investigation has taught us that the whole ovum shows
nothing but a continual formation and differentiation of cells, from the
moment of its appearance up to the time when, through the development of
the serous and mucous layers of the blastoderm, the foundation is given for
all the tissues subsequently appearing: we have found this common parent
of all tissues itself to consist of cells; our next task must be to demonstrate
not only in this general way that tissues originate from cells, but also that
the special formative mass of each tissue is composed of cells, and that all

tissues are either constituted by simple cells or by one or other of the
manifold kinds of modified cells" (p. 71). Five classes of tissue can be
distinguished, according to the extent and manner of the modifications
which the cells composing them have undergone. There are first of all
independent and isolated cells, such as the corpuscles of the blood and
lymph, not forming a coherent tissue in the ordinary sense. Next there are
the assemblages of cells lying in contiguity with one another, but not in any
way fused; examples of this class are the epidermal tissues and the lens of
the eye. In the third class come tissues the cells of which have fused by
their walls, but whose cell-cavities are not in continuity, such as osseous
tissue and cartilage. In the tissues of the fourth class, comprising the most
highly specialised of all, not only are the cell-walls continuous but also the
cell-cavities; to this class belong muscle, nerve and capillary vessels. A fifth
class, of rather a special nature, includes the fibrous tissues of all kinds.
This is the first classification of tissues upon a cellular basis, and it marks
the foundation of a new histology which took the place of the "general
anatomy" of Bichat. The exhaustive account which Schwann gives of the
structure and development of the tissues in this section of his book
constitutes the first systematic treatise on histology in the modern sense,
and it is still worth reading, in spite of many errors in detail.
Schwann found it easy to demonstrate the cellular nature of the tissues of
his first three classes. With the other two classes he had more difficulty.
Fibres of all kinds, he considered, arose by an elongation of cells, which
afterwards split longitudinally into long strips, forming as the case might be
white or elastic fibrous tissue. Muscle-fibres and nerve-fibres were formed
in a totally different way, by coalescence of cells; each separate muscle-
fibre and nerve-fibre was thus a compound cell. Capillaries, Schwann held,
were formed by cells hollowed out like drain-pipes, and set end to end—a
mistaken view soon corrected by Vogt (Embryologie des Salmones, p. 206,
1842).
In this detail part of his book Schwann accumulates material for a general
theory of the cell which he develops in the third and last section. Taking up
the physiological or dynamical standpoint, he points out that one process is
common to all growth and development of tissues both in animals and
plants, namely, the formation of cells, a process which he conceives to take
place in the following manner. There is, first of all, a structureless

substance, the cytoblastem, the matrix in which all cells originate. The
cytoblastem may be either inside the cells, or, more usually, in the spaces
between them. It is not a substance of definite chemical and physical
properties, for the matrix of cartilage and the plasma of the blood alike
come within the definition. It has largely the significance of food material
for the developing cells. In plants, according to Schleiden, cells are never
formed in the intercellular substance—the cytoblastem is within the cells;
but extracellular cell formation seems to be the general rule in animals. An
intracellular formation of cells occurs only in the ovum, in cartilage cells
and chorda cells and in a few others, and even there it is not the exclusive
method of formation; a formation of cells within cells never occurs in
muscles and nerves, nor in fibrous tissue (p. 204). In the cytoblastem
granules appear, which gradually increase in size and take on the
characteristic shape of nuclei; round each of these a young cell is formed.
Sometimes the young cells appear to have no nuclei, as in the intracellular
brood of chorda cells, but, as a rule, a nucleus is clearly visible. The nucleus
is indeed the most characteristic constituent of the cell. "The most important
and most constant criterion of the existence of a cell is the presence or
absence of the nucleus," writes Schwann near the beginning of his book
(p. 43).
As a general rule the nucleolus is formed first, and round it by a sort of
condensation or concretion the nucleus, which is frequently hollow, and
round this again, by a somewhat similar process, the cell. "The whole
process of the formation of a cell consists in the precipitation round a small
previously formed corpuscle (the nucleolus) of first one layer (the nucleus)
and then later round this a second layer (the cell substance)" (p. 213). The
outermost layer of the cell usually thickens to form the membrane, but this
membrane formation does not always occur, and the membrane is not
present in all cells. The nucleus is formed in exactly the same manner as the
cell, and it might with much truth itself be called a cell—a cell of the first
order, while ordinary nucleated cells might be designated cells of the
second order (p. 212). In anucleate cells there is probably only a single
process of layer formation round an infinitely small nucleolus. In almost all
nucleate cells the nucleus is resorbed when the cell reaches its full
development, and it is larger and more important the younger the cell is.

The cell was for Schwann not a morphological concept at all, but a
physiological; the cell was a dynamical, not a statical unit. Cell-formation
was the process at the back of all production of life, and cells were the
centres of all vital activity. Each cell was itself an organism, and its life and
activities were to some extent independent of the lives and activities of all
the other cells. The multicellular organism was a colony of unicellular
organisms, and its life was a sum of the lives of its constituent elements.
This "theory of the organism," which holds so important a place in biology
even at the present day, is developed by Schwann in the concluding pages
of his book.
He begins by contrasting the teleological with the materialistic conception
of living things. In the teleological view, a special force works in the living
organism, guiding and directing its activities towards a purposeful end.
According to the materialistic view there are no other forces at work in the
living organism than those which act in the inorganic realm, or at least there
are none but forces at one with these in their blindness and necessity. True,
the purposiveness of living processes cannot be denied; but its ground lies,
according to this view, not in a vital force which guides and rules the
individual life, but in the original creation and collocation of matter
according to a rational plan. The purposiveness of life is part of the
purposiveness of the universe; just as the stars circle for ever in
harmoniously adjusted paths, so do the processes of life work together
towards a common end. Both are the inevitable result of the original
distribution of matter in the primitive chaos, a distribution fixed by a
rational and foreknowing Being (p. 222).
Which of the two conceptions is to be adopted in biology? Teleological
explanations have long been banished from the physical sciences, and in
biology they are only a last resort when physical explanations have proved
incomplete (p. 223). And if the ground of the purposiveness of living
Nature is the same as the ground of the purposiveness of the universe, is it
not reasonable to suppose that explanations which have proved satisfactory
for inorganic things will in time with sufficient knowledge prove adequate
also for organic things?
The teleological conception, again, leads to difficulties particularly when it
is applied to the facts of reproduction. If we suppose that a vital force

unifies and coordinates the organism and is its very essence, we must also
suppose that this force is divisible and that a part of it—separated in
reproduction—can bring about the same results as the whole. If on the
contrary the forces having play in the organism are the mere result of the
particular combination of the matter composing it, the reconstruction of a
particular combination of molecules in the ovum is all that is necessary to
set development a-going along exactly the course taken by the ovum of the
parent. Another argument against the teleological view is derived from the
facts of the cell-theory. The cell-theory tells us that the molecules of the
living body are not immediately built up in manifold combinations to form
the organism, but are formed first into unit-constructions or cells, and that
these units of composition are invariably formed in all development, of
plants and animals alike, however diverse the goal of development may be.
If there were a vital principle would we not expect to find that, scorning this
roundabout way of reaching its goal, it went straight to the mark, taking a
different and distinctive course for each individual development, building
up the organism direct without the intermediary of cells? But since there is
a universal principle of development, namely, the formation of cells, does it
not seem that the cells must be the true organisms, that the whole
"individual" organism must be an aggregate of cells, and that the concept of
individuality applied to the organism is accordingly a logical fiction? And it
is just upon this notion of the individuality of the organism that the
teleological concept is based. The teleological view can perhaps not be
completely refuted until the adequacy of materialistic explanations has been
finally shown; but it is certain that the most promising method for research
is the materialistic (p. 226).
"We start out then from the assumption that the basis of the organism is not
a force acting according to a definite plan; on the contrary, the organism
arises through the action of blind and necessary laws, of forces which are as
much implicit in matter as those of the inorganic world. Since the chemical
elements in organic Nature differ in no way from those of inorganic Nature,
the ground or cause of organic phenomena can consist only in a different
mode of combination of matter, either in a peculiar mode of combination of
the elementary atoms to form atoms of the second order, or in the particular
arrangement of these compound molecules to form the separate
morphological units of the organism or the whole organism itself" (p. 226).

Accepting then the materialistic conception of the organism, we have to
consider this further problem. Does the ground of organic processes lie in
the whole organism or in its elementary parts? Translated into terms of
metabolism—note the physiological point of view—the question runs, are
metabolic processes the result of the molecular construction of the organism
as a whole, or does the centre of metabolic activity lie in the cell? Is it the
cell rather than the organism that is the immediate agent of assimilatory
processes? In the first alternative the cause of the growth of the constituent
parts lies in the totality of the organism; in the other alternative:—"Growth
is not the result of a force having its ground in the organism as a whole, but
each of the elementary parts possesses a force of its own, a life of its own, if
you will; that is to say, in each elementary part the molecules are so
combined as to set free a force whereby the cell is enabled to attract new
molecules and so to grow, and the whole organism exists only through the
reciprocal action of the single elementary parts.... In this eventuality it is the
elementary parts that form the active element in nutrition, and the totality of
the organism can be indeed a condition, but on this view it cannot be a
cause" (p. 227).
To help in the decision of this question, appeal must be made to the facts
established as to the cellular nature of the organism and of its reproductive
elements. We know that every organism is composed of cells, which are
formed and grow according to the same laws wherever they are found,
whose formation therefore is everywhere due to the same forces. If we find
that certain of these cells—all of which we know to be essentially identical
one with another—have the power when separated from the others of
growing and developing into new organisms, we can infer that not only
such cells but also all other cells have this assimilatory power. The ova of
animals, the spores of plants, the isolated cells of lower organisms in
general, all show the power of separate assimilation and development. "We
must therefore, in general, ascribe to the cell an individual life, that is to
say, the combination of the molecules in the single cell does suffice to
produce the force whereby the cell is enabled to draw to itself new
molecules. The ground of nutrition and growth lies not in the organism as a
whole, but in the separate elementary parts, the cells. The fact that it is not
every cell that can continue to grow when separated from the organism is
not in itself an objection to this theory, any more than it is an objection to

the individual life of a bee that it cannot continue to exist apart from the
swarm. The activation of the forces existing within the cell depends on
conditions which the cell encounters only in connection with the whole"
(pp. 228-9).
Schwann's next step is to discover what are the essential forces active in the
cell, and here he enters the realm of hypothesis. He finds they can be
reduced to two—an attractive force and a metabolic force. The attractive
force is seen in the process of cell-formation, where first of all the nucleolus
is formed by a concentration and precipitation of substances found free in
the cytoblastem, and in the same way the nucleus and later the cell are laid
down as concentric precipitates from the cytoblastem. Cell-formation also
involves the second or metabolic force, by means of which the cell alters
the chemical composition of the medium surrounding it so as to prepare it
for assimilation. Schwann's attractive force brings about the actual taking
up of the prepared substance; his metabolic force is the cause of the
digestion of food substances, and is nearly identical with enzyme action.
With what inorganic process, he now asks (p. 239), can the process of cell-
formation be most nearly compared, and the answer obviously is, with the
process of crystallisation. Cells are, it is true, quite different in shape and
consistency from crystals, and they grow by intussusception, not by
apposition—their plastic or attractive forces seem therefore to be different.
A still more important difference is that the metabolic force is peculiar to
the cell. Yet there are important analogies between crystals and cells. They
agree in the important respect that they both grow in solutions at the cost of
the dissolved substance, according to definite laws, and develop a definite
and characteristic shape. It might even be maintained, Schwann thinks, that
the attractive force of crystals is really identical with that of cells, and that
the difference in result is due merely to the difference between the
substance of the cell and the substance of the crystal. He points out how
organic bodies are remarkable for their powers of imbibition, and he seeks
to show that the cell is the form under which a body capable of imbibition
must necessarily crystallise, and that the organism is an aggregate of such
imbibition-crystals. The analogy between crystallisation and cell-formation
he works out in the following manner:—"The substance of which cells are
composed possesses the power of chemically transforming the substance
with which it is in immediate contact, in somewhat the same way as the

well-known preparation of platinum changes alcohol into acetic acid. Each
part of the cell possesses this property. If now the cytoblastem is altered by
an already formed cell in such a way that a substance is formed that cannot
become part of the cell, it crystallises out first as the nucleolus of a new
cell. This in its turn alters the composition of the cytoblastem. A part of the
transfomed substance may remain in solution in the cytoblastem or may
crystallise out as the beginning of a new cell; another part, the cell-
substance, crystallises round the nucleolus. The cell-substance is either
soluble in the cytoblastem and crystallises out only when the latter is
saturated with it, or it is insoluble and crystallises as soon as it is formed,
according to the aforementioned laws of the crystallisation of imbibition-
bodies; it forms thus one or more layers round the nucleolus, etc. If one
imagines cell-formation to take place in this way, one is led to think of the
plastic force of the cell as identical with the force by means of which a
crystal grows" (pp. 249-50).
Two difficulties have to be faced by this theory—(1) the origin of the
metabolic power of the cells, (2) the reason why the cells arrange
themselves so as to form an organism of complex and definite structure.
Schwann tries to explain the origin of the "metabolic" action, the analogy of
which with the contact-action of colloidal platinum he recognises, by
attributing it to the peculiar structural arrangements of molecules. In
attempting to account for the harmonious structure of the organism he
points to the analogy of ordinary crystals, which often form complex and
regular tree-like arrangements; plants in particular resemble these regularly
shaped crystal-aggregates.
The whole ingenious theory is offered merely as an hypothesis and a guide
to research. It is interesting as one of the most carefully thought-out
attempts ever made to give a thorough-going materialistic account of the
origin and development of organic form, and it arose directly out of the cell-
theory.
Schleiden and Schwann started out from an erroneous theory of the origin
and development of cells, which impaired to some extent the value of their
results. It was not long, however, before their theory of the origin of cells by
"crystallisation" from an intra- or extra-cellular cytoblastem was challenged

and overthrown, and the generalisation that cells originate by division from
pre-existing cells put in its place.
This was established for plant cells by Meyen, Unger, von Mohl, Naegeli
and Hofmeister in or about the forties.[261] Criticism of the Schwann-
Schleiden theory from the zoological side was suggested by the study of the
segmentation of the ovum—the developmental process in which the
multiplication of cells is most easily observed. The segmentation of the
ovum was well known to Schwann, for the process had been described in
the frog by Prévost and Dumas in 1824,[262] in the frog and newt by
Rusconi,[263] and an elaborate study of the process in the frog had been
made by von Baer.[264] Schwann indeed suspected that there must be some
connection between the segmentation of the ovum and the formation of
cells, but he did not realise that the cellular blastoderm of the chick was
formed by the division or segmentation of the egg-cell.
Segmentation was soon found to be of widespread occurrence. Von Siebold
in 1837 described the process in Entozoa,[265] and in the same year Lovén
saw segmentation in Campanularia,[266] and Sars in the starfish and in
Nudibranchs.[267]

In 1838 Bischoff[268] observed segmentation in the mammalian ovum, and
the whole course of segmentation in the ovum of the rabbit from the 2-
celled to the morula stage was carefully described and figured by Barry[269]
in 1839. C. Vogt[270] in 1842 described segmentation in Coregonus and
Alytes. The discovery of segmentation in the ovum of birds was not made
until 1847, by Bergmann,[271] confirmed independently by Coste[272] in
1850. By 1848 segmentation had been noted in Hydra and various hydroids,
in acalephs, in starfish, polyzoa, nematodes, rotifers, leeches, oligochætes,
polychætes, in most groups of molluscs and arthropods, and in all the
vertebrate classes.[273]
The process was at first held to be merely one of yolk-division, or
Dotterfurchung, and its details were by most interpreted in the light of the
Schleiden-Schwann theory of cell-formation.

The first steps towards a truer conception of the process seem to have been
taken by Bergmann, who in 1841[274] called attention to the presence of
nuclei in the segmentation-spheres of the frog's egg, and by Bagge in the
same year, who observed that division of the nuclei preceded the
multiplication of the segmentation spheres.[275] He considered the nuclei to
be anucleate cells, and the same view was taken by Kölliker in 1843.[276]
Next year, however, in his classical paper on Cephalopod development[277]
Kölliker came to the opinion that they were really nuclei. He showed that
segmentation was brought about by cell-division, that between "total" and
"partial" segmentation there was a difference of degree and not of kind, and
that the cells of the body were formed by division of the segmentation
spheres. He held, however, that the nuclei multiplied endogenously and not
by division. The division of nuclei was observed by Coste in 1846.[278]
Leydig in 1848[279] took the necessary step in advance and maintained that
the nuclei as well as the cells increased always by division. He was
supported by Remak, who in a paper of 1852,[280] and more fully in his
monumental Untersuchungen über die Entwickelung der Wirbelthiere
(Berlin, 1850-55), proved that in the frog's egg at least segmentation was a
simple process of cell-division, initiated always by division of the nucleus.
[281]

One point Remak left undecided—the fate of the Keimbläschen or egg-
nucleus. It was generally held, even so late as the 'fifties, that the egg-
nucleus disappeared just before segmentation began—Bischoff clung to this
belief even in 1877.[282] Though Barry had held in 1839 that the egg-
nucleus does not disappear in segmentation, J. Müller seems to have been
the first actually to prove that it forms by division the nuclei of the first two
segmentation spheres. He furnished the demonstration in the egg of
Entoconcha mirabilis,[283] and his paper was known to Remak, who could
not, however, observe a similar division of the egg-nucleus in the frog.
Müller's discovery was confirmed for Oceania armata by Gegenbaur,[284]
and for Notommata sieboldii by Leydig.[285]

In 1854 Virchow,[286] previously a supporter of Schwann, crystallised the
new views in the famous phrase—Omnis cellula e cellula—and gave wide
publicity to them in his classical lectures on Cellular Pathology, delivered in

1858.[287] The new doctrine of cell-formation was also taught by Leydig[288]
in his text-book of histology, published in 1857.
The Schleiden-Schwann theory of the origin of cells by generation in a
cytoblastem was now definitely overthrown.
The importance of the protoplasmic content of the cell was brought into
prominence through the work of Dujardin,[289] Purkinje,[290] Cohen[291] and
Max Schultze.[292] The last-named in 1861 proposed a definition of the cell
which might be accepted at the present day. "A cell," he wrote, "is a little
blob of protoplasm containing a nucleus" (p. 11).
[238] Theoria generationis, Halae, 1759.
[239] See J. v. Sachs, Geschichte der Botanik, book ii., Eng. Trans., 2nd impr.,
1906.
[240] Müller's Archiv, pp. 137-76, 1838.
[241] Trans. Linnean Soc., xvi., p. 710, 1833.
[242] Myxinoiden, i. Theil., p. 89, 1835.
[243] Ann. Sci. nat. (2) (Zool.) ii., pp. 107-18, pl. 11, 1834.
[244] Proc. Phil. Soc. Glasgow, xix., pp. 71-125, 1887-8.
[245] Traité sur le venin de la vipère, 1781.
[246] Müller's Archiv, 1836.
[247] J. Müller, Jahresbericht ü. d. Fortschritte der anat.-physiol.
Wissenschaften im Jahre 1838. Müller's Archiv, 1838.
[248] Symbolæ ad anatomiam villorum imprimis eorum epithelii, Berlin, 1837.
[249] U. d. Ausbreitung des Epitheliums im menschlichen Körper. Müller's
Archiv, 1838.
[250] See Schwann's Bemerkungen at the end of his Mikroskopische
Untersuchungen.
[251] Republished in Ostwald's Klassiker der exakten Wissenschaften, No. 176,
Leipzig, 1910. References in the text are to the original pagination.
[252] Symbolæ ad ovi avium historiam.
[253] De ovi mammalium et hominis genesi.
[254] De mulierum organis, 1672.
[255] Ann. Sci. nat., iii., p. 135, 1842.

[256] Recherches sur la génération des Mammifères. Report by Academy
Committee. Ann. Sci. nat. (2) (Zool.) ii., pp. 1-18, 1834; also Embryogénie
comparée, 1837.
[257] Lond. and Edin. Phil. Mag. (3) vii., 1835; Phil. Trans. 1837.
[258] Handbuch der Enfwickelungsgeschichte, 1835, and Müller's Archiv, 1836.
[259] Prodromus historiæ generationis hominis atque animalium, Lipsiæ, 1836.
[260] Müller's Archiv, 1837.
[261] Sachs, History of Botany, Book ii.
[262] Ann. Sci. nat., i., pp. 110-14, 1824. Swammerdam is said to have observed
the 2-celled stage in the egg of the frog (Bibl. Nat., 1752), and Rösel v. Rosenhof
the same stage in the tree-frog (Hist. nat. ranarum nostratium, 1758).
[263] Développement de la grenouille commune, Milan, 1826. Biblioteca
italiana, lxxix., 1836, and Müller's Archiv, 1836. Agassiz is said by Vogt (1842)
to have seen segmentation in the Perch as early as 1831.
[264] Müller's Archiv, 1836.
[265] In Burdach, Die Physiologie als Erfahrungswissenschaft, 2nd Ed., vol. ii.
[266] Wiegmann's Archiv, 1837.
[267] Bericht Versamml. deutsch. Naturf. in Prag, 1837.
[268] Bericht Versamm. deutsch. Naturf. in Freiburg, 1838. Later in his Entw. d.
Wirbelth., and in his papers on the development of the rabbit.
[269] Phil. Trans., 1839. See particularly Pl. vi., figs. 105-12.
[270] Embryologie des Salmones 1842.
[271] Müller's Archiv, 1847.
[272] C.R. Acad. Sci., xxx., p. 638.
[273] See review by Leydig in Isis, 1848, pp. 161-193.
[274] Müller's Archiv, pp. 89-102, 1841.
[275] De evolution Stronzyli auric. el Ascaridis acum., Erlangen, 1841.
[276] Müller's Archiv, pp. 66-141, 1843.
[277] Entwickelungsgeschichte der Cephalopoden, Zurich, 1844.
[278] Froriep's Notizen, No. 800, 1846.
[279] Isis, 1848.
[280] Müller's Archiv, p. 47, 1852, also 1854 and 1858.
[281] See particularly Plate IX., figs. 3-7.

[282] Hist.-krit. Bemerkungen zu den neuesten Mittheilungen ü. d. erste
Entwickelung d. Säugethiereier, München, 1877.
[283] Monatsber. Akad. Wiss. Berlin, 1851.
[284] Zur Lehre von Generationswechsel u. d. Fortpflanzen d. Medusen u.
Polypen.
[285] U. d. Bau u. d. system. Stellung d. Räderthiere, 1854.
[286] Arch f. path. Anat. Phys., vii., pp. 1-39, 1854. Also in his Beiträge z. spec.
Path. u. Therapie.
[287] Die Cellularpathologie, Berlin, 1858.
[288] Lehrbuch der Histologie, 1857.
[288] Ann, Sci. nat. (2) iii., pp. 108-9 and pp. 312-4, 1835. Also iv, pp 343-77.
[290] 1839 or 1840.
[291] Nova Acta Acad. Leop., xxii., 1850. Trans. in 1853 for Ray Society.
[292] Arch. f. Anat. u. Physiol., pp. 1-27, 1861.

CHAPTER XII

THE CLOSE OF THE PRE-EVOLUTIONARY PERIOD

The influence of the cell-theory on morphology was not altogether happy.
The cell-theory was from the first physiological; cells were looked upon as
centres of force rather than elements of form, and the explanation of all the
activities of the organism was sought in the action of these separate
dynamic centres. There resulted a certain loss of feeling for the problems of
form. The organism was seen no longer as a cunningly constructed complex
of organs, tissues and cells; it had become a mere cell-aggregate; the higher
elements of form were disregarded and ignored.
We have seen this physiological attitude expressed with the utmost
clearness by the founder of the cell-theory himself; we shall see the same
attitude taken up by most of his successors. Thus Vogt, who was later to
become one of the protagonists of materialism in Germany, developed in
his memoir on the embryology of Coregonus[293] the theory of the

independent or individual life of the cell. "Each cell," he wrote, "represents
in some measure a separate organism, and while their development
necessarily conforms to the general plan and the particular tendencies of the
parent organism, they nevertheless each follow their own particular
tendency and do not lose their independence until, by reason of the
metamorphoses which they undergo, they lose their cellular nature"
(p. 275).
And again, "... we are obliged to admit the existence in the cell of an
independent life, which makes its development self-sufficient.... Each cell
consequently represents a little independent organism, which assimilates
foreign substances, builds them up, and rejects those that are useless; from
this point of view the embryo can be compared up to a certain point with a
zoophyte stock, of which each polyp, while living its own independent life,
is yet incorporated in the common corm, which impresses its distinctive
character upon every polyp" (p. 293).
Classical expression was given to the "colonial theory" of the organism by
Virchow in his lectures on "Cellular Pathology."[294] For Virchow the
organism resolves itself into an assemblage of living centres, the cells; the
organism has no real existence as a unity, for there is no one single centre
from which its activities are ruled. Even the nervous system, which appears
to act as a co-ordinating centre, is itself an aggregate of discrete cells. "A
tree is a body of definite and orderly composition, the ultimate elements of
which, in every part of it, in leaf and root, in stem and flower, are cellular
elements—so also are animal forms. Every animal is a sum of vital units,
each of which possesses the full characteristics of life. The character and
the unity of life cannot be found in one definite point of a higher
organisation, for example in the brain of man, but only in the definite,
constantly recurring disposition shown individually by each single element.
It follows that the composition of the major organism, the so-called
individual, must be likened to a kind of social arrangement or society, in
which a number of separate existences are dependent upon one another, in
such a way, however, that each element possesses its own particular
activity, and, although receiving the stimulus to activity from the other
elements, carries out its own task by its own powers" (2nd ed., pp. 12-13).

Analysis, decomposition, or disintegration of the organism is here pushed to
its extreme point, and the problem of recomposition, synthesis and co-
ordination shirked or forgotten.
The harmful influence of the cell-theory upon morphology did not pass
unnoticed by the broader-minded zoologists of the day. Virchow's earlier
paper[295] on the application of the cell-theory to physiology and pathology
called forth a vigorous protest from Reichert,[286] who discussed in a very
instructive way the contrast between the older "systematic" and the newer
"atomistic" attitude to living Nature.
Is it really true, he asks, that the cell is the dominant element in all
organisation; is the cell comparable in importance to the atom of the
chemists; or is it not rather the servant of a higher regulatory power?
Johannes Müller, who was Reichert's master, had in his Physiology[297]
argued splendidly for the existence of a creative force which guides and
rules development, and brings to pass that unity and harmony of
composition which distinguish living things from inorganic products.
Reichert sought in vain in the writings of the biological "atomists" for any
smallest recognition of these broader characteristics of living things upon
which Müller had rightly laid stress. For the atomists the cell was the only
element of form; they ignored the combination of cells to form tissues, of
tissues to form organs, of organs to form an organism. For the
morphologists the cell was one element among many, and the lowest of all.
The difference of attitude is clearly shown if we consider from the two
points of view a complicated organ-system such as the central nervous
system. The atomist sees in this a mere aggregate of cells or at the most of
groups of cells. "The morphologist," on the other hand, "sees in the central
nervous system a proximate element in the composition of the body—a
primitive organ. From this point of view he apprehends and judges its
morphological relations with, in the first place, the other co-ordinated
primitive organs in the system as a whole; in all this the cells remain in the
background, and have nothing to do directly with the determination of these
morphological relations" (p. 6). Within the nervous system there are
separate organs which stand to one another in a definite morphological and
functional relationship. These organs are, it is true, composed of cells; but
between the form and connections of these organs and the cells which

compose them there is no direct and necessary relation (p. 6). It is true that
the cell is the ultimate element of organic form, and that all development
takes place by multiplication and form-change of cells. Yet is the cell in all
this not independent of the unity of the developing embryo, and what the
cells produce, they produce, so to speak, not of their own free will, nor by
chance, but under the guiding influence of the unity of the whole, and in a
certain measure as its agents (p. 7). The atomists will not admit the truth of
this; they see in development nothing more than a process of the form-
change and multiplication of cells. The full meaning of development
escapes them, for they take no cognisance of the increasing complexity of
the embryo, of the separating-out of tissues, of the moulding of organs, of
the harmonious adaptation and adjustment of the parts to form a working
whole.
In general, the fault of the atomists is that they do not respect the limits
which Nature herself has prescribed to the process of logical analysis and
disintegration of the organism; they do not recognise the existence of
natural and rational units or unities; they forget the one great principle of
rational analysis, "that, by universally valid, inductive, logical method,
natural objects must in all cases be accepted and dealt with in the
combination and concatenation in which they are given" (p. 10).
The atomists at least recognised one natural organic element, the cell; the
materialistic physiologists of the time resolved even this unity into an
aggregate of inorganic compounds, and regarded the organism itself as
nothing but a vastly complicated physico-chemical mechanism. From this
point of view morphology had no right of existence, and we find Ludwig,
one of the foremost of the materialistic school, maintaining that
morphology was of no scientific importance, that it was nothing more than
an artistic game, interesting enough, but completely superseded and robbed
of all value by the advance of materialistic physiology.[298]
Naturally enough, morphologists did not accept this rather contemptuous
estimate of their science, but held firmly to the morphological attitude. So
Leuckart in his reply to Ludwig, so Rathke in a letter to Leuckart published
in that reply, so Reichert in his Bericht, so J. V. Carus in his System der
thierischen Morphologie,[299] upheld the validity, the independence, of
morphological methods. Leuckart and Rathke called attention to the

absolute impossibility of explaining by materialistic physiology the unity of
plan underlying the diversity of animal form. J. V.. Carus, who was
convinced of the validity of physiological methods within their proper
sphere, drew a sharp distinction between systematics and morphology on
the one hand, and physiology on the other. Physiology had nothing to do
with the problems of form at all; its business was to study the physical and
chemical processes which lay at the base of all vital activities. Morphology,
on its part, had to accept form as something given, and to study the abstract
relations of forms to one another. "On this point," he writes, "stress is to be
laid, that morphology has to do with animal form as something given by
Nature, that though it follows out the changes taking place during the
development of an animal and tries to explain them, it does not enquire
after the conditions whose necessary and physical consequence this form
actually is" (p. 24). He expressed indeed a pious hope (p. 25) that
physiology might one day be so far advanced that it could attempt with
some hope of success to discover the physico-chemical determinism of
form, but this remained with him merely a pious hope. Reichert, in his
Bericht, applied to the rather wild theorisings of the physiologist Ludwig
the same clear commonsense criticism that he bestowed on the other
"atomists."
It would take too long to describe the great development that materialistic
physiology took at this time, and to show how the separation of morphology
from physiology, which originally took place away back in the 17th century,
had by this time become almost absolute. The years towards the end of the
first half of the century marked indeed the beginning of the classical period
as well of physiology as of dogmatic materialism. Moleschott and Buchner
popularised materialism in Germany in the 'fifties, while Ludwig, du Bois
Reymond and von Helmholtz began to apply the methods of physics to
physiology. In France, Claude Bernard was at the height of his activity,
rivalled by workers almost as great. The doctrine of the conservation of
energy was established about this same time.
Between the cell-theory on the one side, and physiology on the other, it was
a wonder that morphology kept alive at all. The only thing that preserved it
was the return to the sound Cuvierian tradition which had been made by
many zoologists in the 'thirties and 'forties. It is a significant fact that this
return to the functional attitude coincided in the main with the rise of

marine zoology, and that the man who most typically preserved the
Cuvierian attitude, H. Milne-Edwards, was also one of the first and most
consistent of marine biologists. Milne-Edwards describes in his interesting
Rapport sur les Progrès récents des Sciences zoologiques en France (Paris)
1867, how "About the year 1826, two young naturalists, formed in the
schools of Cuvier, Geoffroy and Majendie, considered that zoology, after
having been purely descriptive or systematic and then anatomical, ought to
take on a more physiological character; they considered that it was not
enough to observe living objects in the repose of death, and that it was
desirable to get to understand the organism in action, especially when the
structure of these animals was so different from that of man that the notions
acquired as to the special physiology of man could not properly be applied
to them" (p. 17). The two young naturalists were H. Milne-Edwards and V.
Audouin. In pursuance of these excellent ideas they set to work to study the
animals of the seashore, producing in 1832-4 two volumes of Recherches
pour servir à l'histoire naturelle du littoral de la France. After Audouin's
early death A. de Quatrefages was associated with Milne-Edwards in this
pioneer work, and their valiant struggles with insufficient equipment and
lack of all laboratory accommodation, and the rich harvest they reaped, may
be read of in Quatrefage's fascinating account of their journeyings.[300] Note
that though they called themselves physiologists they meant by physiology
something very different from the mere physical and chemical study of
living things. They were interested, as Cuvier was, primarily in the
problems of form; they sought to penetrate the relation between form and
function; their chief aim was, therefore, the study not of physiology[301] in
the restricted sense, but physiological morphology. As a matter of fact they
produced more taxanomic and anatomical work than work on physiological
morphology, but this was only natural, since such a wealth of new forms
was disclosed to their gaze. Milne-Edwards' masterly Histoire Naturelle des
Crustacés[302] and A. de Quatrefage's Histoire Naturelle des Annelés marins
et d'eau douce[303] were typical products of their activity.
In the North, men like Sars and Lovén were starting to work on the littoral
fauna of the fjords; in Britain, Edward Forbes was opening up new worlds
by the use of the dredge; Johannes Müller was using the tow-net to gather
material for his masterly papers on the metamorphoses of Echinoderms.[304]

Work on the taxonomy and anatomy of marine animals was in general in
full swing by the 'fifties and 'sixties.
This return to Nature and to the sea had a very beneficial effect upon
morphology, bringing it out from the laboratory to the open air and the
seashore. It saved morphology from formalism and aridity, and in particular
from a certain narrowness of outlook born of too close attention paid to the
details of microscopical anatomy. It brought morphologists face to face
again with the wonderful diversity of organic forms, with the unity of plan
underlying that diversity, with the admirable adjustment of organ to
function and of both to the life of the whole.
Milne-Edwards' theoretical views, as expounded in his Introduction à la
zoologie générale (1851), well reflect this Cuvierian attitude.[305] He
acknowledges himself the debt he owes to Cuvier; "the further I advance in
the study of the sciences which he cultivated with so sure a hand," he writes
in 1867, "the more I venerate him."
Milne-Edwards frankly takes up the teleological standpoint, and interprets
organic forms on the assumption that they are purposive and rationally
constructed. "To arrive at an understanding of the harmony of the organic
creation," he writes, "it seemed to me that it would be well to accept the
hypothesis that Nature has gone about her work as we would do ourselves
according to the light of our own intelligence, if it were given us to produce
a similar result. Comparing and studying living things as if they were
machines created by the industry of man, I have tried to grasp the manner in
which they might have been invented, and the principles whose application
would have led to the production of such an assemblage of diversified
instruments" (p. 435). The problem is to discover the laws which rule the
diversity of organic forms. The first and most obvious of these laws is the
"law of economy," or the law of unity of type. Nature, as Cuvier pointed
out, has not had recourse to all the possible forms and combinations of
organs; she appears to work with a limited number of types and to get the
greatest possible diversity out of these by varying the proportions of the
constitutive materials of structure. Within the limits of each type Nature has
brought about diversity by raising her creatures to different degrees of
perfection. This is the second law of organic form, and it is this law that
Milne-Edwards chiefly elaborates. Degrees of perfection mean for him, as

for Aristotle, primarily degrees of perfection of function, but since structure
is necessarily in close relation with function, perfection of function brings
in its train increased perfection of organisation. This can only be attained by
a division of labour[306] among the organs and by their consequent
differentiation. An animal is like a workshop where some complicated
product is manufactured, and the organs are like the workmen. Each
workman has his own special piece of work to do, at which he becomes
thoroughly expert; and the finished product is manufactured more rapidly
and efficiently by the co-operation of workers each skilled in one
department than it would be if each workman had to produce the whole.
Applied to the organism this principle of the division of labour means the
differentiating out of the separate functions, their localisation in different
parts of the organism, and their co-ordination to produce a combined result.
This differentiation of functions implies a corresponding differentiation of
organs, but it is functional differentiation which always takes the lead.
"Where division of labour has not been introduced into the organism there
must exist a great simplicity of structure. But just as uniformity in the
functions of the different parts of the body implies a uniformity in their
mode of constitution, so diversity in function must be accompanied by
particularities in structure; and, in consequence also, the number of
dissimilar parts must be augmented and the complication of the machine
increased" (p. 463). Since function comes before form there is not always a
special organ for every function. "It is a grave error to believe that a
particular function can be performed only by one and the same organ.
Nature can arrive at the desired result by various ways, and when we look
down through the animal kingdom from the highest to the lowest forms we
see that the function does not disappear even when the special instrument
provided for the purpose in the higher types ceases to exist" (p 470).
Nature, holding fast to the law of economy, does not even always create a
new organ for a new function; she may simply adapt an undifferentiated
part to special functions, or she may even convert to other uses an organ
already specialised (p. 464). So, for example, the function of respiration is
in the lowest animals diffused indifferently over the whole surface of the
body, and only as organisation advances is it localised in special organs,
such as gills. Now suppose that Nature wishes to adapt a fish, which
breathes by gills, to life in the air; she does not create an organ specially for

this purpose, but utilises the moist gill-chamber (e.g., in Anabas scandens),
modifying it in certain ways so that the fish can take advantage of the
oxygen it contains. But this gill-chamber lung is at best a makeshift, and
when she comes to the more definitely terrestrial Amphibia Nature gives up
the attempt to use the gill-chamber as a lung, and creates a new organ, the
true vertebrate lung, specially adapted for breathing air (p. 475).
But whatever means Nature adopts, her aim is always the same—to
specialise, to differentiate, to produce diversity from uniformity.
Differentiation not only raises the level of organisation; it usually also takes
the direction of adaptation to particular habits of life, and this is perhaps the
most fruitful cause of diversity. Everywhere we find animals specialised in
adaptation to their environment—to life in air or water, or on land—and
many of their most striking differences are due to this cause. But adaptation
may also act in reducing diversity, for there necessarily occur many
instances of parallel adaptation or convergence. So we get the extraordinary
parallelism between the families of marsupials and the orders of placentals,
[307]
the remarkable similarity between the respiratory organs of land-crabs
and air-breathing fish—to mention only two out of an immense range of
analogous facts.
The last cause of diversity that Milne-Edwards adduces is what he calls a
"borrowing" of peculiarities of structure from another systematic group.
Thus, "among reptiles, the tortoises seem to have borrowed from birds
some of their characteristic features of organisation; and among the sauroid
fishes the piscine type seems to have been influenced by the type from
which reptiles are derived" (p. 479). So many riddles that, a little later on,
stimulated the ingenuity of the evolutionists!
Such, then, were the factors which Milne-Edwards considered adequate to
explain the rich variety of animal forms. We cannot do better than quote his
own summary of his doctrine:—"To sum up, then, the great differences
introduced by Nature into the constitution of animals seem to depend
essentially upon the existence of a certain number of general plans or
distinct types, upon the perfecting in various degrees either of the whole or
of parts of each of these structural plans, upon the adaptation of each type to

varied conditions of existence, and upon the secondary imitation of foreign
types by certain derivatives of each particular type" (p. 480).
We have laid stress on the fact that Milne-Edwards put function before
form, for this is the mark of the true Cuvierian. With it goes the belief that
Nature forms new parts to meet new requirements, that she is not limited, as
Geoffroy thought, to a definite number of "materials of organisation," but
can produce others at need. Cuvier held, for example, that many of the
muscles and even the bones of fish were peculiar to them, and without
homologues in the other Vertebrates, having been created by Nature for
special ends.[308] So, too, Johannes Müller, who in many ways and not least
in his sane vitalism was a follower of the Cuvierian tradition, recognised
that many of the complicated cartilages in the skull of Cyclostomes were
specially formed for the important function of sucking, and had no
equivalent in other fish.[309]
So, too, the embryologists after Cuvier often came across instances of the
special formation of parts to meet temporary needs. Thus Reichert
interpreted the "palatine" and "pterygoid," which are formed in the mouth
of the newt larva by a fusion of conical teeth, as special adaptations to
enable the little larva to lead a carnivorous life.[310]
Not many years after the publication of Milne-Edwards' Introduction à la
zoologie générale (1851) there appeared a book by H. G. Bronn in which
was offered a very similar analysis of organic diversity. The curious thing
was that Bronn approached the problem from quite a different standpoint,
from the standpoint, indeed, of Naturphilosophie. Of this the title of the
book is itself sufficient proof—Morphologische Studien über die
Gestaltungs-gesetze der Naturkörper überhaupt und der organischen
insbesondere (Leipzig and Heidelberg, 1858).[311] The linking up of organic
with inorganic form is characteristic; there is much talk, too, in the book of
Urstoffe and Urkräfte, but underlying the Naturphilosophie we can trace the
same Cuvierian treatment of form, and see crystallise out laws of
progressive development that bear no small analogy with the laws
established by Milne-Edwards.
According to Bronn, the ideal fundamental form of the plant is an ovoid or
strobiloid[312] body, for a plant reaches out in two directions in search of

food—towards the sun and towards the earth. Animals differ from plants in
being endowed with sensation and mobility (cf. Aristotle and Cuvier), and it
is this characteristic that gives them their distinctive form. The main types
of animal form—the Amorphozoa, Actinozoa, and Hemisphenozoa—are
essentially adaptations to particular modes of locomotion. Animals either
are fixed, or they move in all directions without reference to any definite
axis, or they move in one main direction.
The Amorphozoa or shapeless animals include many of the Protozoa and
sponges; they have no typical form, and most of them are sessile. The
Actinozoa include such animals as the Cœlentera, which are fixed, and the
Echinoderms, which have a central point and move indifferently along any
radial axis; their form differs from the strobiloid mainly in having radiate
rather than spiral symmetry. The Hemisphenozoa, or bilaterally symmetrical
animals, include all those that habitually move forward; they have a front
end and a hind end, a dorsal surface and a ventral, and the mouth, sense-
organs and "brain" are concentrated in the front end to form a head—all in
direct adaptation to this forward movement; they make up the vast majority
of animals.
The fundamental forms of living things are, however, merely so many
themes on which a multitude of further variations are woven, through the
action of the laws which rule the detail of organic diversities. These further
laws may be set down under four main heads. Under the first comes the law
of the existence of certain fundamentally distinct structural types, which are
distinguished from one another by their ground-form, by the number of
organ-systems, and by the number of homotypic organs they possess, but
principally by the relative position of the organs to one another (principle of
connections). The form and connections of the nervous system are of
particular importance in distinguishing the types (cf. Cuvier). The second
factor in the diversity of organic form is the action of certain laws of
progressive development[313] (Entwickelungsgesetze), which bear the same
relation to the development of the animal kingdom as the laws of individual
development bear to the development of the embryo, for organs appear in
the different animal series in much the same order and manner as they
develop in the individual. These laws are (1) progressive differentiation of
functions and organs; (2) numerical reduction of serially repeated parts; (3)
concentration of functions and their organs in particular parts of the body;

(4) centralisation of organ-systems and parts of such, so that they come to
depend upon one central organ; (5) internalisation of the "noblest" organs,
unless these are necessarily external, and (6) increase in size of the whole or
of parts. Of these the law of differentiation is by far the most important, and
most of the others are in a sense merely special cases of this fundamental
law. To this law of differentiation is due the increase in complexity or
perfection of organisation which is shown by all the animal series. Bronn
himself recognised the great similarity of this law of progressive
differentiation to Milne-Edwards' principle of the division of labour; he
seems, however, to have arrived at it independently.
Bronn's third factor in the production of variety of form is adaptation to
environment, or better, functional response to environment. Bronn gives an
excellent account of adaptational modifications and calls attention, just as
Milne-Edwards did, to the numerous analogies of structure which
adaptation brings about. He works out the interesting view that there is
some connection between classificatory groups and adaptational forms,
especially such as are connected with the function of locomotion:—"Based
upon a common characteristic method of locomotion are whole or nearly
whole sub-phyla (Hexapoda), classes (mammals and reptiles, birds, fishes,
gastropods, pteropods, brachiopods, Bryozoa, Rotifera, jelly-fish, polypes,
sponges), sub-classes (mobile and immobile lamellibranchs, echinoderms,
walking and swimming Crustacea, parasitic and free-living worms, and so
on), often, however, only orders and quite small groups (snakes, eels, bats,
sepias, medusæ, etc.)" (p. 141).
It was characteristic of the 'forties and 'fifties that transcendental anatomy,
along with Nature-philosophy, went rather out of fashion, its false
simplicities and premature generalisations being overwhelmed by the flood
of new discoveries. A few stalwarts indeed upheld transcendental views. We
have already discussed the morphological system built up by Richard Owen
in the late 'forties, a system transcendental in its main lines. We have seen
the vertebral theory of the skull still maintained in the 'fifties by such men
as Reichert and Kölliker, and we find J. V.. Carus in 1853[314] taking it as
almost conclusively proved.[315]
We may mention, too, as showing clear marks of the influence of
transcendental ideas, L. Agassiz's work on the principles of classification.

[316]
And Serres, who was Geoffroy's chief disciple, recanted not a whit of
his doctrine of recapitulation, but re-affirmed and expanded it from time to
time, and particularly in a lengthy memoir published in 1860.[317] But in
general we may say that pure morphology in the Geoffroyan or Okenian
sense was becoming gradually discredited. A curious indication of this is
seen in the fact that not only the idea but the very word "Archetype" came
to be regarded with suspicion. Thus even J. V.. Carus, who had much
affinity with the transcendentalists, wrote of the vertebrate archetype (which
he took over almost bodily from Owen)—"It may here be observed that this
schema may be used as a methodological help, but it is not to be placed in
the foreground" (loc. cit., p. 395). Huxley, who was definitely a follower of
von Baer, was much more outspoken with regard to ideal types. In an
important memoir on the general anatomy of the Gastropoda and
Cephalopoda,[318] he set himself the task of reducing all their complex
forms to one type. In summing up, he writes:—"From all that has been
stated, I think that it is now possible to form a notion of the archetype of the
Cephalous Mollusca, and I beg it to be understood that in using this term, I
make no reference to any real or imaginary 'ideas' upon which animal forms
are modelled. All that I mean is the conception of a form embodying the
most general propositions that can be affirmed respecting the Cephalous
Mollusca, standing in the same relation to them as the diagram to a
geometrical theorem, and like it, at once imaginary and true" (i., p. 176).
Again, in his Croonian lecture on the theory of the vertebrate skull, he
remarks that a general diagram of the skull could easily be given. "There is
no harm," he continues, "in calling such a convenient diagram the
'Archetype' of the skull, but I prefer to avoid a word whose connotation is
so fundamentally opposed to the spirit of modern science" (Sci. Memoirs,
vol. i., p. 571).
It is instructive to find that between Serres and Milne-Edwards there existed
the same antagonism as between von Baer and the German
transcendentalists. Milne-Edwards was a constant critic of the law of
parallelism which Serres continued to uphold with little modification for
over thirty years, just as von Baer was a critic of that form of the doctrine
which was current in the early part of the century. As early as 1833, Milne-
Edwards, through his studies of crustacean development,[319] had come to
the conclusion, independently of von Baer, that development always

proceeded from the general to the special; that class characters appeared
before family characters, generic characters before specific. In an
interesting paper published in 1844,[320] he discussed the relation of this law
of development to the problems of classification, and arrived at results
almost identical with those set forth by von Baer in his Fifth Scholion.
Like von Baer he rejected completely the theory of parallelism and the
doctrine of the scale of beings; like von Baer he held that the type of
organisation—of which there are several—is manifested in the very earliest
stages and becomes increasingly specialised throughout the course of
further development; like von Baer, too, he sketched a classification based
upon embryological characters.
These views were further developed in his volume of 1851, and also in his
Rapport of 1867.
They brought him into conflict with his confrere in the Academy of
Sciences, Étienne Serres, who in a number of papers published in the
'thirties and 'forties,[321] and particularly in his comprehensive memoir of
1860, still maintained the theory of parallelism and the doctrine of the
absolute unity of type. His memoir of 1860 shows how completely Serres
was under the domination of transcendental ideas. Much of it indeed goes
back to Oken. "The animal kingdom," he writes, "may be considered in its
entirety as a single ideal and complex being" (p. 141). His views have
become a little more complicated since his first exposition of them in 1827,
and he has been forced to modify in some respects the rigour of his
doctrine. But he still holds fast to the main thesis of transcendentalism—the
absolute unity of plan of all animals, vertebrate and invertebrate alike,[322]
the gradual perfecting of organisation from monad to man, the repetition in
the embryogeny of the higher animals of the "zoogeny" of the lower.
He recognised, however, that the idea of a simple scale of beings is only an
abstraction, and that the true repetition is of organs rather than of
organisms. He was willing even to admit, at least in the later pages of his
memoir, that there might be not one animal series but several parallel series,
as had been suggested by Isidore Geoffroy St Hilaire (p. 749). In general,
his views are now less dogmatic than they were in his earlier writings, but
they are not for all that changed in any essential. For, in summing up his

main results, he writes, "The whole animal kingdom can in some measure
be regarded ideally as a single animal, which, in the course of formation
and metamorphosis in its diverse manifestations, here and there arrests its
own development, and thus determines at each point of interruption, by the
very state it has reached, the distinctive characters of the phyla, the classes,
families, genera, and species" (p. 833).[323]
To settle the dispute pending between two of its most illustrious members,
the Academy proposed in 1853, as the subject of one of its prizes, "the
positive determination of the resemblances and differences in the
comparative development of Vertebrates and Invertebrates." A memoir was
presented the next year by Lereboullet[324] which met with the approval of
the Academy in so far as its statements of fact were concerned, but seemed
to them to require amplification in its theoretical part. But even in this
memoir Lereboullet was able to show that the balance of evidence was
greatly in favour of Milne-Edwards' views, and his general conclusions in
1854 were that "in the presence of such fundamental differences, one is
obliged to give up the idea of one single plan in the formation of animals;
while, on the contrary, the existence of diverse plans or types is clearly
demonstrated by all the facts" (p. 79). To fulfil the Academy's requirements,
Lereboullet continued his work, and in 1861-63 he published a series of
elaborate monographs[325] on the embryology of the trout, the lizard and the
pond-snail Lymnæa, and rounded off his work with a full discussion[326] of
the theoretical questions involved. In this considered and authoritative
judgment he completely disposed of Serres' theories of the unity of plan and
the unity of genetic formation. Except in the very earliest stages of
oogenesis there is no real similarity between the development of a
Zoophyte, a Mollusc, an Articulate and a Vertebrate, but each is stamped
from the beginning with the characteristics of its type. The lower animals
are not, and cannot possibly be the permanent embryos of the higher
animals. "The results which I have obtained," he writes, "are diametrically
opposed to the theory of the zoological series constituted by stages of
increasing perfection, a theory which tries to demonstrate in the embryonic
phases of the higher animals a repetition of the forms which characterise the
lower animals, and which has led to the assertion that the latter are
permanent embryos of the former. The embryo of a Vertebrate shows the
vertebrate type from the very beginning, and retains this type throughout

the whole course of its development; it never is, and never can be, either a
Mollusc or an Articulate" (xx., p. 54).
"We are led to establish ... as the general result of our researches, the
existence of several types, and, consequently, of different plans, in the
development of animals. These different types are manifested from the very
beginning of embryonic life; the characters distinguishing them are
therefore primordial, and we can say with M. Milne-Edwards that
everything goes to prove that the distinction established by Nature between
animals belonging to different phyla is a primordial distinction" (p. 58).
In other directions also von Baer's work was confirmed and extended by
later observers—those parts of it particularly that had reference to the germ-
layer theory, and to the concept of histological differentiation. His germ-
layer theory was accepted in its main lines by Rathke, Bischoff and
Lereboullet, and applied by them to the multitude of new facts they
discovered. Rathke, in particular, was a firm upholder of the doctrine, and
made considerable use of it in his writings.[327] Even before the publication
of von Baer's book he had interpreted in terms of the germ-layer theory
sketched by his friend Pander the splitting of the blastoderm which occurs
in the early development of Astacus, whereby there are formed a serous and
a mucous layer, one inside the other—like the coats of an onion, to use his
own expressive phrase.[328]
An ingenious application of the Pander-Baer theory was made by Huxley,
who compared the outer and inner cell-layers which form the groundwork
of the Cœlentera with the serous and mucous layers of the vertebrate germ.
[329]
He laid stress, it is true, rather on the physiological than on the
morphological resemblance. "A complete identity of structure," he writes,
"connects the 'foundation membranes' of the Medusæ with the
corresponding organs in the rest of the series; and it is curious to remark,
that throughout, the outer and inner membranes appear to bear the same
physiological relation to one another as do the serous and mucous layers of
the germ; the outer becoming developed into the muscular system, and
giving rise to the organs of offence and defence; the inner, on the other
hand, appearing to be more closely subservient to the purposes of nutrition
and generation" (p. 24). Von Baer had already hinted at this homology in
the second volume of his Entwickelungsgeschichte (1837), where he says

with reference to the separation of the blastoderm of the chick into two
layers. "Yet originally there are not two distinct or even separable layers, it
is rather the two surfaces of the germ which show this differentiation, just
as polyps show the same contrast of an external surface and an internal
digestive surface. In between the two layers there is in our germ as in the
polyp an indifferent mass" (p. 67). The terms ectoderm and entoderm were
introduced by Allman[330] in 1853 for the two cell-layers in the Hydrozoa.
Remak is the second great name in the history of the germ-layer theory. He
had the great advantage over von Baer of being able to make use of the cell-
theory in interpreting the formation of the germ-layers. Microscopical
technique also had been greatly improved since 1828.[331]
Remak's greatest service was that he put the germ-layer theory in direct
relation with the cell-theory by demonstrating the cellular continuity from
egg-cell to tissue, and by showing that each germ-layer possessed
distinctive histological characteristics. Hardly less important was his clear
marking-off of the "middle layer" as a separate and distinct layer of the
germ. He it was who introduced the modern conception of the mesoderm,
and cleared up the confusion in which Pander and von Baer had left the
organs formed between the serous and the mucous layer. Remak's middle
layer was a different thing from Pander's ill-defined "vessel-layer"; it
included and unified from a new point of view the "vessel" and "muscle"
layers of von Baer.
There are in the unincubated blastoderm of the chick, according to Remak,
[332]
two cell-layers, of which the undermost subsequently splits into two.
Three layers are thus formed—the upper, middle and lower. The upper layer
differentiates into a medullary plate and an epidermic plate (Remak's
Hornblatt), and gives origin to the medullary tube with all its evaginations,
and to the skin with all its derivatives and pockets. It forms such diverse
structures as the brain, the spinal cord, the eye, the ear, the mouth, hairs,
feathers, nails, sweat-glands, lacrymal glands, and so forth. All these parts
are connected directly or indirectly with sensation, and the upper germ-
layer may accordingly be called the sensory layer. The lower layer gives
rise to the epithelium and the proper tissue of the alimentary canal and its
derivatives, as the liver, lungs, pancreas, kidneys, thyroid, thymus, etc.
These parts are all concerned in the processes of assimilation and

dissimilation, and the lower layer may accordingly be called the trophic
layer. Now between the upper or sensory layer and the lower or trophic
layer there exists, in spite of their very different functions, a close
histological likeness, for both are essentially epithelial layers. The
resemblance is particularly strong if we compare the lower layer with the
Hornblatt of the upper layer—both consist of epithelial tissue, and of its
derivative, glandular tissue, and form neither vessels nor nerves. The
middle layer, on the contrary, forms nerves and muscles, vessels and
connective tissue, and little or no epithelium. It does not form all the blood-
vessels without exception (and so cannot be called the vessel-layer), for the
blood-vessels of the central nervous system are in all probability formed
from the upper layer. So, too, it does not form all the nerves and muscles—
the optic and auditory nerves and the nerves and muscles of the iris
probably arise in the upper layer. But, in spite of these exceptions, its
general histological character is so well defined that it may be contrasted
with the other two as preeminently the layer that forms muscular, nervous,
vascular and connective tissue. In view of its functional significance, it may
be called the motory layer, or better, since it forms also the sexual glands,
the motor-germinative layer. The middle layer, early in its history, shows a
division into dorsal plates (Urwirbelplatten) and ventral plates
(Seitenplatten). The former exhibit almost as soon as they are formed the
characteristic proto-vertebral segmentation, the latter split to form the
pleuro-peritoneal or body-cavity. Remak describes the latter process as
follows:—"In the region of the trunk, where a greater independence of the
fate of the alimentary canal and its annexes becomes necessary for the
voluntary executive organs, the ventral plates undergo a process of splitting,
leading to the formation of the sensitive part of the integument (the
Hautplatten), the muscular part of the alimentary tube (the
Darmfaserplatten), and the mother-tissue of the generative organs (the
Mittelplatten). From the Hautplatten there develops, without the dorsal
plates seeming to take any part in the process, the rudiment of the
extremities" (p. 79).
His Darmfaserplatten form the nervous and muscular tissue of the
alimentary canal and its dependencies, and also the heart; the Hautplatten
form the general body-wall (exclusive of the skin) and the appendages. In

the embryo they line the amniotic cavity. The skeleton and peripheral
nerves originate wholly within the middle layer.
Remak's conception of the relations of the three germ-layers to one another
and to the body-cavity is well illustrated in Fig. 12.

Fig. 12.—Transverse Section of Chick Embryo. (After
Remak.)
h. Epidermis. hp. x. Edge of the smniotic
fold.
m. Spinal cord. and "Hautplatte" ph. Pleuro-Peritonial
cavity.
mu. Dorsal plate. um.
ug. Pronephric duct. mp. "Mittelplatte" d. Epithelium of
alimentary canal.
pa. Aortic root. df. "Darmfaser platte."

In his germ-layer theory Remak's standpoint is histological rather than
morphological. The distinction which he draws between the sensory and
trophic layers on the one hand, and the motor-germinative layer on the
other, is entirely a histological one. The greater part of his book, indeed, is
devoted to a study of the histogenesis of the different organs of the body; he
is bent chiefly upon unravelling the part which each germ-layer takes in the
formation of each tissue and organ.
His generalisation that two of the germ-layers give rise exclusively or
almost exclusively to one kind of tissue excited great interest at the time,
and gave the direction to histogenetic research for quite a number of years,
though in the end it turned out to be insufficiently founded.

Though Remak's germ-layer theory had thus principally a histological
orientation, it laid down the main lines of the modern morphological
treatment of the germ-layers.
[293] Embryologie des Salmones, 1842.
[294] Die Cellularpathologie in ihrer Begründung auf physiologische und
pathologische Gewebelehre, Berlin, 2nd ed. 1859; Eng. trans., by Chance, 1860.
[295] Arch. path. Anat. Phys., vii., pp. 1-39 (1854).
[296] Bericht über die Fortschritte der mikroskopischen Anatomie im jahre
1854. Müller's Archiv, 1855. See also 1856.
[297] Hndb. d. Physiol., i., 1835.
[298] See Leuckart's reply to Ludwig's criticism, in Zeit. f. wiss. Zool., ii., p. 271,
1850.
[299] Leipzig, 1853.
[300] Souvenirs d'un Naturaliste, 2 vols., Paris, 1854. Eng. Trans. as Rambles of
a Naturalist on the Coasts of France, Spain, and Italy, 2 vols., 1857.
[301] Milne-Edwards later published a classical textbook on comparative
anatomy and physiology—Leçons sur la Physiologie et l'Anatomie comparées,
14 vols., Paris, 1857-80.
[302] Paris, 1834-40. Three volumes of the Suites à Buffon.
[303] Paris, 1865. Two volumes of the Suites à Buffon.
[304] U. d. Metamorphose der Ophiuren u. Seeigel., Berlin, 1848. U. d.
Metamorphose der Holothurien u. Asterien., Berlin, 1851.
[305] As I have been unable to obtain a copy of the Introduction, the passages
which follow are taken from the Rapport of 1867, where Milne-Edwards gives a
complete exposition of his doctrine, sometimes in the words of the original.
[306] This principle was first developed by Milne-Edwards in 1827, in the
Dictionnaire classique d'Hist. naturelle. It was probably suggested to him by his
studies on the Crustacea, among which the principle is so beautifully exemplified
in the concentration and specialisation of the appendages and the ganglionic
chain.
[307] Studied by Isidore Geoffroy St Hilaire in his paper Classification
parallélique des Mammifères, C. R. Acad. Sci., xx., 1845. Remarked upon by
Cuvier, Règne animal., i., p. 171, 1817, also by de Blainville.
[308] Cuvier et Valenciennes, Hist. nat. des Poissons, i., p. 550, 1828.

[309] Myxinoiden, Th. I. Abh. k. Akad. Wiss. Berlin for 1834, pp. 100, 110, 179,
etc.
[310] Vergl. Entw. Kopf. nackt. Amphibien, p. 101, 1838.
[311] I have not seen the companion volume on palæontological progression,
Unters. ü. d. Entwickelungsgesetze der organischen Welt während der
Bildungszeit unserer Erdoberfläche, Stuttgart, 1858.
[312] "Strobiloid" because of its spiral development. The theory of the spiral
growth of plants played an important part in botanical morphology about this
time.
[313] Cf. Meckel's Principle of progressive Evolution, supra, p. 93.
[314] System der thierischen Morphologie, pp. 33, 457. Also C. Bruch, Die
Wirbeltheorie des Schädels, am Skelette des Lachses geprüft, Frankfort-on-Main,
1862.
[315] In France the vertebral theory was advocated by Lavocat in his Nouvelle
Ostéologie comparée de la tête des animaux domestiques, Toulouse, 1864. It
seems also that Lacaze-Duthiers held fast to it even in 1872—Arch. zool. exp.
gén., i., p. 51, 1872.
[316] An Essay on Classification, Boston, 1857, London, 1859. He considered the
classificatory categories to be the categories of the Creator's thought, and hence
natural, and in no sense mere conventions.
[317] "Principes d'Embryogénie, de Zoogénie et de Teratogénie," Mém. Acad.
Sci., xxv., pp. 1-943, pls. xxv., 1860.
[318] "On the Morphology of the Cephalous Mollusca," Phil. Trans., 1853, Sci.
Memoirs, i., pp. 152-92.
[319] "Observations sur les changements de forme que les divers Crustacés
éprouvent," Ann. Sci. nat. (1) xxx., p. 360, 1833.
[320] "Considérations sur quelques principes relatifs à la classification naturelle
des animaux," Ann. Sci. nat. (3) i., p. 65, 1844.
[321] Supra, pp. 79-83. Also Précis d'anatomie transcendante, principes
d'organogénie, Paris, 1842.
[322] The inversion of the organs shown by Vertebrates as compared with
Invertebrates is due to the reversed position of the embryo relatively to the yolk!
(pp. 821-6).
[323] It is worth while recording that Serres enunciated a "law of symmetry"
according to which the embryo is formed by the union of its two symmetrical
halves—a law which recalls the "concrescence theory" of His and some modern
embryologists.
[324] "Embryologie comparée du Brochet, de la Perche, et de l'Ecrévisse," Ann.
Sci. nat. (4), i., p. 237, 1854; ii., p. 39, 1854. Mém. Savans etrangers, xvii.

[325] Ann. Sci. nat. (4) xvi., p. 113, 1861; xvii., p. 88, 1862; xviii., p. 5, 1862;
xix., p. 5, 1863.
[326] xx., p. 5, 1863.
[327] Particularly in his Blennius (1833) and Natter (1839).
[328] In the "preliminary notice" of his Crayfish paper—Isis, pp 1093-1100,
1825.
[329] "On the Anatomy and the Affinities of the Family of the Medusæ," Phil.
Trans., 1849; Sci. Memoirs, i., pp. 9-32.
[330] Phil. Trans., cxliii., p. 368, 1853.
[331] The principle of achromatism was discovered (by Fraunhofer) and
achromatic microscopes introduced in the early part of the 19th century. The use
of chemical reagents, such as acetic acid, and various hardening fluids, came into
fashion not long after. J. Müller seems to have been one of the first to realise their
importance. Remak himself invented one or two fixing and hardening mixtures
(pp. 87, 127, 1855), which enabled him to cut excellent hand sections. Section-
cutting machines were not invented till later (V. Hensen, 1866, His, 1870).
[332] Untersuchungen über die Entwickelung der Wirbelthiere, folio, pp. xxxvii +
195, 12 plates, Berlin, 1850-1855.

CHAPTER XIII

THE RELATION OF LAMARCK AND DARWIN TO MORPHOLOGY.

It is a remarkable fact that morphology took but a very little part in the
formation of evolution-theory. When one remembers what powerful
arguments for evolution can be drawn from such facts as the unity of plan
and composition and the law of parallelism, one is astonished to find that it
was not the morphologists at all who founded the theory of evolution.
It is true that the noticeable resemblances of animals to one another, the
possibility of arranging them in a system, the vague perception of an all-
pervading plan of structure, did suggest to many minds the thought that
systematic affinities might be due to blood-relationship. Thus Leibniz
considered that the cat tribe might possibly be descended from a common
ancestor,[333] and another great philosopher, Immanuel Kant, was led by his
perception of the unity of type to suggest as possible the derivation of the

whole organic realm from one parent form, or even ultimately from
inorganic matter. In the course of his masterly discussion of mechanism and
teleology,[334] he writes, "The agreement of so many genera of animals in a
certain common schema, which appears to be fundamental not only in the
structure of their bones, but also in the disposition of their remaining parts—
so that with an admirable simplicity of original outline, a great variety of
species has been produced by the shortening of one member and the
lengthening of another, the involution of this part and the evolution of that—
allows a ray of hope, however faint, to penetrate into our minds, that here
something may be accomplished by the aid of the principle of the
mechanism of Nature (without which there can be no natural science in
general). This analogy of forms, which with all their differences seem to
have been produced according to a common original type, strengthens our
suspicions of an actual relationship between them in their production from a
common parent, through the gradual approximation of one animal-genus to
another—from those in which the principle of purposes seems to be best
authenticated, i.e., from man down to the polype, and again from this down
to mosses and lichens, and finally to the lowest stage of Nature noticeable by
us, viz., to crude matter."[335]
So, too, Buffon's evolutionism was suggested by his study of the structural
affinities of animals, and Erasmus Darwin in his Zoonomia (1794) brought
forward as one of the strongest proofs of evolution, "the essential unity of
plan in all warm-blooded animals."[336]
But, as a matter of historical fact, no morphologist, not even Geoffroy,
deduced from the facts of his science any comprehensive theory of
evolution. The pre-Darwinian morphologists were comparatively little
influenced by the evolution-theories current in their day, and it was in the
anatomist Cuvier and the embryologist von Baer that the early evolutionists
found their most uncompromising opponents.
Speaking generally, and excepting for the moment the theory of Lamarck,
we may say that the evolution-theories of the 18th and 19th centuries arose
in connection with the transcendental notion of the Échelle des êtres, or scale
of perfection. This notion, which plays so great a part in the philosophy of
Leibniz, was very generally accepted about the middle of the 18th century,
and received complete and even exaggerated expression from Bonnet and

Robinet. Buffon also was influenced by it. Towards the beginning of the
19th century the idea was taken up eagerly by the transcendental school and
by them given, in their theories of the "one animal," a more morphological
turn. Their recapitulation theory was part and parcel of the same general
idea.
One understands how easily the notion of evolution could arise in minds
filled with the thought of the ideal progression of the whole organic kingdom
towards its crown and microcosm, man. Their theory of recapitulation led
them to conceive evolution as the developmental history of the one great
organism.[337] Many of them wavered between the conception of evolution
as an ideal process, as a Vorstellungsart, and the conception of it as an
historical process. Bonnet, Oken, and the majority of the transcendentalists
seem to have chosen the former alternative; Robinet, Treviranus, Tiedemann,
Meckel, and a few others held evolution to be a real process.

We have already in previous chapters[338] briefly noticed the relation of one
or two of the transcendental evolution-theories to morphology, and there is
little more to be said about them here. They had as good as no influence
upon morphological theory, nor indeed upon biology in general.[339] It is
different with the theory of Lamarck, which, although it had little influence
upon biological thought during and for long after the lifetime of its author, is
still at the present day a living and developing doctrine.
Lamarck's affinity with the transcendentalists was in many ways a close one,
but he differed essentially in being before all a systematist. Nor is the direct
influence of the German transcendentalists traceable in his work—his
spiritual ancestors are the men of his own race, the materialists Condillac
and Cabanis, and Buffon, whose friend he was. The idea of a gradation of all
animals from the lowest to the highest was always present in Lamarck's
mind, and links him up, perhaps through Buffon, with the school of Bonnet.
The idea of the Échelle des êtres had for him much less a morphological
orientation than it had even for the transcendentalists, for he was lacking
almost completely in the sense for morphology. Lamarck's scientific, as
distinguished from his speculative work, was exclusively systematic, and it
was systematics of a very high order. He introduced many reforms into the
general classification of animals. He was the first clearly to separate
Crustacea (1799), and a little later (1800) Arachnids, from insects. He

reduced to a certain orderliness the neglected tribes of the Invertebrates, and
wrote what was for long the standard work on their systematics—the
Histoire naturelle des Animaux sans Vertèbres (1816-22). His speculative
work on biology is contained in three publications, the small book entitled
Considérations sur l'organisation des corps vivants (1802), the larger work
of 1809, the Philosophie zoologique, and the introductory matter to his
Animaux sans Vertèbres (vol. i., 1816).
It is no easy matter to give in short compass an account of Lamarck's
biological philosophy. He is an obscure writer, and often self-contradictory.
In the first part of the Philosophie zoologique Lamarck is largely pre-
occupied with the problem of whether species are really distinct, or do not
rather grade insensibly into one another. As a systematist of vast experience
Lamarck knew how difficult it is in practice to distinguish species from
varieties. "The more," he writes, "we collect the productions of Nature, the
richer our collections become, the more do we see almost all the gaps filled
up and the lines of separation effaced. We find ourselves reduced to an
arbitrary determination, which sometimes leads us to seize upon the slightest
differences of varieties, and form from them the distinctive character of what
we call a species, and at other times leads us to consider as a variety of a
certain species individuals a little bit different, which others regard as
forming a separate species."[340]
For Lamarck, as for Darwin later, the chief problem was not the evolution
and differentiation of types of structure, but the mode of origin of species.
Lamarck is at great pains to show how arbitrary are our determinations of
species, and how artificial the classificatory groups which we distinguish in
Nature. Strictly speaking, there are in Nature only individuals, "... this is
certain, that among her products Nature has in reality formed neither classes,
nor orders, nor families, nor genera, nor constant species, but only
individuals which succeed one another and resemble those that produced
them. Now, these individuals belong to infinitely diversified races, which
shade into one another under all the forms and in all the degrees of
organisation, and each of which maintains itself without change, so long as
no cause of change acts upon it" (p. 41).

But there is a natural order in the animal kingdom, a progression from the
simpler to the more complex organisations, a natural Échelle des êtres.
This order is shown by the relation to one another of the large classificatory
groups, for they can be arranged in series from the simplest to the most
complex, somewhat as follows:—

1. Infusoria. 6. Arachnids. 11. Fishes.
2. Polyps. 7. Crustacea. 12. Reptiles.
3. Radiates. 8. Annelids. 13. Birds.
4. Worms. 9. Cirripedes. 14. Mammals.
5. Insects. 10. Molluscs.

But the order of Nature is essentially continuous, and the limits of even the
best defined of these classes are in reality artificial—"if the order of Nature
were perfectly known in a kingdom, the classes which we should be forced
to establish in it would always constitute entirely artificial sections" (p. 45).
In the same way the lesser classificatory groups represent smaller sections of
the one unique order of Nature. Note that Lamarck's Échelle is in no way a
morphological one, and was not intended to be such. It is a scale of
increasing physiological differentiation, and the stages of it are marked by
the acquirement of this or that new organ (cf. Oken). "Observation of their
state convinces one that in order to produce them successively Nature has
proceeded gradually from the simpler to the more complex. Now Nature,
having had in mind the realisation of a plan of organisation which would
permit of the greatest perfecting (that of the Vertebrates), a plan very
different from those which she has been obliged to form as a preliminary to
reaching it, one understands that, among the multitude of animals, one must
necessarily come across not a single system of organisation which has
become progressively perfected, but diverse very distinct systems, each of
which has come into existence at the moment when each primary organ first
put in its appearance" (p. 171).
For Lamarck this order of Nature was not merely ideal—Nature had actually
formed the classes successively, proceeding from the simpler to the more
complex; she had brought about this evolution by transforming the primitive
species of animals, raising them to higher degrees of organisation, and

modifying them in relation to the environment in which they found
themselves.
Lamarck's theory of evolution is worked out in great detail in his
Philosophie zoologique, but the exposition is diffuse and disconnected; it is
better in giving an account of it to follow the more concise, mature and
general exposition which he gives in the Introduction to his Histoire
naturelle des Animaux sans Vertèbres.[341] Near the beginning of the
Introduction Lamarck gives us in a few short "Fundamental Principles" the
main lines of his general philosophy. He is a confirmed materialist. Every
fact and phenomenon is essentially physical and owes its existence or
production entirely to material bodies or to relations between them. All
change and all movement is in the last resort due to mechanical causes.
Every fact or phenomenon observed in a living body is at once a physical
fact or phenomenon and a product of organisation (p. 19). Life, thought and
sensation are not properties of matter, but result from particular material
combinations.
His thorough-going materialism is most clearly shown in its relation to
living things in the first three of the "Zoological Principles and Axioms,"
which are developed further on in the book.
These are as follows:—"1. No kind or particle of matter can have in itself the
power of moving, living, feeling, thinking, nor of having ideas; and if,
outside of man, we observe bodies endowed with all or one of these
faculties, we ought to consider these faculties as physical phenomena which
Nature has been able to produce, not by employing some particular kind of
matter which itself possesses one or other of these faculties, but by the order
and state of things which she has constituted in each organisation and in
each particular system of organs.
"2. Every animal faculty, of whatever nature it may be, is an organic
phenomenon, and results from a system of organs or an organ-apparatus
which gives rise to it and upon which it is necessarily dependent.
"3. The more highly a faculty is developed the more complex is the system
of organs which produces it, and the higher the general organisation; the
more difficult also does it become to grasp its mechanism. But the faculty is

none the less a phenomenon of organisation, and for that reason purely
physical" (p. 104).
According to these "axioms" function is a direct and mechanical effect of
structure.
The curious thing is that in spite of his avowed materialism, Lamarck's
conception of life and evolution is profoundly psychological, and from the
conflict of his materialism and his vitalism (of which he was himself hardly
conscious), arise most of the obscurities and the irreductible self-
contradiction of his theory.
Lamarck divided animals (psychologically!) into three great groups—
apathetic or insensitive animals, animals endowed with sensation, and
intelligent animals. The first group, which comprise all the lower
Invertebrates, are distinguished from other animals by the fact that their
actions are directly and mechanically due to the excitations of the
environment; they have no principle of reaction to external influences, but
passively prolong into action the excitations they receive from without. They
are irritable merely. The second group are distinguished from the first by
their possessing, in addition to irritability, a power which Lamarck calls the
sentiment intérieur. He has some difficulty in defining exactly what he
means by it:—"I have no term to express this internal power possessed not
only by intelligent animals but also by those that are endowed merely with
the faculty of sensation; it is a power which, when set in action by the
feeling of a need, causes the individual to act at once, i.e., in the very
moment of the sensation it experiences; and if the individual is of those that
are endowed with intelligence it nevertheless acts in such a case entirely
without premeditation and before any mental operation has brought its will
into play" (p. 24).
It is the power we call instinct in animals (p. 25), and it implies neither
consciousness nor will. It acts by transforming external into internal
excitations.
To this second group of animals, possessing the sentiment intérieur, belong
the higher Invertebrates, notably insects and molluscs. Only animals
possessed of a more or less centralised nervous system can manifest this
sentiment, or principle of (unconscious) reaction to external stimuli.

The higher animals, or the four Vertebrate classes, form the group of
"intelligent animals." In virtue of their more complex organisation they
possess in addition to the sentiment intérieur the faculties of intelligence and
will.
Now, broadly put, Lamarck's theory of evolution is that new organs are
formed in direct reaction to needs (besoins) experienced by the sentiment
intérieur. The sentiment intérieur is therefore the cause not only of
instinctive action but also of all morphogenetic processes. Will and
intelligence (which are confined to a relatively small number of animals)
have little or nothing to do directly with evolution.
To understand the working-out of Lamarck's evolution-theory we must
revert to his conception of the Échelle des êtres. What he wrote in the
Philosophie zoologique is here repeated in the work of 1816 with little
modification.
There is a real progression from the simpler to the more complex
organisations; Nature has gradually complicated her creatures by giving
them new organs and therefore new faculties.
It is interesting to note that Lamarck expressly refers to Bonnet (p. 110), but
refuses to accept his view of an Échelle extending down into the inorganic.
Like Bonnet, however, and like the German transcendentalists, Lamarck
makes man the goal of evolution (p. 116). He makes it quite clear that his
Échelle is a functional one, for he links Vertebrates to molluscs even while
expressly admitting that they are not connected by any structural
intermediates (p. 123). He does not fall into the error of the
transcendentalists and assume that Vertebrates and Invertebrates alike are
formed upon one common plan of structure.
The progression of organisation shown by the animal kingdom has not been
altogether regular and uninterrupted:—"The progression in complexity of
organisation shows here and there, in the general animal series, anomalies
induced by the influence of environment and by the influence of the habits
contracted" (Phil. zool., i., p. 145).
There are thus really two causes at work to produce the variety of
organisation as it appears to us, one which tends to produce a regular

increase in complexity, and one which disturbs and diversifies this regular
advance.
The first cause Lamarck calls the vital power (pouvoir de la vie); the other
may be called the influence of circumstance (Anim. s. Vert., p. 134). To the
latter cause are due the lacunæ, the blind alleys, and the complications which
the otherwise simple scale of perfection shows.
To explain both these aspects of evolution Lamarck propounded in his
volume of 1816 four laws, which read as follows:—
"First Law.—Life, by its own forces, tends continually to increase the
volume of every body possessing it, and to extend the dimensions of its
parts, up to a limit which it brings about itself.
"Second Law.—The production of a new organ in an animal body results
from the arisal and continuance of a new need, and from the new movement
which this need brings into being and sustains.
"Third Law.—The degree of development of organs and their force of action
are always proportionate to the use made of these organs.
"Fourth Law.—All that has been acquired, imprinted or changed in the
organisation of the individual during the course of its life is preserved by
generation and transmitted to the new individuals that descend from the
individual so modified" (pp. 151-2).
It is mainly but not entirely by reason of the first of these laws that
organisation tends to progress, and mainly by reason of the second and third
that difference of environment brings about diversity of organisation. In
virtue of the fourth law the acquirements of the individual become the
property of the race.
Lamarck's exposition of his first law, that life tends by its own powers to
enlarge and extend its bodily instrument, is vague and difficult to
understand. He has already explained some pages back how the first
organisms arose by spontaneous generation in the form of minute gelatinous
utricles (cf. Oken). He conceives that it is in the movements of the fluids
proper to the organism that the power resides to enlarge and extend the body.
Nutrition alone is not sufficient to bring about extension; a special force is

required, acting from within outwards (p. 153). In the most primitive
organisms the movements of the vital fluids are weak and slow, but in the
course of evolution they gradually accelerate, and, becoming more rapid,
trace out canals in the delicate tissue which contains them, and finally form
organs.
Subtle fluids play a great part in Lamarck's biology: they take the place of
the soul or entelechy which the vitalists would postulate to explain organic
happenings. Lamarck seems in this to follow certain of the old materialists,
who conceived the soul to be formed of a matter more subtle than the
ordinary.[342]
In his second law Lamarck's essentially vitalistic attitude comes out very
clearly, for it states that a psychological moment enters into all new
production of form, that the ultimate cause of the development of new form
is the need felt by the organism. This need is of course not a conscious one,
it is a need perceived by the sentiment intérieur.
In the large group of apathetic or insensitive animals, which do not possess
this faculty, needs cannot be experienced; accordingly new organs are here
formed directly and mechanically, by the movements of the vital fluids set in
action by excitations from without—the evolution, like the behaviour, of
these animals is due to the direct and physical action of the environment.
"But this is not the case with the more highly organised animals which
possess feeling. They experience needs, and each need felt, acting upon their
'inner feeling,' immediately directs the fluids and the forces to the part of the
body where action can satisfy the need. Now, if there exists at this point an
organ capable of performing the required action, it is quickly stimulated to
act; and if the organ does not exist and the need is pressing and sustained, bit
by bit the organ is produced and developed in proportion to the continuity
and the energy of its use" (p. 155).
In intelligent animals the sentiment intérieur may be moved by thought or
will.
As an example of the way in which the law works Lamarck takes the
hypothetical case of a gastropod mollusc, which as it creeps along
experiences dimly the need to feel the objects in front of it. It makes an
effort (unconscious, be it noted) to touch these objects with the anterior

portions of its head, and sends forward continually to these parts a great
volume of nervous and other fluids. From these efforts and the repeated
afflux of fluids there must result a development of the nerves supplying
these parts. And as, along with the nervous fluids, nutritive juices constantly
flow to the parts, there must result the formation of two or four tentacles in
the places to which these fluids are directed. A curious mixture of
mechanistic "explanations" and vitalistic hypothesis!
In his third law, that use and disuse are powerful to modify organs, Lamarck
is upon more solid ground, and can point to many instances of the visible
effect of these factors of change. It is of course rather closely bound up with
his second law and may even be regarded as an extension of it.
The law has reference to one of the most powerful means employed by
Nature to diversify species, a means which comes into play whenever the
environment changes. The cause of the great diversity shown by animal
species is indeed ultimately to be sought in the environment. As the
imperfect and earliest forms developed they spread over the earth and
invaded the utmost corners of it:—"One can imagine what an enormous
variety of habitats, stations, climates, available foods, environing media, etc.,
animals and plants have had to endure, as the existing species were forced to
change their place of abode. And although these changes have taken place
with extreme slowness ... their reality, necessitated by various causes, has
none the less induced the species affected by them slowly to change their
manner of life and their habitual actions. Through the effects of the second
and third of the laws cited above, these induced activity-changes must have
brought into being new organs, and must have been able to develop them
further if more frequent use was made of them; they must in the same way
have been capable of bringing about the degeneration and finally the
complete disappearance of existing organs which had become useless"
(p. 161).
On the other hand, if the environment does not change, species remain
constant.
It is to be noted that change in environment is rather the occasion than the
cause of modification; the environment induces the organism to change its
habitual way of life; it sets up new needs, to satisfy which the organism must

modify its structure. It is the organism that takes the active part in all this,
the action of the environment is indirect.
Of Lamarck's fourth law, which asserts the transmission of acquired
characters, little need here be said in the way of exposition. Upon the truth of
it depends of course Lamarck's whole theory. He himself never dreamed that
anyone would ever dispute it.
Lamarck sums up as follows:—"By the four laws which I have just
enunciated all the facts of organisation seem to me to be easily explained;
the progression in the complexity of organisation of animals, and in their
faculties, seems to me easy to conceive; so, too, the means which Nature has
employed to diversify animals, and bring them to the state in which we now
see them, become easily determinable" (p. 168).
It is never made quite clear, we may note in passing, how far his second and
third laws tend to bring about an increase in complexity, in addition to
diversifying animals.[343]
"The function creates the organ," this would seem to be the kernel of
Lamarck's doctrine. But how does he reconcile this essentially vitalistic
conception with his strictly materialistic philosophy?
We have seen that irritability, the sentiment intérieur, and intelligence itself,
are the effects of organisation. We are told farther on that both the sentiment
and intelligence are caused by nervous fluids. A great part of both the
Philosophie zoologique and the introduction to the Animaux sans Vertèbres
is given up to the exposition of a materialistic psychology of animals and
man, based entirely upon this hypothesis of nervous fluids. Thus habits are
due to the fluids hollowing out definite paths for themselves.
The sentiment intérieur acts by directing the movements of the subtle fluids
of the body (which are themselves modifications of the nervous fluids) upon
the parts where a new organ is needed. But if it is itself only a result of the
movement of nervous fluids? Again, how can a need be "felt" by a nervous
fluid? This is an entirely psychological notion and cannot be applied to a
purely material system. Whence arises the power of the sentiment intérieur
to canalise the energies of the organism, so to direct and co-ordinate them
that they build up purposive structures, or effect purposive actions (as in all

instinctive behaviour)? Either the sentiment intérieur is a psychological
faculty, or it is nothing.
There is no doubt that, as expressed by Lamarck, the conception conceals a
radical confusion of thought. It is not possible to be a thorough-going
materialist, and at the same time to believe that new organs are formed in
direct response to needs felt by the organism. Lamarck could never resolve
this antinomy, and his speculations were thrown into confusion by it. To this
cause is due the frequent obscurity of his writings.
Should we be right in laying stress upon the psychological side of Lamarck's
theory, and disregarding the materialistic dress in which, perhaps under the
influence of the materialism current in his youth, he clothed his essentially
vitalistic thought? Everything goes to prove it—his constant preoccupation
with psychological questions, his tacit assimilation of organ-formation to
instinctive behaviour, his constant insistence on the importance of besoin
and habitude.
Let us not forget the profundity of his main idea, that, exception made for
the lower forms, the animal is essentially active, that it always reacts to the
external world, is never passively acted upon. Let us not forget that he
pointed out the essentially psychological moment implied in all processes of
individual adaptation. With keen insight he realised that conscious
intelligence counts for little in evolution, and focussed attention upon the
unconscious but obscurely psychical processes of instinct and
morphogenesis.
Not without reason have the later schools of evolutionary thought, who
developed the psychological and vitalistic side of his doctrine, called
themselves Neo-Lamarckians.
We shall say then that Lamarck, in spite of his materialism, was the founder
of the "psychological" theory of evolution.
Lamarck stood curiously aloof and apart from the scientific thought of his
day.[344] He took no interest in the morphological problems that filled the
minds of Cuvier and Geoffroy; he had indeed no feeling at all for
morphology. He did not realise, like Cuvier, the convenance des parties, the
marvellous co-ordination of parts to form a whole; he had little conception

of what is really implied in the word "organism." He was not, like Geoffroy,
imbued with a lively sense of the unity of plan and composition, and of the
significance of vestigial organs as witnesses to that unity. He seems not to
have known of the recapitulation theory, of which he might have made such
good use as powerful evidence for evolution. Even with the German
transcendentalists, with whom in the looseness of his generalisations he
shows some affinity, he seems not to have been specially acquainted.
He was interested more in the problems suggested to him by his daily work
in the museum. He wanted to know why species graded so annoyingly into
one another; he wanted to examine critically his haunting suspicion that
species were really not distinct, and that classification was purely
conventional. The question, too, of the adaptation of species to their
environment, the problem of ecological adaptation, in distinction to that of
functional adaptation which interested Cuvier so greatly, came vividly before
him as he worked through the vast collections of the museum. He was the
first systematist to occupy himself in a philosophical manner with the
problems of general biology. He introduced new problems and a new way of
looking at old. With Lamarck the problem of species and the problem of
ecological adaptation enter into general biology.
The one point in which he does definitely carry on the thought of his
predecessors is his conception of the animal kingdom as forming a scale of
(functional) perfection. He did not go to the same extreme as Bonnet; he did
not even consider that the animal series was a continuation of the vegetable
series; in his opinion they formed two diverging scales. He recognised, too,
that among animals there was no simple and regular gradation from the
lowest to the highest, but that the orderly progression was disturbed and
diverted by the necessity of adaptation to different environments. It is
interesting to note that in developing this idea he arrived at a roughly
accurate distinction between homologous and analogous structures. More
importance, he thought, was to be attributed in classifying animals to
characters which appeared due to the "plan of Nature" than to such as were
produced by an external modifying cause (p. 299). But he did not formulate
the distinction in any strictly morphological way.
As his ideas developed he laid less stress upon the simplicity and continuity
of the scale; in his supplementary remarks to the Introduction of 1816 he
admits that the series is really very much branched, and even that there may

be two distinct series among animals instead of one. His last schema of the
course of evolution shows no little analogy with the genealogical trees of
Darwinian speculation. It is headed "The presumed Order of the formation
of Animals, showing two separate partly-branching series," and it reads as
follows:—

It is interesting to note that Vertebrates are placed between the two series,
and are now not linked on directly to any Invertebrate group.
Lamarck's theory had little success. There is evidence, however, that both
Meckel and Geoffroy owed a good many of their evolutionary ideas to
Lamarck, and Cuvier paid him at least the compliment of criticising his
theory,[345] not distinguishing it, however, very clearly from the evolutionary
theories of the transcendentalists. But, speaking generally, Lamarck's theory
of evolution exercised very little influence upon his contemporaries. This
was probably due partly to the obscurity and confusion of his thought, partly

to his lack of sympathy with the biological thought of his day, which was
preponderatingly morphological.
It was not that men's minds were not ripe for evolution, for in the early
decades of the 19th century evolution was in the air. There were few of von
Baer's contemporaries who had not read Lamarck;[346] Erasmus Darwin's
Zoonomia ran through three editions, and was translated into German,
French and Italian;[347] German philosophy was full of the idea of evolution.
There was no unreadiness to accept the derivation of present-day species
from a primordial form—if only some solid evidence for such derivation
were forthcoming. Cuvier and von Baer, as we have seen, combated the
current evolution theories on the ground that the evidence was insufficient,
but von Baer at least had no rooted objection to evolution. In an essay of
1834, entitled The Most General Law of Nature in all Development,[348] von
Baer expressed belief in a limited amount of evolution. In this paper he did
not admit that all animals have developed from one parent form, and he
refused to believe that man has descended from an ape; but, basing his
supposition upon the facts of variability and upon the evidence of
palæontology, he went so far as to maintain that many species have evolved
from parent stocks. In the absence of conclusive proofs he did not commit
himself to a belief in any extended or comprehensive process of evolution.
Imbued as he was with the idea of development von Baer saw in evolution a
process essentially of the same nature as the development of the individual.
Evolution, like development, was due to a Bildungskraft or formative force.
The ultimate law of all becoming was that "the history of Nature is nothing
but the history of the ever-advancing victory of spirit over matter" (p. 71). In
a later essay (1835) in the same volume he says that all natural science is
nothing but a long commentary on the single phrase Es werde!. (p. 86).
As we shall see, von Baer adopted in later years the same attitude to
Darwinism as he did to the evolution theories in vogue in his youth.
Although in the twenty or thirty years before the publication of the Origin of
Species (1859) no evolution theory of any importance was published, and
although the great majority of biologists believed in the constancy of
species, there were not wanting some who, like von Baer, had an open mind
on the subject, or even believed in the occurrence of evolutionary processes

of small scope. Isidore Geoffroy St Hilaire, the son of the great Etienne
Geoffroy St Hilaire, seems to have held that species might be formed from
varieties. The law which L. Agassiz thought he could establish,[349] of the
parallelism between palæontological succession, systematic rank, and
embryological development, tended to help the progress of evolutionary
ideas. J. V.. Carus, who afterwards became a supporter of Darwin, seems
already, in 1853, to have inferred from Agassiz's law the probability of
evolution.[350]
But no evolution theory was taken very seriously before 1859, when the
Origin of Species was published.
Like Lamarck, Charles Darwin was, neither by inclination nor by training, a
morphologist. In his youth he was a collector, a sportsman and a field
geologist. His voyage round the world on the Beagle aroused in him keen
interest in the problem of species—their variety, their variation according to
place and time, their adaptedness to environment. The conviction gradually
took possession of his mind that the puzzling facts of geographical range and
geological succession which he observed wherever he went were explicable
only on the hypothesis that species change. He was not satisfied with the
theories of evolution that had been proposed by his grandfather, by Lamarck,
and by E. Geoffroy St Hilaire—he did not indeed understand these theories
any too well. He resolved to work out the problem in his own way, for his
own satisfaction. He tells us all this very clearly in his autobiography.
"During the voyage of the Beagle I had been deeply impressed by
discovering in the Pampean formation great fossil animals covered with
armour like that on the existing armadillos; secondly, by the manner in
which closely allied animals replace one another in proceeding southwards
over the continent; and thirdly, by the South American character of most of
the productions of the Galapagos archipelago, and more especially by the
manner in which they differ slightly on each island of the group; some of the
islands appearing to be very ancient in a geological sense.
"It was evident that such facts as these, as well as many others, could only be
explained on the supposition that species gradually become modified; and
the subject haunted me. But it was equally evident that neither the action of
the surrounding conditions, nor the will of the organisms (especially in the
case of plants) could account for the innumerable cases in which organisms

of every kind are beautifully adapted to their habits of life—for instance, a
woodpecker or a tree-frog to climb trees, or a seed for dispersal by hooks or
plumes. I had always been much struck by such adaptations, and until these
could be explained it seemed to me almost useless to endeavour to prove by
indirect evidence that species have been modified."[351]
All Darwin's varied subsequent work revolved round these, for him,
essential problems—How do species change, and how do they become
adapted to their environment? He never ceased to be essentially a field
naturalist, and his theory of natural selection would have been an empty and
abstract thing if his vast knowledge and understanding of the "web of life"
had not given it colour and form. He never lost touch with the living thing in
its living, breathing reality—even plants he rightly regarded as active things,
full of tricks and contrivances for making their way in the world. No one
ever realised more vividly than he the delicacy and complexity of the
adaptations to environment which are the necessary condition of success in
the struggle for existence. Almost his greatest service to biology was that he
made biologists realise as they never did before the vast importance of
environment. He took biology into the open air, away from the museum and
the dissecting-room.
Naturally this attitude was not without its drawbacks. It led him to take only
a lukewarm interest in the problems of morphology. It is true he used the
facts of morphology with great effect as powerful arguments for evolution,
but it was not from such facts that he deduced his theory to account for
evolution. It is questionable indeed whether the theory of natural selection is
properly applicable to the problems of form. It was invented to account for
the evolution of specific differences and of ecological adaptations; it was not
primarily intended as an explanation of the more wonderful and more
mysterious facts of the convenance des parties and the interaction of
structure and function. Perhaps Darwin did not realise this inner aspect of
adaptation quite so vividly as he did the more superficial adaptation of
organisms to their environment. It was, perhaps, his lack of morphological
training and experience that led him to disregard the problems of form, or at
least to realise very insufficiently their difficulty.
It is in any case very significant that only a small part of his Origin of
Species is devoted to the discussion of morphological questions—only one

chapter out of the fourteen contained in the first edition.
Though the theory of natural selection took little account of the problems of
form, Darwin's masterly vindication of the theory of evolution was of
immense service to morphology, and Darwin himself was the first to point
out what a great light evolution threw upon all morphological problems. In a
few pages of the Origin he laid the foundations of evolutionary morphology.
We have here to consider his interpretation of morphological facts and its
relation to the current morphology of his time.

The sketch of his theory, written in 1842,[352] shows a very significant
division into two parts—the first dealing with the positive facts of variability
and the theory of natural selection, the second with the general evidence for
evolution. It is in the second part that the paragraphs on morphological
matters occur. In paragraph 7, on affinities and classification, Darwin points
out that on the theory of evolution homological relationship would be real
relationship, and the natural system would really be genealogical. In the next
paragraph he notes that evolution would account for the unity of type in the
great classes, for the metamorphosis of organs, and for the close
resemblance which early embryos show to one another. It is of special
interest to note that he definitely rejects the Meckel-Serres theory of
recapitulation. "It is not true," he writes, "that one passes through the form of
a lower group, though no doubt fish more nearly related to fœtal state"
(p. 42). The greater divergence which adults show seems to him to be due to
the fact that selection acts more on the later than on the embryonic stages.
He realises very clearly how illuminative the theory of evolution is when
applied to the puzzling facts of embryonic development. "The less
differences of fœtus—this has obvious meaning on this view: otherwise how
strange that a horse, a man, a bat should at one time of life have arteries,
running in a manner which is only intelligibly useful in a fish! The natural
system being on theory genealogical, we can at once see why fœtus,
retaining traces of the ancestral form, is of the highest value in
classification" (p. 45).
Abortive organs, too, gain significance on the evolutionary hypothesis. "The
affinity of different groups, the unity of types of structure, the representative
forms through which fœtus passes, the metamorphosis of organs, the

abortion of others, cease to be metaphorical expressions and become
intelligible facts" (p. 50).
In general, organisms can be understood only if we take into account the
cardinal fact that they are historical beings. "We must look at every
complicated mechanism and instinct as the summary of a long history of
useful contrivances much like a work of art" (p. 51).[353]
Already in 1842 Darwin had seized upon the main principles of evolutionary
morphology: the indications then given are elaborated in the thirteenth
chapter of the Origin of Species (1st ed., 1859). A good part of this chapter is
given up to a discussion of the principles of classification, only a few pages
dealing with morphology proper. But, as Darwin rightly saw, the two things
are inseparable.
We note first that there is no hint of the "scale of beings"—Darwin conceives
the genealogical tree as many branched. Animals can be classed in "groups
under groups," and cannot be arranged in one single series.
He discusses first what kind of characters have the greatest classificatory
value. Certain empirical rules have been recognised, more or less
consciously, by systematists—that analogical characters are less valuable
than homological, that characters of great physiological importance are not
always valuable for classificatory purposes, that rudimentary organs are
often very useful, and so on. He finds that as a general rule "the less any part
of the organisation is concerned with special habits, the more important it
becomes for classification" (p. 414), and adduces in support Owen's remark
that the generative organs afford very clear indications of affinities, since
they are unlikely to be modified by special habits. These rules of
classification can be explained "on the view that the natural system is
founded on descent with modification; that the characters which naturalists
consider as showing true affinity ... are those which have been inherited
from a common parent, and, in so far, all true classification is genealogical;
that community of descent is the hidden bond which naturalists have been
unconsciously seeking, and not some unknown plan of creation, or the
enunciation of general propositions, and the mere putting together and
separating objects more or less alike" (p. 420).

In general, then, homological characters are more valuable for classificatory
purposes because they have a longer pedigree than analogical characters,
which represent recent acquirements of the race.
Coming to morphology proper, Darwin takes up the question of the unity of
type, and the homology of parts, for which the unity of type is but a general
expression.
He treats this on the same lines as E. Geoffroy St Hilaire, and Owen,
referring indeed specifically to Geoffroy's law of connections. "What can be
more curious," he asks, "than that the hand of a man, formed for grasping,
that of a mole for digging, the leg of a horse, the paddle of the porpoise, and
the wing of the bat, should all be constructed on the same pattern, and should
include similar bones, in the same relative positions? Geoffroy St Hilaire has
strongly insisted on the high importance of relative position or connection in
homologous parts; they may differ to almost any extent in form and size, and
yet remain connected together in the same invariable order" (p. 434).
The unity of plan cannot be explained on teleological grounds, as Owen has
admitted in his Nature of Limbs, nor is it explicable on the hypothesis of
special creation (p. 435). It can be understood only on the theory that
animals are descended from one another and retain for innumerable
generations the essential organisation of their ancestors. "The explanation is
to a large extent simple on the theory of the selection of successive slight
modifications—each modification being profitable in some way to the
modified form, but often affecting by correlation other parts of the
organisation. In changes of this nature, there will be little or no tendency to
alter the original pattern or to transpose the parts.... If we suppose that the
ancient progenitor, the archetype as it may be called, of all animals, had its
limbs constructed on the existing general pattern, for whatever purpose they
served, we can at once perceive the plain significance of the homologous
construction of the limbs throughout the whole class" (p. 435).
We may note three important points in this passage—first, the identification
of the archetype with the common progenitor; second, the view that
progressive evolution is essentially adaptive, and dominated by natural
selection; and third, the petitio principii involved in the assumption that
adaptive modification brings inevitably in its train the necessary correlative
changes.

In his section on morphology Darwin shows clearly the influence of Owen,
and through him of the transcendental anatomists. He refers to the
transcendental idea of "metamorphosis," as exemplified in the vertebral
theory of the skull and the theory of the plant appendage, and shows how, on
the hypothesis of descent with modification, "metamorphosis" may now be
interpreted literally, and no longer figuratively merely (p. 439).
Very great interest attaches to Darwin's treatment of development, for post-
Darwinian morphology was based to a very large extent on the presumed
relation between the development of the individual and the evolution of the
race. Just as he kept clear of the notion of the scale of beings, so he avoided
the snare of the Meckel-Serres theory of recapitulation, according to which
the embryo of the highest animal, man, during its development climbs the
ladder upon the rungs of which the whole animal series is distributed, in its
gradual progression from simplicity to complexity. The law of development
which he adopts is that of von Baer, which states that development is
essentially differentiation, and that as a result embryos belonging to the same
group resemble one another the more the less advanced they are in
development. There can be little doubt that he was indebted to von Baer for
the idea, and in the later editions of the Origin he acknowledges this by
quoting the well-known passage in which von Baer tells how he had two
embryos in spirit which he was unable to refer definitely to their proper class
among Vertebrates.[354]
Not only are embryos more alike than adults, because less differentiated, but
it is in points not directly connected with the conditions of existence, not
strictly adaptive, that their resemblance is strongest (p. 440)—think, for
instance, of the arrangement of aortic arches common to all vertebrate
embryos. Larval forms are to some extent exceptions to this rule, for they are
often specially adapted to their particular mode of life, and convergence of
structure may accordingly result. All these facts require an explanation.
"How, then, can we explain these several facts in embryology—namely, the
very general, but not universal, difference in structure between the embryo
and the adult—of parts in the same individual embryo, which ultimately
become very unlike and serve for different purposes, being at this early
period of growth alike—of embryos of different species within the same
class, generally but not universally, resembling each other—of the structure
of the embryo not being closely related to its conditions of existence, except

when the embryo becomes at any period of life active and has to provide for
itself—of the embryo apparently having sometimes a higher organisation
than the mature animal, into which it is developed" (pp. 442-3). Obviously
all these facts are formally explained by the doctrine of descent. But Darwin
goes further, he tries to show exactly how it is that the embryos resemble one
another more than the adults. He thinks that the phenomenon results from
two principles—first, that modifications usually supervene late in the life of
the individual; and second, that such modifications tend to be inherited by
the offspring at a corresponding, not early, age (p. 444).
Thus, applying these principles to a hypothetical case of the origin of new
species of birds from a common stock, he writes:—"... from the many slight
successive steps of variation having supervened at a rather late age and
having been inherited at a corresponding age, the young of the new species
of our supposed genus will manifestly tend to resemble each other much
more closely than do the adults, just as we have seen in the case of pigeons"
[355]
(pp. 446-7).
Since the embryo shows the generalised type, the structure of the embryo is
useful for classificatory purposes. "For the embryo is the animal in its less
modified state; and in so far it reveals the structure of its progenitor" (p. 449)
—the embryological archetype reveals the ancestral form. "Embryology
rises greatly in interest, when we thus look at the embryo as a picture, more
or less complete, of the parent form of each great class of animals" (p. 450)
—a prophetic remark, in view of the enormous subsequent development of
phylogenetic speculation.
We may sum up by saying that Darwin interpreted von Baer's law
phylogenetically.
The rest of the chapter is devoted to a discussion of abortive and vestigial
organs, whose existence Darwin naturally turns to great advantage in his
argument for evolution. Throughout the whole chapter Darwin's
preoccupation with the problems of classification is clearly manifest.
On the question as to whether descent was monophyletic or polyphyletic
Darwin expressed no dogmatic opinion. "I believe that animals have
descended from at most only four or five progenitors, and plants from an
equal or lesser number.... I should infer from analogy that probably all the

organic beings which have ever lived on this earth have descended from one
primordial form, into which life was first breathed" (p. 484).
Darwin rightly laid much stress upon the morphological evidence for
evolution,[356] which he considered to be weighty. It probably contributed
greatly to the success of his theory. Though he himself did little or no work
in pure morphology, he was alive to the importance of such work,[357] and
followed with interest the progress of evolutionary morphology,
incorporating some of its results in later editions of the Origin, and in his
Descent of Man (1871).
In his morphology Darwin was hardly up to date. He does not seem to have
known at first hand the splendid work of the German morphologists, such as
Rathke and Reichert; he pays no attention to the cell-theory, nor to the germ-
layer theory. His sources are, in the main, Geoffroy St Hilaire, Owen, von
Baer, Agassiz, Milne-Edwards, and Huxley.
Perhaps his greatest omission was that he did not give any adequate
treatment of the problem of functional adaptation and the correlation of
parts. It is not too much to say that Darwin not only disregarded these
problems almost entirely, but by his insistence upon ecological adaptation
and upon certain superficial aspects of correlation, succeeded in giving to the
words "adaptation" and "correlation" a new signification, whereby they lost
to a large extent their true and original functional meaning.
It is true that Darwin himself, as well as his successors, believed that natural
selection was all-powerful to account for the evolution of the most
complicated organs, but it may be questioned whether he realised all the
conditions of the problem of which he thus easily disposed. He says, rightly,
in an important passage, that "It is generally acknowledged that all organic
beings have been formed on two great laws—Unity of Type, and the
Conditions of Existence. By unity of type is meant that fundamental
agreement in structure which we see in organic beings of the same class, and
which is quite independent of their habits of life. On my theory, unity of type
is explained by unity of descent. The expression of conditions of existence,
so often insisted upon by the illustrious Cuvier, is fully embraced by the
principle of natural selection. For natural selection acts by either now
adapting the varying parts of each being to its organic and inorganic
conditions of life:[358] or by having adapted them during past periods of time:

the adaptations being aided in many cases by the increased use or disuse of
parts, being affected by the direct action of the external conditions of life,
and subjected in all cases to the several laws of growth and variation. Hence,
in fact, the law of the Conditions of Existence is the higher law; as it
includes, through the inheritance of former variations and adaptations, that
of Unity of Type" (Origin, 6th ed., Pop. Impression, pp. 260-1). It is clear
that Darwin took the phrase "Conditions of Existence" to mean the
environmental conditions, and the law of the Conditions of Existence to
mean the law of adaptation to environment. But that is not what Cuvier
meant by the phrase: he understood by it the principle of the co-ordination of
the parts to form the whole, the essential condition for the existence of any
organism whatsoever (see above, Chap. III., p. 34).
Of this thought there is in Darwin little trace, and that is why he did not
sufficiently appreciate the weight of the argument brought against his theory
that it did not account for the correlation of variations.
Darwin's conception of correlation was singularly incomplete. As examples
of correlation he advanced such trivial cases as the relation between
albinism, deafness and blue eyes in cats, or between the tortoise-shell colour
and the female sex. He used the word only in connection with what he called
"correlated variation," meaning by this expression "that the whole
organisation is so tied together during its growth and development, that
when slight variations in any one part occur, and are accumulated through
natural selection, other parts become modified" (6th ed., p. 177). He took it
for granted that the "correlated variations" would be adapted to the original
variation which was acted upon by natural selection, and he saw no
difficulty in the gradual evolution of a complicated organ like the eye if only
the steps were small enough. "It has been objected," he writes, "that in order
to modify the eye and still preserve it as a perfect instrument, many changes
would have to be effected simultaneously, which, it is assumed, could not be
done through natural selection; but as I have attempted to show in my work
on the variation of domestic animals, it is not necessary to suppose that the
modifications were all simultaneous, if they were extremely slight and
gradual" (6th ed., p. 226).
In post-Darwinian speculation the difficulty of explaining correlated
variation by natural selection alone became more acutely realised, and it was

chiefly this difficulty that led Weismann to formulate his hypothesis of
germinal selection as a necessary supplement to the general selection theory.
The change in the conception of correlation which Darwin's influence
brought about has been very clearly stated by E. von Hartmann,[359] from
whom the following is taken:—"While the correlation of parts in the
organism was before Darwin regarded exclusively from the standpoint of
morphological systematics, Darwin tried to look at it from the standpoint of
physiological and genealogical development, and in so doing he put the
standpoint of morphological systematics in the shade. But the more we are
now beginning to realise that systematic relationship does not necessarily
imply genetic affinity the more must the correlation of parts come back into
favour as a systematic principle. While Darwin only, as it were, against his
will, relied on the law of correlation as a last resort when all other help
failed, this law must be regarded, from the standpoint of the orderly inner
determination of all organic form-change, as having the rank of the highest
principle of all, a principle which rules parallel, divergent and convergent
evolution" (pp. 47-8).
Further on, following Rádl, he characterises Darwin's attitude to the law of
correlation in these terms:—"Darwin's interest is entirely focussed on the
variation, the function, the causes of form-production, in short, upon
evolution. Accordingly he regards correlation essentially as correlative
variation in the sense of a departure from the given type. With
morphological correlation in different types Darwin troubles himself not at
all, nor with correlation in the normal development of a type" (p. 49).
Cuvier's conception of the convenance des parties, essential to all biology,
remained on the whole foreign to Darwin's thought, and to the thought of his
successors.
It was indeed one of their boasts that they had finally eliminated all
teleology from Nature. The great and immediate success which Darwinism
had among the younger generation of biologists and among scientific men in
general was due in large part to the fact that it fitted in well with the
prevailing materialism of the day, and gave solid ground for the hope that in
time a complete mechanistic explanation of life would be forthcoming.
"Darwinismus" became the battle-cry of the militant spirits of that time.