The Origin of Species by Means of Natural Selection Charles Darwin 7018 downloads.pdf

CHAPTER XII.
GEOGRAPHICAL DISTRIBUTION.

Present distribution cannot be accounted for by differences
in physical conditions—Importance of barriers—Affinity of
the productions of the same continent—Centres of creation
—Means of dispersal by changes of climate and of the level
of the land, and by occasional means—Dispersal during the
Glacial period—Alternate Glacial periods in the North and
South.

In considering the distribution of organic beings over the face of the
globe, the first great fact which strikes us is, that neither the similarity nor
the dissimilarity of the inhabitants of various regions can be wholly
accounted for by climatal and other physical conditions. Of late, almost
every author who has studied the subject has come to this conclusion. The
case of America alone would almost suffice to prove its truth; for if we
exclude the arctic and northern temperate parts, all authors agree that one of
the most fundamental divisions in geographical distribution is that between
the New and Old Worlds; yet if we travel over the vast American continent,
from the central parts of the United States to its extreme southern point, we
meet with the most diversified conditions; humid districts, arid deserts,
lofty mountains, grassy plains, forests, marshes, lakes and great rivers,
under almost every temperature. There is hardly a climate or condition in
the Old World which cannot be paralleled in the New—at least so closely as
the same species generally require. No doubt small areas can be pointed out
in the Old World hotter than any in the New World; but these are not
inhabited by a fauna different from that of the surrounding districts; for it is
rare to find a group of organisms confined to a small area, of which the
conditions are peculiar in only a slight degree. Notwithstanding this general

parallelism in the conditions of Old and New Worlds, how widely different
are their living productions!
In the southern hemisphere, if we compare large tracts of land in
Australia, South Africa, and western South America, between latitudes 25°
and 35°, we shall find parts extremely similar in all their conditions, yet it
would not be possible to point out three faunas and floras more utterly
dissimilar. Or, again, we may compare the productions of South America
south of latitude 35° with those north of 25°, which consequently are
separated by a space of ten degrees of latitude, and are exposed to
considerably different conditions; yet they are incomparably more closely
related to each other than they are to the productions of Australia or Africa
under nearly the same climate. Analogous facts could be given with respect
to the inhabitants of the sea.
A second great fact which strikes us in our general review is, that barriers
of any kind, or obstacles to free migration, are related in a close and
important manner to the differences between the productions of various
regions. We see this in the great difference in nearly all the terrestrial
productions of the New and Old Worlds, excepting in the northern parts,
where the land almost joins, and where, under a slightly different climate,
there might have been free migration for the northern temperate forms, as
there now is for the strictly arctic productions. We see the same fact in the
great difference between the inhabitants of Australia, Africa, and South
America under the same latitude; for these countries are almost as much
isolated from each other as is possible. On each continent, also, we see the
same fact; for on the opposite sides of lofty and continuous mountain-
ranges, and of great deserts and even of large rivers, we find different
productions; though as mountain chains, deserts, &c., are not as impassable,
or likely to have endured so long, as the oceans separating continents, the
differences are very inferior in degree to those characteristic of distinct
continents.
Turning to the sea, we find the same law. The marine inhabitants of the
eastern and western shores of South America are very distinct, with
extremely few shells, crustacea, or echinodermata in common; but Dr.
Günther has recently shown that about thirty per cent of the fishes are the
same on the opposite sides of the isthmus of Panama; and this fact has led
naturalists to believe that the isthmus was formerly open. Westward of the

shores of America, a wide space of open ocean extends, with not an island
as a halting-place for emigrants; here we have a barrier of another kind, and
as soon as this is passed we meet in the eastern islands of the Pacific with
another and totally distinct fauna. So that three marine faunas range
northward and southward in parallel lines not far from each other, under
corresponding climate; but from being separated from each other by
impassable barriers, either of land or open sea, they are almost wholly
distinct. On the other hand, proceeding still farther westward from the
eastern islands of the tropical parts of the Pacific, we encounter no
impassable barriers, and we have innumerable islands as halting-places, or
continuous coasts, until, after travelling over a hemisphere, we come to the
shores of Africa; and over this vast space we meet with no well-defined and
distinct marine faunas. Although so few marine animals are common to the
above-named three approximate faunas of Eastern and Western America
and the eastern Pacific islands, yet many fishes range from the Pacific into
the Indian Ocean, and many shells are common to the eastern islands of the
Pacific and the eastern shores of Africa on almost exactly opposite
meridians of longitude.
A third great fact, partly included in the foregoing statement, is the
affinity of the productions of the same continent or of the same sea, though
the species themselves are distinct at different points and stations. It is a law
of the widest generality, and every continent offers innumerable instances.
Nevertheless, the naturalist, in travelling, for instance, from north to south,
never fails to be struck by the manner in which successive groups of beings,
specifically distinct, though nearly related, replace each other. He hears
from closely allied, yet distinct kinds of birds, notes nearly similar, and sees
their nests similarly constructed, but not quite alike, with eggs coloured in
nearly the same manner. The plains near the Straits of Magellan are
inhabited by one species of Rhea (American ostrich), and northward the
plains of La Plata by another species of the same genus; and not by a true
ostrich or emu, like those inhabiting Africa and Australia under the same
latitude. On these same plains of La Plata we see the agouti and bizcacha,
animals having nearly the same habits as our hares and rabbits, and
belonging to the same order of Rodents, but they plainly display an
American type of structure. We ascend the lofty peaks of the Cordillera, and
we find an alpine species of bizcacha; we look to the waters, and we do not
find the beaver or muskrat, but the coypu and capybara, rodents of the

South American type. Innumerable other instances could be given. If we
look to the islands off the American shore, however much they may differ
in geological structure, the inhabitants are essentially American, though
they may be all peculiar species. We may look back to past ages, as shown
in the last chapter, and we find American types then prevailing on the
American continent and in the American seas. We see in these facts some
deep organic bond, throughout space and time, over the same areas of land
and water, independently of physical conditions. The naturalist must be dull
who is not led to inquire what this bond is.
The bond is simply inheritance, that cause which alone, as far as we
positively know, produces organisms quite like each other, or, as we see in
the case of varieties, nearly alike. The dissimilarity of the inhabitants of
different regions may be attributed to modification through variation and
natural selection, and probably in a subordinate degree to the definite
influence of different physical conditions. The degrees of dissimilarity will
depend on the migration of the more dominant forms of life from one
region into another having been more or less effectually prevented, at
periods more or less remote—on the nature and number of the former
immigrants—and on the action of the inhabitants on each other in leading to
the preservation of different modifications; the relation of organism to
organism in the struggle for life being, as I have already often remarked, the
most important of all relations. Thus the high importance of barriers comes
into play by checking migration; as does time for the slow process of
modification through natural selection. Widely-ranging species, abounding
in individuals, which have already triumphed over many competitors in
their own widely-extended homes, will have the best chance of seizing on
new places, when they spread out into new countries. In their new homes
they will be exposed to new conditions, and will frequently undergo further
modification and improvement; and thus they will become still further
victorious, and will produce groups of modified descendants. On this
principle of inheritance with modification we can understand how it is that
sections of genera, whole genera, and even families, are confined to the
same areas, as is so commonly and notoriously the case.
There is no evidence, as was remarked in the last chapter, of the existence
of any law of necessary development. As the variability of each species is
an independent property, and will be taken advantage of by natural
selection, only so far as it profits each individual in its complex struggle for

life, so the amount of modification in different species will be no uniform
quantity. If a number of species, after having long competed with each other
in their old home, were to migrate in a body into a new and afterwards
isolated country, they would be little liable to modification; for neither
migration nor isolation in themselves effect anything. These principles
come into play only by bringing organisms into new relations with each
other and in a lesser degree with the surrounding physical conditions. As we
have seen in the last chapter that some forms have retained nearly the same
character from an enormously remote geological period, so certain species
have migrated over vast spaces, and have not become greatly or at all
modified.
According to these views, it is obvious that the several species of the
same genus, though inhabiting the most distant quarters of the world, must
originally have proceeded from the same source, as they are descended
from the same progenitor. In the case of those species which have
undergone, during whole geological periods, little modification, there is not
much difficulty in believing that they have migrated from the same region;
for during the vast geographical and climatical changes which have
supervened since ancient times, almost any amount of migration is possible.
But in many other cases, in which we have reason to believe that the species
of a genus have been produced within comparatively recent times, there is
great difficulty on this head. It is also obvious that the individuals of the
same species, though now inhabiting distant and isolated regions, must have
proceeded from one spot, where their parents were first produced: for, as
has been explained, it is incredible that individuals identically the same
should have been produced from parents specifically distinct.
Single Centres of supposed Creation.—We are thus brought to the
question which has been largely discussed by naturalists, namely, whether
species have been created at one or more points of the earth’s surface.
Undoubtedly there are many cases of extreme difficulty in understanding
how the same species could possibly have migrated from some one point to
the several distant and isolated points, where now found. Nevertheless the
simplicity of the view that each species was first produced within a single
region captivates the mind. He who rejects it, rejects the vera causa of
ordinary generation with subsequent migration, and calls in the agency of a
miracle. It is universally admitted, that in most cases the area inhabited by a
species is continuous; and that when a plant or animal inhabits two points

so distant from each other, or with an interval of such a nature, that the
space could not have been easily passed over by migration, the fact is given
as something remarkable and exceptional. The incapacity of migrating
across a wide sea is more clear in the case of terrestrial mammals than
perhaps with any other organic beings; and, accordingly, we find no
inexplicable instances of the same mammals inhabiting distant points of the
world. No geologist feels any difficulty in Great Britain possessing the
same quadrupeds with the rest of Europe, for they were no doubt once
united. But if the same species can be produced at two separate points, why
do we not find a single mammal common to Europe and Australia or South
America? The conditions of life are nearly the same, so that a multitude of
European animals and plants have become naturalised in America and
Australia; and some of the aboriginal plants are identically the same at these
distant points of the northern and southern hemispheres? The answer, as I
believe, is, that mammals have not been able to migrate, whereas some
plants, from their varied means of dispersal, have migrated across the wide
and broken interspaces. The great and striking influence of barriers of all
kinds, is intelligible only on the view that the great majority of species have
been produced on one side, and have not been able to migrate to the
opposite side. Some few families, many subfamilies, very many genera, a
still greater number of sections of genera, are confined to a single region;
and it has been observed by several naturalists that the most natural genera,
or those genera in which the species are most closely related to each other,
are generally confined to the same country, or if they have a wide range that
their range is continuous. What a strange anomaly it would be if a directly
opposite rule were to prevail when we go down one step lower in the series,
namely to the individuals of the same species, and these had not been, at
least at first, confined to some one region!
Hence, it seems to me, as it has to many other naturalists, that the view of
each species having been produced in one area alone, and having
subsequently migrated from that area as far as its powers of migration and
subsistence under past and present conditions permitted, is the most
probable. Undoubtedly many cases occur in which we cannot explain how
the same species could have passed from one point to the other. But the
geographical and climatical changes which have certainly occurred within
recent geological times, must have rendered discontinuous the formerly
continuous range of many species. So that we are reduced to consider

whether the exceptions to continuity of range are so numerous, and of so
grave a nature, that we ought to give up the belief, rendered probable by
general considerations, that each species has been produced within one
area, and has migrated thence as far as it could. It would be hopelessly
tedious to discuss all the exceptional cases of the same species, now living
at distant and separated points; nor do I for a moment pretend that any
explanation could be offered of many instances. But, after some preliminary
remarks, I will discuss a few of the most striking classes of facts, namely,
the existence of the same species on the summits of distant mountain
ranges, and at distant points in the Arctic and Antarctic regions; and
secondly (in the following chapter), the wide distribution of fresh water
productions; and thirdly, the occurrence of the same terrestrial species on
islands and on the nearest mainland, though separated by hundreds of miles
of open sea. If the existence of the same species at distant and isolated
points of the earth’s surface can in many instances be explained on the view
of each species having migrated from a single birthplace; then, considering
our ignorance with respect to former climatical and geographical changes,
and to the various occasional means of transport, the belief that a single
birthplace is the law seems to me incomparably the safest.
In discussing this subject we shall be enabled at the same time to
consider a point equally important for us, namely, whether the several
species of a genus which must on our theory all be descended from a
common progenitor, can have migrated, undergoing modification during
their migration from some one area. If, when most of the species inhabiting
one region are different from those of another region, though closely allied
to them, it can be shown that migration from the one region to the other has
probably occurred at some former period, our general view will be much
strengthened; for the explanation is obvious on the principle of descent with
modification. A volcanic island, for instance, upheaved and formed at the
distance of a few hundreds of miles from a continent, would probably
receive from it in the course of time a few colonists, and their descendants,
though modified, would still be related by inheritance to the inhabitants of
that continent. Cases of this nature are common, and are, as we shall
hereafter see, inexplicable on the theory of independent creation. This view
of the relation of the species of one region to those of another, does not
differ much from that advanced by Mr. Wallace, who concludes that “every
species has come into existence coincident both in space and time with a

pre-existing closely allied species.” And it is now well known that he
attributes this coincidence to descent with modification.
The question of single or multiple centres of creation differs from another
though allied question, namely, whether all the individuals of the same
species are descended from a single pair, or single hermaphrodite, or
whether, as some authors suppose, from many individuals simultaneously
created. With organic beings which never intercross, if such exist, each
species, must be descended from a succession of modified varieties, that
have supplanted each other, but have never blended with other individuals
or varieties of the same species, so that, at each successive stage of
modification, all the individuals of the same form will be descended from a
single parent. But in the great majority of cases, namely, with all organisms
which habitually unite for each birth, or which occasionally intercross, the
individuals of the same species inhabiting the same area will be kept nearly
uniform by intercrossing; so that many individuals will go on
simultaneously changing, and the whole amount of modification at each
stage will not be due to descent from a single parent. To illustrate what I
mean: our English race-horses differ from the horses of every other breed;
but they do not owe their difference and superiority to descent from any
single pair, but to continued care in the selecting and training of many
individuals during each generation.
Before discussing the three classes of facts, which I have selected as
presenting the greatest amount of difficulty on the theory of “single centres
of creation,” I must say a few words on the means of dispersal.

Means of Dispersal.

Sir C. Lyell and other authors have ably treated this subject. I can give
here only the briefest abstract of the more important facts. Change of
climate must have had a powerful influence on migration. A region now
impassable to certain organisms from the nature of its climate, might have
been a high road for migration, when the climate was different. I shall,
however, presently have to discuss this branch of the subject in some detail.
Changes of level in the land must also have been highly influential: a
narrow isthmus now separates two marine faunas; submerge it, or let it
formerly have been submerged, and the two faunas will now blend together,
or may formerly have blended. Where the sea now extends, land may at a

former period have connected islands or possibly even continents together,
and thus have allowed terrestrial productions to pass from one to the other.
No geologist disputes that great mutations of level have occurred within the
period of existing organisms. Edward Forbes insisted that all the islands in
the Atlantic must have been recently connected with Europe or Africa, and
Europe likewise with America. Other authors have thus hypothetically
bridged over every ocean, and united almost every island with some
mainland. If, indeed, the arguments used by Forbes are to be trusted, it must
be admitted that scarcely a single island exists which has not recently been
united to some continent. This view cuts the Gordian knot of the dispersal
of the same species to the most distant points, and removes many a
difficulty; but to the best of my judgment we are not authorized in admitting
such enormous geographical changes within the period of existing species.
It seems to me that we have abundant evidence of great oscillations in the
level of the land or sea; but not of such vast changes in the position and
extension of our continents, as to have united them within the recent period
to each other and to the several intervening oceanic islands. I freely admit
the former existence of many islands, now buried beneath the sea, which
may have served as halting-places for plants and for many animals during
their migration. In the coral-producing oceans such sunken islands are now
marked by rings of coral or atolls standing over them. Whenever it is fully
admitted, as it will some day be, that each species has proceeded from a
single birthplace, and when in the course of time we know something
definite about the means of distribution, we shall be enabled to speculate
with security on the former extension of the land. But I do not believe that it
will ever be proved that within the recent period most of our continents
which now stand quite separate, have been continuously, or almost
continuously united with each other, and with the many existing oceanic
islands. Several facts in distribution—such as the great difference in the
marine faunas on the opposite sides of almost every continent—the close
relation of the tertiary inhabitants of several lands and even seas to their
present inhabitants—the degree of affinity between the mammals inhabiting
islands with those of the nearest continent, being in part determined (as we
shall hereafter see) by the depth of the intervening ocean—these and other
such facts are opposed to the admission of such prodigious geographical
revolutions within the recent period, as are necessary on the view advanced
by Forbes and admitted by his followers. The nature and relative

proportions of the inhabitants of oceanic islands are likewise opposed to the
belief of their former continuity of continents. Nor does the almost
universally volcanic composition of such islands favour the admission that
they are the wrecks of sunken continents; if they had originally existed as
continental mountain ranges, some at least of the islands would have been
formed, like other mountain summits, of granite, metamorphic schists, old
fossiliferous and other rocks, instead of consisting of mere piles of volcanic
matter.
I must now say a few words on what are called accidental means, but
which more properly should be called occasional means of distribution. I
shall here confine myself to plants. In botanical works, this or that plant is
often stated to be ill adapted for wide dissemination; but the greater or less
facilities for transport across the sea may be said to be almost wholly
unknown. Until I tried, with Mr. Berkeley’s aid, a few experiments, it was
not even known how far seeds could resist the injurious action of sea-water.
To my surprise I found that out of eighty-seven kinds, sixty-four germinated
after an immersion of twenty-eight days, and a few survived an immersion
of 137 days. It deserves notice that certain orders were far more injured
than others: nine Leguminosæ were tried, and, with one exception, they
resisted the salt-water badly; seven species of the allied orders,
Hydrophyllaceæ and Polemoniaceæ, were all killed by a month’s
immersion. For convenience’ sake I chiefly tried small seeds without the
capsules or fruit; and as all of these sank in a few days, they could not have
been floated across wide spaces of the sea, whether or not they were injured
by salt water. Afterwards I tried some larger fruits, capsules, &c., and some
of these floated for a long time. It is well known what a difference there is
in the buoyancy of green and seasoned timber; and it occurred to me that
floods would often wash into the sea dried plants or branches with seed-
capsules or fruit attached to them. Hence I was led to dry the stems and
branches of ninety-four plants with ripe fruit, and to place them on sea-
water. The majority sank quickly, but some which, whilst green, floated for
a very short time, when dried floated much longer; for instance, ripe hazel-
nuts sank immediately, but when dried they floated for ninety days, and
afterwards when planted germinated; an asparagus plant with ripe berries
floated for twenty-three days, when dried it floated for eighty-five days, and
the seeds afterwards germinated: the ripe seeds of Helosciadium sank in
two days, when dried they floated for above ninety days, and afterwards

germinated. Altogether, out of the ninety-four dried plants, eighteen floated
for above twenty-eight days; and some of the eighteen floated for a very
much longer period. So that as 64/87 kinds of seeds germinated after an
immersion of twenty-eight days; and as 18/94 distinct species with ripe fruit
(but not all the same species as in the foregoing experiment) floated, after
being dried, for above twenty-eight days, we may conclude, as far as
anything can be inferred from these scanty facts, that the seeds of 14/100
kinds of plants of any country might be floated by sea-currents during
twenty-eight days, and would retain their power of germination. In
Johnston’s Physical Atlas, the average rate of the several Atlantic currents is
thirty-three miles per diem (some currents running at the rate of sixty miles
per diem); on this average, the seeds of 14/100 plants belonging to one
country might be floated across 924 miles of sea to another country; and
when stranded, if blown by an inland gale to a favourable spot, would
germinate.
Subsequently to my experiments, M. Martens tried similar ones, but in a
much better manner, for he placed the seeds in a box in the actual sea, so
that they were alternately wet and exposed to the air like really floating
plants. He tried ninety-eight seeds, mostly different from mine, but he chose
many large fruits, and likewise seeds, from plants which live near the sea;
and this would have favoured both the average length of their flotation and
their resistance to the injurious action of the salt-water. On the other hand,
he did not previously dry the plants or branches with the fruit; and this, as
we have seen, would have caused some of them to have floated much
longer. The result was that 18/98 of his seeds of different kinds floated for
forty-two days, and were then capable of germination. But I do not doubt
that plants exposed to the waves would float for a less time than those
protected from violent movement as in our experiments. Therefore, it would
perhaps be safer to assume that the seeds of about 10/100 plants of a flora,
after having been dried, could be floated across a space of sea 900 miles in
width, and would then germinate. The fact of the larger fruits often floating
longer than the small, is interesting; as plants with large seeds or fruit
which, as Alph. de Candolle has shown, generally have restricted ranges,
could hardly be transported by any other means.
Seeds may be occasionally transported in another manner. Drift timber is
thrown up on most islands, even on those in the midst of the widest oceans;
and the natives of the coral islands in the Pacific procure stones for their

tools, solely from the roots of drifted trees, these stones being a valuable
royal tax. I find that when irregularly shaped stones are embedded in the
roots of trees, small parcels of earth are very frequently enclosed in their
interstices and behind them, so perfectly that not a particle could be washed
away during the longest transport: out of one small portion of earth thus
completely enclosed by the roots of an oak about fifty years old, three
dicotyledonous plants germinated: I am certain of the accuracy of this
observation. Again, I can show that the carcasses of birds, when floating on
the sea, sometimes escape being immediately devoured; and many kinds of
seeds in the crops of floating birds long retain their vitality: peas and
vetches, for instance, are killed by even a few days’ immersion in sea-water;
but some taken out of the crop of a pigeon, which had floated on artificial
sea-water for thirty days, to my surprise nearly all germinated.
Living birds can hardly fail to be highly effective agents in the
transportation of seeds. I could give many facts showing how frequently
birds of many kinds are blown by gales to vast distances across the ocean.
We may safely assume that under such circumstances their rate of flight
would often be thirty-five miles an hour; and some authors have given a far
higher estimate. I have never seen an instance of nutritious seeds passing
through the intestines of a bird; but hard seeds of fruit pass uninjured
through even the digestive organs of a turkey. In the course of two months, I
picked up in my garden twelve kinds of seeds, out of the excrement of small
birds, and these seemed perfect, and some of them, which were tried,
germinated. But the following fact is more important: the crops of birds do
not secrete gastric juice, and do not, as I know by trial, injure in the least the
germination of seeds; now, after a bird has found and devoured a large
supply of food, it is positively asserted that all the grains do not pass into
the gizzard for twelve or even eighteen hours. A bird in this interval might
easily be blown to the distance of five hundred miles, and hawks are known
to look out for tired birds, and the contents of their torn crops might thus
readily get scattered. Some hawks and owls bolt their prey whole, and after
an interval of from twelve to twenty hours, disgorge pellets, which, as I
know from experiments made in the Zoological Gardens, include seeds
capable of germination. Some seeds of the oat, wheat, millet, canary, hemp,
clover, and beet germinated after having been from twelve to twenty-one
hours in the stomachs of different birds of prey; and two seeds of beet grew
after having been thus retained for two days and fourteen hours. Fresh-

water fish, I find, eat seeds of many land and water plants; fish are
frequently devoured by birds, and thus the seeds might be transported from
place to place. I forced many kinds of seeds into the stomachs of dead fish,
and then gave their bodies to fishing-eagles, storks, and pelicans; these
birds, after an interval of many hours, either rejected the seeds in pellets or
passed them in their excrement; and several of these seeds retained the
power of germination. Certain seeds, however, were always killed by this
process.
Locusts are sometimes blown to great distances from the land. I myself
caught one 370 miles from the coast of Africa, and have heard of others
caught at greater distances. The Rev. R.T. Lowe informed Sir C. Lyell that
in November, 1844, swarms of locusts visited the island of Madeira. They
were in countless numbers, as thick as the flakes of snow in the heaviest
snowstorm, and extended upward as far as could be seen with a telescope.
During two or three days they slowly careered round and round in an
immense ellipse, at least five or six miles in diameter, and at night alighted
on the taller trees, which were completely coated with them. They then
disappeared over the sea, as suddenly as they had appeared, and have not
since visited the island. Now, in parts of Natal it is believed by some
farmers, though on insufficient evidence, that injurious seeds are introduced
into their grass-land in the dung left by the great flights of locusts which
often visit that country. In consequence of this belief Mr. Weale sent me in a
letter a small packet of the dried pellets, out of which I extracted under the
microscope several seeds, and raised from them seven grass plants,
belonging to two species, of two genera. Hence a swarm of locusts, such as
that which visited Madeira, might readily be the means of introducing
several kinds of plants into an island lying far from the mainland.
Although the beaks and feet of birds are generally clean, earth sometimes
adheres to them: in one case I removed sixty-one grains, and in another case
twenty-two grains of dry argillaceous earth from the foot of a partridge, and
in the earth there was a pebble as large as the seed of a vetch. Here is a
better case: the leg of a woodcock was sent to me by a friend, with a little
cake of dry earth attached to the shank, weighing only nine grains; and this
contained a seed of the toad-rush (Juncus bufonius) which germinated and
flowered. Mr. Swaysland, of Brighton, who during the last forty years has
paid close attention to our migratory birds, informs me that he has often
shot wagtails (Motacillæ), wheatears, and whinchats (Saxicolæ), on their

first arrival on our shores, before they had alighted; and he has several
times noticed little cakes of earth attached to their feet. Many facts could be
given showing how generally soil is charged with seeds. For instance,
Professor Newton sent me the leg of a red-legged partridge (Caccabis rufa)
which had been wounded and could not fly, with a ball of hard earth
adhering to it, and weighing six and a half ounces. The earth had been kept
for three years, but when broken, watered and placed under a bell glass, no
less than eighty-two plants sprung from it: these consisted of twelve
monocotyledons, including the common oat, and at least one kind of grass,
and of seventy dicotyledons, which consisted, judging from the young
leaves, of at least three distinct species. With such facts before us, can we
doubt that the many birds which are annually blown by gales across great
spaces of ocean, and which annually migrate—for instance, the millions of
quails across the Mediterranean—must occasionally transport a few seeds
embedded in dirt adhering to their feet or beaks? But I shall have to recur to
this subject.
As icebergs are known to be sometimes loaded with earth and stones, and
have even carried brushwood, bones, and the nest of a land-bird, it can
hardly be doubted that they must occasionally, as suggested by Lyell, have
transported seeds from one part to another of the arctic and antarctic
regions; and during the Glacial period from one part of the now temperate
regions to another. In the Azores, from the large number of plants common
to Europe, in comparison with the species on the other islands of the
Atlantic, which stand nearer to the mainland, and (as remarked by Mr. H.C.
Watson) from their somewhat northern character, in comparison with the
latitude, I suspected that these islands had been partly stocked by ice-borne
seeds during the Glacial epoch. At my request Sir C. Lyell wrote to M.
Hartung to inquire whether he had observed erratic boulders on these
islands, and he answered that he had found large fragments of granite and
other rocks, which do not occur in the archipelago. Hence we may safely
infer that icebergs formerly landed their rocky burdens on the shores of
these mid-ocean islands, and it is at least possible that they may have
brought thither the seeds of northern plants.
Considering that these several means of transport, and that other means,
which without doubt remain to be discovered, have been in action year after
year for tens of thousands of years, it would, I think, be a marvellous fact if
many plants had not thus become widely transported. These means of

transport are sometimes called accidental, but this is not strictly correct: the
currents of the sea are not accidental, nor is the direction of prevalent gales
of wind. It should be observed that scarcely any means of transport would
carry seeds for very great distances; for seeds do not retain their vitality
when exposed for a great length of time to the action of sea water; nor could
they be long carried in the crops or intestines of birds. These means,
however, would suffice for occasional transport across tracts of sea some
hundred miles in breadth, or from island to island, or from a continent to a
neighbouring island, but not from one distant continent to another. The
floras of distant continents would not by such means become mingled; but
would remain as distinct as they now are. The currents, from their course,
would never bring seeds from North America to Britain, though they might
and do bring seeds from the West Indies to our western shores, where, if not
killed by their very long immersion in salt water, they could not endure our
climate. Almost every year, one or two land-birds are blown across the
whole Atlantic Ocean, from North America to the western shores of Ireland
and England; but seeds could be transported by these rare wanderers only
by one means, namely, by dirt adhering to their feet or beaks, which is in
itself a rare accident. Even in this case, how small would be the chance of a
seed falling on favourable soil, and coming to maturity! But it would be a
great error to argue that because a well-stocked island, like Great Britain,
has not, as far as is known (and it would be very difficult to prove this),
received within the last few centuries, through occasional means of
transport, immigrants from Europe or any other continent, that a poorly-
stocked island, though standing more remote from the mainland, would not
receive colonists by similar means. Out of a hundred kinds of seeds or
animals transported to an island, even if far less well-stocked than Britain,
perhaps not more than one would be so well fitted to its new home, as to
become naturalised. But this is no valid argument against what would be
effected by occasional means of transport, during the long lapse of
geological time, whilst the island was being upheaved, and before it had
become fully stocked with inhabitants. On almost bare land, with few or no
destructive insects or birds living there, nearly every seed which chanced to
arrive, if fitted for the climate, would germinate and survive.

Dispersal during the Glacial Period.

The identity of many plants and animals, on mountain-summits,
separated from each other by hundreds of miles of lowlands, where Alpine
species could not possibly exist, is one of the most striking cases known of
the same species living at distant points, without the apparent possibility of
their having migrated from one point to the other. It is indeed a remarkable
fact to see so many plants of the same species living on the snowy regions
of the Alps or Pyrenees, and in the extreme northern parts of Europe; but it
is far more remarkable, that the plants on the White Mountains, in the
United States of America, are all the same with those of Labrador, and
nearly all the same, as we hear from Asa Gray, with those on the loftiest
mountains of Europe. Even as long ago as 1747, such facts led Gmelin to
conclude that the same species must have been independently created at
many distinct points; and we might have remained in this same belief, had
not Agassiz and others called vivid attention to the Glacial period, which, as
we shall immediately see, affords a simple explanation of these facts. We
have evidence of almost every conceivable kind, organic and inorganic,
that, within a very recent geological period, central Europe and North
America suffered under an Arctic climate. The ruins of a house burnt by fire
do not tell their tale more plainly than do the mountains of Scotland and
Wales, with their scored flanks, polished surfaces, and perched boulders, of
the icy streams with which their valleys were lately filled. So greatly has
the climate of Europe changed, that in Northern Italy, gigantic moraines,
left by old glaciers, are now clothed by the vine and maize. Throughout a
large part of the United States, erratic boulders and scored rocks plainly
reveal a former cold period.
The former influence of the glacial climate on the distribution of the
inhabitants of Europe, as explained by Edward Forbes, is substantially as
follows. But we shall follow the changes more readily, by supposing a new
glacial period slowly to come on, and then pass away, as formerly occurred.
As the cold came on, and as each more southern zone became fitted for the
inhabitants of the north, these would take the places of the former
inhabitants of the temperate regions. The latter, at the same time would
travel further and further southward, unless they were stopped by barriers,
in which case they would perish. The mountains would become covered
with snow and ice, and their former Alpine inhabitants would descend to the
plains. By the time that the cold had reached its maximum, we should have
an arctic fauna and flora, covering the central parts of Europe, as far south

as the Alps and Pyrenees, and even stretching into Spain. The now
temperate regions of the United States would likewise be covered by arctic
plants and animals and these would be nearly the same with those of
Europe; for the present circumpolar inhabitants, which we suppose to have
everywhere travelled southward, are remarkably uniform round the world.
As the warmth returned, the arctic forms would retreat northward, closely
followed up in their retreat by the productions of the more temperate
regions. And as the snow melted from the bases of the mountains, the arctic
forms would seize on the cleared and thawed ground, always ascending, as
the warmth increased and the snow still further disappeared, higher and
higher, whilst their brethren were pursuing their northern journey. Hence,
when the warmth had fully returned, the same species, which had lately
lived together on the European and North American lowlands, would again
be found in the arctic regions of the Old and New Worlds, and on many
isolated mountain-summits far distant from each other.
Thus we can understand the identity of many plants at points so
immensely remote as the mountains of the United States and those of
Europe. We can thus also understand the fact that the Alpine plants of each
mountain-range are more especially related to the arctic forms living due
north or nearly due north of them: for the first migration when the cold
came on, and the re-migration on the returning warmth, would generally
have been due south and north. The Alpine plants, for example, of Scotland,
as remarked by Mr. H.C. Watson, and those of the Pyrenees, as remarked by
Ramond, are more especially allied to the plants of northern Scandinavia;
those of the United States to Labrador; those of the mountains of Siberia to
the arctic regions of that country. These views, grounded as they are on the
perfectly well-ascertained occurrence of a former Glacial period, seem to
me to explain in so satisfactory a manner the present distribution of the
Alpine and Arctic productions of Europe and America, that when in other
regions we find the same species on distant mountain-summits, we may
almost conclude, without other evidence, that a colder climate formerly
permitted their migration across the intervening lowlands, now become too
warm for their existence.
As the arctic forms moved first southward and afterwards backward to
the north, in unison with the changing climate, they will not have been
exposed during their long migrations to any great diversity of temperature;

and as they all migrated in a body together, their mutual relations will not
have been much disturbed. Hence, in accordance with the principles
inculcated in this volume, these forms will not have been liable to much
modification. But with the Alpine productions, left isolated from the
moment of the returning warmth, first at the bases and ultimately on the
summits of the mountains, the case will have been somewhat different; for
it is not likely that all the same arctic species will have been left on
mountain ranges far distant from each other, and have survived there ever
since; they will also, in all probability, have become mingled with ancient
Alpine species, which must have existed on the mountains before the
commencement of the Glacial epoch, and which during the coldest period
will have been temporarily driven down to the plains; they will, also, have
been subsequently exposed to somewhat different climatical influences.
Their mutual relations will thus have been in some degree disturbed;
consequently they will have been liable to modification; and they have been
modified; for if we compare the present Alpine plants and animals of the
several great European mountain ranges, one with another, though many of
the species remain identically the same, some exist as varieties, some as
doubtful forms or sub-species and some as distinct yet closely allied species
representing each other on the several ranges.
In the foregoing illustration, I have assumed that at the commencement
of our imaginary Glacial period, the arctic productions were as uniform
round the polar regions as they are at the present day. But it is also
necessary to assume that many sub-arctic and some few temperate forms
were the same round the world, for some of the species which now exist on
the lower mountain slopes and on the plains of North America and Europe
are the same; and it may be asked how I account for this degree of
uniformity of the sub-arctic and temperate forms round the world, at the
commencement of the real Glacial period. At the present day, the sub-arctic
and northern temperate productions of the Old and New Worlds are
separated from each other by the whole Atlantic Ocean and by the northern
part of the Pacific. During the Glacial period, when the inhabitants of the
Old and New Worlds lived further southwards than they do at present, they
must have been still more completely separated from each other by wider
spaces of ocean; so that it may well be asked how the same species could
then or previously have entered the two continents. The explanation, I
believe, lies in the nature of the climate before the commencement of the

Glacial period. At this, the newer Pliocene period, the majority of the
inhabitants of the world were specifically the same as now, and we have
good reason to believe that the climate was warmer than at the present day.
Hence, we may suppose that the organisms which now live under latitude
60°, lived during the Pliocene period further north, under the Polar Circle,
in latitude 66°–67°; and that the present arctic productions then lived on the
broken land still nearer to the pole. Now, if we look at a terrestrial globe,
we see under the Polar Circle that there is almost continuous land from
western Europe through Siberia, to eastern America. And this continuity of
the circumpolar land, with the consequent freedom under a more favourable
climate for intermigration, will account for the supposed uniformity of the
sub-arctic and temperate productions of the Old and New Worlds, at a
period anterior to the Glacial epoch.
Believing, from reasons before alluded to, that our continents have long
remained in nearly the same relative position, though subjected to great
oscillations of level, I am strongly inclined to extend the above view, and to
infer that during some earlier and still warmer period, such as the older
Pliocene period, a large number of the same plants and animals inhabited
the almost continuous circumpolar land; and that these plants and animals,
both in the Old and New Worlds, began slowly to migrate southwards as the
climate became less warm, long before the commencement of the Glacial
period. We now see, as I believe, their descendants, mostly in a modified
condition, in the central parts of Europe and the United States. On this view
we can understand the relationship with very little identity, between the
productions of North America and Europe—a relationship which is highly
remarkable, considering the distance of the two areas, and their separation
by the whole Atlantic Ocean. We can further understand the singular fact
remarked on by several observers that the productions of Europe and
America during the later tertiary stages were more closely related to each
other than they are at the present time; for during these warmer periods the
northern parts of the Old and New Worlds will have been almost
continuously united by land, serving as a bridge, since rendered impassable
by cold, for the intermigration of their inhabitants.
During the slowly decreasing warmth of the Pliocene period, as soon as
the species in common, which inhabited the New and Old Worlds, migrated
south of the Polar Circle, they will have been completely cut off from each
other. This separation, as far as the more temperate productions are

concerned, must have taken place long ages ago. As the plants and animals
migrated southward, they will have become mingled in the one great region
with the native American productions, and would have had to compete with
them; and in the other great region, with those of the Old World.
Consequently we have here everything favourable for much modification—
for far more modification than with the Alpine productions, left isolated,
within a much more recent period, on the several mountain ranges and on
the arctic lands of Europe and North America. Hence, it has come, that
when we compare the now living productions of the temperate regions of
the New and Old Worlds, we find very few identical species (though Asa
Gray has lately shown that more plants are identical than was formerly
supposed), but we find in every great class many forms, which some
naturalists rank as geographical races, and others as distinct species; and a
host of closely allied or representative forms which are ranked by all
naturalists as specifically distinct.
As on the land, so in the waters of the sea, a slow southern migration of a
marine fauna, which, during the Pliocene or even a somewhat earlier
period, was nearly uniform along the continuous shores of the Polar Circle,
will account, on the theory of modification, for many closely allied forms
now living in marine areas completely sundered. Thus, I think, we can
understand the presence of some closely allied, still existing and extinct
tertiary forms, on the eastern and western shores of temperate North
America; and the still more striking fact of many closely allied crustaceans
(as described in Dana’s admirable work), some fish and other marine
animals, inhabiting the Mediterranean and the seas of Japan—these two
areas being now completely separated by the breadth of a whole continent
and by wide spaces of ocean.
These cases of close relationship in species either now or formerly
inhabiting the seas on the eastern and western shores of North America, the
Mediterranean and Japan, and the temperate lands of North America and
Europe, are inexplicable on the theory of creation. We cannot maintain that
such species have been created alike, in correspondence with the nearly
similar physical conditions of the areas; for if we compare, for instance,
certain parts of South America with parts of South Africa or Australia, we
see countries closely similar in all their physical conditions, with their
inhabitants utterly dissimilar.

Alternate Glacial Periods in the North and South.

But we must return to our more immediate subject. I am convinced that
Forbes’s view may be largely extended. In Europe we meet with the
plainest evidence of the Glacial period, from the western shores of Britain
to the Ural range, and southward to the Pyrenees. We may infer from the
frozen mammals and nature of the mountain vegetation, that Siberia was
similarly affected. In the Lebanon, according to Dr. Hooker, perpetual snow
formerly covered the central axis, and fed glaciers which rolled 4,000 feet
down the valleys. The same observer has recently found great moraines at a
low level on the Atlas range in North Africa. Along the Himalaya, at points
900 miles apart, glaciers have left the marks of their former low descent;
and in Sikkim, Dr. Hooker saw maize growing on ancient and gigantic
moraines. Southward of the Asiatic continent, on the opposite side of the
equator, we know, from the excellent researches of Dr. J. Haast and Dr.
Hector, that in New Zealand immense glaciers formerly descended to a low
level; and the same plants, found by Dr. Hooker on widely separated
mountains in this island tell the same story of a former cold period. From
facts communicated to me by the Rev. W.B. Clarke, it appears also that
there are traces of former glacial action on the mountains of the south-
eastern corner of Australia.
Looking to America: in the northern half, ice-borne fragments of rock
have been observed on the eastern side of the continent, as far south as
latitude 36° and 37°, and on the shores of the Pacific, where the climate is
now so different, as far south as latitude 46°. Erratic boulders have, also,
been noticed on the Rocky Mountains. In the Cordillera of South America,
nearly under the equator, glaciers once extended far below their present
level. In central Chile I examined a vast mound of detritus with great
boulders, crossing the Portillo valley, which, there can hardly be a doubt,
once formed a huge moraine; and Mr. D. Forbes informs me that he found
in various parts of the Cordillera, from latitude 13° to 30° south, at about
the height of 12,000 feet, deeply-furrowed rocks, resembling those with
which he was familiar in Norway, and likewise great masses of detritus,
including grooved pebbles. Along this whole space of the Cordillera true
glaciers do not now exist even at much more considerable heights. Further
south, on both sides of the continent, from latitude 41° to the southernmost

extremity, we have the clearest evidence of former glacial action, in
numerous immense boulders transported far from their parent source.
From these several facts, namely, from the glacial action having extended
all round the northern and southern hemispheres—from the period having
been in a geological sense recent in both hemispheres—from its having
lasted in both during a great length of time, as may be inferred from the
amount of work effected—and lastly, from glaciers having recently
descended to a low level along the whole line of the Cordillera, it at one
time appeared to me that we could not avoid the conclusion that the
temperature of the whole world had been simultaneously lowered during
the Glacial period. But now, Mr. Croll, in a series of admirable memoirs,
has attempted to show that a glacial condition of climate is the result of
various physical causes, brought into operation by an increase in the
eccentricity of the earth’s orbit. All these causes tend towards the same end;
but the most powerful appears to be the indirect influence of the
eccentricity of the orbit upon oceanic currents. According to Mr. Croll, cold
periods regularly recur every ten or fifteen thousand years; and these at long
intervals are extremely severe, owing to certain contingencies, of which the
most important, as Sir C. Lyell has shown, is the relative position of the
land and water. Mr. Croll believes that the last great glacial period occurred
about 240,000 years ago, and endured, with slight alterations of climate, for
about 160,000 years. With respect to more ancient glacial periods, several
geologists are convinced, from direct evidence, that such occurred during
the miocene and eocene formations, not to mention still more ancient
formations. But the most important result for us, arrived at by Mr. Croll, is
that whenever the northern hemisphere passes through a cold period the
temperature of the southern hemisphere is actually raised, with the winters
rendered much milder, chiefly through changes in the direction of the ocean
currents. So conversely it will be with the northern hemisphere, while the
southern passes through a glacial period. This conclusion throws so much
light on geographical distribution that I am strongly inclined to trust in it;
but I will first give the facts which demand an explanation.
In South America, Dr. Hooker has shown that besides many closely allied
species, between forty and fifty of the flowering plants of Tierra del Fuego,
forming no inconsiderable part of its scanty flora, are common to North
America and Europe, enormously remote as these areas in opposite
hemispheres are from each other. On the lofty mountains of equatorial

America a host of peculiar species belonging to European genera occur. On
the Organ Mountains of Brazil some few temperate European, some
Antarctic and some Andean genera were found by Gardner which do not
exist in the low intervening hot countries. On the Silla of Caraccas the
illustrious Humboldt long ago found species belonging to genera
characteristic of the Cordillera.
In Africa, several forms characteristic of Europe, and some few
representatives of the flora of the Cape of Good Hope, occur on the
mountains of Abyssinia. At the Cape of Good Hope a very few European
species, believed not to have been introduced by man, and on the mountains
several representative European forms are found which have not been
discovered in the intertropical parts of Africa. Dr. Hooker has also lately
shown that several of the plants living on the upper parts of the lofty island
of Fernando Po, and on the neighbouring Cameroon Mountains, in the Gulf
of Guinea, are closely related to those on the mountains of Abyssinia, and
likewise to those of temperate Europe. It now also appears, as I hear from
Dr. Hooker, that some of these same temperate plants have been discovered
by the Rev. R.T. Lowe on the mountains of the Cape Verde Islands. This
extension of the same temperate forms, almost under the equator, across the
whole continent of Africa and to the mountains of the Cape Verde
archipelago, is one of the most astonishing facts ever recorded in the
distribution of plants.
On the Himalaya, and on the isolated mountain ranges of the peninsula of
India, on the heights of Ceylon, and on the volcanic cones of Java, many
plants occur either identically the same or representing each other, and at
the same time representing plants of Europe not found in the intervening
hot lowlands. A list of the genera of plants collected on the loftier peaks of
Java, raises a picture of a collection made on a hillock in Europe. Still more
striking is the fact that peculiar Australian forms are represented by certain
plants growing on the summits of the mountains of Borneo. Some of these
Australian forms, as I hear from Dr. Hooker, extend along the heights of the
peninsula of Malacca, and are thinly scattered on the one hand over India,
and on the other hand as far north as Japan.
On the southern mountains of Australia, Dr. F. Müller has discovered
several European species; other species, not introduced by man, occur on
the lowlands; and a long list can be given, as I am informed by Dr. Hooker,

of European genera, found in Australia, but not in the intermediate torrid
regions. In the admirable “Introduction to the Flora of New Zealand,” by
Dr. Hooker, analogous and striking facts are given in regard to the plants of
that large island. Hence, we see that certain plants growing on the more
lofty mountains of the tropics in all parts of the world, and on the temperate
plains of the north and south, are either the same species or varieties of the
same species. It should, however, be observed that these plants are not
strictly arctic forms; for, as Mr. H.C. Watson has remarked, “in receding
from polar toward equatorial latitudes, the Alpine or mountain flora really
become less and less Arctic.” Besides these identical and closely allied
forms, many species inhabiting the same widely sundered areas, belong to
genera not now found in the intermediate tropical lowlands.
These brief remarks apply to plants alone; but some few analogous facts
could be given in regard to terrestrial animals. In marine productions,
similar cases likewise occur; as an example, I may quote a statement by the
highest authority, Prof. Dana, that “it is certainly a wonderful fact that New
Zealand should have a closer resemblance in its crustacea to Great Britain,
its antipode, than to any other part of the world.” Sir J. Richardson, also,
speaks of the reappearance on the shores of New Zealand, Tasmania, &c.,
of northern forms of fish. Dr. Hooker informs me that twenty-five species
of Algæ are common to New Zealand and to Europe, but have not been
found in the intermediate tropical seas.
From the foregoing facts, namely, the presence of temperate forms on the
highlands across the whole of equatorial Africa, and along the Peninsula of
India, to Ceylon and the Malay Archipelago, and in a less well-marked
manner across the wide expanse of tropical South America, it appears
almost certain that at some former period, no doubt during the most severe
part of a Glacial period, the lowlands of these great continents were
everywhere tenanted under the equator by a considerable number of
temperate forms. At this period the equatorial climate at the level of the sea
was probably about the same with that now experienced at the height of
from five to six thousand feet under the same latitude, or perhaps even
rather cooler. During this, the coldest period, the lowlands under the equator
must have been clothed with a mingled tropical and temperate vegetation,
like that described by Hooker as growing luxuriantly at the height of from
four to five thousand feet on the lower slopes of the Himalaya, but with
perhaps a still greater preponderance of temperate forms. So again in the

mountainous island of Fernando Po, in the Gulf of Guinea, Mr. Mann found
temperate European forms beginning to appear at the height of about five
thousand feet. On the mountains of Panama, at the height of only two
thousand feet, Dr. Seemann found the vegetation like that of Mexico, “with
forms of the torrid zone harmoniously blended with those of the temperate.”
Now let us see whether Mr. Croll’s conclusion that when the northern
hemisphere suffered from the extreme cold of the great Glacial period, the
southern hemisphere was actually warmer, throws any clear light on the
present apparently inexplicable distribution of various organisms in the
temperate parts of both hemispheres, and on the mountains of the tropics.
The Glacial period, as measured by years, must have been very long; and
when we remember over what vast spaces some naturalised plants and
animals have spread within a few centuries, this period will have been
ample for any amount of migration. As the cold became more and more
intense, we know that Arctic forms invaded the temperate regions; and from
the facts just given, there can hardly be a doubt that some of the more
vigorous, dominant and widest-spreading temperate forms invaded the
equatorial lowlands. The inhabitants of these hot lowlands would at the
same time have migrated to the tropical and subtropical regions of the
south, for the southern hemisphere was at this period warmer. On the
decline of the Glacial period, as both hemispheres gradually recovered their
former temperature, the northern temperate forms living on the lowlands
under the equator, would have been driven to their former homes or have
been destroyed, being replaced by the equatorial forms returning from the
south. Some, however, of the northern temperate forms would almost
certainly have ascended any adjoining high land, where, if sufficiently lofty,
they would have long survived like the Arctic forms on the mountains of
Europe. They might have survived, even if the climate was not perfectly
fitted for them, for the change of temperature must have been very slow,
and plants undoubtedly possess a certain capacity for acclimatisation, as
shown by their transmitting to their offspring different constitutional
powers of resisting heat and cold.
In the regular course of events the southern hemisphere would in its turn
be subjected to a severe Glacial period, with the northern hemisphere
rendered warmer; and then the southern temperate forms would invade the
equatorial lowlands. The northern forms which had before been left on the
mountains would now descend and mingle with the southern forms. These

latter, when the warmth returned, would return to their former homes,
leaving some few species on the mountains, and carrying southward with
them some of the northern temperate forms which had descended from their
mountain fastnesses. Thus, we should have some few species identically the
same in the northern and southern temperate zones and on the mountains of
the intermediate tropical regions. But the species left during a long time on
these mountains, or in opposite hemispheres, would have to compete with
many new forms and would be exposed to somewhat different physical
conditions; hence, they would be eminently liable to modification, and
would generally now exist as varieties or as representative species; and this
is the case. We must, also, bear in mind the occurrence in both hemispheres
of former Glacial periods; for these will account, in accordance with the
same principles, for the many quite distinct species inhabiting the same
widely separated areas, and belonging to genera not now found in the
intermediate torrid zones.
It is a remarkable fact, strongly insisted on by Hooker in regard to
America, and by Alph. de Candolle in regard to Australia, that many more
identical or slightly modified species have migrated from the north to the
south, than in a reversed direction. We see, however, a few southern forms
on the mountains of Borneo and Abyssinia. I suspect that this preponderant
migration from the north to the south is due to the greater extent of land in
the north, and to the northern forms having existed in their own homes in
greater numbers, and having consequently been advanced through natural
selection and competition to a higher stage of perfection, or dominating
power, than the southern forms. And thus, when the two sets became
commingled in the equatorial regions, during the alternations of the Glacial
periods, the northern forms were the more powerful and were able to hold
their places on the mountains, and afterwards migrate southward with the
southern forms; but not so the southern in regard to the northern forms. In
the same manner, at the present day, we see that very many European
productions cover the ground in La Plata, New Zealand, and to a lesser
degree in Australia, and have beaten the natives; whereas extremely few
southern forms have become naturalised in any part of the northern
hemisphere, though hides, wool, and other objects likely to carry seeds have
been largely imported into Europe during the last two or three centuries
from La Plata and during the last forty or fifty years from Australia. The
Neilgherrie Mountains in India, however, offer a partial exception; for here,

as I hear from Dr. Hooker, Australian forms are rapidly sowing themselves
and becoming naturalised. Before the last great Glacial period, no doubt the
intertropical mountains were stocked with endemic Alpine forms; but these
have almost everywhere yielded to the more dominant forms generated in
the larger areas and more efficient workshops of the north. In many islands
the native productions are nearly equalled, or even outnumbered, by those
which have become naturalised; and this is the first stage towards their
extinction. Mountains are islands on the land; and their inhabitants have
yielded to those produced within the larger areas of the north, just in the
same way as the inhabitants of real islands have everywhere yielded and are
still yielding to continental forms naturalised through man’s agency.
The same principles apply to the distribution of terrestrial animals and of
marine productions, in the northern and southern temperate zones, and on
the intertropical mountains. When, during the height of the Glacial period,
the ocean-currents were widely different to what they now are, some of the
inhabitants of the temperate seas might have reached the equator; of these a
few would perhaps at once be able to migrate southwards, by keeping to the
cooler currents, while others might remain and survive in the colder depths
until the southern hemisphere was in its turn subjected to a glacial climate
and permitted their further progress; in nearly the same manner as,
according to Forbes, isolated spaces inhabited by Arctic productions exist to
the present day in the deeper parts of the northern temperate seas.
I am far from supposing that all the difficulties in regard to the
distribution and affinities of the identical and allied species, which now live
so widely separated in the north and south, and sometimes on the
intermediate mountain ranges, are removed on the views above given. The
exact lines of migration cannot be indicated. We cannot say why certain
species and not others have migrated; why certain species have been
modified and have given rise to new forms, while others have remained
unaltered. We cannot hope to explain such facts, until we can say why one
species and not another becomes naturalised by man’s agency in a foreign
land; why one species ranges twice or thrice as far, and is twice or thrice as
common, as another species within their own homes.
Various special difficulties also remain to be solved; for instance, the
occurrence, as shown by Dr. Hooker, of the same plants at points so
enormously remote as Kerguelen Land, New Zealand, and Fuegia; but

icebergs, as suggested by Lyell, may have been concerned in their dispersal.
The existence at these and other distant points of the southern hemisphere,
of species, which, though distinct, belong to genera exclusively confined to
the south, is a more remarkable case. Some of these species are so distinct,
that we cannot suppose that there has been time since the commencement of
the last Glacial period for their migration and subsequent modification to
the necessary degree. The facts seem to indicate that distinct species
belonging to the same genera have migrated in radiating lines from a
common centre; and I am inclined to look in the southern, as in the northern
hemisphere, to a former and warmer period, before the commencement of
the last Glacial period, when the Antarctic lands, now covered with ice,
supported a highly peculiar and isolated flora. It may be suspected that
before this flora was exterminated during the last Glacial epoch, a few
forms had been already widely dispersed to various points of the southern
hemisphere by occasional means of transport, and by the aid, as halting-
places, of now sunken islands. Thus the southern shores of America,
Australia, and New Zealand may have become slightly tinted by the same
peculiar forms of life.
Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate
throughout the world on geographical distribution. And we have now seen
that Mr. Croll’s conclusion that successive Glacial periods in the one
hemisphere coincide with warmer periods in the opposite hemisphere,
together with the admission of the slow modification of species, explains a
multitude of facts in the distribution of the same and of the allied forms of
life in all parts of the globe. The living waters have flowed during one
period from the north and during another from the south, and in both cases
have reached the equator; but the stream of life has flowed with greater
force from the north than in the opposite direction, and has consequently
more freely inundated the south. As the tide leaves its drift in horizontal
lines, rising higher on the shores where the tide rises highest, so have the
living waters left their living drift on our mountain summits, in a line gently
rising from the Arctic lowlands to a great latitude under the equator. The
various beings thus left stranded may be compared with savage races of
man, driven up and surviving in the mountain fastnesses of almost every
land, which serves as a record, full of interest to us, of the former
inhabitants of the surrounding lowlands.

CHAPTER XIII.
GEOGRAPHICAL DISTRIBUTION—continued.

Distribution of fresh-water productions—On the inhabitants
of oceanic islands—Absence of Batrachians and of
terrestrial Mammals—On the relation of the inhabitants of
islands to those of the nearest mainland—On colonisation
from the nearest source with subsequent modification—
Summary of the last and present chapters.

Fresh-water Productions.

As lakes and river-systems are separated from each other by barriers of
land, it might have been thought that fresh-water productions would not
have ranged widely within the same country, and as the sea is apparently a
still more formidable barrier, that they would never have extended to distant
countries. But the case is exactly the reverse. Not only have many fresh-
water species, belonging to different classes, an enormous range, but allied
species prevail in a remarkable manner throughout the world. When first
collecting in the fresh waters of Brazil, I well remember feeling much
surprise at the similarity of the fresh-water insects, shells, &c., and at the
dissimilarity of the surrounding terrestrial beings, compared with those of
Britain.
But the wide ranging power of fresh-water productions can, I think, in
most cases be explained by their having become fitted, in a manner highly
useful to them, for short and frequent migrations from pond to pond, or
from stream to stream, within their own countries; and liability to wide
dispersal would follow from this capacity as an almost necessary
consequence. We can here consider only a few cases; of these, some of the
most difficult to explain are presented by fish. It was formerly believed that

the same fresh-water species never existed on two continents distant from
each other. But Dr. Günther has lately shown that the Galaxias attenuatus
inhabits Tasmania, New Zealand, the Falkland Islands and the mainland of
South America. This is a wonderful case, and probably indicates dispersal
from an Antarctic centre during a former warm period. This case, however,
is rendered in some degree less surprising by the species of this genus
having the power of crossing by some unknown means considerable spaces
of open ocean: thus there is one species common to New Zealand and to the
Auckland Islands, though separated by a distance of about 230 miles. On
the same continent fresh-water fish often range widely, and as if
capriciously; for in two adjoining river systems some of the species may be
the same and some wholly different.
It is probable that they are occasionally transported by what may be
called accidental means. Thus fishes still alive are not very rarely dropped
at distant points by whirlwinds; and it is known that the ova retain their
vitality for a considerable time after removal from the water. Their dispersal
may, however, be mainly attributed to changes in the level of the land
within the recent period, causing rivers to flow into each other. Instances,
also, could be given of this having occurred during floods, without any
change of level. The wide differences of the fish on the opposite sides of
most mountain-ranges, which are continuous and consequently must, from
an early period, have completely prevented the inosculation of the river
systems on the two sides, leads to the same conclusion. Some fresh-water
fish belong to very ancient forms, and in such cases there will have been
ample time for great geographical changes, and consequently time and
means for much migration. Moreover, Dr. Günther has recently been led by
several considerations to infer that with fishes the same forms have a long
endurance. Salt-water fish can with care be slowly accustomed to live in
fresh water; and, according to Valenciennes, there is hardly a single group
of which all the members are confined to fresh water, so that a marine
species belonging to a fresh-water group might travel far along the shores
of the sea, and could, it is probable, become adapted without much
difficulty to the fresh waters of a distant land.
Some species of fresh-water shells have very wide ranges, and allied
species which, on our theory, are descended from a common parent, and
must have proceeded from a single source, prevail throughout the world.
Their distribution at first perplexed me much, as their ova are not likely to

be transported by birds; and the ova, as well as the adults, are immediately
killed by sea-water. I could not even understand how some naturalised
species have spread rapidly throughout the same country. But two facts,
which I have observed—and many others no doubt will be discovered—
throw some light on this subject. When ducks suddenly emerge from a pond
covered with duck-weed, I have twice seen these little plants adhering to
their backs; and it has happened to me, in removing a little duck-weed from
one aquarium to another, that I have unintentionally stocked the one with
fresh-water shells from the other. But another agency is perhaps more
effectual: I suspended the feet of a duck in an aquarium, where many ova of
fresh-water shells were hatching; and I found that numbers of the extremely
minute and just-hatched shells crawled on the feet, and clung to them so
firmly that when taken out of the water they could not be jarred off, though
at a somewhat more advanced age they would voluntarily drop off. These
just-hatched molluscs, though aquatic in their nature, survived on the duck’s
feet, in damp air, from twelve to twenty hours; and in this length of time a
duck or heron might fly at least six or seven hundred miles, and if blown
across the sea to an oceanic island, or to any other distant point, would be
sure to alight on a pool or rivulet. Sir Charles Lyell informs me that a
Dyticus has been caught with an Ancylus (a fresh-water shell like a limpet)
firmly adhering to it; and a water-beetle of the same family, a Colymbetes,
once flew on board the “Beagle,” when forty-five miles distant from the
nearest land: how much farther it might have been blown by a favouring
gale no one can tell.
With respect to plants, it has long been known what enormous ranges
many fresh-water, and even marsh-species, have, both over continents and
to the most remote oceanic islands. This is strikingly illustrated, according
to Alph. de Candolle, in those large groups of terrestrial plants, which have
very few aquatic members; for the latter seem immediately to acquire, as if
in consequence, a wide range. I think favourable means of dispersal explain
this fact. I have before mentioned that earth occasionally adheres in some
quantity to the feet and beaks of birds. Wading birds, which frequent the
muddy edges of ponds, if suddenly flushed, would be the most likely to
have muddy feet. Birds of this order wander more than those of any other;
and are occasionally found on the most remote and barren islands of the
open ocean; they would not be likely to alight on the surface of the sea, so
that any dirt on their feet would not be washed off; and when gaining the

land, they would be sure to fly to their natural fresh-water haunts. I do not
believe that botanists are aware how charged the mud of ponds is with
seeds: I have tried several little experiments, but will here give only the
most striking case: I took in February three tablespoonfuls of mud from
three different points, beneath water, on the edge of a little pond; this mud
when dry weighed only 6 and 3/4 ounces; I kept it covered up in my study
for six months, pulling up and counting each plant as it grew; the plants
were of many kinds, and were altogether 537 in number; and yet the viscid
mud was all contained in a breakfast cup! Considering these facts, I think it
would be an inexplicable circumstance if water-birds did not transport the
seeds of fresh-water plants to unstocked ponds and streams, situated at very
distant points. The same agency may have come into play with the eggs of
some of the smaller fresh-water animals.
Other and unknown agencies probably have also played a part. I have
stated that fresh-water fish eat some kinds of seeds, though they reject many
other kinds after having swallowed them; even small fish swallow seeds of
moderate size, as of the yellow water-lily and Potamogeton. Herons and
other birds, century after century, have gone on daily devouring fish; they
then take flight and go to other waters, or are blown across the sea; and we
have seen that seeds retain their power of germination, when rejected many
hours afterwards in pellets or in the excrement. When I saw the great size of
the seeds of that fine water-lily, the Nelumbium, and remembered Alph. de
Candolle’s remarks on the distribution of this plant, I thought that the means
of its dispersal must remain inexplicable; but Audubon states that he found
the seeds of the great southern water-lily (probably according to Dr.
Hooker, the Nelumbium luteum) in a heron’s stomach. Now this bird must
often have flown with its stomach thus well stocked to distant ponds, and,
then getting a hearty meal of fish, analogy makes me believe that it would
have rejected the seeds in the pellet in a fit state for germination.
In considering these several means of distribution, it should be
remembered that when a pond or stream is first formed, for instance on a
rising islet, it will be unoccupied; and a single seed or egg will have a good
chance of succeeding. Although there will always be a struggle for life
between the inhabitants of the same pond, however few in kind, yet as the
number even in a well-stocked pond is small in comparison with the
number of species inhabiting an equal area of land, the competition between
them will probably be less severe than between terrestrial species;

consequently an intruder from the waters of a foreign country would have a
better chance of seizing on a new place, than in the case of terrestrial
colonists. We should also remember that many fresh-water productions are
low in the scale of nature, and we have reason to believe that such beings
become modified more slowly than the high; and this will give time for the
migration of aquatic species. We should not forget the probability of many
fresh-water forms having formerly ranged continuously over immense
areas, and then having become extinct at intermediate points. But the wide
distribution of fresh-water plants, and of the lower animals, whether
retaining the same identical form, or in some degree modified, apparently
depends in main part on the wide dispersal of their seeds and eggs by
animals, more especially by fresh-water birds, which have great powers of
flight, and naturally travel from one piece of water to another.

On the Inhabitants of Oceanic Islands.

We now come to the last of the three classes of facts, which I have
selected as presenting the greatest amount of difficulty with respect to
distribution, on the view that not only all the individuals of the same species
have migrated from some one area, but that allied species, although now
inhabiting the most distant points, have proceeded from a single area, the
birthplace of their early progenitors. I have already given my reasons for
disbelieving in continental extensions within the period of existing species
on so enormous a scale that all the many islands of the several oceans were
thus stocked with their present terrestrial inhabitants. This view removes
many difficulties, but it does not accord with all the facts in regard to the
productions of islands. In the following remarks I shall not confine myself
to the mere question of dispersal, but shall consider some other cases
bearing on the truth of the two theories of independent creation and of
descent with modification.
The species of all kinds which inhabit oceanic islands are few in number
compared with those on equal continental areas: Alph. de Candolle admits
this for plants, and Wollaston for insects. New Zealand, for instance, with
its lofty mountains and diversified stations, extending over 780 miles of
latitude, together with the outlying islands of Auckland, Campbell and
Chatham, contain altogether only 960 kinds of flowering plants; if we
compare this moderate number with the species which swarm over equal

areas in Southwestern Australia or at the Cape of Good Hope, we must
admit that some cause, independently of different physical conditions, has
given rise to so great a difference in number. Even the uniform county of
Cambridge has 847 plants, and the little island of Anglesea 764, but a few
ferns and a few introduced plants are included in these numbers, and the
comparison in some other respects is not quite fair. We have evidence that
the barren island of Ascension aboriginally possessed less than half-a-dozen
flowering plants; yet many species have now become naturalised on it, as
they have in New Zealand and on every other oceanic island which can be
named. In St. Helena there is reason to believe that the naturalised plants
and animals have nearly or quite exterminated many native productions. He
who admits the doctrine of the creation of each separate species, will have
to admit that a sufficient number of the best adapted plants and animals
were not created for oceanic islands; for man has unintentionally stocked
them far more fully and perfectly than did nature.
Although in oceanic islands the species are few in number, the proportion
of endemic kinds (i.e. those found nowhere else in the world) is often
extremely large. If we compare, for instance, the number of endemic land-
shells in Madeira, or of endemic birds in the Galapagos Archipelago, with
the number found on any continent, and then compare the area of the island
with that of the continent, we shall see that this is true. This fact might have
been theoretically expected, for, as already explained, species occasionally
arriving, after long intervals of time in the new and isolated district, and
having to compete with new associates, would be eminently liable to
modification, and would often produce groups of modified descendants. But
it by no means follows that, because in an island nearly all the species of
one class are peculiar, those of another class, or of another section of the
same class, are peculiar; and this difference seems to depend partly on the
species which are not modified having immigrated in a body, so that their
mutual relations have not been much disturbed; and partly on the frequent
arrival of unmodified immigrants from the mother-country, with which the
insular forms have intercrossed. It should be borne in mind that the
offspring of such crosses would certainly gain in vigour; so that even an
occasional cross would produce more effect than might have been
anticipated. I will give a few illustrations of the foregoing remarks: in the
Galapagos Islands there are twenty-six land birds; of these twenty-one (or
perhaps twenty-three) are peculiar; whereas of the eleven marine birds only

two are peculiar; and it is obvious that marine birds could arrive at these
islands much more easily and frequently than land-birds. Bermuda, on the
other hand, which lies at about the same distance from North America as
the Galapagos Islands do from South America, and which has a very
peculiar soil, does not possess a single endemic land bird; and we know
from Mr. J.M. Jones’s admirable account of Bermuda, that very many North
American birds occasionally or even frequently visit this island. Almost
every year, as I am informed by Mr. E.V. Harcourt, many European and
African birds are blown to Madeira; this island is inhabited by ninety-nine
kinds, of which one alone is peculiar, though very closely related to a
European form; and three or four other species are confined to this island
and to the Canaries. So that the islands of Bermuda and Madeira have been
stocked from the neighbouring continents with birds, which for long ages
have there struggled together, and have become mutually co-adapted.
Hence, when settled in their new homes, each kind will have been kept by
the others to its proper place and habits, and will consequently have been
but little liable to modification. Any tendency to modification will also have
been checked by intercrossing with the unmodified immigrants, often
arriving from the mother-country. Madeira again is inhabited by a
wonderful number of peculiar land-shells, whereas not one species of sea-
shell is peculiar to its shores: now, though we do not know how sea-shells
are dispersed, yet we can see that their eggs or larvæ, perhaps attached to
seaweed or floating timber, or to the feet of wading birds, might be
transported across three or four hundred miles of open sea far more easily
than land-shells. The different orders of insects inhabiting Madeira present
nearly parallel cases.
Oceanic islands are sometimes deficient in animals of certain whole
classes, and their places are occupied by other classes; thus in the
Galapagos Islands reptiles, and in New Zealand gigantic wingless birds,
take, or recently took, the place of mammals. Although New Zealand is here
spoken of as an oceanic island, it is in some degree doubtful whether it
should be so ranked; it is of large size, and is not separated from Australia
by a profoundly deep sea; from its geological character and the direction of
its mountain ranges, the Rev. W.B. Clarke has lately maintained that this
island, as well as New Caledonia, should be considered as appurtenances of
Australia. Turning to plants, Dr. Hooker has shown that in the Galapagos
Islands the proportional numbers of the different orders are very different

from what they are elsewhere. All such differences in number, and the
absence of certain whole groups of animals and plants, are generally
accounted for by supposed differences in the physical conditions of the
islands; but this explanation is not a little doubtful. Facility of immigration
seems to have been fully as important as the nature of the conditions.
Many remarkable little facts could be given with respect to the
inhabitants of oceanic islands. For instance, in certain islands not tenanted
by a single mammal, some of the endemic plants have beautifully hooked
seeds; yet few relations are more manifest than that hooks serve for the
transportal of seeds in the wool or fur of quadrupeds. But a hooked seed
might be carried to an island by other means; and the plant then becoming
modified would form an endemic species, still retaining its hooks, which
would form a useless appendage, like the shrivelled wings under the
soldered wing-covers of many insular beetles. Again, islands often possess
trees or bushes belonging to orders which elsewhere include only
herbaceous species; now trees, as Alph. de Candolle has shown, generally
have, whatever the cause may be, confined ranges. Hence trees would be
little likely to reach distant oceanic islands; and an herbaceous plant, which
had no chance of successfully competing with the many fully developed
trees growing on a continent, might, when established on an island, gain an
advantage over other herbaceous plants by growing taller and taller and
overtopping them. In this case, natural selection would tend to add to the
stature of the plant, to whatever order it belonged, and thus first convert it
into a bush and then into a tree.

Absence of Batrachians and Terrestrial mammals on Oceanic Islands.

With respect to the absence of whole orders of animals on oceanic
islands, Bory St. Vincent long ago remarked that Batrachians (frogs, toads,
newts) are never found on any of the many islands with which the great
oceans are studded. I have taken pains to verify this assertion, and have
found it true, with the exception of New Zealand, New Caledonia, the
Andaman Islands, and perhaps the Solomon Islands and the Seychelles. But
I have already remarked that it is doubtful whether New Zealand and New
Caledonia ought to be classed as oceanic islands; and this is still more
doubtful with respect to the Andaman and Solomon groups and the
Seychelles. This general absence of frogs, toads and newts on so many true

oceanic islands cannot be accounted for by their physical conditions; indeed
it seems that islands are peculiarly fitted for these animals; for frogs have
been introduced into Madeira, the Azores, and Mauritius, and have
multiplied so as to become a nuisance. But as these animals and their spawn
are immediately killed (with the exception, as far as known, of one Indian
species) by sea-water, there would be great difficulty in their transportal
across the sea, and therefore we can see why they do not exist on strictly
oceanic islands. But why, on the theory of creation, they should not have
been created there, it would be very difficult to explain.
Mammals offer another and similar case. I have carefully searched the
oldest voyages, and have not found a single instance, free from doubt, of a
terrestrial mammal (excluding domesticated animals kept by the natives)
inhabiting an island situated above 300 miles from a continent or great
continental island; and many islands situated at a much less distance are
equally barren. The Falkland Islands, which are inhabited by a wolf-like
fox, come nearest to an exception; but this group cannot be considered as
oceanic, as it lies on a bank in connection with the mainland at a distance of
about 280 miles; moreover, icebergs formerly brought boulders to its
western shores, and they may have formerly transported foxes, as now
frequently happens in the arctic regions. Yet it cannot be said that small
islands will not support at least small mammals, for they occur in many
parts of the world on very small islands, when lying close to a continent;
and hardly an island can be named on which our smaller quadrupeds have
not become naturalised and greatly multiplied. It cannot be said, on the
ordinary view of creation, that there has not been time for the creation of
mammals; many volcanic islands are sufficiently ancient, as shown by the
stupendous degradation which they have suffered, and by their tertiary
strata: there has also been time for the production of endemic species
belonging to other classes; and on continents it is known that new species of
mammals appear and disappear at a quicker rate than other and lower
animals. Although terrestrial mammals do not occur on oceanic islands,
aërial mammals do occur on almost every island. New Zealand possesses
two bats found nowhere else in the world: Norfolk Island, the Viti
Archipelago, the Bonin Islands, the Caroline and Marianne Archipelagoes,
and Mauritius, all possess their peculiar bats. Why, it may be asked, has the
supposed creative force produced bats and no other mammals on remote
islands? On my view this question can easily be answered; for no terrestrial

mammal can be transported across a wide space of sea, but bats can fly
across. Bats have been seen wandering by day far over the Atlantic Ocean;
and two North American species, either regularly or occasionally, visit
Bermuda, at the distance of 600 miles from the mainland. I hear from Mr.
Tomes, who has specially studied this family, that many species have
enormous ranges, and are found on continents and on far distant islands.
Hence, we have only to suppose that such wandering species have been
modified in their new homes in relation to their new position, and we can
understand the presence of endemic bats on oceanic islands, with the
absence of all other terrestrial mammals.
Another interesting relation exists, namely, between the depth of the sea
separating islands from each other, or from the nearest continent, and the
degree of affinity of their mammalian inhabitants. Mr. Windsor Earl has
made some striking observations on this head, since greatly extended by
Mr. Wallace’s admirable researches, in regard to the great Malay
Archipelago, which is traversed near Celebes by a space of deep ocean, and
this separates two widely distinct mammalian faunas. On either side, the
islands stand on a moderately shallow submarine bank, and these islands
are inhabited by the same or by closely allied quadrupeds. I have not as yet
had time to follow up this subject in all quarters of the world; but as far as I
have gone, the relation holds good. For instance, Britain is separated by a
shallow channel from Europe, and the mammals are the same on both sides;
and so it is with all the islands near the shores of Australia. The West Indian
Islands, on the other hand, stand on a deeply submerged bank, nearly one
thousand fathoms in depth, and here we find American forms, but the
species and even the genera are quite distinct. As the amount of
modification which animals of all kinds undergo partly depends on the
lapse of time, and as the islands which are separated from each other, or
from the mainland, by shallow channels, are more likely to have been
continuously united within a recent period than the islands separated by
deeper channels, we can understand how it is that a relation exists between
the depth of the sea separating two mammalian faunas, and the degree of
their affinity, a relation which is quite inexplicable on the theory of
independent acts of creation.
The foregoing statements in regard to the inhabitants of oceanic islands,
namely, the fewness of the species, with a large proportion consisting of
endemic forms—the members of certain groups, but not those of other

groups in the same class, having been modified—the absence of certain
whole orders, as of batrachians and of terrestrial mammals, notwithstanding
the presence of aërial bats, the singular proportions of certain orders of
plants, herbaceous forms having been developed into trees, &c., seem to me
to accord better with the belief in the efficiency of occasional means of
transport, carried on during a long course of time, than with the belief in the
former connection of all oceanic islands with the nearest continent; for on
this latter view it is probable that the various classes would have
immigrated more uniformly, and from the species having entered in a body,
their mutual relations would not have been much disturbed, and
consequently, they would either have not been modified, or all the species
in a more equable manner.
I do not deny that there are many and serious difficulties in understanding
how many of the inhabitants of the more remote islands, whether still
retaining the same specific form or subsequently modified, have reached
their present homes. But the probability of other islands having once existed
as halting-places, of which not a wreck now remains, must not be
overlooked. I will specify one difficult case. Almost all oceanic islands,
even the most isolated and smallest, are inhabited by land-shells, generally
by endemic species, but sometimes by species found elsewhere striking
instances of which have been given by Dr. A.A. Gould in relation to the
Pacific. Now it is notorious that land-shells are easily killed by sea-water;
their eggs, at least such as I have tried, sink in it and are killed. Yet there
must be some unknown, but occasionally efficient means for their
transportal. Would the just-hatched young sometimes adhere to the feet of
birds roosting on the ground and thus get transported? It occurred to me that
land-shells, when hybernating and having a membranous diaphragm over
the mouth of the shell, might be floated in chinks of drifted timber across
moderately wide arms of the sea. And I find that several species in this state
withstand uninjured an immersion in sea-water during seven days. One
shell, the Helix pomatia, after having been thus treated, and again
hybernating, was put into sea-water for twenty days and perfectly
recovered. During this length of time the shell might have been carried by a
marine country of average swiftness to a distance of 660 geographical
miles. As this Helix has a thick calcareous operculum I removed it, and
when it had formed a new membranous one, I again immersed it for
fourteen days in sea-water, and again it recovered and crawled away. Baron

Aucapitaine has since tried similar experiments. He placed 100 land-shells,
belonging to ten species, in a box pierced with holes, and immersed it for a
fortnight in the sea. Out of the hundred shells twenty-seven recovered. The
presence of an operculum seems to have been of importance, as out of
twelve specimens of Cyclostoma elegans, which is thus furnished, eleven
revived. It is remarkable, seeing how well the Helix pomatia resisted with
me the salt-water, that not one of fifty-four specimens belonging to four
other species of Helix tried by Aucapitaine recovered. It is, however, not at
all probable that land-shells have often been thus transported; the feet of
birds offer a more probable method.

On the Relations of the Inhabitants of Islands to those of the nearest
Mainland.

The most striking and important fact for us is the affinity of the species
which inhabit islands to those of the nearest mainland, without being
actually the same. Numerous instances could be given. The Galapagos
Archipelago, situated under the equator, lies at a distance of between 500
and 600 miles from the shores of South America. Here almost every product
of the land and of the water bears the unmistakable stamp of the American
continent. There are twenty-six land-birds. Of these twenty-one, or perhaps
twenty-three, are ranked as distinct species, and would commonly be
assumed to have been here created; yet the close affinity of most of these
birds to American species is manifest in every character in their habits,
gestures, and tones of voice. So it is with the other animals, and with a large
proportion of the plants, as shown by Dr. Hooker in his admirable Flora of
this archipelago. The naturalist, looking at the inhabitants of these volcanic
islands in the Pacific, distant several hundred miles from the continent, feels
that he is standing on American land. Why should this be so? Why should
the species which are supposed to have been created in the Galapagos
Archipelago, and nowhere else, bear so plainly the stamp of affinity to those
created in America? There is nothing in the conditions of life, in the
geological nature of the islands, in their height or climate, or in the
proportions in which the several classes are associated together, which
closely resembles the conditions of the South American coast. In fact, there
is a considerable dissimilarity in all these respects. On the other hand, there
is a considerable degree of resemblance in the volcanic nature of the soil, in

the climate, height, and size of the islands, between the Galapagos and Cape
Verde Archipelagos: but what an entire and absolute difference in their
inhabitants! The inhabitants of the Cape Verde Islands are related to those
of Africa, like those of the Galapagos to America. Facts, such as these,
admit of no sort of explanation on the ordinary view of independent
creation; whereas, on the view here maintained, it is obvious that the
Galapagos Islands would be likely to receive colonists from America,
whether by occasional means of transport or (though I do not believe in this
doctrine) by formerly continuous land, and the Cape Verde Islands from
Africa; such colonists would be liable to modification—the principle of
inheritance still betraying their original birthplace.
Many analogous facts could be given: indeed it is an almost universal
rule that the endemic productions of islands are related to those of the
nearest continent, or of the nearest large island. The exceptions are few, and
most of them can be explained. Thus, although Kerguelen Land stands
nearer to Africa than to America, the plants are related, and that very
closely, as we know from Dr. Hooker’s account, to those of America: but on
the view that this island has been mainly stocked by seeds brought with
earth and stones on icebergs, drifted by the prevailing currents, this
anomaly disappears. New Zealand in its endemic plants is much more
closely related to Australia, the nearest mainland, than to any other region:
and this is what might have been expected; but it is also plainly related to
South America, which, although the next nearest continent, is so
enormously remote, that the fact becomes an anomaly. But this difficulty
partially disappears on the view that New Zealand, South America, and the
other southern lands, have been stocked in part from a nearly intermediate
though distant point, namely, from the antarctic islands, when they were
clothed with vegetation, during a warmer tertiary period, before the
commencement of the last Glacial period. The affinity, which, though
feeble, I am assured by Dr. Hooker is real, between the flora of the south-
western corner of Australia and of the Cape of Good Hope, is a far more
remarkable case; but this affinity is confined to the plants, and will, no
doubt, some day be explained.
The same law which has determined the relationship between the
inhabitants of islands and the nearest mainland, is sometimes displayed on a
small scale, but in a most interesting manner, within the limits of the same
archipelago. Thus each separate island of the Galapagos Archipelago is

tenanted, and the fact is a marvellous one, by many distinct species; but
these species are related to each other in a very much closer manner than to
the inhabitants of the American continent, or of any other quarter of the
world. This is what might have been expected, for islands situated so near
to each other would almost necessarily receive immigrants from the same
original source, and from each other. But how is it that many of the
immigrants have been differently modified, though only in a small degree,
in islands situated within sight of each other, having the same geological
nature, the same height, climate, etc? This long appeared to me a great
difficulty: but it arises in chief part from the deeply-seated error of
considering the physical conditions of a country as the most important;
whereas it cannot be disputed that the nature of the other species with which
each has to compete, is at least as important, and generally a far more
important element of success. Now if we look to the species which inhabit
the Galapagos Archipelago, and are likewise found in other parts of the
world, we find that they differ considerably in the several islands. This
difference might indeed have been expected if the islands have been
stocked by occasional means of transport—a seed, for instance, of one plant
having been brought to one island, and that of another plant to another
island, though all proceeding from the same general source. Hence, when in
former times an immigrant first settled on one of the islands, or when it
subsequently spread from one to another, it would undoubtedly be exposed
to different conditions in the different islands, for it would have to compete
with a different set of organisms; a plant, for instance, would find the
ground best-fitted for it occupied by somewhat different species in the
different islands, and would be exposed to the attacks of somewhat different
enemies. If, then, it varied, natural selection would probably favour
different varieties in the different islands. Some species, however, might
spread and yet retain the same character throughout the group, just as we
see some species spreading widely throughout a continent and remaining
the same.
The really surprising fact in this case of the Galapagos Archipelago, and
in a lesser degree in some analogous cases, is that each new species after
being formed in any one island, did not spread quickly to the other islands.
But the islands, though in sight of each other, are separated by deep arms of
the sea, in most cases wider than the British Channel, and there is no reason
to suppose that they have at any former period been continuously united.

The currents of the sea are rapid and deep between the islands, and gales of
wind are extraordinarily rare; so that the islands are far more effectually
separated from each other than they appear on a map. Nevertheless, some of
the species, both of those found in other parts of the world and of those
confined to the archipelago, are common to the several islands; and we may
infer from the present manner of distribution that they have spread from one
island to the others. But we often take, I think, an erroneous view of the
probability of closely allied species invading each other’s territory, when
put into free intercommunication. Undoubtedly, if one species has any
advantage over another, it will in a very brief time wholly or in part
supplant it; but if both are equally well fitted for their own places, both will
probably hold their separate places for almost any length of time. Being
familiar with the fact that many species, naturalised through man’s agency,
have spread with astonishing rapidity over wide areas, we are apt to infer
that most species would thus spread; but we should remember that the
species which become naturalised in new countries are not generally closely
allied to the aboriginal inhabitants, but are very distinct forms, belonging in
a large proportion of cases, as shown by Alph. de Candolle, to distinct
genera. In the Galapagos Archipelago, many even of the birds, though so
well adapted for flying from island to island, differ on the different islands;
thus there are three closely allied species of mocking-thrush, each confined
to its own island. Now let us suppose the mocking-thrush of Chatham
Island to be blown to Charles Island, which has its own mocking-thrush;
why should it succeed in establishing itself there? We may safely infer that
Charles Island is well stocked with its own species, for annually more eggs
are laid and young birds hatched than can possibly be reared; and we may
infer that the mocking-thrush peculiar to Charles Island is at least as well
fitted for its home as is the species peculiar to Chatham Island. Sir C. Lyell
and Mr. Wollaston have communicated to me a remarkable fact bearing on
this subject; namely, that Madeira and the adjoining islet of Porto Santo
possess many distinct but representative species of land-shells, some of
which live in crevices of stone; and although large quantities of stone are
annually transported from Porto Santo to Madeira, yet this latter island has
not become colonised by the Porto Santo species: nevertheless, both islands
have been colonised by some European land-shells, which no doubt had
some advantage over the indigenous species. From these considerations I
think we need not greatly marvel at the endemic species which inhabit the

several islands of the Galapagos Archipelago not having all spread from
island to island. On the same continent, also, pre-occupation has probably
played an important part in checking the commingling of the species which
inhabit different districts with nearly the same physical conditions. Thus,
the south-east and south-west corners of Australia have nearly the same
physical conditions, and are united by continuous land, yet they are
inhabited by a vast number of distinct mammals, birds, and plants; so it is,
according to Mr. Bates, with the butterflies and other animals inhabiting the
great, open, and continuous valley of the Amazons.
The same principle which governs the general character of the
inhabitants of oceanic islands, namely, the relation to the source whence
colonists could have been most easily derived, together with their
subsequent modification, is of the widest application throughout nature. We
see this on every mountain-summit, in every lake and marsh. For Alpine
species, excepting in as far as the same species have become widely spread
during the Glacial epoch, are related to those of the surrounding lowlands;
thus we have in South America, Alpine humming-birds, Alpine rodents,
Alpine plants, &c., all strictly belonging to American forms; and it is
obvious that a mountain, as it became slowly upheaved, would be colonised
from the surrounding lowlands. So it is with the inhabitants of lakes and
marshes, excepting in so far as great facility of transport has allowed the
same forms to prevail throughout large portions of the world. We see the
same principle in the character of most of the blind animals inhabiting the
caves of America and of Europe. Other analogous facts could be given. It
will, I believe, be found universally true, that wherever in two regions, let
them be ever so distant, many closely allied or representative species occur,
there will likewise be found some identical species; and wherever many
closely-allied species occur, there will be found many forms which some
naturalists rank as distinct species, and others as mere varieties; these
doubtful forms showing us the steps in the process of modification.
The relation between the power and extent of migration in certain
species, either at the present or at some former period, and the existence at
remote points of the world of closely allied species, is shown in another and
more general way. Mr. Gould remarked to me long ago, that in those genera
of birds which range over the world, many of the species have very wide
ranges. I can hardly doubt that this rule is generally true, though difficult of
proof. Among mammals, we see it strikingly displayed in Bats, and in a

lesser degree in the Felidæ and Canidæ. We see the same rule in the
distribution of butterflies and beetles. So it is with most of the inhabitants of
fresh water, for many of the genera in the most distinct classes range over
the world, and many of the species have enormous ranges. It is not meant
that all, but that some of the species have very wide ranges in the genera
which range very widely. Nor is it meant that the species in such genera
have, on an average, a very wide range; for this will largely depend on how
far the process of modification has gone; for instance, two varieties of the
same species inhabit America and Europe, and thus the species has an
immense range; but, if variation were to be carried a little further, the two
varieties would be ranked as distinct species, and their range would be
greatly reduced. Still less is it meant, that species which have the capacity
of crossing barriers and ranging widely, as in the case of certain powerfully-
winged birds, will necessarily range widely; for we should never forget that
to range widely implies not only the power of crossing barriers, but the
more important power of being victorious in distant lands in the struggle for
life with foreign associates. But according to the view that all the species of
a genus, though distributed to the most remote points of the world, are
descended from a single progenitor, we ought to find, and I believe as a
general rule we do find, that some at least of the species range very widely.
We should bear in mind that many genera in all classes are of ancient
origin, and the species in this case will have had ample time for dispersal
and subsequent modification. There is also reason to believe, from
geological evidence, that within each great class the lower organisms
change at a slower rate than the higher; consequently they will have had a
better chance of ranging widely and of still retaining the same specific
character. This fact, together with that of the seeds and eggs of most lowly
organised forms being very minute and better fitted for distant transportal,
probably accounts for a law which has long been observed, and which has
lately been discussed by Alph. de Candolle in regard to plants, namely, that
the lower any group of organisms stands the more widely it ranges.
The relations just discussed—namely, lower organisms ranging more
widely than the higher—some of the species of widely-ranging genera
themselves ranging widely—such facts, as alpine, lacustrine, and marsh
productions being generally related to those which live on the surrounding
low lands and dry lands—the striking relationship between the inhabitants
of islands and those of the nearest mainland—the still closer relationship of

the distinct inhabitants of the islands of the same archipelago—are
inexplicable on the ordinary view of the independent creation of each
species, but are explicable if we admit colonisation from the nearest or
readiest source, together with the subsequent adaptation of the colonists to
their new homes.

Summary of the last and present Chapters.

In these chapters I have endeavoured to show that if we make due
allowance for our ignorance of the full effects of changes of climate and of
the level of the land, which have certainly occurred within the recent
period, and of other changes which have probably occurred—if we
remember how ignorant we are with respect to the many curious means of
occasional transport—if we bear in mind, and this is a very important
consideration, how often a species may have ranged continuously over a
wide area, and then have become extinct in the intermediate tracts—the
difficulty is not insuperable in believing that all the individuals of the same
species, wherever found, are descended from common parents. And we are
led to this conclusion, which has been arrived at by many naturalists under
the designation of single centres of creation, by various general
considerations, more especially from the importance of barriers of all kinds,
and from the analogical distribution of subgenera, genera, and families.
With respect to distinct species belonging to the same genus, which on
our theory have spread from one parent-source; if we make the same
allowances as before for our ignorance, and remember that some forms of
life have changed very slowly, enormous periods of time having been thus
granted for their migration, the difficulties are far from insuperable; though
in this case, as in that of the individuals of the same species, they are often
great.
As exemplifying the effects of climatical changes on distribution, I have
attempted to show how important a part the last Glacial period has played,
which affected even the equatorial regions, and which, during the
alternations of the cold in the north and the south, allowed the productions
of opposite hemispheres to mingle, and left some of them stranded on the
mountain-summits in all parts of the world. As showing how diversified are
the means of occasional transport, I have discussed at some little length the
means of dispersal of fresh-water productions.

If the difficulties be not insuperable in admitting that in the long course
of time all the individuals of the same species, and likewise of the several
species belonging to the same genus, have proceeded from some one
source; then all the grand leading facts of geographical distribution are
explicable on the theory of migration, together with subsequent
modification and the multiplication of new forms. We can thus understand
the high importance of barriers, whether of land or water, in not only
separating but in apparently forming the several zoological and botanical
provinces. We can thus understand the concentration of related species
within the same areas; and how it is that under different latitudes, for
instance, in South America, the inhabitants of the plains and mountains, of
the forests, marshes, and deserts, are linked together in so mysterious a
manner, and are likewise linked to the extinct beings which formerly
inhabited the same continent. Bearing in mind that the mutual relation of
organism to organism is of the highest importance, we can see why two
areas, having nearly the same physical conditions, should often be inhabited
by very different forms of life; for according to the length of time which has
elapsed since the colonists entered one of the regions, or both; according to
the nature of the communication which allowed certain forms and not
others to enter, either in greater or lesser numbers; according or not as those
which entered happened to come into more or less direct competition with
each other and with the aborigines; and according as the immigrants were
capable of varying more or less rapidly, there would ensue in the to or more
regions, independently of their physical conditions, infinitely diversified
conditions of life; there would be an almost endless amount of organic
action and reaction, and we should find some groups of beings greatly, and
some only slightly modified; some developed in great force, some existing
in scanty numbers—and this we do find in the several great geographical
provinces of the world.
On these same principles we can understand, as I have endeavoured to
show, why oceanic islands should have few inhabitants, but that of these, a
large proportion should be endemic or peculiar; and why, in relation to the
means of migration, one group of beings should have all its species
peculiar, and another group, even within the same class, should have all its
species the same with those in an adjoining quarter of the world. We can see
why whole groups of organisms, as batrachians and terrestrial mammals,
should be absent from oceanic islands, whilst the most isolated islands

should possess their own peculiar species of aërial mammals or bats. We
can see why, in islands, there should be some relation between the presence
of mammals, in a more or less modified condition, and the depth of the sea
between such islands and the mainland. We can clearly see why all the
inhabitants of an archipelago, though specifically distinct on the several
islets, should be closely related to each other, and should likewise be
related, but less closely, to those of the nearest continent, or other source
whence immigrants might have been derived. We can see why, if there exist
very closely allied or representative species in two areas, however distant
from each other, some identical species will almost always there be found.
As the late Edward Forbes often insisted, there is a striking parallelism in
the laws of life throughout time and space; the laws governing the
succession of forms in past times being nearly the same with those
governing at the present time the differences in different areas. We see this
in many facts. The endurance of each species and group of species is
continuous in time; for the apparent exceptions to the rule are so few that
they may fairly be attributed to our not having as yet discovered in an
intermediate deposit certain forms which are absent in it, but which occur
above and below: so in space, it certainly is the general rule that the area
inhabited by a single species, or by a group of species, is continuous, and
the exceptions, which are not rare, may, as I have attempted to show, be
accounted for by former migrations under different circumstances, or
through occasional means of transport, or by the species having become
extinct in the intermediate tracts. Both in time and space species and groups
of species have their points of maximum development. Groups of species,
living during the same period of time, or living within the same area, are
often characterised by trifling features in common, as of sculpture or colour.
In looking to the long succession of past ages, as in looking to distant
provinces throughout the world, we find that species in certain classes differ
little from each other, whilst those in another class, or only in a different
section of the same order, differ greatly from each other. In both time and
space the lowly organised members of each class generally change less than
the highly organised; but there are in both cases marked exceptions to the
rule. According to our theory, these several relations throughout time and
space are intelligible; for whether we look to the allied forms of life which
have changed during successive ages, or to those which have changed after
having migrated into distant quarters, in both cases they are connected by

the same bond of ordinary generation; in both cases the laws of variation
have been the same, and modifications have been accumulated by the same
means of natural selection.

CHAPTER XIV.
MUTUAL AFFINITIES OF ORGANIC BEINGS:
MORPHOLOGY: EMBRYOLOGY:
RUDIMENTARY ORGANS.

Classification, groups subordinate to groups—Natural
system—Rules and difficulties in classification, explained
on the theory of descent with modification—Classification
of varieties—Descent always used in classification—
Analogical or adaptive characters—Affinities, general,
complex and radiating—Extinction separates and defines
groups—Morphology, between members of the same class,
between parts of the same individual—Embryology, laws of,
explained by variations not supervening at an early age, and
being inherited at a corresponding age—Rudimentary
organs; their origin explained—Summary.

Classification.

From the most remote period in the history of the world organic beings
have been found to resemble each other in descending degrees, so that they
can be classed in groups under groups. This classification is not arbitrary
like the grouping of the stars in constellations. The existence of groups
would have been of simple significance, if one group had been exclusively
fitted to inhabit the land, and another the water; one to feed on flesh,
another on vegetable matter, and so on; but the case is widely different, for
it is notorious how commonly members of even the same subgroup have
different habits. In the second and fourth chapters, on Variation and on
Natural Selection, I have attempted to show that within each country it is

the widely ranging, the much diffused and common, that is the dominant
species, belonging to the larger genera in each class, which vary most. The
varieties, or incipient species, thus produced, ultimately become converted
into new and distinct species; and these, on the principle of inheritance,
tend to produce other new and dominant species. Consequently the groups
which are now large, and which generally include many dominant species,
tend to go on increasing in size. I further attempted to show that from the
varying descendants of each species trying to occupy as many and as
different places as possible in the economy of nature, they constantly tend
to diverge in character. This latter conclusion is supported by observing the
great diversity of forms, which, in any small area, come into the closest
competition, and by certain facts in naturalisation.
I attempted also to show that there is a steady tendency in the forms
which are increasing in number and diverging in character, to supplant and
exterminate the preceding, less divergent and less improved forms. I request
the reader to turn to the diagram illustrating the action, as formerly
explained, of these several principles; and he will see that the inevitable
result is, that the modified descendants proceeding from one progenitor
become broken up into groups subordinate to groups. In the diagram each
letter on the uppermost line may represent a genus including several
species; and the whole of the genera along this upper line form together one
class, for all are descended from one ancient parent, and, consequently,
have inherited something in common. But the three genera on the left hand
have, on this same principle, much in common, and form a subfamily,
distinct from that containing the next two genera on the right hand, which
diverged from a common parent at the fifth stage of descent. These five
genera have also much in common, though less than when grouped in
subfamilies; and they form a family distinct from that containing the three
genera still further to the right hand, which diverged at an earlier period.
And all these genera, descended from (A), form an order distinct from the
genera descended from (I). So that we here have many species descended
from a single progenitor grouped into genera; and the genera into
subfamilies, families and orders, all under one great class. The grand fact of
the natural subordination of organic beings in groups under groups, which,
from its familiarity, does not always sufficiently strike us, is in my
judgment thus explained. No doubt organic beings, like all other objects,
can be classed in many ways, either artificially by single characters, or

more naturally by a number of characters. We know, for instance, that
minerals and the elemental substances can be thus arranged. In this case
there is of course no relation to genealogical succession, and no cause can
at present be assigned for their falling into groups. But with organic beings
the case is different, and the view above given accords with their natural
arrangement in group under group; and no other explanation has ever been
attempted.
Naturalists, as we have seen, try to arrange the species, genera and
families in each class, on what is called the Natural System. But what is
meant by this system? Some authors look at it merely as a scheme for
arranging together those living objects which are most alike, and for
separating those which are most unlike; or as an artificial method of
enunciating, as briefly as possible, general propositions—that is, by one
sentence to give the characters common, for instance, to all mammals, by
another those common to all carnivora, by another those common to the
dog-genus, and then, by adding a single sentence, a full description is given
of each kind of dog. The ingenuity and utility of this system are
indisputable. But many naturalists think that something more is meant by
the Natural System; they believe that it reveals the plan of the Creator; but
unless it be specified whether order in time or space, or both, or what else is
meant by the plan of the Creator, it seems to me that nothing is thus added
to our knowledge. Expressions such as that famous one by Linnæus, which
we often meet with in a more or less concealed form, namely, that the
characters do not make the genus, but that the genus gives the characters,
seem to imply that some deeper bond is included in our classifications than
mere resemblance. I believe that this is the case, and that community of
descent—the one known cause of close similarity in organic beings—is the
bond, which, though observed by various degrees of modification, is
partially revealed to us by our classifications.
Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification either
gives some unknown plan of creation, or is simply a scheme for enunciating
general propositions and of placing together the forms most like each other.
It might have been thought (and was in ancient times thought) that those
parts of the structure which determined the habits of life, and the general
place of each being in the economy of nature, would be of very high
importance in classification. Nothing can be more false. No one regards the

external similarity of a mouse to a shrew, of a dugong to a whale, of a
whale to a fish, as of any importance. These resemblances, though so
intimately connected with the whole life of the being, are ranked as merely
“adaptive or analogical characters;” but to the consideration of these
resemblances we shall recur. It may even be given as a general rule, that the
less any part of the organisation is concerned with special habits, the more
important it becomes for classification. As an instance: Owen, in speaking
of the dugong, says, “The generative organs, being those which are most
remotely related to the habits and food of an animal, I have always regarded
as affording very clear indications of its true affinities. We are least likely in
the modifications of these organs to mistake a merely adaptive for an
essential character.” With plants how remarkable it is that the organs of
vegetation, on which their nutrition and life depend, are of little
signification; whereas the organs of reproduction, with their product the
seed and embryo, are of paramount importance! So again, in formerly
discussing certain morphological characters which are not functionally
important, we have seen that they are often of the highest service in
classification. This depends on their constancy throughout many allied
groups; and their constancy chiefly depends on any slight deviations not
having been preserved and accumulated by natural selection, which acts
only on serviceable characters.
That the mere physiological importance of an organ does not determine
its classificatory value, is almost proved by the fact, that in allied groups, in
which the same organ, as we have every reason to suppose, has nearly the
same physiological value, its classificatory value is widely different. No
naturalist can have worked at any group without being struck with this fact;
and it has been fully acknowledged in the writings of almost every author. It
will suffice to quote the highest authority, Robert Brown, who, in speaking
of certain organs in the Proteaceæ, says their generic importance, “like that
of all their parts, not only in this, but, as I apprehend in every natural
family, is very unequal, and in some cases seems to be entirely lost.” Again,
in another work he says, the genera of the Connaraceæ “differ in having one
or more ovaria, in the existence or absence of albumen, in the imbricate or
valvular æstivation. Any one of these characters singly is frequently of
more than generic importance, though here even, when all taken together,
they appear insufficient to separate Cnestis from Connarus.” To give an
example among insects: in one great division of the Hymenoptera, the

antennæ, as Westwood has remarked, are most constant in structure; in
another division they differ much, and the differences are of quite
subordinate value in classification; yet no one will say that the antennæ in
these two divisions of the same order are of unequal physiological
importance. Any number of instances could be given of the varying
importance for classification of the same important organ within the same
group of beings.
Again, no one will say that rudimentary or atrophied organs are of high
physiological or vital importance; yet, undoubtedly, organs in this condition
are often of much value in classification. No one will dispute that the
rudimentary teeth in the upper jaws of young ruminants, and certain
rudimentary bones of the leg, are highly serviceable in exhibiting the close
affinity between Ruminants and Pachyderms. Robert Brown has strongly
insisted on the fact that the position of the rudimentary florets is of the
highest importance in the classification of the Grasses.
Numerous instances could be given of characters derived from parts
which must be considered of very trifling physiological importance, but
which are universally admitted as highly serviceable in the definition of
whole groups. For instance, whether or not there is an open passage from
the nostrils to the mouth, the only character, according to Owen, which
absolutely distinguishes fishes and reptiles—the inflection of the angle of
the lower jaw in Marsupials—the manner in which the wings of insects are
folded—mere colour in certain Algæ—mere pubescence on parts of the
flower in grasses—the nature of the dermal covering, as hair or feathers, in
the Vertebrata. If the Ornithorhynchus had been covered with feathers
instead of hair, this external and trifling character would have been
considered by naturalists as an important aid in determining the degree of
affinity of this strange creature to birds.
The importance, for classification, of trifling characters, mainly depends
on their being correlated with many other characters of more or less
importance. The value indeed of an aggregate of characters is very evident
in natural history. Hence, as has often been remarked, a species may depart
from its allies in several characters, both of high physiological importance,
and of almost universal prevalence, and yet leave us in no doubt where it
should be ranked. Hence, also, it has been found that a classification
founded on any single character, however important that may be, has

always failed; for no part of the organisation is invariably constant. The
importance of an aggregate of characters, even when none are important,
alone explains the aphorism enunciated by Linnæus, namely, that the
characters do not give the genus, but the genus gives the character; for this
seems founded on the appreciation of many trifling points of resemblance,
too slight to be defined. Certain plants, belonging to the Malpighiaceæ, bear
perfect and degraded flowers; in the latter, as A. de Jussieu has remarked,
“The greater number of the characters proper to the species, to the genus, to
the family, to the class, disappear, and thus laugh at our classification.”
When Aspicarpa produced in France, during several years, only these
degraded flowers, departing so wonderfully in a number of the most
important points of structure from the proper type of the order, yet M.
Richard sagaciously saw, as Jussieu observes, that this genus should still be
retained among the Malpighiaceæ. This case well illustrates the spirit of our
classifications.
Practically, when naturalists are at work, they do not trouble themselves
about the physiological value of the characters which they use in defining a
group or in allocating any particular species. If they find a character nearly
uniform, and common to a great number of forms, and not common to
others, they use it as one of high value; if common to some lesser number,
they use it as of subordinate value. This principle has been broadly
confessed by some naturalists to be the true one; and by none more clearly
than by that excellent botanist, Aug. St. Hilaire. If several trifling characters
are always found in combination, though no apparent bond of connexion
can be discovered between them, especial value is set on them. As in most
groups of animals, important organs, such as those for propelling the blood,
or for aerating it, or those for propagating the race, are found nearly
uniform, they are considered as highly serviceable in classification; but in
some groups all these, the most important vital organs, are found to offer
characters of quite subordinate value. Thus, as Fritz Müller has lately
remarked, in the same group of crustaceans, Cypridina is furnished with a
heart, while in two closely allied genera, namely Cypris and Cytherea, there
is no such organ; one species of Cypridina has well-developed branchiæ,
while another species is destitute of them.
We can see why characters derived from the embryo should be of equal
importance with those derived from the adult, for a natural classification of
course includes all ages. But it is by no means obvious, on the ordinary

view, why the structure of the embryo should be more important for this
purpose than that of the adult, which alone plays its full part in the economy
of nature. Yet it has been strongly urged by those great naturalists, Milne
Edwards and Agassiz, that embryological characters are the most important
of all; and this doctrine has very generally been admitted as true.
Nevertheless, their importance has sometimes been exaggerated, owing to
the adaptive characters of larvæ not having been excluded; in order to show
this, Fritz Müller arranged, by the aid of such characters alone, the great
class of crustaceans, and the arrangement did not prove a natural one. But
there can be no doubt that embryonic, excluding larval characters, are of the
highest value for classification, not only with animals but with plants. Thus
the main divisions of flowering plants are founded on differences in the
embryo—on the number and position of the cotyledons, and on the mode of
development of the plumule and radicle. We shall immediately see why
these characters possess so high a value in classification, namely, from the
natural system being genealogical in its arrangement.
Our classifications are often plainly influenced by chains of affinities.
Nothing can be easier than to define a number of characters common to all
birds; but with crustaceans, any such definition has hitherto been found
impossible. There are crustaceans at the opposite ends of the series, which
have hardly a character in common; yet the species at both ends, from being
plainly allied to others, and these to others, and so onwards, can be
recognised as unequivocally belonging to this, and to no other class of the
Articulata.
Geographical distribution has often been used, though perhaps not quite
logically, in classification, more especially in very large groups of closely
allied forms. Temminck insists on the utility or even necessity of this
practice in certain groups of birds; and it has been followed by several
entomologists and botanists.
Finally, with respect to the comparative value of the various groups of
species, such as orders, suborders, families, subfamilies, and genera, they
seem to be, at least at present, almost arbitrary. Several of the best botanists,
such as Mr. Bentham and others, have strongly insisted on their arbitrary
value. Instances could be given among plants and insects, of a group first
ranked by practised naturalists as only a genus, and then raised to the rank
of a subfamily or family; and this has been done, not because further

research has detected important structural differences, at first overlooked,
but because numerous allied species, with slightly different grades of
difference, have been subsequently discovered.
All the foregoing rules and aids and difficulties in classification may be
explained, if I do not greatly deceive myself, on the view that the natural
system is founded on descent with modification—that the characters which
naturalists consider as showing true affinity between any two or more
species, are those which have been inherited from a common parent, all true
classification being 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.
But I must explain my meaning more fully. I believe that the
arrangement of the groups within each class, in due subordination and
relation to each other, must be strictly genealogical in order to be natural;
but that the amount of difference in the several branches or groups, though
allied in the same degree in blood to their common progenitor, may differ
greatly, being due to the different degrees of modification which they have
undergone; and this is expressed by the forms being ranked under different
genera, families, sections or orders. The reader will best understand what is
meant, if he will take the trouble to refer to the diagram in the fourth
chapter. We will suppose the letters A to L to represent allied genera
existing during the Silurian epoch, and descended from some still earlier
form. In three of these genera (A, F, and I) a species has transmitted
modified descendants to the present day, represented by the fifteen genera
(a14 to z14) on the uppermost horizontal line. Now, all these modified
descendants from a single species are related in blood or descent in the
same degree. They may metaphorically be called cousins to the same
millionth degree, yet they differ widely and in different degrees from each
other. The forms descended from A, now broken up into two or three
families, constitute a distinct order from those descended from I, also
broken up into two families. Nor can the existing species descended from A
be ranked in the same genus with the parent A, or those from I with parent
I. But the existing genus F14 may be supposed to have been but slightly
modified, and it will then rank with the parent genus F; just as some few
still living organisms belong to Silurian genera. So that the comparative
value of the differences between these organic beings, which are all related

to each other in the same degree in blood, has come to be widely different.
Nevertheless, their genealogical arrangement remains strictly true, not only
at the present time, but at each successive period of descent. All the
modified descendants from A will have inherited something in common
from their common parent, as will all the descendants from I; so will it be
with each subordinate branch of descendants at each successive stage. If,
however, we suppose any descendant of A or of I to have become so much
modified as to have lost all traces of its parentage in this case, its place in
the natural system will be lost, as seems to have occurred with some few
existing organisms. All the descendants of the genus F, along its whole line
of descent, are supposed to have been but little modified, and they form a
single genus. But this genus, though much isolated, will still occupy its
proper intermediate position. The representation of the groups as here given
in the diagram on a flat surface, is much too simple. The branches ought to
have diverged in all directions. If the names of the groups had been simply
written down in a linear series the representation would have been still less
natural; and it is notoriously not possible to represent in a series, on a flat
surface, the affinities which we discover in nature among the beings of the
same group. Thus, the natural system is genealogical in its arrangement,
like a pedigree. But the amount of modification which the different groups
have undergone has to be expressed by ranking them under different so-
called genera, subfamilies, families, sections, orders, and classes.
It may be worth while to illustrate this view of classification, by taking
the case of languages. If we possessed a perfect pedigree of mankind, a
genealogical arrangement of the races of man would afford the best
classification of the various languages now spoken throughout the world;
and if all extinct languages, and all intermediate and slowly changing
dialects, were to be included, such an arrangement would be the only
possible one. Yet it might be that some ancient languages had altered very
little and had given rise to few new languages, whilst others had altered
much owing to the spreading, isolation and state of civilisation of the
several co-descended races, and had thus given rise to many new dialects
and languages. The various degrees of difference between the languages of
the same stock would have to be expressed by groups subordinate to
groups; but the proper or even the only possible arrangement would still be
genealogical; and this would be strictly natural, as it would connect together

all languages, extinct and recent, by the closest affinities, and would give
the filiation and origin of each tongue.
In confirmation of this view, let us glance at the classification of
varieties, which are known or believed to be descended from a single
species. These are grouped under the species, with the subvarieties under
the varieties; and in some cases, as with the domestic pigeon, with several
other grades of difference. Nearly the same rules are followed as in
classifying species. Authors have insisted on the necessity of arranging
varieties on a natural instead of an artificial system; we are cautioned, for
instance, not to class two varieties of the pine-apple together, merely
because their fruit, though the most important part, happens to be nearly
identical; no one puts the Swedish and common turnip together, though the
esculent and thickened stems are so similar. Whatever part is found to be
most constant, is used in classing varieties: thus the great agriculturist
Marshall says the horns are very useful for this purpose with cattle, because
they are less variable than the shape or colour of the body, &c.; whereas
with sheep the horns are much less serviceable, because less constant. In
classing varieties, I apprehend that if we had a real pedigree, a genealogical
classification would be universally preferred; and it has been attempted in
some cases. For we might feel sure, whether there had been more or less
modification, that the principle of inheritance would keep the forms
together which were allied in the greatest number of points. In tumbler
pigeons, though some of the subvarieties differ in the important character of
the length of the beak, yet all are kept together from having the common
habit of tumbling; but the short-faced breed has nearly or quite lost this
habit; nevertheless, without any thought on the subject, these tumblers are
kept in the same group, because allied in blood and alike in some other
respects.
With species in a state of nature, every naturalist has in fact brought
descent into his classification; for he includes in his lowest grade, that of
species, the two sexes; and how enormously these sometimes differ in the
most important characters is known to every naturalist: scarcely a single
fact can be predicated in common of the adult males and hermaphrodites of
certain cirripedes, and yet no one dreams of separating them. As soon as the
three Orchidean forms, Monachanthus, Myanthus, and Catasetum, which
had previously been ranked as three distinct genera, were known to be
sometimes produced on the same plant, they were immediately considered

as varieties; and now I have been able to show that they are the male,
female, and hermaphrodite forms of the same species. The naturalist
includes as one species the various larval stages of the same individual,
however much they may differ from each other and from the adult; as well
as the so-called alternate generations of Steenstrup, which can only in a
technical sense be considered as the same individual. He includes monsters
and varieties, not from their partial resemblance to the parent-form, but
because they are descended from it.
As descent has universally been used in classing together the individuals
of the same species, though the males and females and larvæ are sometimes
extremely different; and as it has been used in classing varieties which have
undergone a certain, and sometimes a considerable amount of modification,
may not this same element of descent have been unconsciously used in
grouping species under genera, and genera under higher groups, all under
the so-called natural system? I believe it has been unconsciously used; and
thus only can I understand the several rules and guides which have been
followed by our best systematists. As we have no written pedigrees, we are
forced to trace community of descent by resemblances of any kind.
Therefore, we choose those characters which are the least likely to have
been modified, in relation to the conditions of life to which each species has
been recently exposed. Rudimentary structures on this view are as good as,
or even sometimes better than other parts of the organisation. We care not
how trifling a character may be—let it be the mere inflection of the angle of
the jaw, the manner in which an insect’s wing is folded, whether the skin be
covered by hair or feathers—if it prevail throughout many and different
species, especially those having very different habits of life, it assumes high
value; for we can account for its presence in so many forms with such
different habits, only by inheritance from a common parent. We may err in
this respect in regard to single points of structure, but when several
characters, let them be ever so trifling, concur throughout a large group of
beings having different habits, we may feel almost sure, on the theory of
descent, that these characters have been inherited from a common ancestor;
and we know that such aggregated characters have especial value in
classification.
We can understand why a species or a group of species may depart from
its allies, in several of its most important characteristics, and yet be safely
classed with them. This may be safely done, and is often done, as long as a

sufficient number of characters, let them be ever so unimportant, betrays the
hidden bond of community of descent. Let two forms have not a single
character in common, yet, if these extreme forms are connected together by
a chain of intermediate groups, we may at once infer their community of
descent, and we put them all into the same class. As we find organs of high
physiological importance—those which serve to preserve life under the
most diverse conditions of existence—are generally the most constant, we
attach especial value to them; but if these same organs, in another group or
section of a group, are found to differ much, we at once value them less in
our classification. We shall presently see why embryological characters are
of such high classificatory importance. Geographical distribution may
sometimes be brought usefully into play in classing large genera, because
all the species of the same genus, inhabiting any distinct and isolated
region, are in all probability descended from the same parents.
Analogical Resemblances.—We can understand, on the above views, the
very important distinction between real affinities and analogical or adaptive
resemblances. Lamarck first called attention to this subject, and he has been
ably followed by Macleay and others. The resemblance in the shape of the
body and in the fin-like anterior limbs between dugongs and whales, and
between these two orders of mammals and fishes, are analogical. So is the
resemblance between a mouse and a shrew-mouse (Sorex), which belong to
different orders; and the still closer resemblance, insisted on by Mr. Mivart,
between the mouse and a small marsupial animal (Antechinus) of Australia.
These latter resemblances may be accounted for, as it seems to me, by
adaptation for similarly active movements through thickets and herbage,
together with concealment from enemies.
Among insects there are innumerable instances; thus Linnæus, misled by
external appearances, actually classed an homopterous insect as a moth. We
see something of the same kind even with our domestic varieties, as in the
strikingly similar shape of the body in the improved breeds of the Chinese
and common pig, which are descended from distinct species; and in the
similarly thickened stems of the common and specifically distinct Swedish
turnip. The resemblance between the greyhound and race-horse is hardly
more fanciful than the analogies which have been drawn by some authors
between widely different animals.

On the view of characters being of real importance for classification,
only in so far as they reveal descent, we can clearly understand why
analogical or adaptive characters, although of the utmost importance to the
welfare of the being, are almost valueless to the systematist. For animals,
belonging to two most distinct lines of descent, may have become adapted
to similar conditions, and thus have assumed a close external resemblance;
but such resemblances will not reveal—will rather tend to conceal their
blood-relationship. We can thus also understand the apparent paradox, that
the very same characters are analogical when one group is compared with
another, but give true affinities when the members of the same group are
compared together: thus the shape of the body and fin-like limbs are only
analogical when whales are compared with fishes, being adaptations in both
classes for swimming through the water; but between the the several
members of the whale family, the shape of the body and the fin-like limbs
offer characters exhibiting true affinity; for as these parts are so nearly
similar throughout the whole family, we cannot doubt that they have been
inherited from a common ancestor. So it is with fishes.
Numerous cases could be given of striking resemblances in quite distinct
beings between single parts or organs, which have been adapted for the
same functions. A good instance is afforded by the close resemblance of the
jaws of the dog and Tasmanian wolf or Thylacinus—animals which are
widely sundered in the natural system. But this resemblance is confined to
general appearance, as in the prominence of the canines, and in the cutting
shape of the molar teeth. For the teeth really differ much: thus the dog has
on each side of the upper jaw four pre-molars and only two molars; while
the Thylacinus has three pre-molars and four molars. The molars also differ
much in the two animals in relative size and structure. The adult dentition is
preceded by a widely different milk dentition. Any one may, of course, deny
that the teeth in either case have been adapted for tearing flesh, through the
natural selection of successive variations; but if this be admitted in the one
case, it is unintelligible to me that it should be denied in the other. I am glad
to find that so high an authority as Professor Flower has come to this same
conclusion.
The extraordinary cases given in a former chapter, of widely different
fishes possessing electric organs—of widely different insects possessing
luminous organs—and of orchids and asclepiads having pollen-masses with
viscid discs, come under this same head of analogical resemblances. But

these cases are so wonderful that they were introduced as difficulties or
objections to our theory. In all such cases some fundamental difference in
the growth or development of the parts, and generally in their matured
structure, can be detected. The end gained is the same, but the means,
though appearing superficially to be the same, are essentially different. The
principle formerly alluded to under the term of analogical variation has
probably in these cases often come into play; that is, the members of the
same class, although only distantly allied, have inherited so much in
common in their constitution, that they are apt to vary under similar
exciting causes in a similar manner; and this would obviously aid in the
acquirement through natural selection of parts or organs, strikingly like
each other, independently of their direct inheritance from a common
progenitor.
As species belonging to distinct classes have often been adapted by
successive slight modifications to live under nearly similar circumstances—
to inhabit, for instance, the three elements of land, air and water—we can
perhaps understand how it is that a numerical parallelism has sometimes
been observed between the subgroups of distinct classes. A naturalist, struck
with a parallelism of this nature, by arbitrarily raising or sinking the value
of the groups in several classes (and all our experience shows that their
valuation is as yet arbitrary), could easily extend the parallelism over a wide
range; and thus the septenary, quinary, quaternary and ternary
classifications have probably arisen.
There is another and curious class of cases in which close external
resemblance does not depend on adaptation to similar habits of life, but has
been gained for the sake of protection. I allude to the wonderful manner in
which certain butterflies imitate, as first described by Mr. Bates, other and
quite distinct species. This excellent observer has shown that in some
districts of South America, where, for instance, an Ithomia abounds in
gaudy swarms, another butterfly, namely, a Leptalis, is often found mingled
in the same flock; and the latter so closely resembles the Ithomia in every
shade and stripe of colour, and even in the shape of its wings, that Mr.
Bates, with his eyes sharpened by collecting during eleven years, was,
though always on his guard, continually deceived. When the mockers and
the mocked are caught and compared, they are found to be very different in
essential structure, and to belong not only to distinct genera, but often to
distinct families. Had this mimicry occurred in only one or two instances, it

might have been passed over as a strange coincidence. But, if we proceed
from a district where one Leptalis imitates an Ithomia, another mocking and
mocked species, belonging to the same two genera, equally close in their
resemblance, may be found. Altogether no less than ten genera are
enumerated, which include species that imitate other butterflies. The
mockers and mocked always inhabit the same region; we never find an
imitator living remote from the form which it imitates. The mockers are
almost invariably rare insects; the mocked in almost every case abounds in
swarms. In the same district in which a species of Leptalis closely imitates
an Ithomia, there are sometimes other Lepidoptera mimicking the same
Ithomia: so that in the same place, species of three genera of butterflies and
even a moth are found all closely resembling a butterfly belonging to a
fourth genus. It deserves especial notice that many of the mimicking forms
of the Leptalis, as well as of the mimicked forms, can be shown by a
graduated series to be merely varieties of the same species; while others are
undoubtedly distinct species. But why, it may be asked, are certain forms
treated as the mimicked and others as the mimickers? Mr. Bates
satisfactorily answers this question by showing that the form which is
imitated keeps the usual dress of the group to which it belongs, while the
counterfeiters have changed their dress and do not resemble their nearest
allies.
We are next led to enquire what reason can be assigned for certain
butterflies and moths so often assuming the dress of another and quite
distinct form; why, to the perplexity of naturalists, has nature condescended
to the tricks of the stage? Mr. Bates has, no doubt, hit on the true
explanation. The mocked forms, which always abound in numbers, must
habitually escape destruction to a large extent, otherwise they could not
exist in such swarms; and a large amount of evidence has now been
collected, showing that they are distasteful to birds and other insect-
devouring animals. The mocking forms, on the other hand, that inhabit the
same district, are comparatively rare, and belong to rare groups; hence, they
must suffer habitually from some danger, for otherwise, from the number of
eggs laid by all butterflies, they would in three or four generations swarm
over the whole country. Now if a member of one of these persecuted and
rare groups were to assume a dress so like that of a well-protected species
that it continually deceived the practised eyes of an entomologist, it would
often deceive predaceous birds and insects, and thus often escape

destruction. Mr. Bates may almost be said to have actually witnessed the
process by which the mimickers have come so closely to resemble the
mimicked; for he found that some of the forms of Leptalis which mimic so
many other butterflies, varied in an extreme degree. In one district several
varieties occurred, and of these one alone resembled, to a certain extent, the
common Ithomia of the same district. In another district there were two or
three varieties, one of which was much commoner than the others, and this
closely mocked another form of Ithomia. From facts of this nature, Mr.
Bates concludes that the Leptalis first varies; and when a variety happens to
resemble in some degree any common butterfly inhabiting the same district,
this variety, from its resemblance to a flourishing and little persecuted kind,
has a better chance of escaping destruction from predaceous birds and
insects, and is consequently oftener preserved; “the less perfect degrees of
resemblance being generation after generation eliminated, and only the
others left to propagate their kind.” So that here we have an excellent
illustration of natural selection.
Messrs. Wallace and Trimen have likewise described several equally
striking cases of imitation in the Lepidoptera of the Malay Archipelago and
Africa, and with some other insects. Mr. Wallace has also detected one such
case with birds, but we have none with the larger quadrupeds. The much
greater frequency of imitation with insects than with other animals, is
probably the consequence of their small size; insects cannot defend
themselves, excepting indeed the kinds furnished with a sting, and I have
never heard of an instance of such kinds mocking other insects, though they
are mocked; insects cannot easily escape by flight from the larger animals
which prey on them; therefore, speaking metaphorically, they are reduced,
like most weak creatures, to trickery and dissimulation.
It should be observed that the process of imitation probably never
commenced between forms widely dissimilar in colour. But, starting with
species already somewhat like each other, the closest resemblance, if
beneficial, could readily be gained by the above means, and if the imitated
form was subsequently and gradually modified through any agency, the
imitating form would be led along the same track, and thus be altered to
almost any extent, so that it might ultimately assume an appearance or
colouring wholly unlike that of the other members of the family to which it
belonged. There is, however, some difficulty on this head, for it is necessary
to suppose in some cases that ancient members belonging to several distinct

groups, before they had diverged to their present extent, accidentally
resembled a member of another and protected group in a sufficient degree
to afford some slight protection, this having given the basis for the
subsequent acquisition of the most perfect resemblance.
On the Nature of the Affinities connecting Organic Beings.—As the
modified descendants of dominant species, belonging to the larger genera,
tend to inherit the advantages which made the groups to which they belong
large and their parents dominant, they are almost sure to spread widely, and
to seize on more and more places in the economy of nature. The larger and
more dominant groups within each class thus tend to go on increasing in
size, and they consequently supplant many smaller and feebler groups.
Thus, we can account for the fact that all organisms, recent and extinct, are
included under a few great orders and under still fewer classes. As showing
how few the higher groups are in number, and how widely they are spread
throughout the world, the fact is striking that the discovery of Australia has
not added an insect belonging to a new class, and that in the vegetable
kingdom, as I learn from Dr. Hooker, it has added only two or three families
of small size.
In the chapter on geological succession I attempted to show, on the
principle of each group having generally diverged much in character during
the long-continued process of modification, how it is that the more ancient
forms of life often present characters in some degree intermediate between
existing groups. As some few of the old and intermediate forms having
transmitted to the present day descendants but little modified, these
constitute our so-called osculant or aberrant groups. The more aberrant any
form is, the greater must be the number of connecting forms which have
been exterminated and utterly lost. And we have evidence of aberrant
groups having suffered severely from extinction, for they are almost always
represented by extremely few species; and such species as do occur are
generally very distinct from each other, which again implies extinction. The
genera Ornithorhynchus and Lepidosiren, for example, would not have been
less aberrant had each been represented by a dozen species, instead of as at
present by a single one, or by two or three. We can, I think, account for this
fact only by looking at aberrant groups as forms which have been
conquered by more successful competitors, with a few members still
preserved under unusually favourable conditions.

Mr. Waterhouse has remarked that when a member belonging to one
group of animals exhibits an affinity to a quite distinct group, this affinity in
most cases is general and not special: thus, according to Mr. Waterhouse, of
all Rodents, the bizcacha is most nearly related to Marsupials; but in the
points in which it approaches this order, its relations are general, that is, not
to any one Marsupial species more than to another. As these points of
affinity are believed to be real and not merely adaptive, they must be due in
accordance with our view to inheritance from a common progenitor.
Therefore, we must suppose either that all Rodents, including the bizcacha,
branched off from some ancient Marsupial, which will naturally have been
more or less intermediate in character with respect to all existing
Marsupials; or that both Rodents and Marsupials branched off from a
common progenitor, and that both groups have since undergone much
modification in divergent directions. On either view we must suppose that
the bizcacha has retained, by inheritance, more of the character of its
ancient progenitor than have other Rodents; and therefore it will not be
specially related to any one existing Marsupial, but indirectly to all or
nearly all Marsupials, from having partially retained the character of their
common progenitor, or of some early member of the group. On the other
hand, of all Marsupials, as Mr. Waterhouse has remarked, the Phascolomys
resembles most nearly, not any one species, but the general order of
Rodents. In this case, however, it may be strongly suspected that the
resemblance is only analogical, owing to the Phascolomys having become
adapted to habits like those of a Rodent. The elder De Candolle has made
nearly similar observations on the general nature of the affinities of distinct
families of plants.
On the principle of the multiplication and gradual divergence in character
of the species descended from a common progenitor, together with their
retention by inheritance of some characters in common, we can understand
the excessively complex and radiating affinities by which all the members
of the same family or higher group are connected together. For the common
progenitor of a whole family, now broken up by extinction into distinct
groups and subgroups, will have transmitted some of its characters,
modified in various ways and degrees, to all the species; and they will
consequently be related to each other by circuitous lines of affinity of
various lengths (as may be seen in the diagram so often referred to),
mounting up through many predecessors. As it is difficult to show the

blood-relationship between the numerous kindred of any ancient and noble
family, even by the aid of a genealogical tree, and almost impossible to do
so without this aid, we can understand the extraordinary difficulty which
naturalists have experienced in describing, without the aid of a diagram, the
various affinities which they perceive between the many living and extinct
members of the same great natural class.
Extinction, as we have seen in the fourth chapter, has played an important
part in defining and widening the intervals between the several groups in
each class. We may thus account for the distinctness of whole classes from
each other—for instance, of birds from all other vertebrate animals—by the
belief that many ancient forms of life have been utterly lost, through which
the early progenitors of birds were formerly connected with the early
progenitors of the other and at that time less differentiated vertebrate
classes. There has been much less extinction of the forms of life which once
connected fishes with Batrachians. There has been still less within some
whole classes, for instance the Crustacea, for here the most wonderfully
diverse forms are still linked together by a long and only partially broken
chain of affinities. Extinction has only defined the groups: it has by no
means made them; for if every form which has ever lived on this earth were
suddenly to reappear, though it would be quite impossible to give
definitions by which each group could be distinguished, still a natural
classification, or at least a natural arrangement, would be possible. We shall
see this by turning to the diagram: the letters, A to L, may represent eleven
Silurian genera, some of which have produced large groups of modified
descendants, with every link in each branch and sub-branch still alive; and
the links not greater than those between existing varieties. In this case it
would be quite impossible to give definitions by which the several members
of the several groups could be distinguished from their more immediate
parents and descendants. Yet the arrangement in the diagram would still
hold good and would be natural; for, on the principle of inheritance, all the
forms descended, for instance from A, would have something in common.
In a tree we can distinguish this or that branch, though at the actual fork the
two unite and blend together. We could not, as I have said, define the
several groups; but we could pick out types, or forms, representing most of
the characters of each group, whether large or small, and thus give a general
idea of the value of the differences between them. This is what we should
be driven to, if we were ever to succeed in collecting all the forms in any

one class which have lived throughout all time and space. Assuredly we
shall never succeed in making so perfect a collection: nevertheless, in
certain classes, we are tending toward this end; and Milne Edwards has
lately insisted, in an able paper, on the high importance of looking to types,
whether or not we can separate and define the groups to which such types
belong.
Finally, we have seen that natural selection, which follows from the
struggle for existence, and which almost inevitably leads to extinction and
divergence of character in the descendants from any one parent-species,
explains that great and universal feature in the affinities of all organic
beings, namely, their subordination in group under group. We use the
element of descent in classing the individuals of both sexes and of all ages
under one species, although they may have but few characters in common;
we use descent in classing acknowledged varieties, however different they
may be from their parents; and I believe that this element of descent is the
hidden bond of connexion which naturalists have sought under the term of
the Natural System. On this idea of the natural system being, in so far as it
has been perfected, genealogical in its arrangement, with the grades of
difference expressed by the terms genera, families, orders, &c., we can
understand the rules which we are compelled to follow in our classification.
We can understand why we value certain resemblances far more than
others; why we use rudimentary and useless organs, or others of trifling
physiological importance; why, in finding the relations between one group
and another, we summarily reject analogical or adaptive characters, and yet
use these same characters within the limits of the same group. We can
clearly see how it is that all living and extinct forms can be grouped
together within a few great classes; and how the several members of each
class are connected together by the most complex and radiating lines of
affinities. We shall never, probably, disentangle the inextricable web of the
affinities between the members of any one class; but when we have a
distinct object in view, and do not look to some unknown plan of creation,
we may hope to make sure but slow progress.
Professor Haeckel in his “Generelle Morphologie” and in another works,
has recently brought his great knowledge and abilities to bear on what he
calls phylogeny, or the lines of descent of all organic beings. In drawing up
the several series he trusts chiefly to embryological characters, but receives
aid from homologous and rudimentary organs, as well as from the

successive periods at which the various forms of life are believed to have
first appeared in our geological formations. He has thus boldly made a great
beginning, and shows us how classification will in the future be treated.

Morphology.

We have seen that the members of the same class, independently of their
habits of life, resemble each other in the general plan of their organisation.
This resemblance is often expressed by the term “unity of type;” or by
saying that the several parts and organs in the different species of the class
are homologous. The whole subject is included under the general term of
Morphology. This is one of the most interesting departments of natural
history, and may almost be said to be its very soul. What can be more
curious than that the hand of a man, formed for grasping, that of a mole for
digging, the leg of the 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? How curious it is, to give a
subordinate though striking instance, that the hind feet of the kangaroo,
which are so well fitted for bounding over the open plains—those of the
climbing, leaf-eating koala, equally well fitted for grasping the branches of
trees—those of the ground-dwelling, insect or root-eating, bandicoots—and
those of some other Australian marsupials—should all be constructed on the
same extraordinary type, namely with the bones of the second and third
digits extremely slender and enveloped within the same skin, so that they
appear like a single toe furnished with two claws. Notwithstanding this
similarity of pattern, it is obvious that the hind feet of these several animals
are used for as widely different purposes as it is possible to conceive. The
case is rendered all the more striking by the American opossums, which
follow nearly the same habits of life as some of their Australian relatives,
having feet constructed on the ordinary plan. Professor Flower, from whom
these statements are taken, remarks in conclusion: “We may call this
conformity to type, without getting much nearer to an explanation of the
phenomenon;” and he then adds “but is it not powerfully suggestive of true
relationship, of inheritance from a common ancestor?”
Geoffroy St. Hilaire has strongly insisted on the high importance of
relative position or connexion in homologous parts; they may differ to
almost any extent in form and size, and yet remain connected together in

the same invariable order. We never find, for instance, the bones of the arm
and forearm, or of the thigh and leg, transposed. Hence the same names can
be given to the homologous bones in widely different animals. We see the
same great law in the construction of the mouths of insects: what can be
more different than the immensely long spiral proboscis of a sphinx-moth,
the curious folded one of a bee or bug, and the great jaws of a beetle? Yet
all these organs, serving for such widely different purposes, are formed by
infinitely numerous modifications of an upper lip, mandibles, and two pairs
of maxillæ. The same law governs the construction of the mouths and limbs
of crustaceans. So it is with the flowers of plants.
Nothing can be more hopeless than to attempt to explain this similarity of
pattern in members of the same class, by utility or by the doctrine of final
causes. The hopelessness of the attempt has been expressly admitted by
Owen in his most interesting work on the “Nature of Limbs.” On the
ordinary view of the independent creation of each being, we can only say
that so it is; that it has pleased the Creator to construct all the animals and
plants in each great class on a uniform plan; but this is not a scientific
explanation.
The explanation is to a large extent simple, on the theory of the selection
of successive slight modifications, each 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. The bones of a limb
might be shortened and flattened to any extent, becoming at the same time
enveloped in thick membrane, so as to serve as a fin; or a webbed hand
might have all its bones, or certain bones, lengthened to any extent, with the
membrane connecting them increased, so as to serve as a wing; yet all these
modifications would not tend to alter the framework of the bones or the
relative connexion of the parts. If we suppose that an early progenitor—the
archetype, as it may be called—of all mammals, birds and reptiles, had its
limbs constructed on the existing general pattern, for whatever purpose they
served, we can at once perceive the plain signification of the homologous
construction of the limbs throughout the class. So with the mouths of
insects, we have only to suppose that their common progenitor had an upper
lip, mandibles, and two pairs of maxillæ, these parts being perhaps very
simple in form; and then natural selection will account for the infinite
diversity in structure and function of the mouths of insects. Nevertheless, it

is conceivable that the general pattern of an organ might become so much
obscured as to be finally lost, by the reduction and ultimately by the
complete abortion of certain parts, by the fusion of other parts, and by the
doubling or multiplication of others, variations which we know to be within
the limits of possibility. In the paddles of the gigantic extinct sea-lizards,
and in the mouths of certain suctorial crustaceans, the general pattern seems
thus to have become partially obscured.
There is another and equally curious branch of our subject; namely, serial
homologies, or the comparison of the different parts or organs in the same
individual, and not of the same parts or organs in different members of the
same class. Most physiologists believe that the bones of the skull are
homologous—that is, correspond in number and in relative connexion—
with the elemental parts of a certain number of vertebræ. The anterior and
posterior limbs in all the higher vertebrate classes are plainly homologous.
So it is with the wonderfully complex jaws and legs of crustaceans. It is
familiar to almost every one, that in a flower the relative position of the
sepals, petals, stamens, and pistils, as well as their intimate structure, are
intelligible on the view that they consist of metamorphosed leaves, arranged
in a spire. In monstrous plants, we often get direct evidence of the
possibility of one organ being transformed into another; and we can actually
see, during the early or embryonic stages of development in flowers, as well
as in crustaceans and many other animals, that organs, which when mature
become extremely different are at first exactly alike.
How inexplicable are the cases of serial homologies on the ordinary view
of creation! Why should the brain be enclosed in a box composed of such
numerous and such extraordinarily shaped pieces of bone apparently
representing vertebræ? As Owen has remarked, the benefit derived from the
yielding of the separate pieces in the act of parturition by mammals, will by
no means explain the same construction in the skulls of birds and reptiles.
Why should similar bones have been created to form the wing and the leg
of a bat, used as they are for such totally different purposes, namely flying
and walking? Why should one crustacean, which has an extremely complex
mouth formed of many parts, consequently always have fewer legs; or
conversely, those with many legs have simpler mouths? Why should the
sepals, petals, stamens, and pistils, in each flower, though fitted for such
distinct purposes, be all constructed on the same pattern?

On the theory of natural selection, we can, to a certain extent, answer
these questions. We need not here consider how the bodies of some animals
first became divided into a series of segments, or how they became divided
into right and left sides, with corresponding organs, for such questions are
almost beyond investigation. It is, however, probable that some serial
structures are the result of cells multiplying by division, entailing the
multiplication of the parts developed from such cells. It must suffice for our
purpose to bear in mind that an indefinite repetition of the same part or
organ is the common characteristic, as Owen has remarked, of all low or
little specialised forms; therefore the unknown progenitor of the Vertebrata
probably possessed many vertebræ; the unknown progenitor of the
Articulata, many segments; and the unknown progenitor of flowering
plants, many leaves arranged in one or more spires. We have also formerly
seen that parts many times repeated are eminently liable to vary, not only in
number, but in form. Consequently such parts, being already present in
considerable numbers, and being highly variable, would naturally afford the
materials for adaptation to the most different purposes; yet they would
generally retain, through the force of inheritance, plain traces of their
original or fundamental resemblance. They would retain this resemblance
all the more, as the variations, which afforded the basis for their subsequent
modification through natural selection, would tend from the first to be
similar; the parts being at an early stage of growth alike, and being
subjected to nearly the same conditions. Such parts, whether more or less
modified, unless their common origin became wholly obscured, would be
serially homologous.
In the great class of molluscs, though the parts in distinct species can be
shown to be homologous, only a few serial homologies; such as the valves
of Chitons, can be indicated; that is, we are seldom enabled to say that one
part is homologous with another part in the same individual. And we can
understand this fact; for in molluscs, even in the lowest members of the
class, we do not find nearly so much indefinite repetition of any one part as
we find in the other great classes of the animal and vegetable kingdoms.
But morphology is a much more complex subject than it at first appears,
as has lately been well shown in a remarkable paper by Mr. E. Ray
Lankester, who has drawn an important distinction between certain classes
of cases which have all been equally ranked by naturalists as homologous.
He proposes to call the structures which resemble each other in distinct

animals, owing to their descent from a common progenitor with subsequent
modification, homogenous; and the resemblances which cannot thus be
accounted for, he proposes to call homoplastic. For instance, he believes
that the hearts of birds and mammals are as a whole homogenous—that is,
have been derived from a common progenitor; but that the four cavities of
the heart in the two classes are homoplastic—that is, have been
independently developed. Mr. Lankester also adduces the close resemblance
of the parts on the right and left sides of the body, and in the successive
segments of the same individual animal; and here we have parts commonly
called homologous which bear no relation to the descent of distinct species
from a common progenitor. Homoplastic structures are the same with those
which I have classed, though in a very imperfect manner, as analogous
modifications or resemblances. Their formation may be attributed in part to
distinct organisms, or to distinct parts of the same organism, having varied
in an analogous manner; and in part to similar modifications, having been
preserved for the same general purpose or function, of which many
instances have been given.
Naturalists frequently speak of the skull as formed of metamorphosed
vertebræ; the jaws of crabs as metamorphosed legs; the stamens and pistils
in flowers as metamorphosed leaves; but it would in most cases be more
correct, as Professor Huxley has remarked, to speak of both skull and
vertebræ, jaws and legs, &c., as having been metamorphosed, not one from
the other, as they now exist, but from some common and simpler element.
Most naturalists, however, use such language only in a metaphorical sense:
they are far from meaning that during a long course of descent, primordial
organs of any kind—vertebræ in the one case and legs in the other—have
actually been converted into skulls or jaws. Yet so strong is the appearance
of this having occurred that naturalists can hardly avoid employing
language having this plain signification. According to the views here
maintained, such language may be used literally; and the wonderful fact of
the jaws, for instance, of a crab retaining numerous characters, which they
probably would have retained through inheritance, if they had really been
metamorphosed from true though extremely simple legs, is in part
explained.

Development and Embryology.

This is one of the most important subjects in the whole round of natural
history. The metamorphoses of insects, with which every one is familiar, are
generally effected abruptly by a few stages; but the transformations are in
reality numerous and gradual, though concealed. A certain ephemerous
insect (Chlöeon) during its development, moults, as shown by Sir J.
Lubbock, above twenty times, and each time undergoes a certain amount of
change; and in this case we see the act of metamorphosis performed in a
primary and gradual manner. Many insects, and especially certain
crustaceans, show us what wonderful changes of structure can be effected
during development. Such changes, however, reach their acme in the so-
called alternate generations of some of the lower animals. It is, for instance,
an astonishing fact that a delicate branching coralline, studded with polypi,
and attached to a submarine rock, should produce, first by budding and then
by transverse division, a host of huge floating jelly-fishes; and that these
should produce eggs, from which are hatched swimming animalcules,
which attach themselves to rocks and become developed into branching
corallines; and so on in an endless cycle. The belief in the essential identity
of the process of alternate generation and of ordinary metamorphosis has
been greatly strengthened by Wagner’s discovery of the larva or maggot of
a fly, namely the Cecidomyia, producing asexually other larvæ, and these
others, which finally are developed into mature males and females,
propagating their kind in the ordinary manner by eggs.
It may be worth notice that when Wagner’s remarkable discovery was
first announced, I was asked how was it possible to account for the larvæ of
this fly having acquired the power of a sexual reproduction. As long as the
case remained unique no answer could be given. But already Grimm has
shown that another fly, a Chironomus, reproduces itself in nearly the same
manner, and he believes that this occurs frequently in the order. It is the
pupa, and not the larva, of the Chironomus which has this power; and
Grimm further shows that this case, to a certain extent, “unites that of the
Cecidomyia with the parthenogenesis of the Coccidæ;” the term
parthenogenesis implying that the mature females of the Coccidæ are
capable of producing fertile eggs without the concourse of the male. Certain
animals belonging to several classes are now known to have the power of
ordinary reproduction at an unusually early age; and we have only to
accelerate parthenogenetic reproduction by gradual steps to an earlier and
earlier age—Chironomus showing us an almost exactly intermediate stage,

viz., that of the pupa—and we can perhaps account for the marvellous case
of the Cecidomyia.
It has already been stated that various parts in the same individual, which
are exactly alike during an early embryonic period, become widely different
and serve for widely different purposes in the adult state. So again it has
been shown that generally the embryos of the most distinct species
belonging to the same class are closely similar, but become, when fully
developed, widely dissimilar. A better proof of this latter fact cannot be
given than the statement by Von Baer that “the embryos of mammalia, of
birds, lizards and snakes, probably also of chelonia, are in the earliest states
exceedingly like one another, both as a whole and in the mode of
development of their parts; so much so, in fact, that we can often
distinguish the embryos only by their size. In my possession are two little
embryos in spirit, whose names I have omitted to attach, and at present I am
quite unable to say to what class they belong. They may be lizards or small
birds, or very young mammalia, so complete is the similarity in the mode of
formation of the head and trunk in these animals. The extremities, however,
are still absent in these embryos. But even if they had existed in the earliest
stage of their development we should learn nothing, for the feet of lizards
and mammals, the wings and feet of birds, no less than the hands and feet of
man, all arise from the same fundamental form.” The larvæ of most
crustaceans, at corresponding stages of development, closely resemble each
other, however different the adults may become; and so it is with very many
other animals. A trace of the law of embryonic resemblance occasionally
lasts till a rather late age: thus birds of the same genus, and of allied genera,
often resemble each other in their immature plumage; as we see in the
spotted feathers in the young of the thrush group. In the cat tribe, most of
the species when adult are striped or spotted in lines; and stripes or spots
can be plainly distinguished in the whelp of the lion and the puma. We
occasionally, though rarely, see something of the same kind in plants; thus
the first leaves of the ulex or furze, and the first leaves of the phyllodineous
acacias, are pinnate or divided like the ordinary leaves of the leguminosæ.
The points of structure, in which the embryos of widely different animals
within the same class resemble each other, often have no direct relation to
their conditions of existence. We cannot, for instance, suppose that in the
embryos of the vertebrata the peculiar loop-like courses of the arteries near
the branchial slits are related to similar conditions—in the young mammal

which is nourished in the womb of its mother, in the egg of the bird which
is hatched in a nest, and in the spawn of a frog under water. We have no
more reason to believe in such a relation than we have to believe that the
similar bones in the hand of a man, wing of a bat, and fin of a porpoise, are
related to similar conditions of life. No one supposes that the stripes on the
whelp of a lion, or the spots on the young blackbird, are of any use to these
animals.
The case, however, is different when an animal, during any part of its
embryonic career, is active, and has to provide for itself. The period of
activity may come on earlier or later in life; but whenever it comes on, the
adaptation of the larva to its conditions of life is just as perfect and as
beautiful as in the adult animal. In how important a manner this has acted,
has recently been well shown by Sir J. Lubbock in his remarks on the close
similarity of the larvæ of some insects belonging to very different orders,
and on the dissimilarity of the larvæ of other insects within the same order,
according to their habits of life. Owing to such adaptations the similarity of
the larvæ of allied animals is sometimes greatly obscured; especially when
there is a division of labour during the different stages of development, as
when the same larva has during one stage to search for food, and during
another stage has to search for a place of attachment. Cases can even be
given of the larvæ of allied species, or groups of species, differing more
from each other than do the adults. In most cases, however, the larvæ,
though active, still obey, more or less closely, the law of common
embryonic resemblance. Cirripedes afford a good instance of this: even the
illustrious Cuvier did not perceive that a barnacle was a crustacean: but a
glance at the larva shows this in an unmistakable manner. So again the two
main divisions of cirripedes, the pedunculated and sessile, though differing
widely in external appearance, have larvæ in all their stages barely
distinguishable.
The embryo in the course of development generally rises in organisation.
I use this expression, though I am aware that it is hardly possible to define
clearly what is meant by organisation being higher or lower. But no one
probably will dispute that the butterfly is higher than the caterpillar. In some
cases, however, the mature animal must be considered as lower in the scale
than the larva, as with certain parasitic crustaceans. To refer once again to
cirripedes: the larvæ in the first stage have three pairs of locomotive organs,
a simple single eye, and a probosciformed mouth, with which they feed

largely, for they increase much in size. In the second stage, answering to the
chrysalis stage of butterflies, they have six pairs of beautifully constructed
natatory legs, a pair of magnificent compound eyes, and extremely complex
antennæ; but they have a closed and imperfect mouth, and cannot feed: their
function at this stage is, to search out by their well-developed organs of
sense, and to reach by their active powers of swimming, a proper place on
which to become attached and to undergo their final metamorphosis. When
this is completed they are fixed for life: their legs are now converted into
prehensile organs; they again obtain a well-constructed mouth; but they
have no antennæ, and their two eyes are now reconverted into a minute,
single, simple eye-spot. In this last and complete state, cirripedes may be
considered as either more highly or more lowly organised than they were in
the larval condition. But in some genera the larvæ become developed into
hermaphrodites having the ordinary structure, or into what I have called
complemental males; and in the latter the development has assuredly been
retrograde; for the male is a mere sack, which lives for a short time and is
destitute of mouth, stomach, and every other organ of importance,
excepting those for reproduction.
We are so much accustomed to see a difference in structure between the
embryo and the adult, that we are tempted to look at this difference as in
some necessary manner contingent on growth. But there is no reason why,
for instance, the wing of a bat, or the fin of a porpoise, should not have been
sketched out with all their parts in proper proportion, as soon as any part
became visible. In some whole groups of animals and in certain members of
other groups this is the case, and the embryo does not at any period differ
widely from the adult: thus Owen has remarked in regard to cuttle-fish,
“there is no metamorphosis; the cephalopodic character is manifested long
before the parts of the embryo are completed.” Land-shells and fresh-water
crustaceans are born having their proper forms, while the marine members
of the same two great classes pass through considerable and often great
changes during their development. Spiders, again, barely undergo any
metamorphosis. The larvæ of most insects pass through a worm-like stage,
whether they are active and adapted to diversified habits, or are inactive
from being placed in the midst of proper nutriment, or from being fed by
their parents; but in some few cases, as in that of Aphis, if we look to the
admirable drawings of the development of this insect, by Professor Huxley,
we see hardly any trace of the vermiform stage.

Sometimes it is only the earlier developmental stages which fail. Thus,
Fritz Müller has made the remarkable discovery that certain shrimp-like
crustaceans (allied to Penoeus) first appear under the simple nauplius-form,
and after passing through two or more zoëa-stages, and then through the
mysis-stage, finally acquire their mature structure: now in the whole great
malacostracan order, to which these crustaceans belong, no other member is
as yet known to be first developed under the nauplius-form, though many
appear as zoëas; nevertheless Müller assigns reasons for his belief, that if
there had been no suppression of development, all these crustaceans would
have appeared as nauplii.
How, then, can we explain these several facts in embryology—namely,
the very general, though not universal, difference in structure between the
embryo and the adult; the various parts in the same individual embryo,
which ultimately become very unlike, and serve for diverse purposes, being
at an early period of growth alike; the common, but not invariable,
resemblance between the embryos or larvæ of the most distinct species in
the same class; the embryo often retaining, while within the egg or womb,
structures which are of no service to it, either at that or at a later period of
life; on the other hand, larvæ which have to provide for their own wants,
being perfectly adapted to the surrounding conditions; and lastly, the fact of
certain larvæ standing higher in the scale of organisation than the mature
animal into which they are developed? I believe that all these facts can be
explained as follows.
It is commonly assumed, perhaps from monstrosities affecting the
embryo at a very early period, that slight variations or individual differences
necessarily appear at an equally early period. We have little evidence on this
head, but what we have certainly points the other way; for it is notorious
that breeders of cattle, horses and various fancy animals, cannot positively
tell, until some time after birth, what will be the merits and demerits of their
young animals. We see this plainly in our own children; we cannot tell
whether a child will be tall or short, or what its precise features will be. The
question is not, at what period of life any variation may have been caused,
but at what period the effects are displayed. The cause may have acted, and
I believe often has acted, on one or both parents before the act of
generation. It deserves notice that it is of no importance to a very young
animal, as long as it is nourished and protected by its parent, whether most
of its characters are acquired a little earlier or later in life. It would not

signify, for instance, to a bird which obtained its food by having a much-
curved beak whether or not while young it possessed a beak of this shape,
as long as it was fed by its parents.
I have stated in the first chapter, that at whatever age any variation first
appears in the parent, it tends to reappear at a corresponding age in the
offspring. Certain variations can only appear at corresponding ages; for
instance, peculiarities in the caterpillar, cocoon, or imago states of the silk-
moth; or, again, in the full-grown horns of cattle. But variations which, for
all that we can see might have appeared either earlier or later in life,
likewise tend to reappear at a corresponding age in the offspring and parent.
I am far from meaning that this is invariably the case, and I could give
several exceptional cases of variations (taking the word in the largest sense)
which have supervened at an earlier age in the child than in the parent.
These two principles, namely, that slight variations generally appear at a
not very early period of life, and are inherited at a corresponding not early
period, explain, as I believe, all the above specified leading facts in
embryology. But first let us look to a few analogous cases in our domestic
varieties. Some authors who have written on Dogs maintain that the
greyhound and bull-dog, though so different, are really closely allied
varieties, descended from the same wild stock, hence I was curious to see
how far their puppies differed from each other. I was told by breeders that
they differed just as much as their parents, and this, judging by the eye,
seemed almost to be the case; but on actually measuring the old dogs and
their six-days-old puppies, I found that the puppies had not acquired nearly
their full amount of proportional difference. So, again, I was told that the
foals of cart and race-horses—breeds which have been almost wholly
formed by selection under domestication—differed as much as the full-
grown animals; but having had careful measurements made of the dams and
of three-days-old colts of race and heavy cart-horses, I find that this is by no
means the case.
As we have conclusive evidence that the breeds of the Pigeon are
descended from a single wild species, I compared the young pigeons within
twelve hours after being hatched. I carefully measured the proportions (but
will not here give the details) of the beak, width of mouth, length of nostril
and of eyelid, size of feet and length of leg, in the wild parent species, in
pouters, fantails, runts, barbs, dragons, carriers, and tumblers. Now, some of

these birds, when mature, differ in so extraordinary a manner in the length
and form of beak, and in other characters, that they would certainly have
been ranked as distinct genera if found in a state of nature. But when the
nestling birds of these several breeds were placed in a row, though most of
them could just be distinguished, the proportional differences in the above
specified points were incomparably less than in the full-grown birds. Some
characteristic points of difference—for instance, that of the width of mouth
—could hardly be detected in the young. But there was one remarkable
exception to this rule, for the young of the short-faced tumbler differed
from the young of the wild rock-pigeon, and of the other breeds, in almost
exactly the same proportions as in the adult stage.
These facts are explained by the above two principles. Fanciers select
their dogs, horses, pigeons, &c., for breeding, when nearly grown up. They
are indifferent whether the desired qualities are acquired earlier or later in
life, if the full-grown animal possesses them. And the cases just given, more
especially that of the pigeons, show that the characteristic differences which
have been accumulated by man’s selection, and which give value to his
breeds, do not generally appear at a very early period of life, and are
inherited at a corresponding not early period. But the case of the short-faced
tumbler, which when twelve hours old possessed its proper characters,
proves that this is not the universal rule; for here the characteristic
differences must either have appeared at an earlier period than usual, or, if
not so, the differences must have been inherited, not at a corresponding, but
at an earlier age.
Now, let us apply these two principles to species in a state of nature. Let
us take a group of birds, descended from some ancient form and modified
through natural selection for different habits. Then, from the many slight
successive variations having supervened in the several species at a not early
age, and having been inherited at a corresponding age, the young will have
been but little modified, and they will still resemble each other much more
closely than do the adults, just as we have seen with the breeds of the
pigeon. We may extend this view to widely distinct structures and to whole
classes. The fore-limbs, for instance, which once served as legs to a remote
progenitor, may have become, through a long course of modification,
adapted in one descendant to act as hands, in another as paddles, in another
as wings; but on the above two principles the fore-limbs will not have been
much modified in the embryos of these several forms; although in each

form the fore-limb will differ greatly in the adult state. Whatever influence
long continued use or disuse may have had in modifying the limbs or other
parts of any species, this will chiefly or solely have affected it when nearly
mature, when it was compelled to use its full powers to gain its own living;
and the effects thus produced will have been transmitted to the offspring at
a corresponding nearly mature age. Thus the young will not be modified, or
will be modified only in a slight degree, through the effects of the increased
use or disuse of parts.
With some animals the successive variations may have supervened at a
very early period of life, or the steps may have been inherited at an earlier
age than that at which they first occurred. In either of these cases the young
or embryo will closely resemble the mature parent-form, as we have seen
with the short-faced tumbler. And this is the rule of development in certain
whole groups, or in certain sub-groups alone, as with cuttle-fish, land-
shells, fresh-water crustaceans, spiders, and some members of the great
class of insects. With respect to the final cause of the young in such groups
not passing through any metamorphosis, we can see that this would follow
from the following contingencies: namely, from the young having to
provide at a very early age for their own wants, and from their following the
same habits of life with their parents; for in this case it would be
indispensable for their existence that they should be modified in the same
manner as their parents. Again, with respect to the singular fact that many
terrestrial and fresh-water animals do not undergo any metamorphosis,
while marine members of the same groups pass through various
transformations, Fritz Müller has suggested that the process of slowly
modifying and adapting an animal to live on the land or in fresh water,
instead of in the sea, would be greatly simplified by its not passing through
any larval stage; for it is not probable that places well adapted for both the
larval and mature stages, under such new and greatly changed habits of life,
would commonly be found unoccupied or ill-occupied by other organisms.
In this case the gradual acquirement at an earlier and earlier age of the adult
structure would be favoured by natural selection; and all traces of former
metamorphoses would finally be lost.
If, on the other hand, it profited the young of an animal to follow habits
of life slightly different from those of the parent-form, and consequently to
be constructed on a slightly different plan, or if it profited a larva already
different from its parent to change still further, then, on the principle of

inheritance at corresponding ages, the young or the larvæ might be rendered
by natural selection more and more different from their parents to any
conceivable extent. Differences in the larva might, also, become correlated
with successive stages of its development; so that the larva, in the first
stage, might come to differ greatly from the larva in the second stage, as is
the case with many animals. The adult might also become fitted for sites or
habits, in which organs of locomotion or of the senses, &c., would be
useless; and in this case the metamorphosis would be retrograde.
From the remarks just made we can see how by changes of structure in
the young, in conformity with changed habits of life, together with
inheritance at corresponding ages, animals might come to pass through
stages of development, perfectly distinct from the primordial condition of
their adult progenitors. Most of our best authorities are now convinced that
the various larval and pupal stages of insects have thus been acquired
through adaptation, and not through inheritance from some ancient form.
The curious case of Sitaris—a beetle which passes through certain unusual
stages of development—will illustrate how this might occur. The first larval
form is described by M. Fabre, as an active, minute insect, furnished with
six legs, two long antennæ, and four eyes. These larvæ are hatched in the
nests of bees; and when the male bees emerge from their burrows, in the
spring, which they do before the females, the larvæ spring on them, and
afterwards crawl on to the females while paired with the males. As soon as
the female bee deposits her eggs on the surface of the honey stored in the
cells, the larvæ of the Sitaris leap on the eggs and devour them. Afterwards
they undergo a complete change; their eyes disappear; their legs and
antennæ become rudimentary, and they feed on honey; so that they now
more closely resemble the ordinary larvæ of insects; ultimately they
undergo a further transformation, and finally emerge as the perfect beetle.
Now, if an insect, undergoing transformations like those of the Sitaris, were
to become the progenitor of a whole new class of insects, the course of
development of the new class would be widely different from that of our
existing insects; and the first larval stage certainly would not represent the
former condition of any adult and ancient form.
On the other hand it is highly probable that with many animals the
embryonic or larval stages show us, more or less completely, the condition
of the progenitor of the whole group in its adult state. In the great class of
the Crustacea, forms wonderfully distinct from each other, namely, suctorial

parasites, cirripedes, entomostraca, and even the malacostraca, appear at
first as larvæ under the nauplius-form; and as these larvæ live and feed in
the open sea, and are not adapted for any peculiar habits of life, and from
other reasons assigned by Fritz Müller, it is probable that at some very
remote period an independent adult animal, resembling the Nauplius,
existed, and subsequently produced, along several divergent lines of
descent, the above-named great Crustacean groups. So again, it is probable,
from what we know of the embryos of mammals, birds, fishes and reptiles,
that these animals are the modified descendants of some ancient progenitor,
which was furnished in its adult state with branchiæ, a swim-bladder, four
fin-like limbs, and a long tail, all fitted for an aquatic life.
As all the organic beings, extinct and recent, which have ever lived, can
be arranged within a few great classes; and as all within each class have,
according to our theory, been connected together by fine gradations, the
best, and, if our collections were nearly perfect, the only possible
arrangement, would be genealogical; descent being the hidden bond of
connexion which naturalists have been seeking under the term of the
Natural System. On this view we can understand how it is that, in the eyes
of most naturalists, the structure of the embryo is even more important for
classification than that of the adult. In two or more groups of animals,
however much they may differ from each other in structure and habits in
their adult condition, if they pass through closely similar embryonic stages,
we may feel assured that they are all descended from one parent-form, and
are therefore closely related. Thus, community in embryonic structure
reveals community of descent; but dissimilarity in embryonic development
does not prove discommunity of descent, for in one of two groups the
developmental stages may have been suppressed, or may have been so
greatly modified through adaptation to new habits of life as to be no longer
recognisable. Even in groups, in which the adults have been modified to an
extreme degree, community of origin is often revealed by the structure of
the larvæ; we have seen, for instance, that cirripedes, though externally so
like shell-fish, are at once known by their larvæ to belong to the great class
of crustaceans. As the embryo often shows us more or less plainly the
structure of the less modified and ancient progenitor of the group, we can
see why ancient and extinct forms so often resemble in their adult state the
embryos of existing species of the same class. Agassiz believes this to be a
universal law of nature; and we may hope hereafter to see the law proved

true. It can, however, be proved true only in those cases in which the
ancient state of the progenitor of the group has not been wholly obliterated,
either by successive variations having supervened at a very early period of
growth, or by such variations having been inherited at an earlier age than
that at which they first appeared. It should also be borne in mind, that the
law may be true, but yet, owing to the geological record not extending far
enough back in time, may remain for a long period, or for ever, incapable of
demonstration. The law will not strictly hold good in those cases in which
an ancient form became adapted in its larval state to some special line of
life, and transmitted the same larval state to a whole group of descendants;
for such larval state will not resemble any still more ancient form in its
adult state.
Thus, as it seems to me, the leading facts in embryology, which are
second to none in importance, are explained on the principle of variations in
the many descendants from some one ancient progenitor, having appeared
at a not very early period of life, and having been inherited at a
corresponding period. Embryology rises greatly in interest, when we look at
the embryo as a picture, more or less obscured, of the progenitor, either in
its adult or larval state, of all the members of the same great class.

Rudimentary, Atrophied, and Aborted Organs.

Organs or parts in this strange condition, bearing the plain stamp of
inutility, are extremely common, or even general, throughout nature. It
would be impossible to name one of the higher animals in which some part
or other is not in a rudimentary condition. In the mammalia, for instance,
the males possess rudimentary mammæ; in snakes one lobe of the lungs is
rudimentary; in birds the “bastard-wing” may safely be considered as a
rudimentary digit, and in some species the whole wing is so far rudimentary
that it cannot be used for flight. What can be more curious than the presence
of teeth in foetal whales, which when grown up have not a tooth in their
heads; or the teeth, which never cut through the gums, in the upper jaws of
unborn calves?
Rudimentary organs plainly declare their origin and meaning in various
ways. There are beetles belonging to closely allied species, or even to the
same identical species, which have either full-sized and perfect wings, or
mere rudiments of membrane, which not rarely lie under wing-covers

firmly soldered together; and in these cases it is impossible to doubt, that
the rudiments represent wings. Rudimentary organs sometimes retain their
potentiality: this occasionally occurs with the mammæ of male mammals,
which have been known to become well developed and to secrete milk. So
again in the udders of the genus Bos, there are normally four developed and
two rudimentary teats; but the latter in our domestic cows sometimes
become well developed and yield milk. In regard to plants, the petals are
sometimes rudimentary, and sometimes well developed in the individuals of
the same species. In certain plants having separated sexes Kölreuter found
that by crossing a species, in which the male flowers included a rudiment of
a pistil, with an hermaphrodite species, having of course a well-developed
pistil, the rudiment in the hybrid offspring was much increased in size; and
this clearly shows that the rudimentary and perfect pistils are essentially
alike in nature. An animal may possess various parts in a perfect state, and
yet they may in one sense be rudimentary, for they are useless: thus the
tadpole of the common salamander or water-newt, as Mr. G.H. Lewes
remarks, “has gills, and passes its existence in the water; but the
Salamandra atra, which lives high up among the mountains, brings forth its
young full-formed. This animal never lives in the water. Yet if we open a
gravid female, we find tadpoles inside her with exquisitely feathered gills;
and when placed in water they swim about like the tadpoles of the water-
newt. Obviously this aquatic organisation has no reference to the future life
of the animal, nor has it any adaptation to its embryonic condition; it has
solely reference to ancestral adaptations, it repeats a phase in the
development of its progenitors.”
An organ, serving for two purposes, may become rudimentary or utterly
aborted for one, even the more important purpose, and remain perfectly
efficient for the other. Thus, in plants, the office of the pistil is to allow the
pollen-tubes to reach the ovules within the ovarium. The pistil consists of a
stigma supported on the style; but in some Compositæ, the male florets,
which of course cannot be fecundated, have a rudimentary pistil, for it is not
crowned with a stigma; but the style remains well developed and is clothed
in the usual manner with hairs, which serve to brush the pollen out of the
surrounding and conjoined anthers. Again, an organ may become
rudimentary for its proper purpose, and be used for a distinct one: in certain
fishes the swim-bladder seems to be rudimentary for its proper function of

giving buoyancy, but has become converted into a nascent breathing organ
or lung. Many similar instances could be given.
Useful organs, however little they may be developed, unless we have
reason to suppose that they were formerly more highly developed, ought
not to be considered as rudimentary. They may be in a nascent condition,
and in progress towards further development. Rudimentary organs, on the
other hand, are either quite useless, such as teeth which never cut through
the gums, or almost useless, such as the wings of an ostrich, which serve
merely as sails. As organs in this condition would formerly, when still less
developed, have been of even less use than at present, they cannot formerly
have been produced through variation and natural selection, which acts
solely by the preservation of useful modifications. They have been partially
retained by the power of inheritance, and relate to a former state of things.
It is, however, often difficult to distinguish between rudimentary and
nascent organs; for we can judge only by analogy whether a part is capable
of further development, in which case alone it deserves to be called nascent.
Organs in this condition will always be somewhat rare; for beings thus
provided will commonly have been supplanted by their successors with the
same organ in a more perfect state, and consequently will have become long
ago extinct. The wing of the penguin is of high service, acting as a fin; it
may, therefore, represent the nascent state of the wing: not that I believe this
to be the case; it is more probably a reduced organ, modified for a new
function: the wing of the Apteryx, on the other hand, is quite useless, and is
truly rudimentary. Owen considers the simple filamentary limbs of the
Lepidosiren as the “beginnings of organs which attain full functional
development in higher vertebrates;” but, according to the view lately
advocated by Dr. Günther, they are probably remnants, consisting of the
persistent axis of a fin, with the lateral rays or branches aborted. The
mammary glands of the Ornithorhynchus may be considered, in comparison
with the udders of a cow, as in a nascent condition. The ovigerous frena of
certain cirripedes, which have ceased to give attachment to the ova and are
feebly developed, are nascent branchiæ.
Rudimentary organs in the individuals of the same species are very liable
to vary in the degree of their development and in other respects. In closely
allied species, also, the extent to which the same organ has been reduced
occasionally differs much. This latter fact is well exemplified in the state of
the wings of female moths belonging to the same family. Rudimentary

organs may be utterly aborted; and this implies, that in certain animals or
plants, parts are entirely absent which analogy would lead us to expect to
find in them, and which are occasionally found in monstrous individuals.
Thus in most of the Scrophulariaceæ the fifth stamen is utterly aborted; yet
we may conclude that a fifth stamen once existed, for a rudiment of it is
found in many species of the family, and this rudiment occasionally
becomes perfectly developed, as may sometimes be seen in the common
snap-dragon. In tracing the homologies of any part in different members of
the same class, nothing is more common, or, in order fully to understand the
relations of the parts, more useful than the discovery of rudiments. This is
well shown in the drawings given by Owen of the leg bones of the horse,
ox, and rhinoceros.
It is an important fact that rudimentary organs, such as teeth in the upper
jaws of whales and ruminants, can often be detected in the embryo, but
afterwards wholly disappear. It is also, I believe, a universal rule, that a
rudimentary part is of greater size in the embryo relatively to the adjoining
parts, than in the adult; so that the organ at this early age is less
rudimentary, or even cannot be said to be in any degree rudimentary. Hence
rudimentary organs in the adult are often said to have retained their
embryonic condition.
I have now given the leading facts with respect to rudimentary organs. In
reflecting on them, every one must be struck with astonishment; for the
same reasoning power which tells us that most parts and organs are
exquisitely adapted for certain purposes, tells us with equal plainness that
these rudimentary or atrophied organs are imperfect and useless. In works
on natural history, rudimentary organs are generally said to have been
created “for the sake of symmetry,” or in order “to complete the scheme of
nature.” But this is not an explanation, merely a restatement of the fact. Nor
is it consistent with itself: thus the boa-constrictor has rudiments of hind
limbs and of a pelvis, and if it be said that these bones have been retained
“to complete the scheme of nature,” why, as Professor Weismann asks, have
they not been retained by other snakes, which do not possess even a vestige
of these same bones? What would be thought of an astronomer who
maintained that the satellites revolve in elliptic courses round their planets
“for the sake of symmetry,” because the planets thus revolve round the sun?
An eminent physiologist accounts for the presence of rudimentary organs,
by supposing that they serve to excrete matter in excess, or matter injurious

to the system; but can we suppose that the minute papilla, which often
represents the pistil in male flowers, and which is formed of mere cellular
tissue, can thus act? Can we suppose that rudimentary teeth, which are
subsequently absorbed, are beneficial to the rapidly growing embryonic calf
by removing matter so precious as phosphate of lime? When a man’s
fingers have been amputated, imperfect nails have been known to appear on
the stumps, and I could as soon believe that these vestiges of nails are
developed in order to excrete horny matter, as that the rudimentary nails on
the fin of the manatee have been developed for this same purpose.
On the view of descent with modification, the origin of rudimentary
organs is comparatively simple; and we can understand to a large extent the
laws governing their imperfect development. We have plenty of cases of
rudimentary organs in our domestic productions, as the stump of a tail in
tailless breeds, the vestige of an ear in earless breeds of sheep—the
reappearance of minute dangling horns in hornless breeds of cattle, more
especially, according to Youatt, in young animals—and the state of the
whole flower in the cauliflower. We often see rudiments of various parts in
monsters; but I doubt whether any of these cases throw light on the origin
of rudimentary organs in a state of nature, further than by showing that
rudiments can be produced; for the balance of evidence clearly indicates
that species under nature do not undergo great and abrupt changes. But we
learn from the study of our domestic productions that the disuse of parts
leads to their reduced size; and that the result is inherited.
It appears probable that disuse has been the main agent in rendering
organs rudimentary. It would at first lead by slow steps to the more and
more complete reduction of a part, until at last it became rudimentary—as
in the case of the eyes of animals inhabiting dark caverns, and of the wings
of birds inhabiting oceanic islands, which have seldom been forced by
beasts of prey to take flight, and have ultimately lost the power of flying.
Again, an organ, useful under certain conditions, might become injurious
under others, as with the wings of beetles living on small and exposed
islands; and in this case natural selection will have aided in reducing the
organ, until it was rendered harmless and rudimentary.
Any change in structure and function, which can be effected by small
stages, is within the power of natural selection; so that an organ rendered,
through changed habits of life, useless or injurious for one purpose, might

be modified and used for another purpose. An organ might, also, be retained
for one alone of its former functions. Organs, originally formed by the aid
of natural selection, when rendered useless may well be variable, for their
variations can no longer be checked by natural selection. All this agrees
well with what we see under nature. Moreover, at whatever period of life
either disuse or selection reduces an organ, and this will generally be when
the being has come to maturity and to exert its full powers of action, the
principle of inheritance at corresponding ages will tend to reproduce the
organ in its reduced state at the same mature age, but will seldom affect it in
the embryo. Thus we can understand the greater size of rudimentary organs
in the embryo relatively to the adjoining parts, and their lesser relative size
in the adult. If, for instance, the digit of an adult animal was used less and
less during many generations, owing to some change of habits, or if an
organ or gland was less and less functionally exercised, we may infer that it
would become reduced in size in the adult descendants of this animal, but
would retain nearly its original standard of development in the embryo.
There remains, however, this difficulty. After an organ has ceased being
used, and has become in consequence much reduced, how can it be still
further reduced in size until the merest vestige is left; and how can it be
finally quite obliterated? It is scarcely possible that disuse can go on
producing any further effect after the organ has once been rendered
functionless. Some additional explanation is here requisite which I cannot
give. If, for instance, it could be proved that every part of the organisation
tends to vary in a greater degree towards diminution than toward
augmentation of size, then we should be able to understand how an organ
which has become useless would be rendered, independently of the effects
of disuse, rudimentary and would at last be wholly suppressed; for the
variations towards diminished size would no longer be checked by natural
selection. The principle of the economy of growth, explained in a former
chapter, by which the materials forming any part, if not useful to the
possessor, are saved as far as is possible, will perhaps come into play in
rendering a useless part rudimentary. But this principle will almost
necessarily be confined to the earlier stages of the process of reduction; for
we cannot suppose that a minute papilla, for instance, representing in a
male flower the pistil of the female flower, and formed merely of cellular
tissue, could be further reduced or absorbed for the sake of economising
nutriment.

Finally, as rudimentary organs, by whatever steps they may have been
degraded into their present useless condition, are the record of a former
state of things, and have been retained solely through the power of
inheritance—we can understand, on the genealogical view of classification,
how it is that systematists, in placing organisms in their proper places in the
natural system, have often found rudimentary parts as useful as, or even
sometimes more useful than, parts of high physiological importance.
Rudimentary organs may be compared with the letters in a word, still
retained in the spelling, but become useless in the pronunciation, but which
serve as a clue for its derivation. On the view of descent with modification,
we may conclude that the existence of organs in a rudimentary, imperfect,
and useless condition, or quite aborted, far from presenting a strange
difficulty, as they assuredly do on the old doctrine of creation, might even
have been anticipated in accordance with the views here explained.

Summary.

In this chapter I have attempted to show that the arrangement of all
organic beings throughout all time in groups under groups—that the nature
of the relationships by which all living and extinct organisms are united by
complex, radiating, and circuitous lines of affinities into a few grand classes
—the rules followed and the difficulties encountered by naturalists in their
classifications—the value set upon characters, if constant and prevalent,
whether of high or of the most trifling importance, or, as with rudimentary
organs of no importance—the wide opposition in value between analogical
or adaptive characters, and characters of true affinity; and other such rules
—all naturally follow if we admit the common parentage of allied forms,
together with their modification through variation and natural selection,
with the contingencies of extinction and divergence of character. In
considering this view of classification, it should be borne in mind that the
element of descent has been universally used in ranking together the sexes,
ages, dimorphic forms, and acknowledged varieties of the same species,
however much they may differ from each other in structure. If we extend
the use of this element of descent—the one certainly known cause of
similarity in organic beings—we shall understand what is meant by the
Natural System: it is genealogical in its attempted arrangement, with the

grades of acquired difference marked by the terms, varieties, species,
genera, families, orders, and classes.
On this same view of descent with modification, most of the great facts
in Morphology become intelligible—whether we look to the same pattern
displayed by the different species of the same class in their homologous
organs, to whatever purpose applied, or to the serial and lateral homologies
in each individual animal and plant.
On the principle of successive slight variations, not necessarily or
generally supervening at a very early period of life, and being inherited at a
corresponding period, we can understand the leading facts in embryology;
namely, the close resemblance in the individual embryo of the parts which
are homologous, and which when matured become widely different in
structure and function; and the resemblance of the homologous parts or
organs in allied though distinct species, though fitted in the adult state for
habits as different as is possible. Larvæ are active embryos, which have
become specially modified in a greater or less degree in relation to their
habits of life, with their modifications inherited at a corresponding early
age. On these same principles, and bearing in mind that when organs are
reduced in size, either from disuse or through natural selection, it will
generally be at that period of life when the being has to provide for its own
wants, and bearing in mind how strong is the force of inheritance—the
occurrence of rudimentary organs might even have been anticipated. The
importance of embryological characters and of rudimentary organs in
classification is intelligible, on the view that a natural arrangement must be
genealogical.
Finally, the several classes of facts which have been considered in this
chapter, seem to me to proclaim so plainly, that the innumerable species,
genera and families, with which this world is peopled, are all descended,
each within its own class or group, from common parents, and have all been
modified in the course of descent, that I should without hesitation adopt this
view, even if it were unsupported by other facts or arguments.

CHAPTER XV.
RECAPITULATION AND CONCLUSION.

Recapitulation of the objections to the theory of Natural
Selection—Recapitulation of the general and special
circumstances in its favour—Causes of the general belief in
the immutability of species—How far the theory of Natural
Selection may be extended—Effects of its adoption on the
study of Natural History—Concluding remarks.

As this whole volume is one long argument, it may be convenient to the
reader to have the leading facts and inferences briefly recapitulated.
That many and serious objections may be advanced against the theory of
descent with modification through variation and natural selection, I do not
deny. I have endeavoured to give to them their full force. Nothing at first
can appear more difficult to believe than that the more complex organs and
instincts have been perfected, not by means superior to, though analogous
with, human reason, but by the accumulation of innumerable slight
variations, each good for the individual possessor. Nevertheless, this
difficulty, though appearing to our imagination insuperably great, cannot be
considered real if we admit the following propositions, namely, that all parts
of the organisation and instincts offer, at least individual differences—that
there is a struggle for existence leading to the preservation of profitable
deviations of structure or instinct—and, lastly, that gradations in the state of
perfection of each organ may have existed, each good of its kind. The truth
of these propositions cannot, I think, be disputed.
It is, no doubt, extremely difficult even to conjecture by what gradations
many structures have been perfected, more especially among broken and
failing groups of organic beings, which have suffered much extinction; but
we see so many strange gradations in nature, that we ought to be extremely

cautious in saying that any organ or instinct, or any whole structure, could
not have arrived at its present state by many graduated steps. There are, it
must be admitted, cases of special difficulty opposed to the theory of
natural selection; and one of the most curious of these is the existence in the
same community of two or three defined castes of workers or sterile female
ants; but I have attempted to show how these difficulties can be mastered.
With respect to the almost universal sterility of species when first
crossed, which forms so remarkable a contrast with the almost universal
fertility of varieties when crossed, I must refer the reader to the
recapitulation of the facts given at the end of the ninth chapter, which seem
to me conclusively to show that this sterility is no more a special
endowment than is the incapacity of two distinct kinds of trees to be grafted
together; but that it is incidental on differences confined to the reproductive
systems of the intercrossed species. We see the truth of this conclusion in
the vast difference in the results of crossing the same two species
reciprocally—that is, when one species is first used as the father and then as
the mother. Analogy from the consideration of dimorphic and trimorphic
plants clearly leads to the same conclusion, for when the forms are
illegitimately united, they yield few or no seed, and their offspring are more
or less sterile; and these forms belong to the same undoubted species, and
differ from each other in no respect except in their reproductive organs and
functions.
Although the fertility of varieties when intercrossed, and of their mongrel
offspring, has been asserted by so many authors to be universal, this cannot
be considered as quite correct after the facts given on the high authority of
Gärtner and Kölreuter. Most of the varieties which have been experimented
on have been produced under domestication; and as domestication (I do not
mean mere confinement) almost certainly tends to eliminate that sterility
which, judging from analogy, would have affected the parent-species if
intercrossed, we ought not to expect that domestication would likewise
induce sterility in their modified descendants when crossed. This
elimination of sterility apparently follows from the same cause which
allows our domestic animals to breed freely under diversified
circumstances; and this again apparently follows from their having been
gradually accustomed to frequent changes in their conditions of life.

A double and parallel series of facts seems to throw much light on the
sterility of species, when first crossed, and of their hybrid offspring. On the
one side, there is good reason to believe that slight changes in the
conditions of life give vigour and fertility to all organic beings. We know
also that a cross between the distinct individuals of the same variety, and
between distinct varieties, increases the number of their offspring, and
certainly gives to them increased size and vigour. This is chiefly owing to
the forms which are crossed having been exposed to somewhat different
conditions of life; for I have ascertained by a labourious series of
experiments that if all the individuals of the same variety be subjected
during several generations to the same conditions, the good derived from
crossing is often much diminished or wholly disappears. This is one side of
the case. On the other side, we know that species which have long been
exposed to nearly uniform conditions, when they are subjected under
confinement to new and greatly changed conditions, either perish, or if they
survive, are rendered sterile, though retaining perfect health. This does not
occur, or only in a very slight degree, with our domesticated productions,
which have long been exposed to fluctuating conditions. Hence when we
find that hybrids produced by a cross between two distinct species are few
in number, owing to their perishing soon after conception or at a very early
age, or if surviving that they are rendered more or less sterile, it seems
highly probable that this result is due to their having been in fact subjected
to a great change in their conditions of life, from being compounded of two
distinct organisations. He who will explain in a definite manner why, for
instance, an elephant or a fox will not breed under confinement in its native
country, whilst the domestic pig or dog will breed freely under the most
diversified conditions, will at the same time be able to give a definite
answer to the question why two distinct species, when crossed, as well as
their hybrid offspring, are generally rendered more or less sterile, while two
domesticated varieties when crossed and their mongrel offspring are
perfectly fertile.
Turning to geographical distribution, the difficulties encountered on the
theory of descent with modification are serious enough. All the individuals
of the same species, and all the species of the same genus, or even higher
group, are descended from common parents; and therefore, in however
distant and isolated parts of the world they may now be found, they must in
the course of successive generations have travelled from some one point to

all the others. We are often wholly unable even to conjecture how this could
have been effected. Yet, as we have reason to believe that some species
have retained the same specific form for very long periods of time,
immensely long as measured by years, too much stress ought not to be laid
on the occasional wide diffusion of the same species; for during very long
periods there will always have been a good chance for wide migration by
many means. A broken or interrupted range may often be accounted for by
the extinction of the species in the intermediate regions. It cannot be denied
that we are as yet very ignorant as to the full extent of the various climatical
and geographical changes which have affected the earth during modern
periods; and such changes will often have facilitated migration. As an
example, I have attempted to show how potent has been the influence of the
Glacial period on the distribution of the same and of allied species
throughout the world. We are as yet profoundly ignorant of the many
occasional means of transport. With respect to distinct species of the same
genus, inhabiting distant and isolated regions, as the process of
modification has necessarily been slow, all the means of migration will
have been possible during a very long period; and consequently the
difficulty of the wide diffusion of the species of the same genus is in some
degree lessened.
As according to the theory of natural selection an interminable number of
intermediate forms must have existed, linking together all the species in
each group by gradations as fine as our existing varieties, it may be asked,
Why do we not see these linking forms all around us? Why are not all
organic beings blended together in an inextricable chaos? With respect to
existing forms, we should remember that we have no right to expect
(excepting in rare cases) to discover directly connecting links between
them, but only between each and some extinct and supplanted form. Even
on a wide area, which has during a long period remained continuous, and of
which the climatic and other conditions of life change insensibly in
proceeding from a district occupied by one species into another district
occupied by a closely allied species, we have no just right to expect often to
find intermediate varieties in the intermediate zones. For we have reason to
believe that only a few species of a genus ever undergo change; the other
species becoming utterly extinct and leaving no modified progeny. Of the
species which do change, only a few within the same country change at the
same time; and all modifications are slowly effected. I have also shown that

the intermediate varieties which probably at first existed in the intermediate
zones, would be liable to be supplanted by the allied forms on either hand;
for the latter, from existing in greater numbers, would generally be modified
and improved at a quicker rate than the intermediate varieties, which
existed in lesser numbers; so that the intermediate varieties would, in the
long run, be supplanted and exterminated.
On this doctrine of the extermination of an infinitude of connecting links,
between the living and extinct inhabitants of the world, and at each
successive period between the extinct and still older species, why is not
every geological formation charged with such links? Why does not every
collection of fossil remains afford plain evidence of the gradation and
mutation of the forms of life? Although geological research has
undoubtedly revealed the former existence of many links, bringing
numerous forms of life much closer together, it does not yield the infinitely
many fine gradations between past and present species required on the
theory, and this is the most obvious of the many objections which may be
urged against it. Why, again, do whole groups of allied species appear,
though this appearance is often false, to have come in suddenly on the
successive geological stages? Although we now know that organic beings
appeared on this globe, at a period incalculably remote, long before the
lowest bed of the Cambrian system was deposited, why do we not find
beneath this system great piles of strata stored with the remains of the
progenitors of the Cambrian fossils? For on the theory, such strata must
somewhere have been deposited at these ancient and utterly unknown
epochs of the world’s history.
I can answer these questions and objections only on the supposition that
the geological record is far more imperfect than most geologists believe.
The number of specimens in all our museums is absolutely as nothing
compared with the countless generations of countless species which have
certainly existed. The parent form of any two or more species would not be
in all its characters directly intermediate between its modified offspring,
any more than the rock-pigeon is directly intermediate in crop and tail
between its descendants, the pouter and fantail pigeons. We should not be
able to recognise a species as the parent of another and modified species, if
we were to examine the two ever so closely, unless we possessed most of
the intermediate links; and owing to the imperfection of the geological
record, we have no just right to expect to find so many links. If two or three,

or even more linking forms were discovered, they would simply be ranked
by many naturalists as so many new species, more especially if found in
different geological substages, let their differences be ever so slight.
Numerous existing doubtful forms could be named which are probably
varieties; but who will pretend that in future ages so many fossil links will
be discovered, that naturalists will be able to decide whether or not these
doubtful forms ought to be called varieties? Only a small portion of the
world has been geologically explored. Only organic beings of certain
classes can be preserved in a fossil condition, at least in any great number.
Many species when once formed never undergo any further change but
become extinct without leaving modified descendants; and the periods
during which species have undergone modification, though long as
measured by years, have probably been short in comparison with the
periods during which they retained the same form. It is the dominant and
widely ranging species which vary most frequently and vary most, and
varieties are often at first local—both causes rendering the discovery of
intermediate links in any one formation less likely. Local varieties will not
spread into other and distant regions until they are considerably modified
and improved; and when they have spread, and are discovered in a
geological formation, they appear as if suddenly created there, and will be
simply classed as new species. Most formations have been intermittent in
their accumulation; and their duration has probably been shorter than the
average duration of specific forms. Successive formations are in most cases
separated from each other by blank intervals of time of great length, for
fossiliferous formations thick enough to resist future degradation can, as a
general rule, be accumulated only where much sediment is deposited on the
subsiding bed of the sea. During the alternate periods of elevation and of
stationary level the record will generally be blank. During these latter
periods there will probably be more variability in the forms of life; during
periods of subsidence, more extinction.
With respect to the absence of strata rich in fossils beneath the Cambrian
formation, I can recur only to the hypothesis given in the tenth chapter;
namely, that though our continents and oceans have endured for an
enormous period in nearly their present relative positions, we have no
reason to assume that this has always been the case; consequently
formations much older than any now known may lie buried beneath the
great oceans. With respect to the lapse of time not having been sufficient

since our planet was consolidated for the assumed amount of organic
change, and this objection, as urged by Sir William Thompson, is probably
one of the gravest as yet advanced, I can only say, firstly, that we do not
know at what rate species change, as measured by years, and secondly, that
many philosophers are not as yet willing to admit that we know enough of
the constitution of the universe and of the interior of our globe to speculate
with safety on its past duration.
That the geological record is imperfect all will admit; but that it is
imperfect to the degree required by our theory, few will be inclined to
admit. If we look to long enough intervals of time, geology plainly declares
that species have all changed; and they have changed in the manner
required by the theory, for they have changed slowly and in a graduated
manner. We clearly see this in the fossil remains from consecutive
formations invariably being much more closely related to each other than
are the fossils from widely separated formations.
Such is the sum of the several chief objections and difficulties which may
justly be urged against the theory; and I have now briefly recapitulated the
answers and explanations which, as far as I can see, may be given. I have
felt these difficulties far too heavily during many years to doubt their
weight. But it deserves especial notice that the more important objections
relate to questions on which we are confessedly ignorant; nor do we know
how ignorant we are. We do not know all the possible transitional
gradations between the simplest and the most perfect organs; it cannot be
pretended that we know all the varied means of Distribution during the long
lapse of years, or that we know how imperfect is the Geological Record.
Serious as these several objections are, in my judgment they are by no
means sufficient to overthrow the theory of descent with subsequent
modification.

Now let us turn to the other side of the argument. Under domestication
we see much variability, caused, or at least excited, by changed conditions
of life; but often in so obscure a manner, that we are tempted to consider the
variations as spontaneous. Variability is governed by many complex laws,
by correlated growth, compensation, the increased use and disuse of parts,
and the definite action of the surrounding conditions. There is much
difficulty in ascertaining how largely our domestic productions have been

modified; but we may safely infer that the amount has been large, and that
modifications can be inherited for long periods. As long as the conditions of
life remain the same, we have reason to believe that a modification, which
has already been inherited for many generations, may continue to be
inherited for an almost infinite number of generations. On the other hand
we have evidence that variability, when it has once come into play, does not
cease under domestication for a very long period; nor do we know that it
ever ceases, for new varieties are still occasionally produced by our oldest
domesticated productions.
Variability is not actually caused by man; he only unintentionally exposes
organic beings to new conditions of life and then nature acts on the
organisation and causes it to vary. But man can and does select the
variations given to him by nature, and thus accumulates them in any desired
manner. He thus adapts animals and plants for his own benefit or pleasure.
He may do this methodically, or he may do it unconsciously by preserving
the individuals most useful or pleasing to him without any intention of
altering the breed. It is certain that he can largely influence the character of
a breed by selecting, in each successive generation, individual differences
so slight as to be inappreciable except by an educated eye. This unconscious
process of selection has been the great agency in the formation of the most
distinct and useful domestic breeds. That many breeds produced by man
have to a large extent the character of natural species, is shown by the
inextricable doubts whether many of them are varieties or aboriginally
distinct species.
There is no reason why the principles which have acted so efficiently
under domestication should not have acted under nature. In the survival of
favoured individuals and races, during the constantly recurrent Struggle for
Existence, we see a powerful and ever-acting form of Selection. The
struggle for existence inevitably follows from the high geometrical ratio of
increase which is common to all organic beings. This high rate of increase
is proved by calculation—by the rapid increase of many animals and plants
during a succession of peculiar seasons, and when naturalised in new
countries. More individuals are born than can possibly survive. A grain in
the balance may determine which individuals shall live and which shall die
—which variety or species shall increase in number, and which shall
decrease, or finally become extinct. As the individuals of the same species
come in all respects into the closest competition with each other, the

struggle will generally be most severe between them; it will be almost
equally severe between the varieties of the same species, and next in
severity between the species of the same genus. On the other hand the
struggle will often be severe between beings remote in the scale of nature.
The slightest advantage in certain individuals, at any age or during any
season, over those with which they come into competition, or better
adaptation in however slight a degree to the surrounding physical
conditions, will, in the long run, turn the balance.
With animals having separated sexes, there will be in most cases a
struggle between the males for the possession of the females. The most
vigorous males, or those which have most successfully struggled with their
conditions of life, will generally leave most progeny. But success will often
depend on the males having special weapons or means of defence or
charms; and a slight advantage will lead to victory.
As geology plainly proclaims that each land has undergone great physical
changes, we might have expected to find that organic beings have varied
under nature, in the same way as they have varied under domestication. And
if there has been any variability under nature, it would be an unaccountable
fact if natural selection had not come into play. It has often been asserted,
but the assertion is incapable of proof, that the amount of variation under
nature is a strictly limited quantity. Man, though acting on external
characters alone and often capriciously, can produce within a short period a
great result by adding up mere individual differences in his domestic
productions; and every one admits that species present individual
differences. But, besides such differences, all naturalists admit that natural
varieties exist, which are considered sufficiently distinct to be worthy of
record in systematic works. No one has drawn any clear distinction between
individual differences and slight varieties; or between more plainly marked
varieties and subspecies and species. On separate continents, and on
different parts of the same continent, when divided by barriers of any kind,
and on outlying islands, what a multitude of forms exist, which some
experienced naturalists rank as varieties, others as geographical races or sub
species, and others as distinct, though closely allied species!
If, then, animals and plants do vary, let it be ever so slightly or slowly,
why should not variations or individual differences, which are in any way
beneficial, be preserved and accumulated through natural selection, or the

survival of the fittest? If man can by patience select variations useful to
him, why, under changing and complex conditions of life, should not
variations useful to nature’s living products often arise, and be preserved or
selected? What limit can be put to this power, acting during long ages and
rigidly scrutinising the whole constitution, structure, and habits of each
creature, favouring the good and rejecting the bad? I can see no limit to this
power, in slowly and beautifully adapting each form to the most complex
relations of life. The theory of natural selection, even if we look no further
than this, seems to be in the highest degree probable. I have already
recapitulated, as fairly as I could, the opposed difficulties and objections:
now let us turn to the special facts and arguments in favour of the theory.

On the view that species are only strongly marked and permanent
varieties, and that each species first existed as a variety, we can see why it is
that no line of demarcation can be drawn between species, commonly
supposed to have been produced by special acts of creation, and varieties
which are acknowledged to have been produced by secondary laws. On this
same view we can understand how it is that in a region where many species
of a genus have been produced, and where they now flourish, these same
species should present many varieties; for where the manufactory of species
has been active, we might expect, as a general rule, to find it still in action;
and this is the case if varieties be incipient species. Moreover, the species of
the larger genera, which afford the greater number of varieties or incipient
species, retain to a certain degree the character of varieties; for they differ
from each other by a less amount of difference than do the species of
smaller genera. The closely allied species also of a larger genera apparently
have restricted ranges, and in their affinities they are clustered in little
groups round other species—in both respects resembling varieties. These
are strange relations on the view that each species was independently
created, but are intelligible if each existed first as a variety.
As each species tends by its geometrical rate of reproduction to increase
inordinately in number; and as the modified descendants of each species
will be enabled to increase by as much as they become more diversified in
habits and structure, so as to be able to seize on many and widely different
places in the economy of nature, there will be a constant tendency in natural
selection to preserve the most divergent offspring of any one species. Hence

during a long-continued course of modification, the slight differences
characteristic of varieties of the same species, tend to be augmented into the
greater differences characteristic of the species of the same genus. New and
improved varieties will inevitably supplant and exterminate the older, less
improved and intermediate varieties; and thus species are rendered to a
large extent defined and distinct objects. Dominant species belonging to the
larger groups within each class tend to give birth to new and dominant
forms; so that each large group tends to become still larger, and at the same
time more divergent in character. But as all groups cannot thus go on
increasing in size, for the world would not hold them, the more dominant
groups beat the less dominant. This tendency in the large groups to go on
increasing in size and diverging in character, together with the inevitable
contingency of much extinction, explains the arrangement of all the forms
of life in groups subordinate to groups, all within a few great classes, which
has prevailed throughout all time. This grand fact of the grouping of all
organic beings under what is called the Natural System, is utterly
inexplicable on the theory of creation.
As natural selection acts solely by accumulating slight, successive,
favourable variations, it can produce no great or sudden modifications; it
can act only by short and slow steps. Hence, the canon of “Natura non facit
saltum,” which every fresh addition to our knowledge tends to confirm, is
on this theory intelligible. We can see why throughout nature the same
general end is gained by an almost infinite diversity of means, for every
peculiarity when once acquired is long inherited, and structures already
modified in many different ways have to be adapted for the same general
purpose. We can, in short, see why nature is prodigal in variety, though
niggard in innovation. But why this should be a law of nature if each
species has been independently created no man can explain.
Many other facts are, as it seems to me, explicable on this theory. How
strange it is that a bird, under the form of a woodpecker, should prey on
insects on the ground; that upland geese, which rarely or never swim, would
possess webbed feet; that a thrush-like bird should dive and feed on sub-
aquatic insects; and that a petrel should have the habits and structure fitting
it for the life of an auk! and so in endless other cases. But on the view of
each species constantly trying to increase in number, with natural selection
always ready to adapt the slowly varying descendants of each to any

unoccupied or ill-occupied place in nature, these facts cease to be strange,
or might even have been anticipated.
We can to a certain extent understand how it is that there is so much
beauty throughout nature; for this may be largely attributed to the agency of
selection. That beauty, according to our sense of it, is not universal, must be
admitted by every one who will look at some venomous snakes, at some
fishes, and at certain hideous bats with a distorted resemblance to the
human face. Sexual selection has given the most brilliant colours, elegant
patterns, and other ornaments to the males, and sometimes to both sexes of
many birds, butterflies and other animals. With birds it has often rendered
the voice of the male musical to the female, as well as to our ears. Flowers
and fruit have been rendered conspicuous by brilliant colours in contrast
with the green foliage, in order that the flowers may be easily seen, visited
and fertilised by insects, and the seeds disseminated by birds. How it comes
that certain colours, sounds and forms should give pleasure to man and the
lower animals, that is, how the sense of beauty in its simplest form was first
acquired, we do not know any more than how certain odours and flavours
were first rendered agreeable.
As natural selection acts by competition, it adapts and improves the
inhabitants of each country only in relation to their co-inhabitants; so that
we need feel no surprise at the species of any one country, although on the
ordinary view supposed to have been created and specially adapted for that
country, being beaten and supplanted by the naturalised productions from
another land. Nor ought we to marvel if all the contrivances in nature be
not, as far as we can judge, absolutely perfect; as in the case even of the
human eye; or if some of them be abhorrent to our ideas of fitness. We need
not marvel at the sting of the bee, when used against the enemy, causing the
bee’s own death; at drones being produced in such great numbers for one
single act, and being then slaughtered by their sterile sisters; at the
astonishing waste of pollen by our fir-trees; at the instinctive hatred of the
queen-bee for her own fertile daughters; at ichneumonidæ feeding within
the living bodies of caterpillars; and at other such cases. The wonder,
indeed, is, on the theory of natural selection, that more cases of the want of
absolute perfection have not been detected.
The complex and little known laws governing the production of varieties
are the same, as far as we can judge, with the laws which have governed the

production of distinct species. In both cases physical conditions seem to
have produced some direct and definite effect, but how much we cannot
say. Thus, when varieties enter any new station, they occasionally assume
some of the characters proper to the species of that station. With both
varieties and species, use and disuse seem to have produced a considerable
effect; for it is impossible to resist this conclusion when we look, for
instance, at the logger-headed duck, which has wings incapable of flight, in
nearly the same condition as in the domestic duck; or when we look at the
burrowing tucu-tucu, which is occasionally blind, and then at certain moles,
which are habitually blind and have their eyes covered with skin; or when
we look at the blind animals inhabiting the dark caves of America and
Europe. With varieties and species, correlated variation seems to have
played an important part, so that when one part has been modified other
parts have been necessarily modified. With both varieties and species,
reversions to long-lost characters occasionally occur. How inexplicable on
the theory of creation is the occasional appearance of stripes on the
shoulders and legs of the several species of the horse-genus and of their
hybrids! How simply is this fact explained if we believe that these species
are all descended from a striped progenitor, in the same manner as the
several domestic breeds of the pigeon are descended from the blue and
barred rock-pigeon!
On the ordinary view of each species having been independently created,
why should specific characters, or those by which the species of the same
genus differ from each other, be more variable than the generic characters in
which they all agree? Why, for instance, should the colour of a flower be
more likely to vary in any one species of a genus, if the other species
possess differently coloured flowers, than if all possessed the same coloured
flowers? If species are only well-marked varieties, of which the characters
have become in a high degree permanent, we can understand this fact; for
they have already varied since they branched off from a common progenitor
in certain characters, by which they have come to be specifically distinct
from each other; therefore these same characters would be more likely
again to vary than the generic characters which have been inherited without
change for an immense period. It is inexplicable on the theory of creation
why a part developed in a very unusual manner in one species alone of a
genus, and therefore, as we may naturally infer, of great importance to that
species, should be eminently liable to variation; but, on our view, this part

has undergone, since the several species branched off from a common
progenitor, an unusual amount of variability and modification, and therefore
we might expect the part generally to be still variable. But a part may be
developed in the most unusual manner, like the wing of a bat, and yet not be
more variable than any other structure, if the part be common to many
subordinate forms, that is, if it has been inherited for a very long period; for
in this case it will have been rendered constant by long-continued natural
selection.
Glancing at instincts, marvellous as some are, they offer no greater
difficulty than do corporeal structures on the theory of the natural selection
of successive, slight, but profitable modifications. We can thus understand
why nature moves by graduated steps in endowing different animals of the
same class with their several instincts. I have attempted to show how much
light the principle of gradation throws on the admirable architectural
powers of the hive-bee. Habit no doubt often comes into play in modifying
instincts; but it certainly is not indispensable, as we see in the case of neuter
insects, which leave no progeny to inherit the effects of long-continued
habit. On the view of all the species of the same genus having descended
from a common parent, and having inherited much in common, we can
understand how it is that allied species, when placed under widely different
conditions of life, yet follow nearly the same instincts; why the thrushes of
tropical and temperate South America, for instance, line their nests with
mud like our British species. On the view of instincts having been slowly
acquired through natural selection, we need not marvel at some instincts
being not perfect and liable to mistakes, and at many instincts causing other
animals to suffer.
If species be only well-marked and permanent varieties, we can at once
see why their crossed offspring should follow the same complex laws in
their degrees and kinds of resemblance to their parents—in being absorbed
into each other by successive crosses, and in other such points—as do the
crossed offspring of acknowledged varieties. This similarity would be a
strange fact, if species had been independently created and varieties had
been produced through secondary laws.
If we admit that the geological record is imperfect to an extreme degree,
then the facts, which the record does give, strongly support the theory of
descent with modification. New species have come on the stage slowly and

at successive intervals; and the amount of change after equal intervals of
time, is widely different in different groups. The extinction of species and
of whole groups of species, which has played so conspicuous a part in the
history of the organic world, almost inevitably follows from the principle of
natural selection; for old forms are supplanted by new and improved forms.
Neither single species nor groups of species reappear when the chain of
ordinary generation is once broken. The gradual diffusion of dominant
forms, with the slow modification of their descendants, causes the forms of
life, after long intervals of time, to appear as if they had changed
simultaneously throughout the world. The fact of the fossil remains of each
formation being in some degree intermediate in character between the
fossils in the formations above and below, is simply explained by their
intermediate position in the chain of descent. The grand fact that all extinct
beings can be classed with all recent beings, naturally follows from the
living and the extinct being the offspring of common parents. As species
have generally diverged in character during their long course of descent and
modification, we can understand why it is that the more ancient forms, or
early progenitors of each group, so often occupy a position in some degree
intermediate between existing groups. Recent forms are generally looked
upon as being, on the whole, higher in the scale of organisation than ancient
forms; and they must be higher, in so far as the later and more improved
forms have conquered the older and less improved forms in the struggle for
life; they have also generally had their organs more specialised for different
functions. This fact is perfectly compatible with numerous beings still
retaining simple and but little improved structures, fitted for simple
conditions of life; it is likewise compatible with some forms having
retrograded in organisation, by having become at each stage of descent
better fitted for new and degraded habits of life. Lastly, the wonderful law
of the long endurance of allied forms on the same continent—of marsupials
in Australia, of edentata in America, and other such cases—is intelligible,
for within the same country the existing and the extinct will be closely
allied by descent.
Looking to geographical distribution, if we admit that there has been
during the long course of ages much migration from one part of the world
to another, owing to former climatical and geographical changes and to the
many occasional and unknown means of dispersal, then we can understand,
on the theory of descent with modification, most of the great leading facts

in Distribution. We can see why there should be so striking a parallelism in
the distribution of organic beings throughout space, and in their geological
succession throughout time; for in both cases the beings have been
connected by the bond of ordinary generation, and the means of
modification have been the same. We see the full meaning of the wonderful
fact, which has struck every traveller, namely, that on the same continent,
under the most diverse conditions, under heat and cold, on mountain and
lowland, on deserts and marshes, most of the inhabitants within each great
class are plainly related; for they are the descendants of the same
progenitors and early colonists. On this same principle of former migration,
combined in most cases with modification, we can understand, by the aid of
the Glacial period, the identity of some few plants, and the close alliance of
many others, on the most distant mountains, and in the northern and
southern temperate zones; and likewise the close alliance of some of the
inhabitants of the sea in the northern and southern temperate latitudes,
though separated by the whole intertropical ocean. Although two countries
may present physical conditions as closely similar as the same species ever
require, we need feel no surprise at their inhabitants being widely different,
if they have been for a long period completely sundered from each other;
for as the relation of organism to organism is the most important of all
relations, and as the two countries will have received colonists at various
periods and in different proportions, from some other country or from each
other, the course of modification in the two areas will inevitably have been
different.
On this view of migration, with subsequent modification, we see why
oceanic islands are inhabited by only few species, but of these, why many
are peculiar or endemic forms. We clearly see why species belonging to
those groups of animals which cannot cross wide spaces of the ocean, as
frogs and terrestrial mammals, do not inhabit oceanic islands; and why, on
the other hand, new and peculiar species of bats, animals which can traverse
the ocean, are often found on islands far distant from any continent. Such
cases as the presence of peculiar species of bats on oceanic islands and the
absence of all other terrestrial mammals, are facts utterly inexplicable on
the theory of independent acts of creation.
The existence of closely allied representative species in any two areas,
implies, on the theory of descent with modification, that the same parent-
forms formerly inhabited both areas; and we almost invariably find that

wherever many closely allied species inhabit two areas, some identical
species are still common to both. Wherever many closely allied yet distinct
species occur, doubtful forms and varieties belonging to the same groups
likewise occur. It is a rule of high generality that the inhabitants of each
area are related to the inhabitants of the nearest source whence immigrants
might have been derived. We see this in the striking relation of nearly all the
plants and animals of the Galapagos Archipelago, of Juan Fernandez, and of
the other American islands, to the plants and animals of the neighbouring
American mainland; and of those of the Cape de Verde Archipelago, and of
the other African islands to the African mainland. It must be admitted that
these facts receive no explanation on the theory of creation.
The fact, as we have seen, that all past and present organic beings can be
arranged within a few great classes, in groups subordinate to groups, and
with the extinct groups often falling in between the recent groups, is
intelligible on the theory of natural selection with its contingencies of
extinction and divergence of character. On these same principles we see
how it is that the mutual affinities of the forms within each class are so
complex and circuitous. We see why certain characters are far more
serviceable than others for classification; why adaptive characters, though
of paramount importance to the beings, are of hardly any importance in
classification; why characters derived from rudimentary parts, though of no
service to the beings, are often of high classificatory value; and why
embryological characters are often the most valuable of all. The real
affinities of all organic beings, in contradistinction to their adaptive
resemblances, are due to inheritance or community of descent. The Natural
System is a genealogical arrangement, with the acquired grades of
difference, marked by the terms, varieties, species, genera, families, &c.;
and we have to discover the lines of descent by the most permanent
characters, whatever they may be, and of however slight vital importance.
The similar framework of bones in the hand of a man, wing of a bat, fin
of the porpoise, and leg of the horse—the same number of vertebræ forming
the neck of the giraffe and of the elephant—and innumerable other such
facts, at once explain themselves on the theory of descent with slow and
slight successive modifications. The similarity of pattern in the wing and in
the leg of a bat, though used for such different purpose—in the jaws and
legs of a crab—in the petals, stamens, and pistils of a flower, is likewise, to
a large extent, intelligible on the view of the gradual modification of parts

or organs, which were aboriginally alike in an early progenitor in each of
these classes. On the principle of successive variations not always
supervening at an early age, and being inherited at a corresponding not
early period of life, we clearly see why the embryos of mammals, birds,
reptiles, and fishes should be so closely similar, and so unlike the adult
forms. We may cease marvelling at the embryo of an air-breathing mammal
or bird having branchial slits and arteries running in loops, like those of a
fish which has to breathe the air dissolved in water by the aid of well-
developed branchiæ.
Disuse, aided sometimes by natural selection, will often have reduced
organs when rendered useless under changed habits or conditions of life;
and we can understand on this view the meaning of rudimentary organs. But
disuse and selection will generally act on each creature, when it has come to
maturity and has to play its full part in the struggle for existence, and will
thus have little power on an organ during early life; hence the organ will not
be reduced or rendered rudimentary at this early age. The calf, for instance,
has inherited teeth, which never cut through the gums of the upper jaw,
from an early progenitor having well-developed teeth; and we may believe,
that the teeth in the mature animal were formerly reduced by disuse owing
to the tongue and palate, or lips, having become excellently fitted through
natural selection to browse without their aid; whereas in the calf, the teeth
have been left unaffected, and on the principle of inheritance at
corresponding ages have been inherited from a remote period to the present
day. On the view of each organism with all its separate parts having been
specially created, how utterly inexplicable is it that organs bearing the plain
stamp of inutility, such as the teeth in the embryonic calf or the shrivelled
wings under the soldered wing-covers of many beetles, should so frequently
occur. Nature may be said to have taken pains to reveal her scheme of
modification, by means of rudimentary organs, of embryological and
homologous structures, but we are too blind to understand her meaning.

I have now recapitulated the facts and considerations which have
thoroughly convinced me that species have been modified, during a long
course of descent. This has been effected chiefly through the natural
selection of numerous successive, slight, favourable variations; aided in an
important manner by the inherited effects of the use and disuse of parts; and

in an unimportant manner, that is, in relation to adaptive structures, whether
past or present, by the direct action of external conditions, and by variations
which seem to us in our ignorance to arise spontaneously. It appears that I
formerly underrated the frequency and value of these latter forms of
variation, as leading to permanent modifications of structure independently
of natural selection. But as my conclusions have lately been much
misrepresented, and it has been stated that I attribute the modification of
species exclusively to natural selection, I may be permitted to remark that in
the first edition of this work, and subsequently, I placed in a most
conspicuous position—namely, at the close of the Introduction—the
following words: “I am convinced that natural selection has been the main
but not the exclusive means of modification.” This has been of no avail.
Great is the power of steady misrepresentation; but the history of science
shows that fortunately this power does not long endure.
It can hardly be supposed that a false theory would explain, in so
satisfactory a manner as does the theory of natural selection, the several
large classes of facts above specified. It has recently been objected that this
is an unsafe method of arguing; but it is a method used in judging of the
common events of life, and has often been used by the greatest natural
philosophers. The undulatory theory of light has thus been arrived at; and
the belief in the revolution of the earth on its own axis was until lately
supported by hardly any direct evidence. It is no valid objection that science
as yet throws no light on the far higher problem of the essence or origin of
life. Who can explain what is the essence of the attraction of gravity? No
one now objects to following out the results consequent on this unknown
element of attraction; notwithstanding that Leibnitz formerly accused
Newton of introducing “occult qualities and miracles into philosophy.”
I see no good reasons why the views given in this volume should shock
the religious feelings of any one. It is satisfactory, as showing how transient
such impressions are, to remember that the greatest discovery ever made by
man, namely, the law of the attraction of gravity, was also attacked by
Leibnitz, “as subversive of natural, and inferentially of revealed, religion.”
A celebrated author and divine has written to me that “he has gradually
learned to see that it is just as noble a conception of the Deity to believe that
He created a few original forms capable of self-development into other and
needful forms, as to believe that He required a fresh act of creation to
supply the voids caused by the action of His laws.”

Why, it may be asked, until recently did nearly all the most eminent
living naturalists and geologists disbelieve in the mutability of species? It
cannot be asserted that organic beings in a state of nature are subject to no
variation; it cannot be proved that the amount of variation in the course of
long ages is a limited quantity; no clear distinction has been, or can be,
drawn between species and well-marked varieties. It cannot be maintained
that species when intercrossed are invariably sterile and varieties invariably
fertile; or that sterility is a special endowment and sign of creation. The
belief that species were immutable productions was almost unavoidable as
long as the history of the world was thought to be of short duration; and
now that we have acquired some idea of the lapse of time, we are too apt to
assume, without proof, that the geological record is so perfect that it would
have afforded us plain evidence of the mutation of species, if they had
undergone mutation.
But the chief cause of our natural unwillingness to admit that one species
has given birth to other and distinct species, is that we are always slow in
admitting any great changes of which we do not see the steps. The difficulty
is the same as that felt by so many geologists, when Lyell first insisted that
long lines of inland cliffs had been formed, and great valleys excavated, by
the agencies which we still see at work. The mind cannot possibly grasp the
full meaning of the term of even a million years; it cannot add up and
perceive the full effects of many slight variations, accumulated during an
almost infinite number of generations.
Although I am fully convinced of the truth of the views given in this
volume under the form of an abstract, I by no means expect to convince
experienced naturalists whose minds are stocked with a multitude of facts
all viewed, during a long course of years, from a point of view directly
opposite to mine. It is so easy to hide our ignorance under such expressions
as the “plan of creation,” “unity of design,” &c., and to think that we give
an explanation when we only restate a fact. Any one whose disposition
leads him to attach more weight to unexplained difficulties than to the
explanation of a certain number of facts will certainly reject the theory. A
few naturalists, endowed with much flexibility of mind, and who have
already begun to doubt the immutability of species, may be influenced by
this volume; but I look with confidence to the future, to young and rising
naturalists, who will be able to view both sides of the question with
impartiality. Whoever is led to believe that species are mutable will do good

service by conscientiously expressing his conviction; for thus only can the
load of prejudice by which this subject is overwhelmed be removed.
Several eminent naturalists have of late published their belief that a
multitude of reputed species in each genus are not real species; but that
other species are real, that is, have been independently created. This seems
to me a strange conclusion to arrive at. They admit that a multitude of
forms, which till lately they themselves thought were special creations, and
which are still thus looked at by the majority of naturalists, and which
consequently have all the external characteristic features of true species—
they admit that these have been produced by variation, but they refuse to
extend the same view to other and slightly different forms. Nevertheless,
they do not pretend that they can define, or even conjecture, which are the
created forms of life, and which are those produced by secondary laws.
They admit variation as a vera causa in one case, they arbitrarily reject it in
another, without assigning any distinction in the two cases. The day will
come when this will be given as a curious illustration of the blindness of
preconceived opinion. These authors seem no more startled at a miraculous
act of creation than at an ordinary birth. But do they really believe that at
innumerable periods in the earth’s history certain elemental atoms have
been commanded suddenly to flash into living tissues? Do they believe that
at each supposed act of creation one individual or many were produced?
Were all the infinitely numerous kinds of animals and plants created as eggs
or seed, or as full grown? and in the case of mammals, were they created
bearing the false marks of nourishment from the mother’s womb?
Undoubtedly some of these same questions cannot be answered by those
who believe in the appearance or creation of only a few forms of life or of
some one form alone. It has been maintained by several authors that it is as
easy to believe in the creation of a million beings as of one; but Maupertuis’
philosophical axiom “of least action” leads the mind more willingly to
admit the smaller number; and certainly we ought not to believe that
innumerable beings within each great class have been created with plain,
but deceptive, marks of descent from a single parent.
As a record of a former state of things, I have retained in the foregoing
paragraphs, and elsewhere, several sentences which imply that naturalists
believe in the separate creation of each species; and I have been much
censured for having thus expressed myself. But undoubtedly this was the
general belief when the first edition of the present work appeared. I

formerly spoke to very many naturalists on the subject of evolution, and
never once met with any sympathetic agreement. It is probable that some
did then believe in evolution, but they were either silent or expressed
themselves so ambiguously that it was not easy to understand their
meaning. Now, things are wholly changed, and almost every naturalist
admits the great principle of evolution. There are, however, some who still
think that species have suddenly given birth, through quite unexplained
means, to new and totally different forms. But, as I have attempted to show,
weighty evidence can be opposed to the admission of great and abrupt
modifications. Under a scientific point of view, and as leading to further
investigation, but little advantage is gained by believing that new forms are
suddenly developed in an inexplicable manner from old and widely
different forms, over the old belief in the creation of species from the dust
of the earth.
It may be asked how far I extend the doctrine of the modification of
species. The question is difficult to answer, because the more distinct the
forms are which we consider, by so much the arguments in favour of
community of descent become fewer in number and less in force. But some
arguments of the greatest weight extend very far. All the members of whole
classes are connected together by a chain of affinities, and all can be classed
on the same principle, in groups subordinate to groups. Fossil remains
sometimes tend to fill up very wide intervals between existing orders.
Organs in a rudimentary condition plainly show that an early progenitor
had the organ in a fully developed condition, and this in some cases implies
an enormous amount of modification in the descendants. Throughout whole
classes various structures are formed on the same pattern, and at a very
early age the embryos closely resemble each other. Therefore I cannot doubt
that the theory of descent with modification embraces all the members of
the same great class or kingdom. I believe that animals are descended from
at most only four or five progenitors, and plants from an equal or lesser
number.
Analogy would lead me one step further, namely, to the belief that all
animals and plants are descended from some one prototype. But analogy
may be a deceitful guide. Nevertheless all living things have much in
common, in their chemical composition, their cellular structure, their laws
of growth, and their liability to injurious influences. We see this even in so

trifling a fact as that the same poison often similarly affects plants and
animals; or that the poison secreted by the gall-fly produces monstrous
growths on the wild rose or oak-tree. With all organic beings, excepting
perhaps some of the very lowest, sexual reproduction seems to be
essentially similar. With all, as far as is at present known, the germinal
vesicle is the same; so that all organisms start from a common origin. If we
look even to the two main divisions—namely, to the animal and vegetable
kingdoms—certain low forms are so far intermediate in character that
naturalists have disputed to which kingdom they should be referred. As
Professor Asa Gray has remarked, “the spores and other reproductive bodies
of many of the lower algæ may claim to have first a characteristically
animal, and then an unequivocally vegetable existence.” Therefore, on the
principle of natural selection with divergence of character, it does not seem
incredible that, from some such low and intermediate form, both animals
and plants may have been developed; and, if we admit this, we must
likewise admit that all the organic beings which have ever lived on this
earth may be descended from some one primordial form. But this inference
is chiefly grounded on analogy, and it is immaterial whether or not it be
accepted. No doubt it is possible, as Mr. G.H. Lewes has urged, that at the
first commencement of life many different forms were evolved; but if so,
we may conclude that only a very few have left modified descendants. For,
as I have recently remarked in regard to the members of each great
kingdom, such as the Vertebrata, Articulata, &c., we have distinct evidence
in their embryological, homologous, and rudimentary structures, that within
each kingdom all the members are descended from a single progenitor.
When the views advanced by me in this volume, and by Mr. Wallace or
when analogous views on the origin of species are generally admitted, we
can dimly foresee that there will be a considerable revolution in natural
history. Systematists will be able to pursue their labours as at present; but
they will not be incessantly haunted by the shadowy doubt whether this or
that form be a true species. This, I feel sure and I speak after experience,
will be no slight relief. The endless disputes whether or not some fifty
species of British brambles are good species will cease. Systematists will
have only to decide (not that this will be easy) whether any form be
sufficiently constant and distinct from other forms, to be capable of
definition; and if definable, whether the differences be sufficiently
important to deserve a specific name. This latter point will become a far

more essential consideration than it is at present; for differences, however
slight, between any two forms, if not blended by intermediate gradations,
are looked at by most naturalists as sufficient to raise both forms to the rank
of species.
Hereafter we shall be compelled to acknowledge that the only distinction
between species and well-marked varieties is, that the latter are known, or
believed to be connected at the present day by intermediate gradations,
whereas species were formerly thus connected. Hence, without rejecting the
consideration of the present existence of intermediate gradations between
any two forms, we shall be led to weigh more carefully and to value higher
the actual amount of difference between them. It is quite possible that forms
now generally acknowledged to be merely varieties may hereafter be
thought worthy of specific names; and in this case scientific and common
language will come into accordance. In short, we shall have to treat species
in the same manner as those naturalists treat genera, who admit that genera
are merely artificial combinations made for convenience. This may not be a
cheering prospect; but we shall at least be freed from the vain search for the
undiscovered and undiscoverable essence of the term species.
The other and more general departments of natural history will rise
greatly in interest. The terms used by naturalists, of affinity, relationship,
community of type, paternity, morphology, adaptive characters, rudimentary
and aborted organs, &c., will cease to be metaphorical and will have a plain
signification. When we no longer look at an organic being as a savage looks
at a ship, as something wholly beyond his comprehension; when we regard
every production of nature as one which has had a long history; when we
contemplate every complex structure and instinct as the summing up of
many contrivances, each useful to the possessor, in the same way as any
great mechanical invention is the summing up of the labour, the experience,
the reason, and even the blunders of numerous workmen; when we thus
view each organic being, how far more interesting—I speak from
experience—does the study of natural history become!
A grand and almost untrodden field of inquiry will be opened, on the
causes and laws of variation, on correlation, on the effects of use and
disuse, on the direct action of external conditions, and so forth. The study of
domestic productions will rise immensely in value. A new variety raised by
man will be a far more important and interesting subject for study than one

more species added to the infinitude of already recorded species. Our
classifications will come to be, as far as they can be so made, genealogies;
and will then truly give what may be called the plan of creation. The rules
for classifying will no doubt become simpler when we have a definite
object in view. We possess no pedigree or armorial bearings; and we have to
discover and trace the many diverging lines of descent in our natural
genealogies, by characters of any kind which have long been inherited.
Rudimentary organs will speak infallibly with respect to the nature of long-
lost structures. Species and groups of species which are called aberrant, and
which may fancifully be called living fossils, will aid us in forming a
picture of the ancient forms of life. Embryology will often reveal to us the
structure, in some degree obscured, of the prototypes of each great class.
When we can feel assured that all the individuals of the same species,
and all the closely allied species of most genera, have, within a not very
remote period descended from one parent, and have migrated from some
one birth-place; and when we better know the many means of migration,
then, by the light which geology now throws, and will continue to throw, on
former changes of climate and of the level of the land, we shall surely be
enabled to trace in an admirable manner the former migrations of the
inhabitants of the whole world. Even at present, by comparing the
differences between the inhabitants of the sea on the opposite sides of a
continent, and the nature of the various inhabitants of that continent in
relation to their apparent means of immigration, some light can be thrown
on ancient geography.
The noble science of geology loses glory from the extreme imperfection
of the record. The crust of the earth, with its embedded remains, must not
be looked at as a well-filled museum, but as a poor collection made at
hazard and at rare intervals. The accumulation of each great fossiliferous
formation will be recognised as having depended on an unusual occurrence
of favourable circumstances, and the blank intervals between the successive
stages as having been of vast duration. But we shall be able to gauge with
some security the duration of these intervals by a comparison of the
preceding and succeeding organic forms. We must be cautious in attempting
to correlate as strictly contemporaneous two formations, which do not
include many identical species, by the general succession of the forms of
life. As species are produced and exterminated by slowly acting and still
existing causes, and not by miraculous acts of creation; and as the most

important of all causes of organic change is one which is almost
independent of altered and perhaps suddenly altered physical conditions,
namely, the mutual relation of organism to organism—the improvement of
one organism entailing the improvement or the extermination of others; it
follows, that the amount of organic change in the fossils of consecutive
formations probably serves as a fair measure of the relative, though not
actual lapse of time. A number of species, however, keeping in a body
might remain for a long period unchanged, whilst within the same period,
several of these species, by migrating into new countries and coming into
competition with foreign associates, might become modified; so that we
must not overrate the accuracy of organic change as a measure of time.
In the future I see open fields for far more important researches.
Psychology will be securely based on the foundation already well laid by
Mr. Herbert Spencer, that of the necessary acquirement of each mental
power and capacity by gradation. Much light will be thrown on the origin of
man and his history.
Authors of the highest eminence seem to be fully satisfied with the view
that each species has been independently created. To my mind it accords
better with what we know of the laws impressed on matter by the Creator,
that the production and extinction of the past and present inhabitants of the
world should have been due to secondary causes, like those determining the
birth and death of the individual. When I view all beings not as special
creations, but as the lineal descendants of some few beings which lived long
before the first bed of the Cambrian system was deposited, they seem to me
to become ennobled. Judging from the past, we may safely infer that not
one living species will transmit its unaltered likeness to a distinct futurity.
And of the species now living very few will transmit progeny of any kind to
a far distant futurity; for the manner in which all organic beings are
grouped, shows that the greater number of species in each genus, and all the
species in many genera, have left no descendants, but have become utterly
extinct. We can so far take a prophetic glance into futurity as to foretell that
it will be the common and widely spread species, belonging to the larger
and dominant groups within each class, which will ultimately prevail and
procreate new and dominant species. As all the living forms of life are the
lineal descendants of those which lived long before the Cambrian epoch, we
may feel certain that the ordinary succession by generation has never once
been broken, and that no cataclysm has desolated the whole world. Hence,

we may look with some confidence to a secure future of great length. And
as natural selection works solely by and for the good of each being, all
corporeal and mental endowments will tend to progress towards perfection.
It is interesting to contemplate a tangled bank, clothed with many plants
of many kinds, with birds singing on the bushes, with various insects
flitting about, and with worms crawling through the damp earth, and to
reflect that these elaborately constructed forms, so different from each
other, and dependent upon each other in so complex a manner, have all been
produced by laws acting around us. These laws, taken in the largest sense,
being Growth with reproduction; Inheritance which is almost implied by
reproduction; Variability from the indirect and direct action of the
conditions of life, and from use and disuse; a Ratio of Increase so high as to
lead to a Struggle for Life, and as a consequence to Natural Selection,
entailing Divergence of Character and the Extinction of less improved
forms. Thus, from the war of nature, from famine and death, the most
exalted object which we are capable of conceiving, namely, the production
of the higher animals, directly follows. There is grandeur in this view of
life, with its several powers, having been originally breathed by the Creator
into a few forms or into one; and that, whilst this planet has gone circling
on according to the fixed law of gravity, from so simple a beginning endless
forms most beautiful and most wonderful have been, and are being evolved.

GLOSSARY OF THE PRINCIPAL SCIENTIFIC
TERMS USED IN THE PRESENT VOLUME.*
* I am indebted to the kindness of Mr. W.S. Dallas for this Glossary, which has
been given because several readers have complained to me that some of the
terms used were unintelligible to them. Mr. Dallas has endeavoured to give the
explanations of the terms in as popular a form as possible.

ABERRANT.—Forms or groups of animals or plants which deviate in
important characters from their nearest allies, so as not to be easily included
in the same group with them, are said to be aberrant.
ABERRATION (in Optics).—In the refraction of light by a convex lens
the rays passing through different parts of the lens are brought to a focus at
slightly different distances—this is called spherical aberration; at the same
time the coloured rays are separated by the prismatic action of the lens and
likewise brought to a focus at different distances—this is chromatic
aberration.
ABNORMAL.—Contrary to the general rule.
ABORTED.—An organ is said to be aborted, when its development has
been arrested at a very early stage.
ALBINISM.—Albinos are animals in which the usual colouring matters
characteristic of the species have not been produced in the skin and its
appendages. Albinism is the state of being an albino.
ALGÆ.—A class of plants including the ordinary sea-weeds and the
filamentous fresh-water weeds.
ALTERNATION OF GENERATIONS.—This term is applied to a
peculiar mode of reproduction which prevails among many of the lower
animals, in which the egg produces a living form quite different from its
parent, but from which the parent-form is reproduced by a process of
budding, or by the division of the substance of the first product of the egg.

AMMONITES.—A group of fossil, spiral, chambered shells, allied to the
existing pearly Nautilus, but having the partitions between the chambers
waved in complicated patterns at their junction with the outer wall of the
shell.
ANALOGY.—That resemblance of structures which depends upon
similarity of function, as in the wings of insects and birds. Such structures
are said to be analogous, and to be analogues of each other.
ANIMALCULE.—A minute animal: generally applied to those visible
only by the microscope.
ANNELIDS.—A class of worms in which the surface of the body
exhibits a more or less distinct division into rings or segments, generally
provided with appendages for locomotion and with gills. It includes the
ordinary marine worms, the earth-worms, and the leeches.
ANTENNÆ.—Jointed organs appended to the head in Insects, Crustacea
and Centipedes, and not belonging to the mouth.
ANTHERS.—The summits of the stamens of flowers, in which the
pollen or fertilising dust is produced.
APLACENTALIA, APLACENTATA or Aplacental Mammals.—See
mammalia.
ARCHETYPAL.—Of or belonging to the Archetype, or ideal primitive
form upon which all the beings of a group seem to be organised.
ARTICULATA.—A great division of the Animal Kingdom characterised
generally by having the surface of the body divided into rings called
segments, a greater or less number of which are furnished with jointed legs
(such as Insects, Crustaceans and Centipedes).
ASYMMETRICAL.—Having the two sides unlike.
ATROPHIED.—Arrested in development at a very early stage.
BALANUS.—The genus including the common Acorn-shells which live
in abundance on the rocks of the sea-coast.
BATRACHIANS.—A class of animals allied to the Reptiles, but
undergoing a peculiar metamorphosis, in which the young animal is
generally aquatic and breathes by gills. (Examples, Frogs, Toads, and
Newts.)

BOULDERS.—Large transported blocks of stone generally embedded in
clays or gravels.
BRACHIOPODA.—A class of marine Mollusca, or soft-bodied animals,
furnished with a bivalve shell, attached to submarine objects by a stalk
which passes through an aperture in one of the valves, and furnished with
fringed arms, by the action of which food is carried to the mouth.
BRANCHIÆ.—Gills or organs for respiration in water.
BRANCHIAL.—Pertaining to gills or branchiæ.
CAMBRIAN SYSTEM.—A series of very ancient Palæozoic rocks,
between the Laurentian and the Silurian. Until recently these were regarded
as the oldest fossiliferous rocks.
CANIDÆ.—The Dog-family, including the Dog, Wolf, Fox, Jackal, &c.
CARAPACE.—The shell enveloping the anterior part of the body in
Crustaceans generally; applied also to the hard shelly pieces of the
Cirripedes.
CARBONIFEROUS.—This term is applied to the great formation which
includes, among other rocks, the coal-measures. It belongs to the oldest, or
Palæozoic, system of formations.
CAUDAL.—Of or belonging to the tail.
CEPHALOPODS.—The highest class of the Mollusca, or soft-bodied
animals, characterised by having the mouth surrounded by a greater or less
number of fleshy arms or tentacles, which, in most living species, are
furnished with sucking-cups. (Examples, Cuttle-fish, Nautilus.)
CETACEA.—An order of Mammalia, including the Whales, Dolphins,
&c., having the form of the body fish-like, the skin naked, and only the fore
limbs developed.
CHELONIA.—An order of Reptiles including the Turtles, Tortoises, &c.
CIRRIPEDES.—An order of Crustaceans including the Barnacles and
Acorn-shells. Their young resemble those of many other Crustaceans in
form; but when mature they are always attached to other objects, either
directly or by means of a stalk, and their bodies are enclosed by a
calcareous shell composed of several pieces, two of which can open to give
issue to a bunch of curled, jointed tentacles, which represent the limbs.

COCCUS.—The genus of Insects including the Cochineal. In these the
male is a minute, winged fly, and the female generally a motionless, berry-
like mass.
COCOON.—A case usually of silky material, in which insects are
frequently enveloped during the second or resting-stage (pupa) of their
existence. The term “cocoon-stage” is here used as equivalent to “pupa-
stage.”
CŒLOSPERMOUS.—A term applied to those fruits of the Umbelliferæ
which have the seed hollowed on the inner face.
COLEOPTERA.—Beetles, an order of Insects, having a biting mouth
and the first pair of wings more or less horny, forming sheaths for the
second pair, and usually meeting in a straight line down the middle of the
back.
COLUMN.—A peculiar organ in the flowers of Orchids, in which the
stamens, style and stigma (or the reproductive parts) are united.
COMPOSITÆ or COMPOSITOUS PLANTS.—Plants in which the
inflorescence consists of numerous small flowers (florets) brought together
into a dense head, the base of which is enclosed by a common envelope.
(Examples, the Daisy, Dandelion, &c.)
CONFERVÆ.—The filamentous weeds of fresh water.
CONGLOMERATE.—A rock made up of fragments of rock or pebbles,
cemented together by some other material.
COROLLA.—The second envelope of a flower usually composed of
coloured, leaf-like organs (petals), which may be united by their edges
either in the basal part or throughout.
CORRELATION.—The normal coincidence of one phenomenon,
character, &c., with another.
CORYMB.—A bunch of flowers in which those springing from the lower
part of the flower stalks are supported on long stalks so as to be nearly on a
level with the upper ones.
COTYLEDONS.—The first or seed-leaves of plants.
CRUSTACEANS.—A class of articulated animals, having the skin of the
body generally more or less hardened by the deposition of calcareous
matter, breathing by means of gills. (Examples, Crab, Lobster, Shrimp, &c.)

CURCULIO.—The old generic term for the Beetles known as Weevils,
characterised by their four-jointed feet, and by the head being produced into
a sort of beak, upon the sides of which the antennæ are inserted.
CUTANEOUS.—Of or belonging to the skin.
DEGRADATION.—The wearing down of land by the action of the sea
or of meteoric agencies.
DENUDATION.—The wearing away of the surface of the land by water.
DEVONIAN SYSTEM or FORMATION.—A series of Palæozoic rocks,
including the Old Red Sandstone.
DICOTYLEDONS, or DICOTYLEDONOUS PLANTS.—A class of
plants characterised by having two seed-leaves, by the formation of new
wood between the bark and the old wood (exogenous growth) and by the
reticulation of the veins of the leaves. The parts of the flowers are generally
in multiples of five.
DIFFERENTATION.—The separation or discrimination of parts or
organs which in simpler forms of life are more or less united.
DIMORPHIC.—Having two distinct forms.—DIMORPHISM is the
condition of the appearance of the same species under two dissimilar forms.
DIOECIOUS.—Having the organs of the sexes upon distinct individuals.
DIORITE.—A peculiar form of Greenstone.
DORSAL.—Of or belonging to the back.
EDENTATA.—A peculiar order of Quadrupeds, characterised by the
absence of at least the middle incisor (front) teeth in both jaws. (Examples,
the Sloths and Armadillos.)
ELYTRA.—The hardened fore-wings of Beetles, serving as sheaths for
the membranous hind-wings, which constitute the true organs of flight.
EMBRYO.—The young animal undergoing development within the egg
or womb.
EMBRYOLOGY.—The study of the development of the embryo.
ENDEMIC.—Peculiar to a given locality.
ENTOMOSTRACA.—A division of the class Crustacea, having all the
segments of the body usually distinct, gills attached to the feet or organs of
the mouth, and the feet fringed with fine hairs. They are generally of small
size.

EOCENE.—The earliest of the three divisions of the Tertiary epoch of
geologists. Rocks of this age contain a small proportion of shells identical
with species now living.
EPHEMEROUS INSECTS.—Insects allied to the May-fly.
FAUNA.—The totality of the animals naturally inhabiting a certain
country or region, or which have lived during a given geological period.
FELIDÆ.—The Cat-family.
FERAL.—Having become wild from a state of cultivation or
domestication.
FLORA.—The totality of the plants growing naturally in a country, or
during a given geological period.
FLORETS.—Flowers imperfectly developed in some respects, and
collected into a dense spike or head, as in the Grasses, the Dandelion, &c.
FOETAL.—Of or belonging to the foetus, or embryo in course of
development.
FORAMINIFERA.—A class of animals of very low organisation and
generally of small size, having a jelly-like body, from the surface of which
delicate filaments can be given off and retracted for the prehension of
external objects, and having a calcareous or sandy shell, usually divided
into chambers and perforated with small apertures.
FOSSILIFEROUS.—Containing fossils.
FOSSORIAL.—Having a faculty of digging. The Fossorial Hymenoptera
are a group of Wasp-like Insects, which burrow in sandy soil to make nests
for their young.
FRENUM (pl. FRENA).—A small band or fold of skin.
FUNGI (sing. FUNGUS).—A class of cellular plants, of which
Mushrooms, Toadstools, and Moulds, are familiar examples.
FURCULA.—The forked bone formed by the union of the collar-bones
in many birds, such as the common Fowl.
GALLINACEOUS BIRDS.—An order of birds of which the common
Fowl, Turkey, and Pheasant, are well-known examples.
GALLUS.—The genus of birds which includes the common Fowl.
GANGLION.—A swelling or knot from which nerves are given off as
from a centre.

GANOID FISHES.—Fishes covered with peculiar enamelled bony
scales. Most of them are extinct.
GERMINAL VESICLE.—A minute vesicle in the eggs of animals, from
which the development of the embryo proceeds.
GLACIAL PERIOD.—A period of great cold and of enormous extension
of ice upon the surface of the earth. It is believed that glacial periods have
occurred repeatedly during the geological history of the earth, but the term
is generally applied to the close of the Tertiary epoch, when nearly the
whole of Europe was subjected to an arctic climate.
GLAND.—An organ which secretes or separates some peculiar product
from the blood or sap of animals or plants.
GLOTTIS.—The opening of the windpipe into the œsophagus or gullet.
GNEISS.—A rock approaching granite in composition, but more or less
laminated, and really produced by the alteration of a sedimentary deposit
after its consolidation.
GRALLATORES.—The so-called wading-birds (storks, cranes, snipes,
&c.), which are generally furnished with long legs, bare of feathers above
the heel, and have no membranes between the toes.
GRANITE.—A rock consisting essentially of crystals of felspar and mica
in a mass of quartz.
HABITAT.—The locality in which a plant or animal naturally lives.
HEMIPTERA.—An order or sub-order of insects, characterised by the
possession of a jointed beak or rostrum, and by having the fore-wings horny
in the basal portion and membranous at the extremity, where they cross
each other. This group includes the various species of bugs.
HERMAPHRODITE.—Possessing the organs of both sexes.
HOMOLOGY.—That relation between parts which results from their
development from corresponding embryonic parts, either in different
animals, as in the case of the arm of man, the fore-leg of a quadruped, and
the wing of a bird; or in the same individual, as in the case of the fore and
hind legs in quadrupeds, and the segments or rings and their appendages of
which the body of a worm, a centipede, &c., is composed. The latter is
called serial homology. The parts which stand in such a relation to each
other are said to be homologous, and one such part or organ is called the
homologue of the other. In different plants the parts of the flower are

homologous, and in general these parts are regarded as homologous with
leaves.
HOMOPTERA.—An order or sub-order of insects having (like the
Hemiptera) a jointed beak, but in which the fore-wings are either wholly
membranous or wholly leathery, The Cicadæ, frog-hoppers, and Aphides,
are well-known examples.
HYBRID.—The offspring of the union of two distinct species.
HYMENOPTERA.—An order of insects possessing biting jaws and
usually four membranous wings in which there are a few veins. Bees and
wasps are familiar examples of this group.
HYPERTROPHIED.—Excessively developed.
ICHNEUMONIDÆ.—A family of hymenopterous insects, the members
of which lay their eggs in the bodies or eggs of other insects.
IMAGO.—The perfect (generally winged) reproductive state of an
insect.
INDIGENES.—The aboriginal animal or vegetable inhabitants of a
country or region.
INFLORESCENCE.—The mode of arrangement of the flowers of plants.
INFUSORIA.—A class of microscopic animalcules, so called from their
having originally been observed in infusions of vegetable matters. They
consist of a gelatinous material enclosed in a delicate membrane, the whole
or part of which is furnished with short vibrating hairs (called cilia), by
means of which the animalcules swim through the water or convey the
minute particles of their food to the orifice of the mouth.
INSECTIVOROUS.—Feeding on insects.
INVERTEBRATA, or INVERTEBRATE ANIMALS.—Those animals
which do not possess a backbone or spinal column.
LACUNÆ.—Spaces left among the tissues in some of the lower animals
and serving in place of vessels for the circulation of the fluids of the body.
LAMELLATED.—Furnished with lamellæ or little plates.
LARVA (pl. LARVÆ).—The first condition of an insect at its issuing
from the egg, when it is usually in the form of a grub, caterpillar, or maggot.
LARYNX.—The upper part of the windpipe opening into the gullet.

LAURENTIAN.—A group of greatly altered and very ancient rocks,
which is greatly developed along the course of the St. Laurence, whence the
name. It is in these that the earliest known traces of organic bodies have
been found.
LEGUMINOSÆ.—An order of plants represented by the common peas
and beans, having an irregular flower in which one petal stands up like a
wing, and the stamens and pistil are enclosed in a sheath formed by two
other petals. The fruit is a pod (or legume).
LEMURIDÆ.—A group of four-handed animals, distinct from the
monkeys and approaching the insectivorous quadrupeds in some of their
characters and habits. Its members have the nostrils curved or twisted, and a
claw instead of a nail upon the first finger of the hind hands.
LEPIDOPTERA.—An order of insects, characterised by the possession
of a spiral proboscis, and of four large more or less scaly wings. It includes
the well-known butterflies and moths.
LITTORAL.—Inhabiting the seashore.
LOESS.—A marly deposit of recent (Post-Tertiary) date, which occupies
a great part of the valley of the Rhine.
MALACOSTRACA.—The higher division of the Crustacea, including
the ordinary crabs, lobsters, shrimps, &c., together with the woodlice and
sand-hoppers.
MAMMALIA.—The highest class of animals, including the ordinary
hairy quadrupeds, the whales and man, and characterised by the production
of living young which are nourished after birth by milk from the teats
(Mammæ, Mammary glands) of the mother. A striking difference in
embryonic development has led to the division of this class into two great
groups; in one of these, when the embryo has attained a certain stage, a
vascular connection, called the placenta, is formed between the embryo and
the mother; in the other this is wanting, and the young are produced in a
very incomplete state. The former, including the greater part of the class,
are called Placental Mammals; the latter, or Aplacental Mammals, include
the Marsupials and Monotremes (Ornithorhynchus).
MAMMIFEROUS.—Having mammæ or teats (see MAMMALIA).
MANDIBLES.—in insects, the first or uppermost pair of jaws, which are
generally solid, horny, biting organs. In birds the term is applied to both

jaws with their horny coverings. In quadrupeds the mandible is properly the
lower jaw.
MARSUPIALS.—An order of Mammalia in which the young are born in
a very incomplete state of development, and carried by the mother, while
sucking, in a ventral pouch (marsupium), such as the kangaroos, opossums,
&c. (see MAMMALIA).
MAXILLÆ.—in insects, the second or lower pair of jaws, which are
composed of several joints and furnished with peculiar jointed appendages
called palpi, or feelers.
MELANISM.—The opposite of albinism; an undue development of
colouring material in the skin and its appendages.
METAMORPHIC ROCKS.—Sedimentary rocks which have undergone
alteration, generally by the action of heat, subsequently to their deposition
and consolidation.
MOLLUSCA.—One of the great divisions of the animal kingdom,
including those animals which have a soft body, usually furnished with a
shell, and in which the nervous ganglia, or centres, present no definite
general arrangement. They are generally known under the denomination of
“shellfish”; the cuttle-fish, and the common snails, whelks, oysters,
mussels, and cockles, may serve as examples of them.
MONOCOTYLEDONS, or MONOCOTYLEDONOUS PLANTS.—
Plants in which the seed sends up only a single seed-leaf (or cotyledon);
characterised by the absence of consecutive layers of wood in the stem
(endogenous growth), by the veins of the leaves being generally straight,
and by the parts of the flowers being generally in multiples of three.
(Examples, Grasses, Lilies, Orchids, Palms, &c.)
MORAINES.—The accumulations of fragments of rock brought down
by glaciers.
MORPHOLOGY.—The law of form or structure independent of
function.
MYSIS-STAGE.—A stage in the development of certain crustaceans
(prawns), in which they closely resemble the adults of a genus (Mysis)
belonging to a slightly lower group.
NASCENT.—Commencing development.
NATATORY.—Adapted for the purpose of swimming.

NAUPLIUS-FORM.—The earliest stage in the development of many
Crustacea, especially belonging to the lower groups. In this stage the animal
has a short body, with indistinct indications of a division into segments, and
three pairs of fringed limbs. This form of the common fresh-water Cyclops
was described as a distinct genus under the name of Nauplius.
NEURATION.—The arrangement of the veins or nervures in the wings
of insects.
NEUTERS.—Imperfectly developed females of certain social insects
(such as ants and bees), which perform all the labours of the community.
Hence, they are also called workers.
NICTITATING MEMBRANE.—A semi-transparent membrane, which
can be drawn across the eye in birds and reptiles, either to moderate the
effects of a strong light or to sweep particles of dust, &c., from the surface
of the eye.
OCELLI.—The simple eyes or stemmata of insects, usually situated on
the crown of the head between the great compound eyes.
ŒSOPHAGUS.—The gullet.
OOLITIC.—A great series of secondary rocks, so called from the texture
of some of its members, which appear to be made up of a mass of small
EGG-LIKE calcareous bodies.
OPERCULUM.—A calcareous plate employed by many Molluscæ to
close the aperture of their shell. The OPERCULAR VALVES of Cirripedes
are those which close the aperture of the shell.
ORBIT.—The bony cavity for the reception of the eye.
ORGANISM.—An organised being, whether plant or animal.
ORTHOSPERMOUS.—A term applied to those fruits of the Umbelliferæ
which have the seed straight.
OSCULANT.—Forms or groups apparently intermediate between and
connecting other groups are said to be osculant.
OVA.—Eggs.
OVARIUM or OVARY (in plants).—The lower part of the pistil or
female organ of the flower, containing the ovules or incipient seeds; by
growth after the other organs of the flower have fallen, it usually becomes
converted into the fruit.

OVIGEROUS.—Egg-bearing.
OVULES (of plants).—The seeds in the earliest condition.
PACHYDERMS.—A group of Mammalia, so called from their thick
skins, and including the elephant, rhinoceros, hippopotamus, &c.
PALÆOZOIC.—The oldest system of fossiliferous rocks.
PALPI.—Jointed appendages to some of the organs of the mouth in
insects and Crustacea.
PAPILIONACEÆ.—An order of plants (see LEGUMINOSÆ), The
flowers of these plants are called papilionaceous, or butterfly-like, from the
fancied resemblance of the expanded superior petals to the wings of a
butterfly.
PARASITE.—An animal or plant living upon or in, and at the expense
of, another organism.
PARTHENOGENESIS.—The production of living organisms from
unimpregnated eggs or seeds.
PEDUNCULATED.—Supported upon a stem or stalk. The pedunculated
oak has its acorns borne upon a footstool.
PELORIA or PELORISM.—The appearance of regularity of structure in
the flowers of plants which normally bear irregular flowers.
PELVIS.—The bony arch to which the hind limbs of vertebrate animals
are articulated.
PETALS.—The leaves of the corolla, or second circle of organs in a
flower. They are usually of delicate texture and brightly coloured.
PHYLLODINEOUS.—Having flattened, leaf-like twigs or leafstalks
instead of true leaves.
PIGMENT.—The colouring material produced generally in the
superficial parts of animals. The cells secreting it are called pigment-cells.
PINNATE.—Bearing leaflets on each side of a central stalk.
PISTILS.—The female organs of a flower, which occupy a position in
the centre of the other floral organs. The pistil is generally divisible into the
ovary or germen, the style and the stigma.
PLACENTALIA, PLACENTATA.—or PLACENTAL MAMMALS, See
MAMMALIA.

PLANTIGRADES.—Quadrupeds which walk upon the whole sole of the
foot, like the bears.
PLASTIC.—Readily capable of change.
PLEISTOCENE PERIOD.—The latest portion of the Tertiary epoch.
PLUMULE (in plants).—The minute bud between the seed-leaves of
newly-germinated plants.
PLUTONIC ROCKS.—Rocks supposed to have been produced by
igneous action in the depths of the earth.
POLLEN.—The male element in flowering plants; usually a fine dust
produced by the anthers, which, by contact with the stigma effects the
fecundation of the seeds. This impregnation is brought about by means of
tubes (pollen-tubes) which issue from the pollen-grains adhering to the
stigma, and penetrate through the tissues until they reach the ovary.
POLYANDROUS (flowers).—Flowers having many stamens.
POLYGAMOUS PLANTS.—Plants in which some flowers are unisexual
and others hermaphrodite. The unisexual (male and female) flowers, may be
on the same or on different plants.
POLYMORPHIC.—Presenting many forms.
POLYZOARY.—The common structure formed by the cells of the
Polyzoa, such as the well-known seamats.
PREHENSILE.—Capable of grasping.
PREPOTENT.—Having a superiority of power.
PRIMARIES.—The feathers forming the tip of the wing of a bird, and
inserted upon that part which represents the hand of man.
PROCESSES.—Projecting portions of bones, usually for the attachment
of muscles, ligaments, &c.
PROPOLIS.—A resinous material collected by the hivebees from the
opening buds of various trees.
PROTEAN.—Exceedingly variable.
PROTOZOA.—The lowest great division of the animal kingdom. These
animals are composed of a gelatinous material, and show scarcely any trace
of distinct organs. The Infusoria, Foraminifera, and sponges, with some
other forms, belong to this division.

PUPA (pl. PUPÆ).—The second stage in the development of an insect,
from which it emerges in the perfect (winged) reproductive form. In most
insects the pupal stage is passed in perfect repose. The chrysalis is the
pupal state of butterflies.
RADICLE.—The minute root of an embryo plant.
RAMUS.—One half of the lower jaw in the Mammalia. The portion
which rises to articulate with the skull is called the ascending ramus.
RANGE.—The extent of country over which a plant or animal is
naturally spread. Range in time expresses the distribution of a species or
group through the fossiliferous beds of the earth’s crust.
RETINA.—The delicate inner coat of the eye, formed by nervous
filaments spreading from the optic nerve, and serving for the perception of
the impressions produced by light.
RETROGRESSION.—Backward development. When an animal, as it
approaches maturity, becomes less perfectly organised than might be
expected from its early stages and known relationships, it is said to undergo
a retrogade development or metamorphosis.
RHIZOPODS.—A class of lowly organised animals (Protozoa), having a
gelatinous body, the surface of which can be protruded in the form of root-
like processes or filaments, which serve for locomotion and the prehension
of food. The most important order is that of the Foraminifera.
RODENTS.—The gnawing Mammalia, such as the rats, rabbits, and
squirrels. They are especially characterised by the possession of a single
pair of chisel-like cutting teeth in each jaw, between which and the grinding
teeth there is a great gap.
RUBUS.—The bramble genus.
RUDIMENTARY.—Very imperfectly developed.
RUMINANTS.—The group of quadrupeds which ruminate or chew the
cud, such as oxen, sheep, and deer. They have divided hoofs, and are
destitute of front teeth in the upper jaw.
SACRAL.—Belonging to the sacrum, or the bone composed usually of
two or more united vertebræ to which the sides of the pelvis in vertebrate
animals are attached.

SARCODE.—The gelatinous material of which the bodies of the lowest
animals (Protozoa) are composed.
SCUTELLÆ.—The horny plates with which the feet of birds are
generally more or less covered, especially in front.
SEDIMENTARY FORMATIONS.—Rocks deposited as sediments from
water.
SEGMENTS.—The transverse rings of which the body of an articulate
animal or annelid is composed.
SEPALS.—The leaves or segments of the calyx, or outermost envelope
of an ordinary flower. They are usually green, but sometimes brightly
coloured.
SERRATURES.—Teeth like those of a saw.
SESSILE.—Not supported on a stem or footstalk.
SILURIAN SYSTEM.—A very ancient system of fossiliferous rocks
belonging to the earlier part of the Palæozoic series.
SPECIALISATION.—The setting apart of a particular organ for the
performance of a particular function.
SPINAL CORD.—The central portion of the nervous system in the
Vertebrata, which descends from the brain through the arches of the
vertebræ, and gives off nearly all the nerves to the various organs of the
body.
STAMENS.—The male organs of flowering plants, standing in a circle
within the petals. They usually consist of a filament and an anther, the
anther being the essential part in which the pollen, or fecundating dust, is
formed.
STERNUM.—The breast-bone.
STIGMA.—The apical portion of the pistil in flowering plants.
STIPULES.—Small leafy organs placed at the base of the footstalks of
the leaves in many plants.
STYLE.—The middle portion of the perfect pistil, which rises like a
column from the ovary and supports the stigma at its summit.
SUBCUTANEOUS.—Situated beneath the skin.
SUCTORIAL.—Adapted for sucking.

SUTURES (in the skull).—The lines of junction of the bones of which
the skull is composed.
TARSUS (pl. TARSI).—The jointed feet of articulate animals, such as
insects.
TELEOSTEAN FISHES.—Fishes of the kind familiar to us in the
present day, having the skeleton usually completely ossified and the scales
horny.
TENTACULA or TENTACLES.—Delicate fleshy organs of prehension
or touch possessed by many of the lower animals.
TERTIARY.—The latest geological epoch, immediately preceding the
establishment of the present order of things.
TRACHEA.—The windpipe or passage for the admission of air to the
lungs.
TRIDACTYLE.—Three-fingered, or composed of three movable parts
attached to a common base.
TRILOBITES.—A peculiar group of extinct crustaceans, somewhat
resembling the woodlice in external form, and, like some of them, capable
of rolling themselves up into a ball. Their remains are found only in the
Palæozoic rocks, and most abundantly in those of Silurian age.
TRIMORPHIC.—Presenting three distinct forms.
UMBELLIFERÆ.—An order of plants in which the flowers, which
contain five stamens and a pistil with two styles, are supported upon
footstalks which spring from the top of the flower stem and spread out like
the wires of an umbrella, so as to bring all the flowers in the same head
(umbel) nearly to the same level. (Examples, Parsley and Carrot.)
UNGULATA.—Hoofed quadrupeds.
UNICELLULAR.—Consisting of a single cell.
VASCULAR.—Containing blood-vessels.
VERMIFORM.—Like a worm.
VERTEBRATA or VERTEBRATE ANIMALS.—The highest division of
the animal kingdom, so called from the presence in most cases of a
backbone composed of numerous joints or vertebræ, which constitutes the
centre of the skeleton and at the same time supports and protects the central
parts of the nervous system.

WHORLS.—The circles or spiral lines in which the parts of plants are
arranged upon the axis of growth.
WORKERS.—See neuters.
ZOËA-STAGE.—The earliest stage in the development of many of the
higher Crustacea, so called from the name of Zoëa applied to these young
animals when they were supposed to constitute a peculiar genus.
ZOOIDS.—In many of the lower animals (such as the Corals, Medusæ,
&c.) reproduction takes place in two ways, namely, by means of eggs and
by a process of budding with or without separation from the parent of the
product of the latter, which is often very different from that of the egg. The
individuality of the species is represented by the whole of the form
produced between two sexual reproductions; and these forms, which are
apparently individual animals, have been called zooids.

INDEX.

Aberrant groups, 379.

Abyssinia, plants of, 340.

Acclimatisation, 112.

Adoxa, 173.

Affinities of extinct species, 301.
—, of organic beings, 378.

Agassiz on Amblyopsis, 112.
—, on groups of species suddenly appearing, 289.
—, on prophetic forms, 301.
—, on embryological succession, 310.
—, on the Glacial period, 330.
—, on embryological characters, 368.
—, on the latest tertiary forms, 278.
—, on parallelism of embryological development and geological
succession, 396.
—, Alex., on pedicellariæ, 191.

Algæ of New Zealand, 338.

Alligators, males, fighting, 69.

Alternate generations, 387.

Amblyopsis, blind fish, 112.

America, North, productions allied to those of Europe, 333.
—, boulders and glaciers of, 335.
—, South, no modern formations on west coast, 272.

Ammonites, sudden extinction of, 297.

Anagallis, sterility of, 236.

Analogy of variations, 127.

Andaman Islands inhabited by a toad, 350.

Ancylus, 345.

Animals, not domesticated from being variable, 13.
—, domestic; descended from several stocks, 14.
—, acclimatisation of, 112.

Animals of Australia, 90.
—, with thicker fur in cold climates, 107.
—, blind, in caves, 110.
—, extinct, of Australia, 310.

Anomma, 232.

Antarctic islands, ancient flora of, 355.

Antechinus, 373.

Ants attending aphides, 207.
—, slave-making instinct, 217.
—, neuters, structure of, 230.

Apes, not having acquired intellectual powers, 181.

Aphides attended by ants, 207.

Aphis, development of, 390.

Apteryx, 140.

Arab horses, 26.

Aralo-Caspian Sea, 311.

Archeopteryx, 284.

Archiac, M. de, on the succession of species, 299.

Artichoke, Jerusalem, 114.

Ascension, plants of, 347.

Asclepias, pollen of, 151.

Asparagus, 325.

Aspicarpa, 367.

Asses, striped, 127.
—, improved by selection, 30.

Ateuchus, 109.

Aucapitaine, on land-shells, 353.

Audubon, on habits of frigate-bird, 142.
—, on variation in birds’ nests, 208.
—, on heron eating seeds, 346.

Australia, animals of, 90.
—, dogs of, 211.

—, extinct animals of, 310.
—, European plants in, 337.
—, glaciers of, 335.

Azara, on flies destroying cattle, 56.

Azores, flora of, 328.

Babington, Mr., on British plants, 37.

Baer, Von, standard of Highness, 97.
—, comparison of bee and fish, 308.
—, embryonic similarity of the Vertebrata, 387.

Baker, Sir S., on the giraffe, 178.

Balancement of growth, 117.

Baleen, 182.

Barberry, flowers of, 77.

Barrande, M., on Silurian colonies, 291.
—, on the succession of species, 299.
—, on parallelism of palæozoic formations, 301.
—, on affinities of ancient species, 302.

Barriers, importance of, 317.

Bates, Mr., on mimetic butterflies, 375, 376.

Batrachians on islands, 350.

Bats, how structure acquired, 140.
—, distribution of, 351.

Bear, catching water-insects, 141.

Beauty, how acquired, 159, 414.

Bee, sting of, 163.
—, queen, killing rivals, 164.
—, Australian, extermination of, 59.

Bees, fertilizing flowers, 57.
—, hive, not sucking the red clover, 75.
—, Ligurian, 75.
—, hive, cell-making instinct, 220.
—, variation in habits, 208.
—, parasitic, 216.
—, humble, cells of, 220.

Beetles, wingless, in Madeira, 109.
—, with deficient tarsi, 109.

Bentham, Mr., on British plants, 37.
—, on classification, 369.

Berkeley, Mr., on seeds in salt-water, 324.

Bermuda, birds of, 348.

Birds acquiring fear, 208.
—, beauty of, 161.
—, annually cross the Atlantic, 329.
—, colour of, on continents, 107.
—, footsteps, and remains of, in secondary rocks, 284.
—, fossil, in caves of Brazil, 310.
—, of Madeira, Bermuda, and Galapagos, 349, 349.
—, song of males, 70.
—, transporting seeds, 328.
—, waders, 345.
—, wingless, 108, 140.

Bizcacha, 318.
—, , affinities of, 379.

Bladder for swimming, in fish, 147.

Blindness of cave animals, 110.

Blyth, Mr., on distinctness of Indian cattle, 14.
—, on striped Hemionus, 128.
—, on crossed geese, 240.

Borrow, Mr., on the Spanish pointer, 26.

Bory St. Vincent, on Batrachians, 350.

Bosquet, M., on fossil Chthamalus, 284.

Boulders, erratic, on the Azores, 328.

Branchiæ, 148, 149.
—, of crustaceans, 152.

Braun, Prof., on the seeds of Fumariaceæ, 174.

Brent, Mr., on house-tumblers, 210.

Britain, mammals of, 352.

Broca, Prof., on Natural Selection, 170.

Bronn, Prof., on duration of specific forms, 275.
—, various objections by, 170.

Brown, Robert, on classification, 366.

Brown-Sequard, on inherited mutilations, 108.

Busk, Mr., on the Polyzoa, 193.

Butterflies, mimetic, 375, 376.

Buzareingues, on sterility of varieties, 258.

Cabbage, varieties of, crossed, 78.

Calceolaria, 239.

Canary-birds, sterility of hybrids, 240.

Cape de Verde Islands, productions of, 354.
—, plants of, on mountains, 337.

Cape of Good Hope, plants of, 101, 347.

Carpenter, Dr., on foraminifera, 308.

Carthemus, 173.

Catasetum, 155, 372.

Cats, with blue eyes, deaf, 9.
—, variation in habits of, 209.
—, curling tail when going to spring, 162.

Cattle destroying fir-trees, 56.
—, destroyed by flies in Paraguay, 56.
—, breeds of, locally extinct, 86.
—, fertility of Indian and European breeds, 241.
—, Indian, 14, 241.

Cave, inhabitants of, blind, 110.

Cecidomyia, 387.

Celts, proving antiquity of man, 13.

Centres of creation, 320.

Cephalopodæ, structures of eyes, 151.
—, development of, 390.

Cercopithecus, tail of, 189.

Ceroxylus laceratus, 182.

Cervulus, 240.

Cetacea, teeth and hair, 115.
—, development of the whalebone, 182.

Cetaceans, 182.

Ceylon, plants of, 338.

Chalk formation, 297.

Characters, divergence of, 86.
—, sexual, variable, 119, 123.
—, adaptive or analogical, 373.

Charlock, 59.

Checks to increase, 53.
—, mutual, 55.

Chelæ of Crustaceans, 193.

Chickens, instinctive tameness of, 211.

Chironomus, its asexual reproduction, 387.

Chthamalinæ, 271.

Chthamalus, cretacean species of, 384.

Circumstances favourable to selection of domestic products, 29.
—, to natural selection, 80.

Cirripedes capable of crossing, 79.
—, carapace aborted, 118.
—, their ovigerous frena, 148.
—, fossil, 284.
—, larvæ of, 389.

Claparède, Prof., on the hair-claspers of the Acaridæ, 153.

Clarke, Rev. W.B., on old glaciers in Australia, 335.

Classification, 363.

Clift, Mr., on the succession of types, 310.

Climate, effects of, in checking increase of beings, 54.
—, adaptation of, to organisms, 112.

Climbing plants, 147.
—, development of, 96.

Clover visited by bees, 75.

Cobites, intestine of, 147.

Cockroach, 59.

Collections, palæontological, poor, 270.

Colour, influenced by climate, 107.
—, in relation to attacks by flies, 159.

Columba livia, parent of domestic pigeons, 17.

Colymbetes, 345.

Compensation of growth, 117.

Compositæ, flowers and seeds of, 116.
—, outer and inner florets of, 173.
—, male flowers of, 398.

Conclusion, general, 421.

Conditions, slight changes in, favourable to fertility, 251.

Convergence of genera, 100.

Coot, 142.

Cope, Prof., on the acceleration or retardation of the period of
reproduction, 149.

Coral-islands, seeds drifted to, 326.
—, reefs, indicating movements of earth, 326.

Corn-crake, 143.

Correlated variation in domestic productions, 9.

Coryanthes, 154.

Creation, single centres of, 320.

Crinum, 238.

Croll, Mr., on subaërial denudation, 267, 269.
—, on the age of our oldest formations, 286.

—, on alternate Glacial periods in the North and South, 336.

Crosses, reciprocal, 244.

Crossing of domestic animals, importance in altering breeds, 15.
—, advantages of, 76, 77.
—, unfavourable to selection, 80.

Crüger, Dr., on Coryanthes, 154.

Crustacea of New Zealand, 338.

Crustacean, blind, 110.
air-breathers, 152.

Crustaceans, their chelæ, 193.

Cryptocerus, 231.

Ctenomys, blind, 110.

Cuckoo, instinct of, 205, 212.

Cunningham, Mr., on the flight of the logger-headed duck, 108.

Currants, grafts of, 246.

Currents of sea, rate of, 325.

Cuvier on conditions of existence, 205.
—, on fossil monkeys, 283, 284.

Cuvier, Fred., on instinct, 205.

Cyclostoma, resisting salt water, 353.

Dana, Prof., on blind cave-animals, 111.

—, on relations of crustaceans of Japan, 334.
—, on crustaceans of New Zealand, 338.

Dawson, Dr., on eozoon, 287.

De Candolle, Aug. Pyr., on struggle for existence, 49.
—, on umbelliferæ, 116.
—, on general affinities, 379.

De Candolle, Alph., on the variability of oaks, 40.
—, on low plants, widely dispersed, 359.
—, on widely-ranging plants being variable, 43.
—, on naturalisation, 89.
—, on winged seeds, 117.
—, on Alpine species suddenly becoming rare, 135.
—, on distribution of plants with large seeds, 326.
—, on vegetation of Australia, 340.
—, on fresh-water plants, 345.
—, on insular plants, 347.

Degradation of rocks, 266.

Denudation, rate of, 268.
—, of oldest rocks, 287.
—, of granitic areas, 274.

Development of ancient forms, 307.

Devonian system, 305.

Dianthus, fertility of crosses, 243.

Dimorphism in plants, 35, 252.

Dirt on feet of birds, 328.

Dispersal, means of, 323.

—, during Glacial period, 330.

Distribution, geographical, 316.
—, means of, 323.

Disuse, effect of, under nature, 108.

Diversification of means for same general purpose, 153.

Division, physiological, of labour, 89.

Divergence of character, 86.

Dog, resemblance of jaw to that of the Thylacinus, 374.

Dogs, hairless, with imperfect teeth, 9.
—, descended from several wild stocks, 15.
—, domestic instincts of, 210.
—, inherited civilisation of, 210.
—, fertility of breeds together, 241.
—, of crosses, 256.
—, proportions of body in different breeds, when young, 392.

Domestication, variation under, 5.

Double flowers, 230.

Downing, Mr., on fruit-trees in America, 66.

Dragon-flies, intestines of, 147.

Drift-timber, 326.

Driver-ant, 232.

Drones killed by other bees, 164.

Duck, domestic, wings of, reduced, 8.
—, beak of, 183.
—, logger-headed, 140.

Duckweed, 344.

Dugong, affinities of, 365.

Dung-beetles with deficient tarsi, 108.

Dyticus, 345.

Earl, Mr., W., on the Malay Archipelago, 351.

Ears, drooping, in domestic animals, 8.
—, rudimentary, 400.

Earth, seeds in roots of trees, 326.
—, charged with seeds, 328.

Echinodermata, their pedicellariæ, 191.

Eciton, 230.

Economy of organisation, 117.

Edentata, teeth and hair, 115.
—, fossil species of, 417.

Edwards, Milne, on physiological division of labour, 89.
—, on gradations of structure, 156.

Edwards, on embryological characters, 368.

Eggs, young birds escaping from, 68.

Egypt, productions of, not modified, 169.

Electric organs, 150.

Elephant, rate of increase, 51.
—, of Glacial period, 113.

Embryology, 386.

Eozoon Canadense, 287.

Epilipsy inherited, 108.

Existence, struggle for, 48.
—, condition of, 167.

Extinction, as bearing on natural selection, 96.
—, of domestic varieties, 93.
—, , 293.

Eye, structure of, 144.
—, correction for aberration, 163.

Eyes, reduced, in moles, 110.

Fabre, M., on hymenoptera fighting, 69.
—, on parasitic sphex, 216.
—, on Sitaris, 394.

Falconer, Dr., on naturalisation of plants in India, 51.
—, on elephants and mastodons, 306.
—, and Cautley on mammals of sub-Himalayan beds, 311.

Falkland Islands, wolf of, 351.

Faults, 268.

Faunas, marine, 317.

Fear, instinctive, in birds, 211.

Feet of birds, young molluscs adhering to, 345.

Fertilisation variously effected, 154, 161.

Fertility of hybrids, 238.
—, from slight changes in conditions, 252.
—, of crossed varieties, 255.

Fir-trees destroyed by cattle, 56.
—, pollen of, 164.

Fish, flying, 140.
—, teleostean, sudden appearance of, 285.
—, eating seeds, 327, 346.
—, fresh-water, distribution of, 343.

Fishes, ganoid, now confined to fresh water, 83.
—, electric organs of, 150.
—, ganoid, living in fresh water, 296.
—, of southern hemisphere, 338.

Flat-fish, their structure, 186.

Flight, powers of, how acquired, 140.

Flint-tools, proving antiquity of man, 13.

Flower, Prof., on the larynx, 190.
—, on Halitherium, 302.
—, on the resemblance between the jaws of the dog and Thylacinus, 375.
—, on the homology of the feet of certain marsupials, 382.

Flowers, structure of in relation to crossing, 73.
—, of compositæ and umbelliferæ, 116, 173.

—, beauty of, 161.
—, double, 230.

Flysch formation, destitute of organic remains, 271.

Forbes, Mr. D., on glacial action in the Andes, 335.

Forbes, E., on colours of shells, 107.
—, on abrupt range of shells in depth, 135.
—, on poorness of palæontological collections, 270.
—, on continuous succession of genera, 293.
—, on continental extensions, 323.
—, on distribution during Glacial period, 330.
—, on parallelism in time and space, 361.

Forests, changes in, in America, 58.

Formation, Devonian, 305.
—, Cambrian, 287.
Formations, thickness of, in Britain, 268.
—, intermittent, 277.

Formica rufescens, 216.
—, sanguinea, 217.
—, flava, neuter of, 231.

Forms, lowly organised, long enduring, 99.

Frena, ovigerous, of cirripedes, 148.

Fresh-water productions, dispersal of, 343.

Fries on species in large genera being closely allied to other species, 45.

Frigate-bird, 142.

Frogs on islands, 350.

Fruit-trees, gradual improvement of, 27.
—, in United States, 66.
—, varieties of, acclimatised in United States, 114.

Fuci, crossed, 249, 343.

Fur, thicker in cold climates, 107.

Furze, 388.

Galapagos Archipelago, birds of, 348.
—, productions of, 353, 355.

Galaxias, its wide range, 343.

Galeopithecus, 139.

Game, increase of, checked by vermin, 55.

Gärtner on sterility of hybrids, 237, 241.
—, on reciprocal crosses, 243.
—, on crossed maize and verbascum, 257, 258.
—, on comparison of hybrids and mongrels, 259, 260.

Gaudry, Prof., on intermediate genera of fossil mammals in Attica, 301.

Geese, fertility when crossed, 307.
—, upland, 142.

Geikie, Mr., on subaërial denudation, 267.

Genealogy, important in classification, 369.

Generations, alternate, 387.

Geoffroy St. Hilaire, on balancement, 117.

—, on homologous organs, 382.
—, , Isidore, on variability of repeated parts, 118.
—, on correlation, in monstrosities, 9.
—, on correlation, 115.
—, on variable parts being often monstrous, 122.

Geographical distribution, 316.

Geography, ancient, 427.

Geology, future progress of, 427.
—, imperfection of the record, 427.

Gervais, Prof., on Typotherium, 302.

Giraffe, tail of, 157.
—, structure of, 177.

Glacial period, 330.
—, affecting the North and South, 335.

Glands, mammary, 189.

Gmelin, on distribution, 330.

Godwin-Austin, Mr., on the Malay Archipelago, 280.

Goethe, on compensation of growth, 117.

Gomphia, 174.

Gooseberry, grafts of, 246.

Gould, Dr. Aug. A., on land-shells, 353.

Gould, Mr., on colours of birds, 107.
—, on instincts of cuckoo, 214.

—, on distribution of genera of birds, 358.

Gourds, crossed, 258.

Graba, on the Uria lacrymans, 72.

Grafting, capacity of, 245, 246.

Granite, areas of denuded, 274.

Grasses, varieties of, 88.

Gray, Dr. Asa, on the variability of oaks, 40.
—, on man not causing variability, 62.
—, on sexes of the holly, 74.
—, on trees of the United States, 79.
—, on naturalised plants in the United States, 89.
—, on æstivation, 174.
—, on Alpine plants, 330.
—, on rarity of intermediate varieties, 136.

Gray, Dr. J.E., on striped mule, 128.

Grebe, 142.

Grimm, on asexual reproduction, 387.

Groups, aberrant, 378.

Grouse, colours of, 66.
—, red, a doubtful species, 38.

Growth, compensation of, 117.

Günther, Dr., on flat-fish, 187.
—, on prehensile tails, 189.
—, on the fishes of Panama, 317.

—, on the range of fresh-water fishes, 343.
—, on the limbs of Lepidosiren, 399.

Haast, Dr., on glaciers of New Zealand, 335.

Habit, effect of, under domestication, 8.
—, effect of, under nature, 108.
—, diversified, of same species, 141.

Häckel, Prof., on classification and the lines of descent, 381.

Hair and teeth, correlated, 115.

Halitherium, 302.

Harcourt, Mr. E.V., on the birds of Madeira, 348.

Hartung, M., on boulders in the Azores, 328.

Hazel-nuts, 325.

Hearne, on habits of bears, 141.

Heath, changes in vegetation, 55.

Hector, Dr., on glaciers of New Zealand, 335.

Heer, Oswald, on ancient cultivated plants, 13.
—, on plants of Madeira, 83.

Helianthemum, 174.

Helix, resisting salt water, 353.

Helix pomatia, 353.

Helmholtz, M., on the imperfection of the human eye, 163.

Helosciadium, 325.

Hemionus, striped, 128.

Hensen, Dr., on the eyes of Cephalopods, 152.

Herbert, W., on struggle for existence, 49.
—, on sterility of hybrids, 238.

Hermaphrodites crossing, 76.

Heron eating seed, 346.

Heron, Sir R., on peacocks, 70.

Heusinger, on white animals poisoned by certain plants, 9.

Hewitt, Mr., on sterility of first crosses, 249.

Hildebrand, Prof., on the self-sterility of Corydalis, 238.

Hilgendorf, on intermediate varieties, 275.

Himalaya, glaciers of, 335.
—, plants of, 337.

Hippeastrum, 238.

Hippocampus, 189.

Hofmeister, Prof., on the movements of plants, 197.

Holly-trees, sexes of, 73.

Hooker, Dr., on trees of New Zealand, 78.
—, on acclimatisation of Himalayan trees, 112.

—, on flowers of umbelliferæ, 116.
—, on the position of ovules, 172.
—, on glaciers of Himalaya, 335.
—, on algæ of New Zealand, 338.
—, on vegetation at the base of the Himalaya, 338.
—, on plants of Tierra del Fuego, 336.
—, on Australian plants, 337, 355.
—, on relations of flora of America, 340.
—, on flora of the Antarctic lands, 341, 354.
—, on the plants of the Galapagos, 349, 354.
—, on glaciers of the Lebanon, 335.
—, on man not causing variability, 62.
—, on plants of mountains of Fernando Po, 337.

Hooks on palms, 158.
—, on seeds, on islands, 349.

Hopkins, Mr., on denudation, 274.

Hornbill, remarkable instinct of, 234.

Horns, rudimentary, 400.

Horse, fossil in La Plata, 294.
—, proportions of, when young, 392.

Horses destroyed by flies in Paraguay, 56.
—, striped, 128.

Horticulturists, selection applied by, 23.

Huber on cells of bees, 224.

Huber, P., on reason blended with instinct, 205.
—, on habitual nature of instincts, 206.
—, on slave-making ants, 216.
—, on Melipona domestica, 220.

Hudson, Mr., on the Ground-woodpecker of La Plata, 142.
—, on the Molothrus, 215.

Humble-bees, cells of, 221.

Hunter, J., on secondary sexual characters, 119.

Hutton, Captain, on crossed geese, 240.

Huxley, Prof., on structure of hermaphrodites, 79.
—, on the affinities of the Sirenia, 302.
—, on forms connecting birds and reptiles, 302.
—, on homologous organs, 386.
—, on the development of aphis, 390.

Hybrids and mongrels compared, 259.

Hybridism, 235.

Hydra, structure of, 147.

Hymenoptera, fighting, 69.

Hymenopterous insect, diving, 142.

Hyoseris, 173.

Ibla, 118.

Icebergs transporting seeds, 329.

Increase, rate of, 50.

Individuals, numbers favourable to selection, 80.
—, many, whether simultaneously created, 322.

Inheritance, laws of, 10.
—, at corresponding ages, 10, 67.

Insects, colour of, fitted for their stations, 66.
—, sea-side, colours of, 107.
—, blind, in caves, 110.
—, luminous, 151.
—, their resemblance to various objects, 181.
—, neuter, 2320.

Instinct, 205.
—, , not varying simultaneously with structure, 229.

Instincts, domestic, 209.

Intercrossing, advantages of, 76, 251.

Islands, oceanic, 347.

Isolation favourable to selection, 81.

Japan, productions of, 334.

Java, plants of, 337.

Jones, Mr. J.M., on the birds of Bermuda, 348.

Jordain, M., on the eye-spots of star fishes, 144.

Jukes, Prof., on subaërial denudation, 267.

Jussieu on classification, 367.

Kentucky, caves of, 111.

Kerguelen-land, flora of, 341, 354.

Kidney-bean, acclimatisation of, 114.

Kidneys of birds, 115.

Kirby, on tarsi deficient in beetles, 108.

Knight, Andrew, on cause of variation, 5.

Kölreuter, on intercrossing, 76.
—, on the barberry, 77.
—, on sterility of hybrids, 237.
—, on reciprocal crosses, 243.
—, on crossed varieties of nicotiana, 258.
—, on crossing male and hermaphrodite flowers, 397.

Lamarck, on adaptive characters, 373.

Lancelet, 99.
—, , eyes of, 145.

Landois, on the development of the wings of insects, 148.

Land-shells, distribution of, 353.
—, of Madeira, naturalised, 357.
—, resisting salt water, 353.

Languages, classification of, 371.

Lankester, Mr. E. Ray, on longevity, 169.
—, on homologies, 385.

Lapse, great, of time, 266.

Larvæ, 388, 389.

Laurel, nectar secreted by the leaves, 73.

Laurentian formation, 287.

Laws of variation, 106.

Leech, varieties of, 59.

Leguminosæ, nectar secreted by glands, 73.

Leibnitz’ attack on Newton, 421.

Lepidosiren, 83, 303.
—, , limbs in a nascent condition, 398, 399.

Lewes, Mr. G.H., on species not having changed in Egypt, 169.
—, on the Salamandra atra, 397.
—, on many forms of life having been at first evolved, 425.

Life, struggle for, 49.

Lingula, Silurian, 286.

Linnæus, aphorism of, 365.

Lion, mane of, 69.
—, young of, striped, 388.

Lobelia fulgens, 57, 77.

Lobelia, sterility of crosses, 238.

Lockwood, Mr., on the ova of the Hippocampus, 189.

Locusts transporting seeds, 327.

Logan, Sir W., on Laurentian formation, 287.

Lowe, Rev. R.T., on locusts visiting Madeira, 327.

Lowness, of structure connected with variability, 118.
—, related to wide distribution, 359.

Lubbock, Sir J., on the nerves of coccus, 35.
—, on secondary sexual characters, 124.
—, on a diving hymenopterous insect, 142.
—, on affinities, 280.
—, on metamorphoses, 386, 389.

Lucas, Dr. P., on inheritance, 9.
—, on resemblance of child to parent, 261.

Lund and Clausen, on fossils of Brazil, 310.

Lyell, Sir C., on the struggle for existence, 49.
—, on modern changes of the earth, 75.
—, on terrestrial animals not having been developed on islands, 180.
—, on a carboniferous land-shell, 271.
—, on strata beneath Silurian system, 287.
—, on the imperfection of the geological record, 289.
—, on the appearance of species, 289.
—, on Barrande’s colonies, 291.
—, on tertiary formations of Europe and North America, 298.
—, on parallelism of tertiary formations, 301.
—, on transport of seeds by icebergs, 328.
—, on great alternations of climate, 342.
—, on the distribution of fresh-water shells, 345.
—, on land-shells of Madeira, 357.

Lyell and Dawson, on fossilized trees in Nova Scotia, 278.

Lythrum salicaria, trimorphic, 254.

Macleay, on analogical characters, 373.

Macrauchenia, 302.

McDonnell, Dr., on electric organs, 150.

Madeira, plants of, 83.
—, beetles of, wingless, 109.
—, fossil land-shells of, 311.
—, birds of, 348.

Magpie tame in Norway, 209.

Males, fighting, 69.

Maize, crossed, 257.

Malay Archipelago, compared with Europe, 280.
—, mammals of, 352.

Malm, on flat-fish, 186.

Malpighiaceæ, small imperfect flowers of, 173.

Malpighiaceæ, 367.

Mammæ, their development, 189.
—, rudimentary, 397.

Mammals, fossil, in secondary formation, 283.
—, insular, 351.

Man, origin of, 428.

Manatee, rudimentary nails of, 400.

Marsupials of Australia, 90.
—, , fossil species of, 382.
—, , structure of their feet, 310.

Martens, M., experiment on seeds, 325.

Martin, Mr. W.C., on striped mules, 129.

Masters, Dr., on Saponaria, 174.

Matteucci, on the electric organs of rays, 150.

Matthiola, reciprocal crosses of, 244.

Maurandia, 197.

Means of dispersal, 323.

Melipona domestica, 220.

Merrill, Dr., on the American cuckoo, 212.

Metamorphism of oldest rocks, 287.

Mice destroying bees, 56.
—, acclimatisation of, 113.
—, tails of, 189.

Miller, Prof., on the cells of bees, 221, 224.

Mirabilis, crosses of, 243.

Missel-thrush, 59.

Mistletoe, complex relations of, 2.

Mivart, Mr., on the relation of hair and teeth, 115.
—, on the eyes of cephalopods, 151.
—, various objections to Natural Selection, 174.
—, on abrupt modifications, 201.
—, on the resemblance of the mouse and antechinus, 373.

Mocking-thrush of the Galapagos, 357.

Modification of species, not abrupt, 424.

Moles, blind, 110.

Molothrus, habits of, 215.

Mongrels, fertility and sterility of, 255.
—, and hybrids compared, 259.

Monkeys, fossil, 284, 285.

Monachanthus, 372.

Mons, Van, on the origin of fruit-trees, 21.

Monstrosities, 33.

Moquin-Tandon, on sea-side plants, 107.

Morphology, 382.

Morren, on the leaves of Oxalis, 197.

Moths, hybrid, 240.

Mozart, musical powers of, 206.

Mud, seeds in, 345.

Mules, striped, 129.

Müller, Adolph, on the instincts of the cuckoo, 213.

Müller, Dr. Ferdinand, on Alpine Australian plants, 337.

Müller, Fritz, on dimorphic crustaceans, 35, 233.
—, on the lancelet, 99.
—, on air-breathing crustaceans, 152.
—, on climbing plants, 197.
—, on the self-sterility of orchids, 238.
—, on embryology in relation to classification, 368.
—, on the metamorphoses of crustaceans, 390, 395.
—, on terrestrial and fresh-water organisms not undergoing any
metamorphosis, 394.

Multiplication of species not indefinite, 101.

Murchison, Sir, R., on the formations of Russia, 272.
—, on azoic formations, 286.
—, on extinction, 293.

Murie, Dr., on the modification of the skull in old age, 149.

Murray, Mr. A., on cave-insects, 111.

Mustela vison, 138.

Myanthus, 372.

Myrmecocystus, 231.

Myrmica, eyes of, 232.

Nägeli, on morphological characters, 170.

Nails, rudimentary, 400.

Nathusius, Von, on pigs, 159.

Natural history, future progress of, 426.
—, selection, 62.

—, system, 364.

Naturalisation of forms distinct from the indigenous species, 89.
—, in New Zealand, 163.

Naudin, on analagous variations in gourds, 125.
—, on hybrid gourds, 258.
—, on reversion, 260.

Nautilus, Silurian, 286.

Nectar of plants, 73.

Nectaries, how formed, 73.

Nelumbium luteum, 346.

Nests, variation in, 208, 228, 234.

Neuter insects, 230, 231.

New Zealand, productions of, not perfect, 163.
—, naturalised products of, 309.
—, fossil birds of, 310.
—, glaciers of, 335.
—, crustaceans of, 338.
—, algæ of, 338.
—, flora of, 354.
—, number of plants of, 374.

Newman, Col., on humble-bees, 57.

Newton, Prof., on earth attached to a partridge’s foot, 328.

Newton, Sir I., attacked for irreligion, 421.

Nicotiana, crossed varieties of, 258.

—, certain species very sterile, 243.

Nitsche, Dr., on the Polyzoa, 193.

Noble, Mr., on fertility of Rhododendron, 239.

Nodules, phosphatic, in azoic rocks, 287.

Oaks, variability of, 40.

Œnonis, small imperfect flowers of, 173.

Onites apelles, 108.

Orchids, fertilisation of, 154.
—, the development of their flowers, 195.
—, forms of, 372.

Orchis, pollen of, 151.

Organisation, tendency to advance, 97.

Organs of extreme perfection, 143.
—, electric, of fishes, 150.
—, of little importance, 156.
—, homologous, 382.
—, rudiments of, and nascent, 397.

Ornithorhynchus, 83, 367.
—, mammæ of, 190.

Ostrich not capable of flight, 180.
—, habit of laying eggs together, 215.
—, American, two species of, 318.

Otter, habits of, how acquired, 138.

Ouzel, water, 142.

Owen, Prof., on birds not flying, 108.
—, on vegetative repetition, 118.
—, on variability of unusually developed parts, 119.
—, on the eyes of fishes, 145.
—, on the swim-bladder of fishes, 148.
—, on fossil horse of La Plata, 294.
—, on generalised form, 301.
—, on relation of ruminants and pachyderms, 303.
—, on fossil birds of New Zealand, 310.
—, on succession of types, 310.
—, on affinities of the dugong, 365.
—, on homologous organs, 383.
—, on the metamorphosis of cephalopods, 390.

Pacific Ocean, faunas of, 317.

Pacini, on electric organs, 151.

Paley, on no organ formed to give pain, 163.

Pallas, on the fertility of the domesticated descendants of wild stocks, 241.

Palm with hooks, 158.

Papaver bracteatum, 174.

Paraguay, cattle destroyed by flies, 56.

Parasites, 215.

Partridge, with ball of dirt attached to foot, 328.

Parts greatly developed, variable, 119.

Parus major, 141.

Passiflora, 238.

Peaches in United States, 66.

Pear, grafts of, 246.

Pedicellariæ, 191.

Pelargonium, flowers of, 166.
—, sterility of, 239.

Peloria, 116.

Pelvis of women, 115.

Period, glacial, 330.

Petrels, habits of, 142.

Phasianus, fertility of hybrids, 240.

Pheasant, young, wild, 211.

Pictet, Prof., on groups of species suddenly appearing, 282.
—, on rate of organic change, 291.
—, on continuous succession of genera, 293.
—, on change in latest tertiary forms, 278.
—, on close alliance of fossils in consecutive formations, 306.
—, on early transitional links, 283.

Pierce, Mr., on varieties of wolves, 71.

Pigeons with feathered feet and skin between toes, 9.
—, breeds described, and origin of, 15.
—, breeds of, how produced, 28, 30.
—, tumbler, not being able to get out of egg, 68.

—, reverting to blue colour, 127.
—, instinct of tumbling, 210.
—, young of, 392.

Pigs, black, not affected by the paint-root, 9.
—, modified by want of exercise, 159.

Pistil, rudimentary, 397.

Plants, poisonous, not affecting certain coloured animals, 9.
—, selection, applied to, 27.
—, gradual improvement of, 27.
—, not improved in barbarous countries, 27.
—, dimorphic, 35, 253.
—, destroyed by insects, 53.
—, in midst of range, have to struggle with other plants, 60.
—, nectar of, 73.
—, fleshy, on sea-shores, 107.
—, climbing, 147, 196.
—, fresh-water, distribution of, 345.
—, low in scale, widely distributed, 359.

Pleuronectidæ, their structure, 186.

Plumage, laws of change in sexes of birds, 70.

Plums in the United States, 66.

Pointer dog, origin of, 25.
—, habits of, 210.

Poison not affecting certain coloured animals, 9.

Poison, similar effect of, on animals and plants, 425.

Pollen of fir-trees, 164.
—, transported by various means, 154, 161.

Pollinia, their development, 195.

Polyzoa, their avicularia, 193.

Poole, Col., on striped hemionus, 128.

Potemogeton, 346.

Pouchet, on the colours of flat-fish, 188.

Prestwich, Mr., on English and French eocene formations, 300.

Proctotrupes, 142.

Proteolepas, 118.

Proteus, 112.

Psychology, future progress of, 428.

Pyrgoma, found in the chalk, 284.

Quagga, striped, 129.

Quatrefages, M., on hybrid moths, 240.

Quercus, variability of, 40.

Quince, grafts of, 246.

Rabbit, disposition of young, 211.

Races, domestic, characters of, 12.

Race-horses, Arab, 26.
—, English, 323.

Radcliffe, Dr., the electrical organs of the torpedo, 150.

Ramond, on plants of Pyrenees, 331.

Ramsay, Prof., on subaërial denudation, 267.
—, on thickness of the British formations, 268, 269.
—, on faults, .

Ramsay, Mr., on instincts of cuckoo, 213.

Ratio of increase, 50.

Rats, supplanting each other, 59.
—, acclimatisation of, 113.
—, blind, in cave, 110.

Rattle-snake, 162.

Reason and instinct, 205.

Recapitulation, general, 404.

Reciprocity of crosses, 243.

Record, geological, imperfect, 264.

Rengger, on flies destroying cattle, 56.

Reproduction, rate of, 50.

Resemblance, protective, of insects, 181.
—, to parents in mongrels and hybrids, 260.

Reversion, law of inheritance, 11.
—, in pigeons, to blue colour, 127.

Rhododendron, sterility of, 239.

Richard, Prof., on Aspicarpa, 367.

Richardson, Sir J., on structure of squirrels, 139.
—, on fishes of the southern hemisphere, 338.

Robinia, grafts of, 246.

Rodents, blind, 110.

Rogers, Prof., Map of N. America, 274.

Rudimentary organs, 397.

Rudiments important for classification, 367.

Rütimeyer, on Indian cattle, 14, 241.

Sageret, on grafts, 246.

Salamandra atra, 397.

Saliva used in nests, 228.

Salmons, males fighting, and hooked jaws of, 69.

Salt-water, how far injurious to seeds, 325.
—, not destructive to land-shells, 353.

Salter, Mr., on early death of hybrid embryos, 249.

Salvin, Mr., on the beaks of ducks, 184.

Saurophagus sulphuratus, 141.

Schacht, Prof., on Phyllotaxy, 173.

Schiödte, on blind insects, 110.
—, on flat-fish, 186.

Schlegel, on snakes, 115.

Schöbl, Dr., on the ears of mice, 172.

Scott, Mr. J., on the self-sterility of orchids, 238.
—, on the crossing of varieties of verbascum, 258.

Sea-water, how far injurious to seeds, 325.
—, not destructive to land-shells, 325.

Sebright, Sir J., on crossed animals, 15.

Sedgwick, Prof., on groups of species suddenly appearing, 282.

Seedlings destroyed by insects, 53.

Seeds, nutriment in, 60.
—, winged, 117.
—, means of dissemination, 154, 161, 327, 328.
—, power of resisting salt-water, 325.
—, in crops and intestines of birds, 326, 327.
—, eaten by fish, 327, 346.
—, in mud, 345.
—, hooked, on islands, 349.

Selection of domestic products, 22.
—, principle not of recent origin, 27.
—, unconscious, 27.
—, natural, 62.
—, sexual, 69.
—, objections to term, 63.
—, natural, has not induced sterility, 247.

Sexes, relations of, 69.

Sexual characters variable, 123.
—, selection, 69.

Sheep, Merino, their selection, 23.
—, two sub-breeds, unintentionally produced, 26.
—, mountain, varieties of, 59.

Shells, colours of, 107.
—, hinges of, 154.
—, littoral, seldom embedded, 270.
—, fresh-water, long retain the same forms, 308.
—, fresh-water, dispersal of, 344.
—, of Madeira, 349.
—, land, distribution of, 349.
—, land, resisting salt water, 325.

Shrew-mouse, 373.

Silene, infertility of crosses, 243.

Silliman, Prof., on blind rat, 110.

Sirenia, their affinities, 302.

Sitaris, metamorphosis of, 394.

Skulls of young mammals, 159, 384.

Slave-making instinct, 216.

Smith, Col. Hamilton, on striped horses, 129.

Smith, Dr., on the Polyzoa, 193.

Smith, Mr. Fred., on slave-making ants, 217.

—, on neuter ants, 231.

Snake with tooth for cutting through egg-shell, 214.

Somerville, Lord, on selection of sheep, 23.

Sorbus, grafts of, 246.

Sorex, 373.

Spaniel, King Charles’ breed, 25.

Specialisation of organs, 98.

Species, polymorphic, 35.
—, dominant, 43.
—, common, variable, 42.
—, in large genera variable, 44.
—, groups of, suddenly appearing, 282, 285.
—, beneath Silurian formations, 287.
—, successively appearing, 290.
—, changing simultaneously throughout the world, 297.

Spencer, Lord, on increase in size of cattle, 26.

Spencer, Mr. Herbert, on the first steps in differentiation, 100.
—, on the tendency to an equilibrium in all forces, 252.

Sphex, parasitic, 216.

Spiders, development of, 390.

Sports in plants, 8.

Sprengel, C.C., on crossing, 76.
—, on ray-florets, 116.

Squalodon, 302.

Squirrels, gradations in structure, 139.

Staffordshire, heath, changes in, 55.

Stag-beetles, fighting, 69.

Star fishes, eyes of, 144.
—, their pedicellariæ, 192.

Sterility from changed conditions of life, 7.
—, of hybrids, 236.
—, laws of, 241.
—, causes of, 247.
—, from unfavourable conditions, 250.
—, not induced through natural selection, 247.

St. Helena, productions of, 347.

St. Hilaire, Aug., on variability of certain plants, 174.
—, on classification, 368.

St. John, Mr., on habits of cats, 209.

Sting of bee, 163.

Stocks, aboriginal, of domestic animals, 14.

Strata, thickness of, in Britain, 268, 269.

Stripes on horses, 128.

Structure, degrees of utility of, 159.

Struggle for existence, 48.

Succession, geological, 290.
—, of types in same areas, 310.

Swallow, one species supplanting another, 59.

Swaysland, Mr., on earth adhering to the feet of migratory birds, 328.

Swifts, nests of, 228.

Swim-bladder, 148.

Switzerland, lake habitations of, 13.

System, natural, 364.

Tail of giraffe, 157.
—, of aquatic animals, 157.
—, prehensile, 188.
—, rudimentary, 400.

Tanais, dimorphic, 36.

Tarsi deficient, 108.

Tausch, Dr., on umbelliferæ, 173.

Teeth and hair correlated, 115.
—, rudimentary, in embryonic calf, 397, 420.

Tegetmeier, Mr., on cells of bees, 222, 226.

Temminck, on distribution aiding classification, 369.

Tendrils, their development, 196.

Thompson, Sir W., on the age of the habitable world, 286.
—, on the consolidation of the crust of the earth, 409.

Thouin, on grafts, 246.

Thrush, aquatic species of, 142.
—, mocking, of the Galapagos, 356.
—, young of, spotted, 388.
—, nest of, 234.

Thuret, M., on crossed fuci, 243.

Thwaites, Mr., on acclimatisation, 112.

Thylacinus, 374.

Tierra del Fuego, dogs of, 211.
—, plants of, 341.

Timber-drift, 326.

Time, lapse of, 266.
—, by itself not causing modification, 81.

Titmouse, 141.

Toads on islands, 350.

Tobacco, crossed varieties of, 258.

Tomes, Mr., on the distribution of bats, 351.

Transitions in varieties rare, 134.

Traquair, Dr., on flat-fish, 188.

Trautschold, on intermediate varieties, 275.

Trees on islands belong to peculiar orders, 350.

—, with separated sexes, 78.

Trifolium pratense, 57, 75.
—, incarnatum, 75.

Trigonia, 296.

Trilobites, 286.
—, sudden extinction of, 297.

Trimen, Mr., on imitating-insects, 377.

Trimorphism in plants, 35, 252.

Troglodytes, 234.

Tuco-tuco, blind, 110.

Tumbler pigeons, habits of, hereditary, 210.
—, young of, 392.

Turkey-cock, tuft of hair on breast, 70.

—, naked skin on head, 158.
—, young of, instinctively wild, 265.

Turnip and cabbage, analogous variations of, 125.

Type, unity of, 166, 167.

Types, succession of, in same areas, 310.

Typotherium, 302.

Udders enlarged by use, 8.
—, rudimentary, 397.

Ulex, young leaves of, 388.

Umbelliferæ, flowers and seeds of, 116.
—, outer and inner florets of, 173.

Unity of type, 166, 167.

Uria lacrymans, 72.

Use, effects of, under domestication, 8.
—, effects of, in a state of nature, 108.

Utility, how far important in the construction of each part, 159.

Valenciennes, on fresh-water fish, 344.

Variability of mongrels and hybrids, 259.

Variation, under domestication, 5.
—, caused by reproductive system being affected by conditions of life, 7.
—, under nature, 33.
—, laws of, 106.
—, correlated, 9, 114, 159.

Variations appear at corresponding ages, 10, 67.
—, analogous in distinct species, 124.

Varieties, natural, 32.
—, struggle between, 59.
—, domestic, extinction of, 86.
—, transitional, rarity of, 134.
—, when crossed, fertile, 257.
—, when crossed, sterile, 256.
—, classification of, 371.

Verbascum, sterility of, 238.
—, varieties of, crossed, 258.

Verlot, M., on double stocks, 230.

Verneuil, M. de, on the succession of species, 299.

Vibracula of the Polyzoa, 193.

Viola, small imperfect flowers of, 173.
—, tricolor, 57.

Virchow, on the structure of the crystalline lens, 145.

Virginia, pigs of, 66.

Volcanic islands, denudation of, 268.

Vulture, naked skin on head, 158.

Wading-birds, 375.

Wagner, Dr., on Cecidomyia, 387.

Wagner, Moritz, on the importance of isolation, 81.

Wallace, Mr., on origin of species, 1.
—, on the limit of variation under domestication, 31.
—, on dimorphic lepidoptera, 36, 232.
—, on races in the Malay Archipelago, 37.
—, on the improvement of the eye, 145.
—, on the walking-stick insect, 182.
—, on laws of geographical distribution, 322.
—, on the Malay Archipelago, 351.
—, on mimetic animals, 377.

Walsh, Mr. B.D., on phytophagic forms, 38.
—, on equal variability, 125.

Water, fresh, productions of, 343.

Water-hen, 143.

Waterhouse, Mr., on Australian marsupials, 90.
—, on greatly developed parts being variable, 119.
—, on the cells of bees, 220.
—, on general affinities, 379.

Water-ouzel, 142.

Watson, Mr. H.C., on range of varieties of British plants, 37, 46.
—, on acclimatisation, 112.
—, on flora of Azores, 328.
—, on rarity of intermediate varieties, 136.
—, on Alpine plants, 331.
—, on convergence, 100.
—, on the indefinite multiplication of species, 101.

Weale, Mr., on locusts transporting seeds, 327.

Web of feet in water-birds, 142.

Weismann, Prof., on the causes of variability, 6.
—, on rudimentary organs, 400.

West Indian islands, mammals of, 352.

Westwood, on species in large genera being closely allied to others, 45.
—, on the tarsi of Engidæ, 124.
—, on the antennæ of hymenopterous insects, 366.

Whales, 182.

Wheat, varieties of, 88.

White Mountains, flora of, 330.

Whittaker, Mr., on lines of escarpment, 267.

Wichura, Max, on hybrids, 249, 251, 260.

Wings, reduction of size, 109.
—, of insects homologous with branchiæ, 148.
—, rudimentary, in insects, 397.

Wolf crossed with dog, 210.
—, of Falkland Isles, 351.

Wollaston, Mr., on varieties of insects, 38.
—, on fossil varieties of shells in Madeira, 42.
—, on colours of insects on sea-shore, 107.
—, on wingless beetles, 109.
—, on rarity of intermediate varieties, 136.
—, on insular insects, 347.
—, on land-shells of Madeira naturalised, 357.

Wolves, varieties of, 71.

Woodcock with earth attached to leg, 328.

Woodpecker, habits of, 141.
—, green colour of, 158.

Woodward, Mr., on the duration of specific forms, 276.
—, on Pyrgoma, 284.
—, on the continuous succession of genera, 293.
—, on the succession of types, 311.

World, species changing simultaneously throughout, 297.

Wrens, nest of, 234.

Wright, Mr. Chauncey, on the giraffe, 178.

—, on abrupt modifications, 203.

Wyman, Prof., on correlation of colour and effects of poison, 9.
—, on the cells of the bee, 22.

Youatt, Mr., on selection, 23.
—, on sub-breeds of sheep, 26.
—, on rudimentary horns in young cattle, 400.

Zanthoxylon, 174.

Zebra, stripes on, 128.

Zeuglodon, 302.

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