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Title: The Story of the Living Machine

Author: H. W. Conn

Release date: August 8, 2005 [eBook #16487]
Most recently updated: December 12, 2020

Language: English

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*** START OF THE PROJECT GUTENBERG EBOOK THE STORY OF THE
LIVING MACHINE ***

THE STORY OF THE LIVING
MACHINE
A REVIEW OF THE CONCLUSIONS OF MODERN BIOLOGY
IN REGARD TO THE MECHANISM WHICH CONTROLS THE
PHENOMENA OF LIVING ACTIVITY

BY

H.W. CONN
PROFESSOR OF BIOLOGY IN WESLEYAN UNIVERSITY

AUTHOR OF THE STORY OF GERM LIFE, EVOLUTION OF TO-DAY,
THE LIVING WORLD, ETC.

WITH FIFTY ILLUSTRATIONS
NEW YORK D. APPLETON AND COMPANY 1903
Copyright, 1899,
By D. APPLETON AND COMPANY.

PREFACE.
That the living body is a machine is a statement that is frequently made without
any very accurate idea as to what it means. On the one hand it is made with a
belief that a strict comparison can be made between the body and an ordinary,
artificial machine, and that living beings are thus reduced to simple mechanisms;
on the other hand it is made loosely, without any special thought as to its
significance, and certainly with no conception that it reduces life to a
mechanism. The conclusion that the living body is a machine, involving as it
does a mechanical conception of life, is one of most extreme philosophical
importance, and no one interested in the philosophical conception of nature can
fail to have an interest in this problem of the strict accuracy of the statement that
the body is a machine. Doubtless the complete story of the living machine can
not yet be told; but the studies of the last fifty years have brought us so far along
the road toward its completion that a review of the progress made and a glance at
the yet unexplored realms and unanswered questions will be profitable. For this
purpose this work is designed, with the hope that it may give a clear idea of the
trend of recent biological science and of the advances made toward the solution
of the problem of life.
Middletown, Conn., U.S.A.

October 1, 1898.

CONTENTS.

PREFACE.
LIST OF ILLUSTRATIONS.
THE STORY OF THE LIVING MACHINE.
PART I.
CHAPTER I.
CHAPTER II.
PART II.
CHAPTER III.
THE LIBRARY OF USEFUL STORIES.
NEW EDITION OF HUXLEY'S ESSAYS.
BOOKS FOR NATURE LOVERS.

introduction—Biology a new science—Historical
biology—Conservation of energy—Evolution—Cytology—New
aspects of biology—The mechanical
nature of living organisms—Significance of the new
biological problems—Outline of the subject 1

PART I.

THE RUNNING OF THE LIVING MACHINE.

CHAPTER I.

IS THE BODY A MACHINE?

What is a machine?—A general comparison of a body and
a machine—Details of the action of the machine—Physical
explanation of the chief vital functions—The

living body is a machine—The living machine
constructive as well as destructive—The vital factor 19

CHAPTER II.

THE CELL AND PROTOPLASM.

Vital properties—The discovery of cells—The cell doctrine—The
cell—The cellular structure of organisms—The
cell wall—Protoplasm—The reign of protoplasm—The
decline of the reign of protoplasm—The
structure of protoplasm—The nucleus—Centrosome—Function
of the nucleus—Cell division or karyokinesis—Fertilization
of the egg—The significance of
fertilization—What is protoplasm?—Reaction against
the cell doctrine—Fundamental vital activities as
located in cells—Summary 54

PART II.

THE BUILDING OF THE LIVING MACHINE.

CHAPTER III.

THE FACTORS CONCERNED IN THE BUILDING OF THE LIVING
MACHINE.

History of the living machine—Evidence for this
history—Historical—Embryological—Anatomical—Significance
of these sources of history—Forces at work in
the building of the living machine—Reproduction—Heredity—Variation—
Inheritance of variations—Method of machine building—Migration and
isolation—Direct influence of environment—Consciousness—Summary
of Nature's power of building machines—The origin of the cell
machine—General summary 131

LIST OF ILLUSTRATIONS.
Figure_illustrating_osmosis
Figure_illustrating_osmosis
Diagram_of_the_intestinal_walls
Diagram_of_a_single_villus
Enlarged_figure_of_four_cells_in_the_villus_membrane
A_bit_of_muscle_showing blood-vessels
A_bit_of_bark_showing_cellular_structure
Successive_stages_in_the_division_of_the_developing_egg
A_typical_cell
Cells_at_a_root_tip
Section_of_a_leaf_showing_cells_of_different_shapes
Plant_cells_with_thick_walls_from_a_fern
Section_of_potato
Various_shaped_wood_cells_from_plant_tissue
A_bit_of_cartilage
Frogs_blood
A_bit_of_bone
Connective_tissue
A_piece_of_nerve_fibre
A_muscle_fibre
A_complex_cell_vorticella
An_amœba
A_cell_as_it_appears_to_the_modern_microscope
A_cell_cut_into_pieces_each_containing_a_bit_of_nucleus
A_cell_cut_in_pieces_only_one_of_which_contains_any_nucleus
Different_forms_of_nucleii
Two_stages_in_cell_division
Stages_in_cell_division
Latest_stages_in_cell_division
An_egg
Stages_in_the_process_of_fertilization_of_the_egg_1
Stages_in_the_process_of_fertilization_of_the_egg_2

Stages_in_fertilization_of_the_egg
Latest_stages_in_the_fertilization_of_the_egg
Two_stages_in_the_division_of_the_egg
A_group_of_cells_resulting_from_division_the_first_step_in_machine_building
A_later_step_in_machine_building_the_gastrula
The_arm_of_a_monkey
The_arm_of_a_bird
The_arm_of_an_ancient_half-bird_half-reptile_animal
Diagram_to_illustrate_the_principle_of_heredity

THE STORY OF THE LIVING MACHINE.
INTRODUCTION.

Biology a New Science.—In recent years biology has been spoken of as a new
science. Thirty years ago departments of biology were practically unknown in
educational institutions. To-day none of our higher institutions of learning
considers itself equipped without such a department. This seems to be somewhat
strange. Biology is simply the study of living things; and living nature has been
studied as long as mankind has studied anything. Even Aristotle, four hundred
years before Christ, classified living things. From this foundation down through
the centuries living phenomena have received constant attention. Recent
centuries have paid more attention to living things than to any other objects in
nature. Linnæus erected his systems of classification before modern chemistry
came into existence; the systematic study of zoology antedated that of physics;
and long before geology had been conceived in its modern form, the animal and
vegetable kingdoms had been comprehended in a scientific system. How, then,
can biology be called a new science When it is older than all the others?
There must be some reason why this, the oldest of all, has been recently called a
new science, and some explanation of the fact that it has only recently advanced
to form a distinct department in our educational system. The reason is not
difficult to find. Biology is a new science, not because the objects it studies are
new, but because it has adopted a new relation to those objects and is studying
them from a new standpoint. Animals and plants have been studied long enough,
but not as we now study them. Perhaps the new attitude adopted toward living
nature may be tersely expressed by saying that in the past it has been studied as
at rest, while to-day it is studied as in motion. The older zoologists and botanists
confined themselves largely to the study of animals and plants simply as so
many museum specimens to be arranged on shelves with appropriate names. The
modern biologist is studying these same objects as intensely active beings and as
parts of an ever-changing history. To the student of natural history fifty years
ago, animals and plants were objects to be classified; to the biologist of to-day,
they are objects to be explained.
To understand this new attitude, a brief review of the history of the fundamental
features of philosophical thought will be necessary. When, long ago, man began

to think upon the phenomena of nature, he was able to understand almost
nothing. In his inability to comprehend the activities going on around him he
came to regard the forces of nature as manifestations of some supernatural
beings. This was eminently natural. He had a direct consciousness of his own
power to act, and it was natural for him to assume that the activities going on
around him were caused by similar powers on the part of some being like
himself, only superior to him. Thus he came to fill the unseen universe with gods
controlling the forces of nature. The wind was the breath of one god, and the
lightning a bolt thrown from the hands of another.
With advancing thought the ideas of polytheism later gave place to the nobler
conception of monotheism. But for a long time yet the same ideas of the
supernatural, as related to the natural, retained their place in man's philosophy.
Those phenomena which he thought he could understand were looked upon as
natural, while those which he could not understand were looked upon as
supernatural, and as produced by the direct personal activity of some divine
agency. As the centuries passed, and man's power of observation became keener
and his thinking more logical, many of the hitherto mysterious phenomena
became intelligible and subject to simple explanations. As fast as this occurred
these phenomena were unconsciously taken from the realm of the supernatural
and placed among natural phenomena which could be explained by natural laws.
Among the first mysteries to be thus comprehended by natural law were those of
astronomy. The complicated and yet harmonious motions of the heavenly bodies
had hitherto been inexplicable. To explain them many a sublime conception of
almighty power had arisen, and the study of the heavenly bodies ever gave rise
to the highest thoughts of Deity. But Newton's law of gravitation reduced the
whole to the greatest simplicity. Through the law and force of gravitation these
mysteries were brought within the grasp of human understanding. They ceased to
be looked upon as supernatural, and became natural phenomena as soon as the
force of gravitation was accepted as a part of nature.
In other branches of natural phenomena the same history followed. The forces
and laws of chemical affinity were formulated and studied, and physical laws
and forces were comprehended. As these natural forces were grasped it became,
little by little, evident that the various phenomena of nature were simply the
result of nature's forces acting in accordance with nature's laws. Phenomena
hitherto mysterious were one after another brought within the realm of law, and
as this occurred a smaller and smaller portion of them were left within the realm
of the so-called supernatural. By the middle of this century this advance had
reached a point where scientists, at least, were ready to believe that nature's

forces were all-powerful to account for nature's phenomena. Science had passed
from the reign of mysticism to the reign of law.
But after chemistry and physics, with all the forces that they could muster, had
exhausted their powers in explaining natural phenomena, there apparently
remained one class of facts which was still left in the realm of the supernatural
and the unexplained. The phenomena associated with living things remained
nearly as mysterious as ever. Life appeared to be the most inexplicable
phenomena of nature, and none of the forces and laws which had been found
sufficient to account for other departments of nature appeared to have much
influence in rendering intelligible the phenomena of life. Living organisms
appeared to be actuated by an entirely unique force. Their shapes and structure
showed so many marvellous adaptations to their surroundings as to render it
apparently certain that their adjustment must have been the result of some
intelligent planning, and not the outcome of blind force. Who could look upon
the adaptation of the eye to light without seeing in It the result of intelligent
design? Adaptation to conditions is seen in all animals and plants. These
organisms are evidently complicated machines with their parts intricately
adapted to each other and to surrounding conditions. Apart from animals and
plants the only other similarly adjusted machines are those which have been
made by human intelligence; and the inference seemed to be clear that a similar
intelligence was needed to account for the living machine. The blind action of
physical forces seemed inadequate. Thus the phenomena of life, which had been
studied longer than any other phase of nature, continued to stand aloof from the
rest and refused to fall into line with the general drift of thought. The living
world seemed to give no promise of being included among natural phenomena,
but still persisted in retaining its supernatural aspect.
It is the attempt to explain the phenomena of the living world by the same kind
of natural forces that have been adequate to account for other phenomena, that
has created modern Biology. So long as students simply studied animals and
plants as objects for classification, as museum objects, or as objects which had
been stationary in the history of nature, so long were they simply following
along the same lines in which their predecessors had been travelling. But when
once they began to ask if living nature were not perhaps subject to an intelligent
explanation, to study living things as part of a general history and to look upon
them as active moving objects whose motion and whose history might perhaps
be accounted for, then at once was created a new department of thought and a
new science inaugurated.

Historical Geology.—Preparation had been made for this new method of
studying life by the formulation of a number of important scientific discoveries.
Prominent among these stood historical geology. That the earth had left a record
of her history in the rocks in language plain enough to be read appears to have
been impressed upon scientists in the last of the century. That the earth has had a
history and that man could read it became more and more thoroughly understood
as the first decades of this century passed. The reading of that history proved a
somewhat difficult task. It was written in a strange language, and it required
many years to discover the key to the record. But under the influence of the
writings of Lyell, just before the middle of the century, it began to appear that the
key to this language is to be found by simply opening the eyes and observing
what is going on around us to-day. A more extraordinary and more important
discovery has hardly ever been made, for it contained the foundation of nearly
all scientific discoveries which have been made since. This discovery proclaimed
that an application of the forces still at work to-day on the earth's surface, but
continued throughout long ages, will furnish the interpretation of the history
written in the rocks, and thus an explanation of the history of the earth itself. The
slow elevation of the earth's crust, such as is still going on to-day, would, if
continued, produce mountains; and the washing away of the land by rains and
floods, such as we see all around us, would, if continued through the long
centuries, produce the valleys and gorges which so astound us. The explanation
of the past is to be found in the present. But this geological history told of a
history of life as well as a history of rocks. The history of the rocks has indeed
been bound up in the history of life, and no sooner did it appear that the earth's
crust has had a readable history than it appeared that living nature had a parallel
history. If the present is a key to the past in interpreting geological history,
should not the same be true of this history of life? It was inevitable that problems
of life should come to the front, and that the study of life from the dynamical
standpoint, rather than a statical, should ensue. Modern biology was the child of
historical geology.
But historical geology alone could never have led to the dynamical phase of
modern biology. Three other conceptions have contributed in an even greater
degree to the development of this science.
Conservation of Energy.—The first of these was the doctrine of conservation
of energy and the correlation of forces. This doctrine is really quite simple, and
may be outlined as follows: In the universe, as we know it, there exists a certain
amount of energy or power of doing work. This amount of energy can neither be
increased nor decreased; energy can no more be created or destroyed than matter.

It exists, however, in a variety of forms, which may be either active or passive.
In the active state it takes some form of motion. The various forces which we
recognize in nature—heat, light, electricity, chemism, etc.—are simply forms of
motion, and thus forms of this energy. These various types of energy, being only
expressions of the universal energy, are convertible into each other in such a way
that when one disappears another appears. A cannon ball flying through the air
exhibits energy of motion; but it strikes an obstacle and stops. The motion has
apparently stopped, but an examination shows that this is not the case. The
cannon ball and the object it strikes have been heated, and thus the motion of the
ball has simply been transformed into a different form of motion, which we call
heat. Or, again, the heat set free under the locomotive boiler is converted by
machinery into the motion of the locomotive. By still different mechanism it
may be converted into electric force. All forms of motion are readily convertible
into each other, and each form in which energy appears is only a phase of the
total energy of nature.
A second condition of energy is energy at rest, or potential energy. A stone on the
roof of a house is at rest, but by virtue of its position it has a certain amount of
potential energy, since, if dislodged, it will fall to the ground, and thus develop
energy of motion. Moreover, it required to raise the stone to the roof the
expenditure of an amount of energy exactly equal to that which will reappear if
the stone is allowed to fall to the ground. So in a chemical molecule, like fat,
there is a store of potential energy which may be made active by simply breaking
the molecule to pieces and setting it free. This occurs when the fat burns and the
energy is liberated as heat. But it required at some time the expenditure of an
equal amount of energy to make the molecule. When the molecule of fat was
built in the plant which produced it, there was used in its construction an amount
of solar energy exactly equivalent to the energy which may be liberated by
breaking the molecule to pieces. The total sum of the active and potential energy
in the universe is thus at all times the same.
This magnificent conception has become the cornerstone of modern science. As
soon as conceived it brought at once within its grasp all forms of energy in
nature. It is primarily a physical doctrine, and has been developed chiefly in
connection with the physical sciences. But it shows at once a possible connection
between living and non-living nature. The living organism also exhibits motion
and heat, and, if the doctrine of the conservation of energy be true, this energy
must be correlated with other forms of energy. Here is a suggestion that the same
laws control the living and the non-living world; and a suspicion that if we can
find a natural explanation of the burning of a piece of coal and the motion of a

locomotive, so, too, we may find a natural explanation of the motion of a living
machine.
Evolution—A second conception, whose influence upon-the development of
biology was even greater, was the doctrine of evolution. It is true that the
doctrine of evolution was no new doctrine with the middle of this century, for it
had been conceived somewhat vaguely before. But until historical geology had
been formulated, and until the idea of the unity of nature had dawned upon the
minds of scientists, the doctrine of evolution had little significance. It made little
difference in our philosophy whether the living organisms were regarded as
independent creations or as descended from each other, so long as they were
looked upon as a distinct realm of nature without connection with the rest of
nature's activity. If they are distinct from the rest of nature, and therefore require
a distinct origin, it makes little difference whether we looked upon that origin as
a single originating point or as thousands of independent creations. But so soon
as it appeared that the present condition of the earth's crust was formed by the
action of forces still in existence, and so soon as it appeared that the forces
outside of living forces, including astronomical, physical and chemical forces,
are all correlated with each other as parts of the same store of energy, then the
problem of the origin of living things assumed a new meaning. Living things
became then a part of nature, and demanded to be included in the same general
category. The reign of law, which was claiming that all nature's phenomena are
the result of natural rather than supernatural powers, demanded some
explanation of the origin of living things. Consequently, when Darwin pointed
out a possible way in which living phenomena could thus be included in the
realm of natural law, science was ready and anxious to receive his explanation.
Cytology.—A third conception which contributed to the formulation of modern
biology was derived from the facts discovered in connection with the organic
cell and protoplasm. The significance of these facts we shall notice later, but here
we may simply state that these discoveries offered to students simplicity in the
place of complexity. The doctrine of cells and protoplasm appeared to offer to
biologists no longer the complicated problems which were associated with
animals and plants, but the same problems stripped of all side issues and reduced
to their lowest terms. This simplifying of the problems proved to be an
extraordinary stimulus to the students who were trying to find some way of
understanding life.
New Aspects of Biology.—These three conceptions seized hold of the scientific
world at periods not very distant from each other, and their influence upon the
study of living nature was immediate and extraordinary. Living things now came

to be looked upon not simply as objects to be catalogued, but as objects which
had a history, and a history which was of interest not merely in itself, but as a
part of a general plan. They were no longer studied as stationary, but as moving
phases of nature. Animals were no longer looked upon simply as beings now
existing, but as the results of the action of past forces and as the foundation of a
different series of beings in the future. The present existing animals and plants
came to be regarded simply as a step in the long history of the universe. It
appeared at once that the study of the present forms of life would offer us a
means of interpreting the past and perhaps predicting the future.
In a short time the entire attitude which the student assumed toward living
phenomena had changed. Biological science assumed new guises and adopted
new methods. Even the problems which it tried to solve were radically changed.
Hitherto the attempt had been made to find instances of purpose in nature. The
marvellous adaptations of living beings to their conditions had long been felt,
and the study of the purposes of these adaptations had inspired many a
magnificent conception. But now the scientist lost sight of the purpose in hunting
for the cause. Natural law is blind and can have no purpose. To the scientist,
filled with the thought of the reign of law, purpose could not exist in nature.
Only cause and effect appeal to him. The present phenomena are the result of
forces acting in the past, and the scientist's search should be not for the purpose
of an adaptation, but for the action of the forces which produced it. To discover
the forces and laws which led to the development of the present forms of animals
and plants, to explain the method by which these forces of nature have acted to
bring about present results, these became the objects of scientific research. It no
longer had any meaning to find that a special organ was adapted to its
conditions; but it was necessary to find out how it became adapted. The
difference in the attitude of these two points of view is world-wide. The former
fixes the attention upon the end, the latter upon the means by which the end was
attained; the former is what we sometimes call teleological, the latter scientific;
the former was the attitude of the study of animals and plants before the middle
of this century, the latter the spirit which actuates modern biology.
The Mechanical Nature of Living Organisms.—This new attitude forced
many new problems to the front. Foremost among them and fundamental to them
all were the questions as to the mechanical nature of living organisms. The law
of the correlation of force told that the various forms of energy which appear
around us—light, heat, electricity, etc.—are all parts of one common store of
energy and convertible into each other. The question whether vital energy is in
like manner correlated with other forms of energy was now extremely

significant. Living forces had been considered as standing apart from the rest of
nature. Vital force, or vitality, had been thought of as something distinct in itself;
and that there was any measurable relation between the powers of the living
organism and the forces of heat and chemical affinity was of course unthinkable
before the formulation of the doctrine of the correlation of forces. But as soon as
that doctrine was understood it began to appear at once that, to a certain extent at
least, the living body might be compared to a machine whose function is simply
to convert one kind of energy into another. A steam engine is fed with fuel. In
that fuel is a store of energy deposited there perhaps centuries ago. The rays of
the sun, shining on the world in earlier ages, were seized upon by the growing
plants and stored away in a potential form in the wood which later became coal.
This coal is placed in the furnace of the steam engine and is broken to pieces so
that it can no longer hold its store of energy, which is at once liberated in its
active form as heat. The engine then takes the energy thus liberated, and as a
result of its peculiar mechanism converts it into the motion of its great fly-wheel.
With this notion clearly in mind the question forces itself to the front whether the
same facts are not true of the living animal organism. It, too, is fed with food
containing a store of energy; and should we not regard it, like the steam engine,
simply a machine for converting this potential energy into motion, heat, or some
other active form? This problem of the correlation of vital and physical forces is
inevitably forced upon us with the doctrine of the correlation of forces. Plainly,
however, such questions were inconceivable before about the middle of the
nineteenth century.
This mechanical conception of living activity was carried even farther. Under the
lead of Huxley there arose in the seventh decade of the century a view of life
which reduced it to a pure mechanism. The microscope had, at that time, just
disclosed the universal presence in living things of that wonderful substance,
protoplasm. This material appeared to be a homogeneous substance, and a
chemical study showed it to be made of chemical elements united in such a way
as to show close relation to albumens. It appeared to be somewhat more complex
than ordinary albumen, but it was looked upon as a definite chemical compound,
or, perhaps, as a simple mixture of compounds. Chemists had shown that the
properties of compounds vary with their composition, and that the more complex
the compound the more varied its properties. It was a natural conception,
therefore, that protoplasm was a complex chemical compound, and that its vital
properties were simply the chemical properties resulting from its composition.
Just as water possesses the power of becoming solid at certain temperatures, so
protoplasm possesses the power of assimilating food and growing; and, since we
do not doubt that the properties of water are the result of its chemical

composition, so we may also assume that the vital properties of protoplasm are
the result of its chemical composition. It followed from this conclusion that if
chemists ever succeeded in manufacturing the chemical compound, protoplasm,
it would be alive. Vital phenomena were thus reduced to chemical and
mechanical problems.
These ideas arose shortly after the middle of the century, and have dominated the
development of biological science up to the present time. It is evident that the
aim of biological study must be to test these conceptions and carry them out into
details. The chemical and mechanical laws of nature must be applied to vital
phenomena in order to see whether they can furnish a satisfactory explanation of
life. Are the laws and forces of chemistry sufficient to explain digestion? Are the
laws of electricity applicable to an understanding of nervous phenomena? Are
physical and chemical forces together sufficient to explain life? Can the animal
body be properly regarded as a machine controlled by mechanical laws? Or, on
the other hand, are there some phases of life which the forces of chemistry and
physics cannot account for? Are there limits to the application of natural law to
explain life? Can there be found something connected with living beings which
is force but not correlated with the ordinary forms of energy? Is there such a
thing as vital energy, or is the so-called vital force simply a name which we have
given to the peculiar manifestations of ordinary energy as shown in the substance
protoplasm? These are some of the questions that modern biology is trying to
answer, and it is the existence of such questions which has made modern biology
a new science. Such questions not only did not, but could not, have arisen before
the doctrines of the conservation of energy and evolution had made their
impression upon the thought of the world.
Significance of the New Biological Problems—It is further evident that the
answers to these questions will have a significance reaching beyond the domain
of biology proper and affecting the fundamental philosophy of nature. The
answer will determine whether or not we can accept in entirety the doctrines of
the conservation of energy and evolution. Plainly if it should be found that the
energy of animate nature was not correlated with other forms of energy, this
would demand either a rejection or a complete modification of our doctrine of
the conservation of energy. If an animal can create any energy within itself, or
can destroy any energy, we can no longer regard the amount of energy of the
universe as constant. Even if that subtile form of force which we call nervous
energy should prove to be uncorrelated with other forms of energy, the idea of
the conservation of energy must be changed. It is even possible that we must
insist that the still more subtile form of force, mental force, must be brought

within the scope of this great law in order that it be implicitly accepted. This law
has proved itself strictly applicable to the inanimate world, and has then thrust
upon us the various questions in regard to vital force, and we must recognize that
the real significance of this great law must rest upon the possibility of its
application to vital phenomena.
No less intimate is the relation of these problems to the doctrine of evolution.
Evolution tries to account for each moment in the history of the world as the
result of the conditions of the moment before. Such a theory loses its meaning
unless it can be shown that natural forces are sufficient to account for living
phenomena. If the supernatural must be brought in here and there to account for
living phenomena, then evolution ceases to have much meaning. It is
undoubtedly a fact that the rapidly developing ideas along the above mentioned
lines of dynamical biology have, been potent factors in bringing about the
adoption of evolution. Certain it is that, had it been found that no correlation
could be traced between vital and non-vital forces, the doctrine of evolution
could not have stood, and even now the special significance which we shall in
the end give to evolution will depend upon how we succeed in answering the
questions above outlined. The fact is that this problem of the mechanical
explanation of vital phenomena forms the capstone of the arch, the sides of
which are built of the doctrines of the conservation of energy and the theory of
evolution. To the presentation of these problems the following pages will be
devoted. The fact that both the doctrine of the conservation of energy and that of
evolution are practically everywhere accepted indicates that the mechanical
nature of vital forces is regarded as proved. But there are still many questions
which are not so easily answered. It will be our purpose in the following
discussion to ascertain just what are these problems in dynamical biology and
how far they have been answered. Our object will be then in brief to discover to
what extent the conception of the living organism as a machine is borne out by
the facts which have been collected in the last quarter century, and to learn
where, if anywhere, limits have been found to our possibility of applying the
forces of chemistry and physics to an explanation of life. In other words, we
shall try to see how far we have been able to understand living phenomena in
terms of natural force.
Outline of the Subject.—The subject, as thus presented, resolves itself at once
into two parts. That the living organism is a machine is everywhere recognized,
although some may still doubt as to the completeness of the comparison. In the
attempt to explain the phenomena of life we have two entirely different
problems. The first is manifestly to account for the existence of this machine, for

such a completed piece of mechanism as a man or a tree cannot be explained as a
result of simple accident, as the existence of a rough piece of rock might be
explained. Its intricacy of parts and their purposeful interrelation demands
explanation, and therefore the fundamental problem is to explain how this
machine came into existence. The second problem is simpler, for it is simply to
explain the running of the machine after it is made. If the organism is really a
machine, we ought to be able to find some way of explaining its actions as we
can those of a steam engine.
Of these two problems the first is the more fundamental, for if we fail to find an
explanation for the existence of the machine, our explanation of its method of
action is only partly satisfactory. But the second question is the simpler, and
must be answered first. We cannot hope to explain the more puzzling matter of
the origin of the machine unless we can first understand how it acts. In our
treatment of the subject, therefore, we shall divide it into two parts:
I. The Running of the Living Machine.
II. The Origin of the Living Machine.

PART I.
THE RUNNING OF THE LIVING MACHINE.

CHAPTER I.

IS THE BODY A MACHINE?

The problem before us in this section is to find out to what extent animals and
plants are machines. We wish to determine whether the laws and forces which
regulate their activities are the same as the laws and forces with which we
experiment in the chemical and physical laboratory, and whether the principles
of mechanics and the doctrine of the conservation of energy apply equally well
in the living machine and the steam engine.
It might be inferred that the proper method of study would be to confine our
attention largely to the simplest forms of life, since the problems would be here
less complicated, and therefore of easier solution. This, however, has not been
nor can it be the method of study. Our knowledge of the processes of life have
been derived largely from the most rather than the least complex forms. We have
a better knowledge of the physiology of man and his allies than any other
animals. The reason for this is plain enough. In the first place, there is a value in
the knowledge of the life activities of man entirely apart from any theoretical
aspects, and hence human physiology has demanded attention for its own sake.
The practical utility of human physiology has stimulated its study for centuries;
and in the last fifty years of scientific progress it has been human physiology and
that of allied animals that has attracted the chief attention of physiologists. The
result is that while the physiology of man is tolerably well known, that of other
animals is less understood the farther we get away from man and his allies. For
this reason most of our knowledge of the living body as a machine must be
derived from the study of man. This is, however, fortunate rather than otherwise.
In the first place, it enables us to proceed from the known to the unknown; and in
the second place, more interest attaches to the problem as connected with human
physiology than along any other line. In our discussion, therefore, we shall refer

chiefly to the physiology of man. If we find that the functions of human life are
amenable to a mechanical explanation we cannot hesitate to believe that this will
be equally true of the lower orders of nature. For similar reasons little reference
will be made to the mechanism of plant life. The structure of the plant is simpler
and its activities are much more easily referable to mechanical principles than
are those of animals. For these reasons it will only be necessary for us to turn our
attention to the life activities of the higher animals.
What is a Machine?—Turning now to our more immediate subject of the
accuracy of the statement that the body is a machine, we must first ask what is
meant by a machine? A brief definition of a machine might be as follows: A
machine is a piece of apparatus so designed that it can change one kind of
energy into another for a definite purpose. Energy, as already noticed, is the
power of doing work, and its ordinary active forms are heat, motion, electricity,
light, etc.; but it may be in a passive or potential form, and in this form stored
within a chemical molecule. These various forms of energy are readily
convertible into each other; and any form of apparatus designed for the purpose
of producing such a conversion is called a machine. A dynamo is thus a machine
so adjusted that when mechanical motion is supplied to it the energy of motion is
converted into electricity; while an electromotor, on the other hand, is a piece of
apparatus so designed that when electricity is applied to it, it is converted into
motion. A steam engine, again, is designed to convert potential or passive energy
into active energy. Potential energy in the form of chemical composition (coal) is
supplied to the engine, and this energy is first liberated in the active form of heat
and then is converted into the motion of the great fly-wheel. In all these cases
there is no energy or power created, for the machine must be always supplied
with an amount of energy equal to that which it gives back in another form.
Indeed, a larger amount of energy must be furnished the machine than is
expected back, for there is always an actual loss of available energy. In the
process of the conversion of one form of energy into another some of the energy,
from friction or other cause, takes the form of heat, and is then radiated into
space beyond our reach. It is, of course, not destroyed, for energy cannot be
destroyed; but it has assumed a form called radiant heat, which is not available
for our uses. A machine thus neither creates nor destroys energy. It receives it in
one form and gives it back in another form, with an inevitable loss of a portion
of the energy as radiant heat. With this understanding, we may now ask if the
living body can be properly compared with a machine.
A General Comparison of a Body and a Machine.--That the living body
exhibits the ordinary types of energy is of course clear enough when we

remember that it is always in motion and is always radiating heat—two of the
most common types of physical energy. That this energy is supplied to the body
as it is to other machines, in the form of the energy of chemical composition,
will also need no further proof when it is remembered that it is necessary to
supply the body with appropriate food in order that it may do work. The food we
eat, like coal, represents so much solar energy which is stored up by the agency
of plant life, and the close comparison between feeding the body to enable it to
work and feeding the engine to enable it to develop energy is so evident that it
demands no further demonstration. The details of the problem may, however,
present some difficulties.
The first question which presents itself is whether the only power the body
possesses is, as in the case with other machines, to transform energy without
being able to create or destroy it? Can every bit of energy shown by the living
organism be accounted for by energy furnished in the food, and conversely can
all the energy furnished in the food be found manifested in the living organism?
The theoretical answer to this question in terms of the law of the conservation of
energy is clear enough, but it is by no means so easy to answer it by
experimental data. To obtain experimental demonstration it would be necessary
to make an accurate determination of the amount of energy an individual
receives during a given period, and at the same time a similar measurement of
the amount of energy liberated in his body either as motion or heat. If the body is
a machine, these two should exactly balance, and if they do not balance it would
indicate that the living organism either creates or destroys energy, and is
therefore not a machine. Such experiments are exceedingly difficult. They must
be performed usually upon man rather than other animals, and it is necessary to
inclose an individual in an absolutely sealed space with arrangements for
furnishing him with air and food in measured quantity, and with appliances for
measuring accurately the work he does and the heat given off from his body. In
addition, it is necessary to measure the exact amount of material he eliminates in
the form of carbonic acid and other excretions. Such experiments present many
difficulties which have not yet been thoroughly overcome, but they have been
attempted by several investigators. For the purpose of such an experiment
scientists have allowed themselves to be shut up in a small chamber six or eight
feet in length, in which their only communication with the outer world is by
telephone and through a small opening in the side of the chamber, occasionally
opened for a second or two to supply the prisoner with food. In such a chamber
they have remained as long as twelve days. In these experiments it is necessary
to take account not only of the food eaten, but of the actual amount of this food

which is used by the body. If the person gains in weight, this must mean that he
is storing up in his body material for future use; while if he loses in weight, this
means that he is consuming his own tissues for fuel. Careful daily records of his
weight must therefore be taken. Estimates of the solids, liquids, and gases given
off from his body must be obtained, for to carry out the experiment an exact
balance must be made between the income and the outgo. The apparatus devised
for such experiments has been made very delicate; so delicate, indeed, that the
rising of the individual in the box from his chair is immediately seen in a rise in
temperature of the apparatus. But even with this delicacy the apparatus is
comparatively coarse, and can measure only the most apparent forms of energy.
The more subtle types of energy, such as nervous force, if this is to be regarded
as energy, do not make any impression on the apparatus.
The obstacles in the way of these experiments do not particularly concern us, but
the general results are of the greatest significance for our purpose. While, for
manifest reasons, it has not been possible to carry on these experiments for any
great length of time, and while the results have not yet been very accurately
refined, they are all of one kind and teach unhesitatingly one conclusion. So far
as concerns measurable energy or measurable material, the body behaves just
like any other machine. If the body is to do work in this respiration apparatus, it
does so only by breaking to pieces a certain amount of food and using the energy
thus liberated, and the amount of food needed is proportional to the amount of
work done. When the individual simply walks across the floor, or even rises
from his chair, this is accompanied by an increase in the amount of food material
broken up and a consequent increase in the amount of refuse matter eliminated
and the heat given off. The income and outgo of the body in both matter and
energy is balanced. If, during the experimental period, it is found that less energy
is liberated than that contained in the food assimilated, it is also found that the
body has gained in weight, which simply means that the extra energy has been
stored in the body for future use. No more energy can be obtained from the body
than is furnished, and for all furnished in the food an equivalent amount is
regained. There is no trace of any creation or destruction of energy. While, on
account of the complexity of the experimenting, an absolutely strict balance
sheet cannot be made, all the results are of the same nature. So far as concerns
measurable energy, all the facts collected bear out the theoretical conception that
the living body is to be regarded as a machine which converts the potential
energy of chemical composition, stored passively in its food, into active energy
of motion and heat.

It is found, however, that the body is a machine of a somewhat superior grade,
since it is able to convert this potential energy into motion with less loss than the
ordinary machine. As noticed above, in all machines a portion of the energy is
converted into heat and rendered unavailable by radiating into space. In an
ordinary engine only about one-fifteenth of the energy furnished in the coal can
be regained in the form of motive power, the rest being radiated from the
machine as heat. Some of our better engines to-day utilize a somewhat larger
part, but most of them utilize less than one-tenth. The experiments with the
living body in the respiration apparatus above described, give a means of
determining the proportion of the energy furnished in the form of food which can
be utilized in the form of motive force. This figure appears to be decidedly larger
than that obtained by any machine yet devised by man.
The conclusion of the matter up to this point is then clear. If we leave out of
account the phenomena of the nervous system, which we shall consider
presently, the general income and outgo of the body as concerns matter and
energy is such that the body must be regarded as a machine, which, like other
machines, simply transforms energy without creating or destroying it. To this
extent, at least, animals conform to the law of the conservation of energy and are
veritable machines.
Details of the Action of the Machine.—We turn next to some of the subordinate
problems concerning the details of the action of the living machine. We have a
clear understanding of the method of action of a steam engine. Its mechanism is
simple, and, moreover, it was designed by human intelligence. We can
understand how the force of chemical affinity breaks up the chemical
composition of the coal, how the heat thus liberated is applied to the water to
vapourize it; how the vapour is collected in the boiler under pressure; how this
pressure is applied to the piston in the cylinder, and how this finally results in the
revolution of the fly-wheel. It is true that we do not understand the underlying
forces of chemism, etc., but these forces certainly exist and are the foundation of
science. But the mechanism of the engine is intelligible. Our understanding of it
is such that, with the forces of chemistry and physics as a foundation, we can
readily explain the running of the machine. Our next problem, therefore, is to see
if we can in the same way reach an understanding of the phenomena of the living
machine. Can we, by the use of these same chemical and physical forces, explain
the activities taking place in the living organism? Can the motion of the body, for
example, be made as intelligible as the motion of the steam engine?
Physical Explanation of the Chief Vital Functions.—The living machine is, of
course, vastly more complicated than the steam engine, and there are many

different processes which must be considered separately. There is not space in a
work of this size to consider them all carefully, but we may select a few of the
vital functions as illustrations of the method which is pursued. It will be assumed
that the fundamental processes of human physiology are understood by the
reader, and we shall try to interpret some of them in terms of chemical and
physical force.
Digestion.—The first step in this transformation of fuel is the process of
digestion. Now this process of digestion is nothing mysterious, nor does it
involve any peculiar or special forces. Digestion of food is simply a chemical
change therein. The food which is taken into the body in the form of sugar,
starch, fat or protein, is acted upon by the digestive juices in such a way that its
chemical nature is slightly changed. But the changes that thus occur are not
peculiar to the living body, since they will take place equally well in the
chemist's laboratory. They are simply changes in the molecular structure of the
food material, and only such changes as are simple and familiar to the chemist.
The forces which effect the change are undoubtedly those of chemical affinity.
The only feature of the process which is not perfectly intelligible in terms of
chemical law is the nature of the digestive juices. The digestive fluids of the
mouth and stomach contain certain substances which possess a somewhat
remarkable power, inasmuch as they are able to bring about the chemical
changes which occur in the digestion of food. An example will make this clearer.
One of the digestive processes is the conversion of starch into sugar. The relation
of these two bodies is a very simple one, starch being readily converted into
sugar by the addition to its molecule of a molecule of water. The change can not
be produced by simply adding starch to water, but the water must be introduced
into the starch molecule. This change can be brought about in a variety of ways,
and is undoubtedly effected by the forces of chemical affinity. Chemists have
found simple methods of producing this chemical union, and the manufacture of
sugar out of starchy material has even become something of a commercial
industry. One of the methods by which this change can be produced is by adding
to the starch, along with some water, a little saliva. The saliva has the power of
causing the chemical change to occur at once, and the molecule of water enters
into the starch molecule and forms sugar. Now we do not understand how this
saliva possesses this power to induce the chemical change. But apparently the
process is of the simplest character and involves no greater mystery than
chemical affinity. We know that the saliva contains a certain material called a
ferment, which is the active agent in bringing about the change. This ferment is
not alive, nor does it need any living environment for its action. It can be
separated from the saliva in the form of a dry amorphous powder, and in this

form can be preserved almost indefinitely, retaining its power to effect the
change whenever put under proper conditions. The change of starch into sugar is
thus a simple chemical change occurring under the influence of chemical affinity
under certain conditions. One of the conditions is the presence of this saliva
ferment. If we can not exactly understand how the ferment produces this action,
neither do we exactly understand how a spark causes a bit of gunpowder to
explode. But we can not doubt that the latter is a purely natural result of the
relation of chemical and physical forces, and there is no more reason for
doubting it in the former case.
What is true of the digestion of starch by saliva is equally true of the digestion of
other foods in the stomach and intestine. Each of the digestive juices contains a
ferment which brings about a chemical change in the food. The changes are
always chemical changes and are the result of chemical forces. Apart from the
presence of these ferments there is really little difference between laboratory
chemistry and living chemistry.
Absorption of food.—The next function
of this machine to attract our attention is
the absorption of food from the intestine
into the blood. The digested food is
carried down the alimentary canal in a
purely mechanical fashion by muscular
action, and when it reaches the intestine
FIG. 1.—To illustrate osmosis. In the it begins to pass through its walls into
vessel A is a solution of sugar; in B the blood. In this absorption we find
is pure water. The two are separated engaged another set of forces, the chief
by the mebrane C. The of which appears to be the physical force
sugar passes through the membrane of osmosis. The force of osmosis has no
into B. special connection with life. If a
membrane separates two liquids of
different composition (Fig. i), a force is exerted on the liquids which cause them
to pass through the membrane, each passing through the membrane into the other
compartment. The force which drives these liquids through the membrane is
considerable, and may sometimes be exerted against considerable pressure. A
simple experiment will illustrate this force. In Fig. 2 is represented a
membranous bag tightly fastened to a glass tube. The bag is filled with a strong
solution of sugar, and is immersed in a vessel containing pure water. Under these
conditions some of the sugar solution passes through the bag into the water, and
some of the water passes from the vessel into the bag. But if the solution of sugar

is inside the bag and the pure water outside, the amount of liquid passing into the
bag is greater than the amount passing out; the bag soon becomes distended and
the water even rises in the tube to a considerable height at a(Fig. 2). The force
here concerned is a force known as osmosis or dialysis, and is always exerted
when two different solutions of certain substances are separated from each other
by a membrane. The substances in solution will, under these conditions, pass
from the dense to the weaker solution. The process is a purely physical one.
This process of osmosis lies at the basis of the
absorption of food from the alimentary canal. In the
first place, most of the food when swallowed is not
soluble, and therefore not capable of osmosis. But the
process of digestion, as we have seen, changes the
chemical nature of the food. The food, as the result of
chemical change, has become soluble, and after being
dissolved it is dialyzable—i.e., capable of osmosis.
After digestion, therefore, the food is dissolved in the FIG. 2.—In the bladder
liquids in the stomach and intestine, and is in proper A is a sugar solution
condition for dialysis. Furthermore, the structure of the In the vessel B
intestine is such as to produce conditions adapted for is pure water.
dialysis. This can be understood from Fig. 3, which Sugar passes out
represents diagrammatically a cross section through and water into
the intestinal wall. Within the intestinal wall, at A, is the bladder until it
the food mass in solution. At B are shown little rises in the tube
projections of the intestinal wall, called villi extending to a.
into this food and covered by a membrane. One of
these villi is shown more highly magnified in Fig. 4, in which B shows this
membrane. Inside of these villi are blood-vessels, C, and it will be thus seen that
the membrane, B, separates two liquids, one containing the dissolved food
outside the villus, and the other containing blood inside the villus. Here are
proper conditions for osmosis, and this process of dialysis will take place
whenever the intestinal contents holds more dialyzable material than the blood.
Under these conditions, which will always occur after food has been digested by
the digestive juices, the food will begin to pass through this membranous wall of
the intestine into the blood under the influence of the physical force of osmosis.
Thus the primary factor in food absorption is a physical one.
We must notice, however, that the physical force of osmosis is not the only
factor concerned in absorption. In the first place, it is found that the food during
its passage through the intestinal wall, or shortly afterwards, undergoes a further

change, so that by the time it has fairly
reached the blood it has again changed its
chemical nature. These changes are,
however, of a chemical nature, and, while
we do not yet know very much about them,
they are of the same sort as those of
digestion, and involve probably nothing
more than chemical processes.
Secondly, we notice that there is one phase
FIG. 3—Diagram of the intestinal of absorption which is still obscure. Part of
walls. A, lumen of intestine the food is composed of fat, and this fat, as
filled with digested food. B, the result of digestion, is mechanically
villi, containing blood vessels. broken up into extremely minute droplets.
C, larger blood vessel, which Although these droplets are of microscopic
carries blood with absorbed food size they are not actually in solution, and
away from the intestine. therefore not subject to the force of osmosis
which only affects solutions. The osmotic
force will not force fat drops through membranes, and to explain their passage
through the walls of the intestine requires something additional. We are as yet,
however, able to give only a partial explanation of this matter. The inner wall of
the intestine is not an inert, lifeless membrane, but is made of active bits of
living matter. These bits of living matter appear to seize hold of the droplets of
oil by means of little processes which they thrust out, and then pass them
through their own bodies to excrete them on their inner surface into the blood
vessels. Fig. 5 shows a few of these living bits of the membrane, each containing
several such fat droplets. This fat absorption thus appears to be a vital process,
and not one simply controlled by physical forces like osmosis. Here our
explanation runs against what we call vital power of the ultimate elements of the
body. The consideration of this vital feature we must, of course, investigate
further; but this will be done later. At present our purpose is a general
comparison of the body and a machine, and we may for a little postpone the
consideration of this vital phenomenon.
Circulation.—The next piece of mechanism for us to consider in this machine is
the device for distributing this fuel to the various parts of the machine where it is
to be used as a source of energy, corresponding in a sense to the fireman of a
locomotive. This mechanism we call the circulatory system. It consists of a
series of tubes, or blood vessels, running to every part of the body and supplying
every bit of tissue. Within the tubes is the blood, which, from its liquid nature, is

easily forced around the body
through the tubes. At the
centre of the system is a pump
which keeps the blood in
motion. The tubes form a
closed system, such that the
pump, or heart, may suck the
blood in from one side to
force it out into the tubes on
the other side; and the blood, FIG. 4.—Diagram of a single villus enlarged.
after passing over the body in B represents the membranous
this closed set of tubes, is surface covering the villus; C, the blood-vessels
finally brought back again to within the villus.
be forced once more over the FIG. 5.—An enlarged figure of four cells of
same path. As this blood is the membrane B in Fig. 4. The free
carried around the body it surface is at a; f shows fat droplets in
conveys from one part of the process of passage through the cells.
machine to another all
material that needs distribution. While in the intestine, as already noticed (Fig.
3), it receives the food, and now this food is carried by the circulation to the
muscles or the other organs that need it. While in the lungs the blood receives
oxygen, and this oxygen is then carried to those parts of the body that need it.
The circulatory system is thus simply a medium by which each part of the
machine may receive its proper share of the supplies needed for its action.
Now in this circulation we have again to do with chemical and physical forces.
All of its general phenomena are based upon purely mechanical principles. The
action of the heart—leaving out of consideration for a moment its muscular
power—is that of a simple pump. It is provided with valves whose action is as
simple and as easy to understand as those of any water pump. By the action of
these valves the blood is kept circulating in one direction. The blood vessels are
elastic, and the study of the effect of a liquid pumped rhythmically into elastic
tubes explains with simplicity the various phenomena associated with the
circulation. For example, the rhythmically contracting heart forces a small
quantity of blood into the arteries at short intervals. These tubes are large near
the heart, but smaller at their ends, where they flow into the veins, so that the
blood does not flow out into the veins so readily as it flows in from the heart.
The jet of blood that is sent in with every beat of the heart slightly stretches the
artery, and the tension thus produced causes the blood to continue to flow
between the beats. But the heart continues beating, and there is an accumulation

of the blood in the arteries until it exists under some pressure—a pressure
sufficient to force it rapidly through the small ends of the arteries into the veins.
After passing into the veins the pressure is at once removed, since the veins are
larger than the arteries, and there is no resistance to the flow of the blood. Hence
the blood in the arteries is under pressure, while there is little or no pressure in
the veins. Into the details of this matter we need not go, but this will be sufficient
to indicate that the whole process is a mechanical one.
We must not fail to see, however, that in this problem of circulation there are two
points at least where once more we meet with that class of phenomena which we
still call vital. The beating of the heart is the first of these, for this is active
muscular power. The second is a contraction of the smaller blood-vessels which
regulates the blood supply. Both of these phenomena are phases of muscular
activity, and will be included under the discussion of other similar phenomena
later.
We next notice that
not only is the
distribution of the
blood explained
upon mechanical
principles, but the
supplying of the
active parts of the
body with food is
in the same way
FIG. 6.—A bit of muscle with its blood-vessels: intelligible. As we
a, the muscle fibres; b, the minute blood-vessels. have seen, the
The fibres and vessels are bathed in lymph blood coming from
(not shown in the figure), and food material passes through the intestine
the walls of the blood-vessels into this lymph. contains the food
material received
from the digested food. Now when this blood in its circulation flows through the
active tissues—for instance, the muscles—it is again placed under conditions
where osmosis is sure to occur. In the muscles the thin-walled blood-vessels are
surrounded and bathed by a liquid called lymph. Figure 6 shows a bit of muscle
tissue, with its blood-vessels, which are surrounded by lymph. The lymph, which
is not shown, fills all the space outside the blood-vessels, thus bathing both
muscles and blood-vessels. Here again we have a membrane (i.e., the wall of the
blood-vessel) separating two liquids, and since the lymph is of a different

composition from the blood, dialysis between them is sure to occur, and the
materials which passed into the blood in the intestine through the influence of
the osmotic force, now pass out into the lymph under the influence of the same
force. The food is thus brought into the lymph; and since the lymph lies in actual
contact with the living muscle fibres, these fibres are now able to take directly
from the lymph the material needed for their use. The power which enables the
muscle fibre to take the material it needs, discarding the rest, is, again, one of the
vital processes which we defer for a moment.
Respiration.—Pursuing the same line of study, we turn for a moment to the
relation of the circulatory system to the function of supplying the body with
oxygen gas. Oxygen is absolutely needed to carry on the functions of life; for
these, like those of the engine, are based upon the oxidation of the fuel. The
oxygen is derived from the air in the simplest manner. During its circulation the
blood is brought for a fraction of a second into practical contact with air. This
occurs in the lungs, where there are great numbers of air cells, in the walls of
which the blood-vessels are distributed in great profusion. While the blood is in
these vessels it is not indeed in actual contact with the air, but is separated from
it by only a very thin membrane—so thin that it forms no hindrance to the
interchange of gases. These air-cells are kept filled with air by simple muscular
action. By the contraction of the muscles of the thorax the thoracic cavity is
enlarged, and as a result air is sucked in in exactly the same way that it is sucked
into a pair of bellows when expanded. Then the contraction of another set of
muscles decreases the size of the thoracic cavity, and the air is squeezed out
again. The action is just as truly mechanical as is that of the blacksmith's
bellows.
The relation of the air to the blood is just as simple. In the blood there are
various chemical ingredients, among which is one known as hæmoglobin. It does
not concern us at present to ask where this material comes from, since this
question is part of the broader question, the origin of the machine, to be
discussed in the second part of this work. The hæmoglobin is a normal
constituent of the blood, and, being red in colour, gives the red colour to the
blood. This hæmoglobin has peculiar relations to oxygen. It can be separated
from the blood and experimented upon by the chemist in his laboratory. It is
found that when hæmoglobin is brought in contact with oxygen, under sufficient
pressure it will form a chemical union with it. This chemical union is, however,
what the chemist calls a loose combination, since it is readily broken up. If the
oxygen is above a certain rather low pressure, the union will take place; while if
the pressure be below this point the union is at once destroyed, and the oxygen

leaves the hæmoglobin to become free. All of this is a purely chemical matter,
and can be demonstrated at will in a test tube in the laboratory. But this union
and disassociation is just what occurs as the foundation of respiration. The blood
coming to the lungs contains hæmoglobin, and since the oxygen pressure in the
air is quite high, this hæmoglobin unites at once with a quantity of oxygen while
the blood is flowing through the air-vessels. The blood is then carried off in the
circulation to the active tissues like the muscles. These tissues are constantly
using oxygen to carry on their life processes, and consequently at all times use
up about all the oxygen within their reach. The result is that in these tissues the
oxygen pressure is very low, and when the oxygen-laden hæmoglobin reaches
them the association of the hæmoglobin with oxygen is at once broken up and
the oxygen set free in the tissue. It passes at once to the lymph, from which the
active tissues seize it for the purpose of carrying on the oxidizing processes of
the body. This whole matter of supplying the body with oxygen is thus
fundamentally a chemical one, controlled by chemical laws.
Removal of Waste.—The next step in this life process is one of difficulty. After
the food and oxygen have reached the tissues it is seized by the living cell. The
food material is now oxidized by the oxygen and its latent energy is liberated,
and appears in the form of motion or heat or some other vital function. Herein is
the really mysterious part of the life process; but for the present we will overlook
the mystery of this action, and consider the results from a purely material
standpoint.
In a steam engine the fundamental process by which the latent energy of the fuel
is liberated is that of oxidation. The oxygen of the air unites with the chemical
elements of the fuel, and breaks up that fuel into simple compounds—which may
be chiefly considered as three—carbonic dioxide (CO2), water (H2O), and ash.
The energy contained in the original compound can not be held by these simpler
bodies, and it therefore escapes as heat. Just the same process, with of course
difference in details, is found in the living machine. The food, after reaching the
living cell, is united with the oxygen, and, so far as chemical results are
concerned, the process is much the same as if it occurred outside the body. The
food is broken into simpler compounds and the contained energy is liberated.
The energy is, by the mechanism of the machine, changed into motion or
nervous impulse, etc. The food is broken into simple compounds, which are
chiefly carbonic dioxide, water, and ash; the ash being, however, quite different
from the ash obtained from burning coal. Now the engine must have its chimney
to remove the gases and vapours (the CO2 and H2O) and its ashpit for the ashes.
In the same way the living machine has its excretory system for removing

wastes. In the removal of the carbonic acid and water we have to do once more
with the respiratory system, and the process is simply a repetition of the story of
gas diffusion, chemical union, and osmosis. It is sufficient here to say that the
process is just as simple and as easily explained as those already described. The
elimination of these wastes is simply a problem of chemistry and mechanics.
In the removal of the ash, however, we have something more, for here again we
are brought up against the vital action of the cell. This ash takes chiefly the form
of a compound known as urea, which finds its way into the general circulatory
system. From the blood it is finally removed by the kidneys. In the kidneys are a
large number of bits of living matter (kidney cells), which have the power of
seizing hold of the urea as the blood is flowing over them, and after thus taking it
out of the blood they deposit it in a series of tubes which lead to the bladder and
hence to the exterior. The bringing of this ash to the kidney cell is a mechanical
matter, based simply upon the flow of the blood. The seizing of the urea by the
kidney cell is a vital phenomenon which we must waive for the moment.
Up to this point in the analysis there has been no difficulty, and no one can fail to
agree with the conclusions. The position we reach is as follows: So far as relates
to the general problems of energy in the universe the body is a machine. It
neither creates nor destroys energy, but simply transforms one form into another.
In attempting to explain the action of the machine, we find that for the functions
thus far considered (sometimes called the vegetative functions) the laws of
chemistry and physics furnish adequate explanation.
We must now look a little further, and question some of the functions the
mechanical nature of which is less obvious. The whole operation thus far
described is under the control of the nervous system, which acts somewhat like
the engineer of an engine. Can this phase of living activity be included within the
conception of the body as a machine?
Nervous System.—When we come to try to apply mechanical principles to the
nervous system, we meet with what seems at first to be no thoroughfare. While
dealing with the grosser questions of chemical compounds, heat, and motion,
there is little difficulty in applying natural laws to the explanation of living
phenomena. But the problem with the nervous system is very different. It is only
to-day that we are finding that the problem is open to study, to say nothing of
solution. It is true that mental and other nervous phenomena have been studied
for a long time, but this study has been simply the study of these phenomena by
themselves without a thought of their correlation with other phenomena of

nature. It is a matter of quite recent conception that nervous phenomena have
any direct relation to the other realms of nature.
Our first question must be whether we can find any correlation between nervous
energy and other types of energy. For our purpose it will be convenient to
distinguish between the phenomena of simple nervous transmission and the
phenomena of mental activity. The former are the simpler, and offer the greatest
hope of solution. If we are to find any correlation between nervous energy and
other physical energy, we must do so by finding some way of measuring nervous
energy and comparing it with the latter. This has been very difficult, for we have
no way of measuring a nervous impulse directly. In the larger experiments upon
the income and outgo of the body, in the respiration apparatus mentioned above,
nervous phenomena apparently leave no trace. So far as experiments have gone
as yet, there is no evidence of an expenditure of extra physical energy when the
nervous system is in action. This is not surprising, however, for this apparatus is
entirely too coarse to measure such delicate factors.
That there is a correlation between nervous energy and physical energy is,
however, pretty definitely proved by experiments along different lines. The first
step in this direction was to find that a nervous stimulus can be measured at least
indirectly. When the nerve is stimulated there passes from one end to the other
an impulse, and the rapidity with which it travels can be accurately measured.
When such an impulse reaches the brain it may give rise to a conscious
sensation, and a somewhat definite estimation can be made of the amount of
time required for this. The periods are very short, of course, but they are not
instantaneous. The nervous impulse, can be studied in still other ways. We find
that the impulse can be started by ordinary forms of energy. A mechanical shock,
a chemical or an electrical shock will develop nervous energy. Now these are
ordinary forms of physical energy, and if, when they are applied to a nerve, they
give rise to a nervous stimulus, the inference is certainly a legitimate one that the
nerve is simply a bit of machinery adapted to the conversion of certain kinds of
physical energy into nervous energy. If this is the case, then it is necessary to
regard nervous energy as correlated with other forms of energy.
Other facts point in the same direction. Not only can the nervous stimulus be
developed by an electric shock, but the strength of the stimulus is within certain
limits proportional to the strength of the shock which produces it. Again, not
only is it found that an electrical shock can develop a nervous stimulus, but
conversely a nervous stimulus develops electrical energy. In ordinary nerves,
even when not active, slight electric currents can be detected. They are extremely
slight, and require the most delicate instruments for their detection. Now when a

nerve is stimulated these currents are immediately affected in such a way that
under proper conditions they are increased in intensity. The increase is sufficient
to make itself easily seen by the motion of a galvanometer. The motion of the
galvanometer under these conditions gives a ready means of studying the
character of the nervous impulse. By its use it can be determined that the nerve
impulse travels along the nerve like a wave, and we can approximately
determine the length and shape of the wave and its relative height at various
points.
Now what is the significance of all these facts for our discussion? Together they
point clearly to the conclusion that nervous energy is correlated with other forms
of physical energy. Since the nervous stimulus is started by other forms of
energy, and since it can, in turn, modify ordinary forms of energy, we can not
avoid the conclusion that the nervous impulse is only a special form of energy
developed within the nerve. It is a form of wave motion peculiar to the nerve
substance, but correlated with and developed from other types of energy. This, of
course, makes the nerve simply a bit of machinery.
If this conclusion is true, the development of a nerve impulse would mean that a
certain portion of food is broken to pieces in the body to liberate energy, and this
should be accompanied by an elimination of carbonic dioxide and heat. This is
easily shown to be true of muscle action. When we remove a muscle from the
body it may remain capable of contracting for some time. By studying it under
these conditions we find that it gives rise to carbonic dioxide and other
substances, and liberates heat whenever it contracts. As already noticed, in the
respiration experiments, whenever the individual experimented upon makes any
motions, there is an accompanying elimination of waste products and a
development of heat. But this does not appear to be demonstrable for the actions
of the nervous system. Although very careful experiments have been made, it has
as yet been found impossible to detect any rise in temperature when a nerve
impulse is passing through a nerve, nor is there any demonstrable excretion of
waste products. This would be a serious objection to the conception of the nerve
as a machine were it not for the fact that the nerve is so small that the total sum
of its nervous energy must be very slight. The total energy of this minute
machine is so slight that it can not be detected by our comparatively rough
instruments of measurement.
In short, all evidence goes to show that the nerve impulse is a form of motion,
and hence of energy, correlated with other forms of physical energy. The nerve
is, however, a very delicate machine, and its total amount of energy is very
small. A tiny watch is a more delicate machine than a water-wheel, and its

actions are more dependent upon the accuracy of its adjustment. The water-
wheel may be made very coarse and yet be perfectly efficacious, while the watch
must be fashioned with extreme delicacy. Yet the water-wheel transforms vastly
more energy than the watch. It may drive the many machines in a factory, while
the watch can do no more than move itself. But who can doubt that the watch, as
well as the water-wheel, is governed by the law of the correlation of forces? So
the nervous system of the living machine is delicately adjusted and easily put out
of order, and its action involves only a small amount of energy; but it is just as
truly subject to the law of the conservation of energy as is the more massive
muscle.
Sensations.—Pursuing this subject further, we next notice that it is possible to
trace a connection between physical energy and sensations. Sensations are
excited by certain external forms of motion. The living machine has, for
example, one piece of apparatus capable of being affected by rapidly vibrating
waves of air. This bit of the machine we call the ear. It is made of parts delicately
adjusted, so that vibrating waves of air set them in motion, and their motion
starts a nervous stimulus travelling along the auditory nerve. As a result this
apparatus will be set in motion, and an impulse sent along the auditory nerve
whenever that external type of motion which we call sound strikes the ear. In
other words, the ear is a piece of apparatus for changing air vibrations into
nervous stimulation, and is therefore a machine. Apparently the material in the
ear is like a bit of gunpowder, capable of being exploded by certain kinds of
external excitation; but neither the gunpowder nor the material in the ear
develops any energy other than that in it at the outset. In the same way the optic
nerve has, at its end, a bit of mechanism readily excited by light vibrations of the
ether, and hence the optic nerve will always be excited when ether vibrations
chance to have an opportunity of setting the optic machinery in motion. And so
on with the other senses. Each sensory nerve has, at its end, a bit of machinery
designed for the transformation of certain kinds of external energy into nervous
energy, just as a dynamo is a machine for transforming motion into electricity. If
the machine is broken, the external force has no longer any power of acting upon
it, and the individual becomes deaf or blind.
Mental Phenomena.—Thus far in our analysis we need not hesitate in
recognizing a correlation between physical and nervous energy. Even though
nervous energy is very subtle and only affects our instruments of measurements
under exceptional conditions, the fact that nervous forces are excited by physical
forces, and are themselves directly measurable, indicates that they are correlated

with physical forces. Up to this point, then, we may confidently say that the
nervous system is part of the machine.
But when we turn to the more obscure parts of the nervous phenomena, those
which we commonly call mental, we find ourselves obliged to stop abruptly. We
may trace the external force to the sensory organ, we may trace this force into a
nervous stimulus, and may follow this stimulus to the brain as a wave motion,
and therefore as a form of physical energy. But there we must stop. We have no
idea of how the nervous impulse is converted into a sensation. The mental side
of the sensation appears to stand in a category by itself, and we can not look
upon it as a form of energy. It is true that many brave attempts have been made
to associate the two. Sensations can be measured as to intensity, and the intensity
of a sensation is to a certain extent dependent upon the intensity of the stimulus
exciting it. The mental sensation is undoubtedly excited by the physical wave of
nervous impulse. In the growth of the individual the development of its mental
powers are found to be parallel to the development of its nerves and brain—a
fact which, of course, proves that mental power is dependent upon brain
structure. Further, it is found that certain visible changes occur in certain parts of
the brain—the brain cells—when they are excited into mental activity. Such
series of facts point to an association between the mental side of sensations and
physical structure of the machine. But they do not prove any correlation between
them. The unlikeness of mental and physical phenomena is so absolute that we
must hesitate about drawing any connection between them. It is impossible to
conceive the mental side of a sensation as a form of wave motion. If, further, we
take into consideration the other phenomena associated with the nervous system,
the more distinctly mental processes, we have absolutely no data for any
comparison. We can not imagine thought measured by units, and until we can
conceive of such measurement we can get no meaning from any attempt to find a
correlation between mental and physical phenomena. It is true that certain
psychologists have tried to build up a conception of the physical nature of mind;
but their attempts have chiefly resulted in building up a conception of the
physical nature of the brain, and then ignoring the radical chasm that exists
between mind and matter. The possibility of describing a complex brain as
growing parallel to the growth of a complex mind has been regarded as
equivalent to proving their identity. All attempts in this direction thus far have
simply ignored the fact that the stimulation of a nerve, a purely physical process,
is not the same thing as a mental action. What the future may disclose it is
hazardous to say, but at present the mental side of the living machine has not
been included within the conception of the mechanical nature of the organism.

The Living Body is a Machine.—Reviewing the subject up to this point, what
must be our verdict as to our ability to understand the running of the living
machine? In the first place, we are justified in regarding the body as a machine,
since, so far as concerns its relations to energy, it is simply a piece of mechanism
—complicated, indeed, beyond any other machine, but still a machine for
changing one kind of energy into another. It receives the energy in the form of
chemical composition and converts it into heat, motion, nervous wave motion,
etc. All of this is sure enough. Whether other forms of nervous and mental
activity can be placed under the same category, or whether these must be
regarded as belonging to a realm by themselves and outside of the scope of
energy in the physical sense, can not perhaps be yet definitely decided. We can
simply say that as yet no one has been able even to conceive how thought can be
commensurate with physical energy. The utter unlikeness of thought and wave
motion of any kind leads us at present to feel that on the side of mentality the
comparison of the body with a machine fails of being complete.
In regard to the second half of the question, whether natural forces are adequate
to explain the running of the machine, we have again been able to reach a
satisfactory positive answer. Digestion, assimilation, circulation, respiration,
excretion, the principal categories of physiological action, and at least certain
phases of the action of the nervous system are readily understood as controlled
by the action of chemical and physical forces. In the accomplishment of these
actions there is no need for the supposition of any force other than those which
are at our command in the scientific laboratory.
The Living Machine Constructive as well as Destructive.—In one respect the
living machine differs from all others. The action of all other machines results in
the destruction of organized material, and thus in a degradation of matter. For
example, a steam engine receives coal, a substance of high chemical
composition, and breaks it into more simple compounds, in this way liberating its
stored energy. Now if we examine all forms of artificial machines, we find in the
same way that there is always a destruction of compounds of high chemical
composition. In such machines it is common to start with heat as a source of
energy, and this heat is always produced by the breaking of chemical compounds
to pieces. In all chemical processes going on in the chemist's laboratory there is
similarly a destruction of organic compounds. It is true that the chemist
sometimes makes complex compounds out of simpler ones; but in order to do
this he is obliged to use heat to bring about the combination, and this heat is
obtained from the destruction of a much larger quantity of high compounds than
he manufactures. The total result is therefore destruction rather than manufacture

of high compounds. Thus it is a fact, that in all artificial machines and in all
artificial chemical processes there is, as a total result, a degradation of matter
toward the simpler from the more complex compounds.
As a result of the action of the living machine, however, we have the opposite
process of construction going on. All high chemical compounds are to be traced
to living beings as their source. When green plants grow in sunlight they take
simple compounds and combine them together to form more complex ones in
such a way that the total result is an increase of chemical compounds of high
complexity. In doing this they use the energy of sunlight, which they then store
away in the compounds formed. They thus produce starches, oils, proteids,
woods, etc., and these stores of energy now may be used by artificial machines.
The living machine builds up, other machines pull down. The living machine
stores sunlight in complex compounds, other machines take it out and use it. The
living organism is therefore to be compared to a sun engine, which obtains its
energy directly from the sun, rather than to the ordinary engine. While this does
not in the slightest militate against the idea of the living body as a machine, it
does indicate that it is a machine of quite a different character from any other,
and has powers possessed by no other machine. Living machines alone increase
the amount of chemical compounds of high complexity.
We must notice, however, that this power of construction in distinction from
destruction, is possessed only by one special class of living machines. Green
plants alone can thus increase the store of organic compounds in the world. All
colourless plants and all animals, on the other hand, live by destroying these
compounds and using the energy thus liberated; in this respect being more like
ordinary artificial machines. The animal does indeed perform certain
constructive operations, manufacturing complex material out of simpler bodies;
as, for example, making fats out of starches. But in this operation it destroys a
large amount of organic material to furnish the energy for the construction, so
that the total result is a degradation of chemical compounds rather than a
construction. Constructive processes, which increase the amount of high
compounds in nature, are confined to the living machine, and indeed to one
special form of it, viz., the green plant. This constructive power radically
separates the living from other machines; for while constructive processes are
possible to the chemist, and while engines making use of sunlight are possible,
the living machine is the only machine that increases the amount of high
chemical compounds in the world.
The Vital Factor.—With all this explanation of life processes it can not fail to
be apparent that we have not really reached the centre of the problem. We have

explained many secondary processes, but the primary ones are still unsolved. In
studying digestion we reach an understanding of everything until we come to the
active vital property of the gland-cells in secreting. In studying absorption we
understand the process until we come to what we have called the vital powers of
the absorptive cells of the alimentary canal. The circulation is intelligible until
we come to the beating of the heart and the contraction of the muscles of the
blood-vessels. Excretion is also partly explained, but here again we finally must
refer certain processes to the vital powers of active cells. And thus wherever we
probe the problem we find ourselves able to explain many secondary problems,
while the fundamental ones we still attribute to the vital properties of the active
tissues. Why a muscle contracts or a gland secretes we have certainly not yet
answered. The relation of the actions to the general problems of correlation of
force is simple enough. That a muscle is a machine in the sense of our definition
is beyond question. But the problem of why a muscle acts is not answered by
showing that it derives its energy from broken food material. There are plainly
still left for us a number of fundamental problems, although the secondary ones
are soluble.
What can we say in regard to these fundamental vital powers of the active
tissues? Firstly, we must notice that many of the processes which we now
understand were formerly classed as vital, and we only retain under this term
those which are not yet explained. This, of course, suggests to us that perhaps we
may some day find an explanation for all the so-called vital powers by the
application of simple physical forces. Is it a fact that the only significance to the
term vital is that we have not yet been able to explain these processes to our
entire satisfaction? Is the difference between what we have called the secondary
processes and the primary ones only one of degree? Is there a probability that the
actions which we now call vital will some day be as readily understood as those
which have already been explained?
Is there any method by which we can approach these fundamental problems of
muscle action, heart beat, gland secretion, etc.? Evidently, if this is to be done, it
must be by resolving the body into its simple units and studying these units. Our
study thus far has been a study of the machinery of the body as a whole; but we
have found that the various parts of the machine are themselves active, that apart
from the action of the general machine as a whole, the separate parts have vital
powers. We must, therefore, get rid of this complicated machinery, which
confuses the problem, and see if we can find the fundamental units which show
these properties, unencumbered by the secondary machinery which has hitherto
attracted our attention. We must turn now to the problem connected with

protoplasm and the living cell, since here, if anywhere, can we find the life
substance reduced to its lowest terms.

CHAPTER II.
THE CELL AND PROTOPLASM.

Vital Properties.—We have seen that the general activities of the body are
intelligible according to chemical and mechanical laws, provided we can assume
as their foundation the simple vital properties of living phenomena. We must
now approach closer to the centre of the problem, and ask whether we can trace
these fundamental properties to their source and find an explanation of them.
In the first place, what are these properties? The vital powers are varied, and lie
at the basis of every form of living activity. When we free them from
complications, however, they may all be reduced to four. These are: (1)
Irritability, or the property possessed by living matter of reacting when
stimulated. (2) Movement, or the power of contracting when stimulated. (3)
Metabolism, or the power of absorbing extraneous food and producing in it
certain chemical changes, which either convert it into more living tissue or break
it to pieces to liberate the inclosed energy. (4) Reproduction, or the power of
producing new individuals. From these four simple vital activities all other vital
actions follow; and if we can find an explanation of these, we have explained the
living machine. If we grant that certain parts of the body can assimilate food and
multiply, having the power of contraction when irritated, we can readily explain
the other functions of the living machine by the application of these properties to
the complicated machinery of the body. But these properties are fundamental,
and unless we can grasp them we have failed to reach the centre of the problem.
As we pass from the more to the less complicated animals we find a gradual
simplification of the machinery until the machinery apparently disappears. With
this simplification of the machinery we find the animals provided with less
varied powers and with less delicate adaptations to conditions. But withal we
find the fundamental powers of the living organisms the same. For the
performance of these fundamental activities there is apparently needed no
machinery. The simple types of living bodies are simple in number of parts, but
they possess essentially the same powers of assimilation and growth that
characterize the higher forms. It is evident that in our attempt to trace the vital
properties to their source we may proceed in two ways. We may either direct our
attention to the simplest organisms where all secondary machinery is wanting, or

to the smallest parts into which the tissues of higher organisms can be resolved
and yet retain their life properties. In either way we may hope to find living
phenomena in its simplest form independent of secondary machinery.
But the fact is, when we turn our attention in these two directions, we find the
result is the same. If we look for the lowest organisms we find them among
forms that are made of a single cell, and if we analyze the tissues of higher
animals we find the ultimate parts to be cells. Thus, in either direction, the study
of the cell is forced upon us.
Before beginning the study of the cell it will be well for us to try to get a clear
notion of the exact nature of the problems we are trying to solve. We wish to
explain the activities of life phenomena in such a way as to make them
intelligible through the application of natural forces. That these processes are
fundamentally chemical ones is evident enough. A chemical oxidation of food
lies at the basis of all vital activity, and it is thus through the action of chemical
forces that the vital powers are furnished with their energy. But the real problem
is what it is in the living machine that controls these chemical processes. Fat and
starch may be oxidized in a chemist's test tubes, and will there liberate energy;
but they do not, under these conditions, manifest vital phenomena. Proteid may
be brought in contact with oxygen without any oxidation occurring, and even if
it is oxidized no motion or assimilation or reproduction occurs under ordinary
conditions. These phenomena occur only when the oxidation takes place in the
living machine. Our problem is then to determine, if possible, what it is in the
living machine that regulates the oxidations and other changes in such a way as
to produce from them vital activities. Why is it that the oxidation of starch in the
living machine gives rise to motion, growth, and reproduction, while if the
oxidation occurs in the chemist's laboratory, or even in a bit of dead protoplasm,
it simply gives rise to heat?
One of the primary questions to demand attention in this search is whether we
are to find the explanation, at the bottom, a chemical or a mechanical one. In the
simplest form of life in which vital manifestations are found are we to attribute
these properties simply to chemical forces of the living substance, or must we
here too attribute them to the action of a complicated machinery? This question
is more than a formal one. That it is one of most profound significance will
appear from the following considerations:
Chemical affinity is a well recognized force. Under the action of this force
chemical compounds are produced and different compounds formed under
different conditions. The properties of the different compounds differ with their

composition, and the more complex are the compounds the more varied their
properties. Now it might be assumed as an hypothesis that there could be a
chemical compound so complex as to possess, among other properties, that of
causing the oxidation of food to occur in such a way as to produce assimilation
and growth. Such a compound would, of course, be alive, and it would be just as
true that its power of assimilating food would be one of its physical properties as
it is that freezing is a physical property of water. If such an hypothesis should
prove to be the true one, then the problem of explaining life would be a chemical
one, for all vital properties would be reducible to the properties of a chemical
compound. It would then only be necessary to show how such a compound came
into existence and we should have explained life. Nor would this be a hopeless
task. We are well acquainted with forces adequate to the formation of chemical
compounds. If the force of chemical affinity is adequate under certain conditions
to form some compounds, it is easy to conceive it as a possibility under other
conditions to produce this chemical living substance. Our search would need
then to be for a set of conditions under which our living compound could have
been produced by the known forces of chemical affinity.
But suppose, on the other hand, that we find this simplest bit of living matter is
not a chemical compound, but is in itself a complicated machine. Suppose that,
after reducing this vital substance to its simplest type, we find that the substance
with which we are dealing not only has complex chemical structure, but that it
also possesses a large number of structural parts adapted to each other in such a
way as to work together in the form of an intricate mechanism. The whole
problem would then be changed. To explain such a machine we could no longer
call upon chemical forces. Chemical affinity is adequate to the explanation of
chemical compounds however complicated, but it cannot offer any explanation
for the adaptation of parts which make a machine. The problem of the origin of
the simplest form of life would then be no longer one of chemical but one of
mechanical evolution. It is plain then that the question of whether we can
attribute the properties of the simplest type of life to chemical composition or to
mechanical structure is more than a formal one.
The Discovery of Cells.—It is difficult for us to-day to have any adequate idea
of the wonderful flood of light that was thrown upon scientific and philosophical
study by the discoveries which are grouped around the terms cells and
protoplasm. Cells and protoplasm have become so thoroughly a part of modern
biology that we can hardly picture to ourselves the vagueness of knowledge
before these facts were recognized. Perhaps a somewhat crude comparison will
illustrate the relation which the discovery of cells had to the study of life.

Imagine for a moment, some intelligent being located on the moon and trying to
study the phenomena on the earth's surface. Suppose that he is provided with a
telescope sufficiently powerful to disclose moderately large objects on the earth,
but not smaller ones. He would see cities in various parts of the world with wide
differences in appearance, size, and shape. He would see railroad trains on the
earth rushing to and fro. He would see new cities arising and old ones increasing
in size, and we may imagine him speculating as to their method of origin and the
reasons why they adopt this or that shape. But in spite of his most acute
observations and his most ingenious speculation, he could never understand the
real significance of the cities, since he is not acquainted with the actual living
unit. Imagine now, if you will, that this supramundane observer invents a
telescope which enables him to perceive more minute objects and thus discovers
human beings. What a complete revolution this would make in his knowledge of
mundane affairs! We can imagine how rapidly discovery would follow
discovery; how it would be found that it was the human beings that build the
houses, construct and run the railroads, and control the growth of the cities
according to their fancy; and, lastly, how it would be learned that it is the human
being alone that grows and multiplies and that all else is the result of his
activities. Such a supramundane observer would find himself entering into a new
era, in which all his previous knowledge would sink into oblivion.
Something of this same sort of revolution was inaugurated in the study of living
things by the discovery of cells and protoplasms. Animals and plants had been
studied for centuries and many accurate and painstaking observations had been
made upon them. Monumental masses of evidence had been collected bearing
upon their shapes, sizes, distribution, and relations. Anatomy had long occupied
the attention of naturalists, and the general structure of animals and plants was
already well known. But the discoveries starting in the fourth decade of the
century by disclosing the unity of activity changed the aspect of biological
science.
The Cell Doctrine.—The cell doctrine is, in brief, the theory that the bodies of
animals and plants are built up entirely of minute elementary units, more or less
independent of each other, and all capable of growth and multiplication. This
doctrine is commonly regarded as being inaugurated in 1839 by Schwann. Long
before this, however, many microscopists had seen that the bodies of plants are
made up of elementary units. In describing the bark of a tree in 1665, Robert
Hooke had stated that it was composed of little boxes or cells, and regarded it as
a sort of honeycomb structure with its cells filled with air. The term cell quite
aptly describes the compartments of such a structure, as can be seen by a glance

at Fig. 7, and this term has been retained even till to-day in spite of the fact that
its original significance has entirely disappeared. During the last century not a
few naturalists observed and described these little vesicles, always regarding
them as little spaces and never looking upon them as having any significance in
the activities of plants. In one or two instances similar bodies were noticed in
animals, although no connection was drawn between them and the cells of
plants. In the early part of the century observations upon various kinds of
animals and plant tissues multiplied, and many microscopists independently
announced the discovery of similar small corpuscular bodies. Finally, in 1839,
these observations were combined together by Schwann into one general theory.
According to the cell doctrine then formulated, the parts of all animals and plants
are either composed of cells or of material derived from cells. The bark, the
wood, the roots, the leaves of plants are all composed of little vesicles similar to
those already described under the name of cells. In animals the cellular structure
is not so easy to make out; but here too the muscle, the bone, the nerve, the gland
are all made up of similar vesicles or of material made from them. The cells are
of wonderfully different shapes and widely different sizes, but in general
structure they are alike. These cells, thus found in animals and plants alike,
formed the first connecting link between animals and plants. This discovery was
like that of our supposed supramundane observer when he first found the human
being that brought into connection the widely different cities in the various parts
of the world.
Schwann and his immediate
followers, while recognizing
that the bodies of animals
and plants were composed of
cells, were at a loss to
explain how these cells
arose. The belief held at first
was that there existed in the
bodies of animals and plants
a structureless substance FIG. 7.—A bit of bark showing cellular structure.
which formed the basis out
of which the cells develop, in somewhat the same way that crystals arise from a
mother liquid. This supposed substance Schwann called the cytoblastema, and he
thought it existed between the cells or sometimes within them. For example, the
fluid part of the blood is the cytoblastema, the blood corpuscles being the cells.
From this structureless fluid the cells were supposed to arise by a process akin to
crystallization. To be sure, the cells grow in a manner very different from that of

a crystal. A crystal always grows by layers being added upon its outside, while
the cells grow by additions within its body. But this was a minor detail, the
essential point being that from a structureless liquid containing proper materials
the organized cell separated itself.
This idea of the cytoblastema was early thrown into suspicion, and almost at the
time of the announcement of the cell doctrine certain microscopists made the
claim that these cells did not come from any structureless medium, but by
division from other cells like themselves. This claim, and its demonstration, was
of even greater importance than the discovery of the cells. For a number of
years, however, the matter was in dispute, evidence being collected which about
equally attested each view. It was a Scotchman, Dr. Barry, who finally produced
evidence which settled the question from the study of the developing egg.
The essence of his discovery was as follows: The ovum of an animal is a single
cell, and when it begins to develop into an embryo it first simply divides into
two halves, producing two cells (Fig, 8, a and b). Each of these in turn divides,
giving four, and by repeated divisions of this kind there arises a solid mass of
smaller cells (Fig. 8, b to f,) called the mulberry stage, from its resemblance to a
berry. This is, of course, simply a mass of cells, each derived by division from
the original. As the cells increase in number, the mass also increases in size by
the absorption of nutriment, and the cells continue dividing until the mass
contains thousands of cells. Meantime the body of the animal is formed out of
these cells, and when it is adult it consists of millions of cells, all of which have
been derived by division from the original cell. In such a history each cell comes
from pre-existing cells and a cytoblastema plays no part.

FIG. 8.—Successive stages in the division of the developing egg.
It was impossible, however, for Barry or any other person to follow the
successive divisions of the egg cell through all the stages to the adult. The

divisions can be followed for a short time under the microscope, but the rest
must be a matter of simple inference. It was argued that since cell origin begins
in this way by simple division, and since the same process can be observed in the
adult, it is reasonable to assume that the same process has continued
uninterruptedly, and that this is the only method of cell origin. But a final
demonstration of this conclusion was not forthcoming for a long time. For many
years some biologists continued to believe that cells can have other origin than
from pre-existing cells. Year by year has the evidence for such "free cell" origin
become less, until the view has been entirely abandoned, and to-day it is
everywhere admitted that new cells always arise from old ones by direct descent,
and thus every cell in the body of an animal or plant is a direct descendant by
division from the original egg cell.
The Cell.—But what is this cell which
forms the unit of life, and to which all the
fundamental vital properties can be traced?
We will first glance at the structure of the
cell as it was understood by the earlier
microscopists. A typical cell is shown in
Fig. 9. It will be seen that it consists of
three quite distinct parts. There is first the
cell wall (cw) which is a limiting FIG. 9.—A cell; cw is the cell wall;
membrane of varying thickness and shape. pr, the cell substance; n, the
This is in reality lifeless material, and is nucleus.
secreted by the rest of the cell. Being thus
produced by the other active parts of the cell, we will speak of it as formed
material in distinction from the rest, which is active material. Inside this vesicle
is contained a somewhat transparent semifluid material which has received
various names, but which for the present we will call cell substance (Fig. 9, pr).
It may be abundant or scanty, and has a widely varying consistency from a very
liquid mass to a decidedly thick jellylike substance. Lying within the cell
substance is a small body, usually more or less spherical in shape, which is
called the nucleus (Fig. 9, n). It appears to the microscope similar to the cell
substance in character, and has frequently been described as a bit of the cell
substance more dense than the remainder. Lying within the nucleus there are
usually to be seen one or more smaller rounded bodies which have been called
nucleoli. From the very earliest period that cells have been studied, these three
parts, cell wall, cell substance, and nucleus have been recognized, but as to their
relations to each other and to the general activities of the cell there has been the
widest variety of opinion.

Cellular Structure of Organisms.—It will be well to notice next just what is
meant by saying that all living bodies are composed of cells. This can best be
understood by referring to the accompanying figures. Figs. 10-14, for instance,
show the microscopic appearance of several plant tissues.

FIG. 10.—Cells at a root tip.
At Fig. 10 will be seen the tip of a root, plainly made of cells quite similar to the
typical cell described. At Fig. 11 will be seen a bit of a leaf showing the same
general structure. At Fig. 12 is a bit of plant tissue of which the cell walls are
very thick, so that a very dense structure is formed.
At Fig. 13 is a bit of a potato showing its
cells filled with small granules of starch
which the cells have produced by their
activities and deposited within their own
bodies. At Fig. 14 are several wood cells
showing cell walls of different shape
which, having become dead, have lost
their contents and simply remain as dead
FIG. 11.—Section of a leaf showing cell walls. Each was in its earlier history
cells of different shapes. filled with cell substance and contained a
nucleus. In a similar way any bit of
vegetable tissue would readily show itself to be made of similar cells.
In animal tissues the cellular structure is not so easily seen, largely because the
products made by the cells, the formed products, become relatively more
abundant and the cells themselves not so prominent. But the cellular structure is
none the less demonstrable. In Fig. 15, for instance, will be seen a bit of cartilage
where the cells themselves are rather small, while the material deposited
between them is abundant. This material between the cells is really to be
regarded as an excessively thickened cell wall and has been secreted by the cell
substance lying within the cells, so that a bit of cartilage is really a mass of cells
with an exceptionally thick cell wall.

At Fig. 16 is shown a little blood.
Here the cells are to be seen
floating in a liquid. The liquid is
colourless and it is the red colour
in the blood cells which gives the
FIG. 12.—Plant cells with thick walls, from blood its red colour. The liquid
a fern. may here again be regarded as
material produced by cells. At Fig.
17 is a bit of bone showing small irregular cells imbedded within a large mass of
material which has been deposited by the cell.
In this case the formed
material has been hardened by
calcium phosphate, which
gives the rigid consistency to
the bone. In some animal
tissues the formed material is
still greater in amount. At Fig. FIG. 13.—Section of a potato showing different
18, for example, is a bit of shaped cells, the inner and larger ones being
connective tissue, made up of filled with grains of starch.
a mass of fine fibres which
have no resemblance to cells, and indeed are not cells.These fibres have,
however, been made by cells, and a careful study of such tissue at proper places
will show the cells within it. The cells shown in Fig. 18 (c) have secreted the
fibrous material. Fig. 19 shows a cell composing a bit of nerve. At Fig. 20 is a bit
of muscle; the only trace of cellular structure that it shows is in the nuclei (n),
but if the muscle be studied in a young condition its cellular structure is more
evident.
Thus it
happens
in adult
animals
that the
cells
which are
large and
FIG. 14.—Various shaped wood cells clear at FIG. 15.—A bit of cartilage.
from plant tissue. first,
become less and less evident, until the

adult tissue seems sometimes to be composed mostly of what we have called
formed material.
It must
not be
imagin
ed,
howeve
r, that a
very
rigid FIG. 16.—Frog's blood: a and
FIG. 17.—A bit of bone, showing line can b are the cells; c is the liquid.
the cells imbedded in the bony matter. be
drawn between the cell itself and the
material it forms. The formed material is in many cases simply a thickened cell
wall, and this we commonly regard as part of the cell. In many cases the formed
material is simply the old dead cell walls from which the living substance has
been withdrawn (Fig. 14). In other cases the cell substance acquires peculiar
functions, so that what seems to be the formed material is really a modified cell
body and is still active and alive. Such is the case in the muscle. In other cases
the formed material appears to be manufactured within the cell and secreted, as
in the case of bone. No sharp lines can be drawn, however, between the various
types. But the distinction between formed material and cell body is a convenient
one and may well be retained in the discussion of cells. In our discussion of the
fundamental vital properties we are only concerned in the cell substance, the
formed material having nothing to do with fundamental activities of life,
although it forms largely the secondary machinery which we have already
studied.
In all higher animals and plants the life of the individual
begins as a single ovum or a single cell, and as it grows
the cells increase rapidly until the adult is formed out of
hundreds of millions of cells. As these cells become
numerous they cease, after a little, to be alike. They
assume different shapes which are adapted to the
different duties they are to perform. Thus, those cells
which are to form bone soon become different from
FIG. 18.—Connective those which are to form muscle, and those which are to
tissue. The cells form the blood are quite unlike those which are to
of the tissue are produce the hairs. By means of such a differentiation
shown at c, and the

fibres or formed there arises a very complex mass of cells, with great
matter at f. variety in shape and function.
It should be noticed
further that there are some animals and plants
in which the whole animal is composed of a
single cell. These organisms are usually of
extremely minute size, and they comprise most
of the so-called animalculæ which are found in
water. In such animals the different parts of the
cell are modified to perform different FIG. 19. A piece of nerve fibre,
functions. The different organs appear within showing the cell with its
the cell, and the cell is more complex than the nucleus at n.
typical cell described. Fig. 21 shows such a
cell. Such an animal possesses several organs, but, since it consists of a single
mass of protoplasm and a single nucleus, it is still only a single cell. In the
multicellular organisms the organs of the body are made up of cells, and the
different organs are produced by a differentiation of cells, but in the unicellular
organisms the organs are the results of the differentiation of the parts of a single
cell. In the one case there is a differentiation of cells, and in the other of the parts
of a cell.
Such, in brief, is the cell to whose activities it is
possible to trace the fundamental properties of
all living things. Cells are endowed with the
properties of irritability, contractibility,
assimilation and reproduction, and it is thus
plainly to the study of cells that we must look
for an interpretation of life phenomena. If we
can reach an intelligible understanding of the
activities of the cell our problem is solved, for FIG. 20.—A muscle fibre. The
the activities of the fully formed animal or nucleii are shown at n.
plant, however complex, are simply the
application of mechanical and chemical principles among the groups of such
cells. But wherein does this knowledge of cells help us? Are we any nearer to
understanding how these vital processes arise? In answer to this question we
may first ask whether it is possible to determine whether any one part of the cell
is the seat of its activities.
The Cell Wall.—The first suggestion which arose was that the cell wall was the
important part of the cell, the others being secondary. This was not an unnatural

conclusion. The cell wall is the most persistent
part of the cell. It was the part first discovered
by the microscope and is the part which
remains after the other parts are gone. Indeed,
in many of the so-called cells the cell wall is
all that is seen, the cell contents having
disappeared (Fig. 14). It was not strange, then,
FIG. 21.—A complex cell. It is that this should at first have been looked upon
an entire animal, but composed as the primary part. The idea was that the cell
of only one cell. wall in some way changed the chemical
character of the substances in contact with its
two sides, and thus gave rise to vital activities
which, as we have seen, are fundamentally chemical. Thus the cell wall was
regarded as the most essential part of the cell, since it controlled its activities.
This the belief of Schwann, although he also regarded the other parts of the cell
as of importance.
This conception, however, was
quite temporary. It was much as
if our hypothetical
supramundane observer looked
upon the clothes of his newly
discovered human being as
forming the essential part of his
nature. It was soon evident that
this position could not be
maintained. It was found that
many bits of living matter were FIG. 22.—An amœba. A single
entirely destitute of cell wall. cell without cell wall. n is the nucleus; f, a bit
This is especially true of animal of food which the cell has absorbed.
cells. While among plants the
cell wall is almost always well developed, it is very common for animal cells to
be entirely lacking in this external covering—as, for example, the white blood-
cells. Fig. 22 shows an amœba, a cell with very active powers of motion and
assimilation, but with no cell wall. Moreover, young cells are always more active
than older ones, and they commonly possess either no cell wall or a very slight
one, this being deposited as the cell becomes older and remaining long after it is
dead. Such facts soon disproved the notion that the cell wall is a vital part of the
cell, and a new conception took its place which was to have a more profound

influence upon the study of living things than any discovery hitherto made. This
was the formulation of the doctrine of the nature of protoplasm.
Protoplasm.—(a) Discovery. As it became evident that the cell wall is a
somewhat inactive part of the cell, more attention was put on the cell contents.
For twenty years after the formulation of the cell doctrine both the cell substance
and the nucleus had been looked upon as essential to its activities. This was more
especially true of the nucleus, which had been thought of as an organ of
reproduction. These suggestions appeared indefinitely in the writings of one
scientist and another, and were finally formulated in 1860 into a general theory
which formed what has sometimes been called the starting point of modern
biology. From that time the material known as protoplasm was elevated into a
prominent position in the discussion of all subjects connected with living
phenomena. The idea of protoplasm was first clearly defined by Schultze, who
claimed that the real active part of the cell was the cell substance within the cell
wall. This substance he proved to be endowed with powers of motion and
powers of inducing chemical changes associated with vital phenomena. He
showed it to be the most abundant in the most active cells, becoming less
abundant as the cells lose their activity, and disappearing when the cells lose
their vitality. This cell substance was soon raised into a position of such
importance that the smaller body within it was obscured, and for some twenty
years more the nucleus was silently ignored in biological discussion. According
to Schultze, the cell substance itself constituted the cell, the other parts being
entirely subordinate, and indeed frequently absent. A cell was thus a bit of
protoplasm, and nothing more. But the more important feature of this doctrine
was not the simple conclusion that the cell substance constitutes the cell, but the
more sweeping conclusion that this cell substance is in all cells essentially
identical. The study of all animals, high and low, showed all active cells filled
with a similar material, and more important still, the study of plant cells
disclosed a material strikingly similar. Schultze experimented with this material
by all means at his command, and finding that the cell substance in all animals
and plants obeys the same tests, reached the conclusion that the cell substance in
animals and plants is always identical. To this material he now gave the name
protoplasm, choosing a name hitherto given to the cell contents of plant cells.
From this time forth this term protoplasm was applied to the living material
found in all cells, and became at once the most important factor in the discussion
of biological problems.
The importance of this newly formulated doctrine it is difficult to appreciate.
Here, in protoplasm had been apparently found the foundation of living

phenomena. Here was a substance universally present in animals and plants,
simple and uniform—a substance always present in living parts and disappearing
with death. It was the simplest thing that had life, and indeed the only thing that
had life, for there is no life outside of cells and protoplasm. But simple as it was
it had all the fundamental properties of living things—irritability, contractibility,
assimilation, and reproduction. It was a compound which seemingly deserved
the name of "physical basis of life", which was soon given to it by Huxley. With
this conception of protoplasm as the physical basis of life the problems
connected with the study of life became more simplified. In order to study the
nature of life it was no longer necessary to study the confusing mass of complex
organs disclosed to us by animals and plants, or even the somewhat less
confusing structures shown by individual cells. Even the simple cell has several
separate parts capable of undergoing great modifications in different types of
animals. This confusion now appeared to vanish, for only one thing was found to
be alive, and that was apparently very simple. But that substance exhibited all
the properties of life. It moved, it could grow, and reproduce itself, so that it was
necessary only to explain this substance and life would be explained.
(b) Nature of Protoplasm.—What is this material, protoplasm? As disclosed by
the early microscope it appeared to be nothing more than a simple mass of jelly,
usually transparent, more or less consistent, sometimes being quite fluid, and at
others more solid. Structure it appeared to have none. Its chief peculiarity, so far
as physical characters were concerned, was a wonderful and never-ceasing
activity. This jellylike material appeared to be endowed with wonderful powers,
and yet neither physical nor microscopical study revealed at first anything more
than a uniform homogeneous mass of jelly. Chemical study of the same
substance was of no less interest than the microscopical study. Of course it was
no easy matter to collect this protoplasm in sufficient quantity and pure enough
to make a careful analysis. The difficulties were in time, however, overcome, and
chemical study showed protoplasm to be a proteid, related to other proteids like
albumen, but one which was more complex than any other known. It was for a
long time looked upon by many as a single definite chemical compound, and
attempts were made to determine its chemical formula. Such an analysis
indicated a molecule made up of several hundred atoms. Chemists did not,
however, look with much confidence upon these results, and it is not surprising
that there was no very close agreement among them as to the number of atoms in
this supposed complex molecule. Moreover, from the very first, some biologists
thought protoplasm to be not one, but more likely a mixture of several
substances. But although it was more complex than any other substance studied,
its general characters were so like those of albumen that it was uniformly

regarded as a proteid; but one which was of a higher complexity than others,
forming perhaps the highest number of a series of complex chemical
compounds, of which ordinary proteids, such as albumen, formed lower
members. Thus, within a few years following the discovery of protoplasm there
had developed a theory that living phenomena are due to the activities of a
definite though complex chemical compound, composed chiefly of the elements
carbon, oxygen, hydrogen, and nitrogen, and closely related to ordinary proteids.
This substance was the basis of living activity, and to its modification under
different conditions were due the miscellaneous phenomena of life.
(c) Significance of Protoplasm.—The philosophical significance of this
conception was very far-reaching. The problem of life was so simplified by
substituting the simple protoplasm for the complex organism that its solution
seemed to be not very difficult. This idea of a chemical compound as the basis of
all living phenomena gave rise in a short time to a chemical theory of life which
was at least tenable, and which accounted for the fundamental properties of life.
That theory, the chemical theory of life, may be outlined somewhat as follows:
The study of the chemical nature of substances derived from living organisms
has developed into what has been called organic chemistry. Organic chemistry
has shown that it is possible to manufacture artificially many of the compounds
which are called organic, and which had been hitherto regarded as produced only
by living organisms. At the beginning of the century, it was supposed to be
impossible to manufacture by artificial means any of the compounds which
animals and plants produce as the result of their life. But chemists were not long
in showing that this position is untenable. Many of the organic products were
soon shown capable of production by artificial means in the chemist's laboratory.
These organic compounds form a series beginning with such simple bodies as
carbonic acid (CO2), water (H2O), and ammonia (NH3), and passing up through
a large number of members of greater and greater complexity, all composed,
however, chiefly of the elements carbon, oxygen, hydrogen, and nitrogen. Our
chemists found that starting with simple substances they could, by proper means,
combine them into molecules of greater complexity, and in so doing could make
many of the compounds that had hitherto been produced only as a result of living
activities. For example, urea, formic acid, indigo, and many other bodies,
hitherto produced only by animals and plants, were easily produced by the
chemist by purely chemical methods. Now when protoplasm had been
discovered as the "physical basis of life," and, when it was further conceived that
this substance is a proteid related to albumens, it was inevitable that a theory

should arise which found the explanation of life in accordance with simple
chemical laws.
If, as chemists and biologists then believe, protoplasm is a compound which
stands at the head of the organic series, and if, as is the fact, chemists are each
year succeeding in making higher and higher members of the series, it is an easy
assumption that some day they will be able to make the highest member of the
series. Further, it is a well-known fact that simple chemical compounds have
simple physical properties, while the higher ones have more varied properties.
Water has the property of being liquid at certain temperatures and solid at others,
and of dividing into small particles (i.e., dissolving) certain bodies brought in
contact with it. The higher compound albumen has, however, a great number of
properties and possibilities of combination far beyond those of water. Now if the
properties increase in complexity with the complexity of the compound, it is
again an easy assumption that when we reach a compound as complex as
protoplasm, it will have properties as complex as those of the simple life
substance. Nor was this such a very wild hypothesis. After all, the fundamental
life activities may all be traced to the simple oxidation of food, for this results in
movement, assimilation, and growth, and the result of growth is reproduction. It
was therefore only necessary for our biological chemists to suppose that their
chemical compound protoplasm possessed the power of causing certain kinds of
oxidation to take place, just as water itself induces a simpler kind of oxidation,
and they would have a mechanical explanation of the life activities. It was
certainly not a very absurd assumption to make, that this substance protoplasm
could have this power, and from this the other vital activities are easily derived.
In other words, the formulation of the doctrine of protoplasm made it possible to
assume that life is not a distinct force, but simply a name given to the properties
possessed by that highly complex chemical compound protoplasm. Just as we
might give the name aquacity to the properties possessed by water, so we have
actually given the name vitality to the properties possessed by protoplasm. To be
sure, vitality is more marvelous than aquacity, but so is protoplasm a more
complex compound than water. This compound was a very unstable compound,
just as is a mass of gunpowder, and hence it is highly irritable, also like
gunpowder, and any disturbance of its condition produces motion, just as a spark
will do in a mass of gunpowder. It is capable of inducing oxidation in foods,
something as water induces oxidation in a bit of iron. The oxidation is, however,
of a different kind, and results in the formation of different chemical
combinations; but it is the basis of assimilation. Since now assimilation is the
foundation of growth and reproduction, this mechanical theory of life thus

succeeded in tracing to the simple properties of the chemical compound
protoplasm, all the fundamental properties of life. Since further, as we have seen
in our first chapter, the more complex properties of higher organisms are easily
deduced from these simple ones by the application of the laws of mechanics, we
have here in this mechanical theory of life the complete reduction of the body to
a machine.
The Reign of Protoplasm.—This substance protoplasm became now naturally
the centre of biological thought. The theory of protoplasm arose at about the
same time that the doctrine of evolution began to be seriously discussed under
the stimulus of Darwin, and naturally these two great conceptions developed side
by side. Evolution was constantly teaching that natural forces are sufficient to
account for many of the complex phenomena which had hitherto been regarded
as insolvable; and what more natural than the same kind of thinking should be
applied to the vital activities manifested by this substance protoplasm. While the
study of plants and animals was showing scientists that natural forces would
explain the origin of more complex types from simpler ones through the law of
natural selection, here in this conception of protoplasm was a theory which
promised to show how the simplest forms may have been derived from the non-
living. For an explanation of the origin of life by natural means appeared now to
be a simple matter.
It required now no violent stretch of the imagination to explain the origin of life
something as follows: We know that the chemical elements have certain
affinities for each other, and will unite with each other under proper conditions.
We know that the methods of union and the resulting compounds vary with the
conditions under which the union takes place. We know further that the elements
carbon, hydrogen, oxygen, and nitrogen have most remarkable properties, and
unite to form an almost endless series of remarkable bodies when brought into
combination under different conditions. We know that by varying the conditions
the chemist can force these elements to unite into a most extraordinary variety of
compounds with an equal variety of properties. What more natural, then, than the
assumption that under certain conditions these same elements would unite in
such a way as to form this compound protoplasm; and then, if the ideas
concerning protoplasm were correct, this body would show the properties of
protoplasm, and therefore be alive. Certainly such a supposition was not absurd,
and viewed in the light of the rapid advance in the manufacture of organic
compounds could hardly be called improbable. Chemists beginning with simple
bodies like CO2 and H2O were climbing the ladder, each round of which was
represented by compounds of higher complexity. At the top was protoplasm, and

each year saw our chemists nearer the top of the ladder, and thus approaching
protoplasm as their final goal. They now began to predict that only a few more
years would be required for chemists to discover the proper conditions, and thus
make protoplasm. As late as 1880 the prediction was freely made that the next
great discovery would be the manufacture of a bit of protoplasm by artificial
means, and thus in the artificial production of life. The rapid advance in organic
chemistry rendered this prediction each year more and more probable. The
ability of chemists to manufacture chemical compounds appeared to be
unlimited, and the only question in regard to their ability to make protoplasm
thus resolved itself into the question of whether protoplasm is really a chemical
compound.
We can easily understand how eager biologists became now in pursuit of the
goal which seemed almost within their reach; how interested they were in any
new discovery, and how eagerly they sought for lower and simpler types of
protoplasm since these would be a step nearer to the earliest undifferentiated life
substance. Indeed so eager was this pursuit for pure undifferentiated protoplasm,
that it led to one of those unfounded discoveries which time showed to be purely
imaginary. When this reign of protoplasm was at its height and biologists were
seeking for even greater simplicity a most astounding discovery was announced.
The British exploring ship Challenger had returned from its voyage of discovery
and collection, and its various treasures were turned over to the different
scientists for study. The brilliant Prof. Huxley, who had first formulated the
mechanical theory of life, now startled the biological world with the statement
that these collections had shown him that at the bottom of the deep sea, in certain
parts of the world, there exists a diffused mass of living undifferentiated
protoplasm. So simple and undifferentiated was it that it was not divided into
cells and contained no nucleii. It was, in short, exactly the kind of primitive
protoplasm which the evolutionist wanted to complete his chain of living
structures, and the biologist wanted to serve as a foundation for his mechanical
theory of life. If such a diffused mass of undifferentiated protoplasm existed at
the bottom of the sea, one could hardly doubt that it was developed there by
some purely natural forces. The discovery was a startling one, for it seemed that
the actual starting point of life had been reached. Huxley named his substance
Bathybias, and this name became in a short time familiar to every one who was
thinking of the problems of life. But the discovery was suspected from the first,
because it was too closely in accord with speculation, and it was soon disproved.
Its discoverer soon after courageously announced to the world that he had been
entirely mistaken, and that the Bathybias, so far from being undifferentiated
protoplasm, was not an organic product at all, but simply a mineral deposit in the

sea water made by purely artificial means. Bathybias stands therefore as an
instance of a too precipitate advance in speculation, which led even such a
brilliant man as Prof. Huxley into an unfortunate error of observation; for,
beyond question, he would never have made such a mistake had he not been
dominated by his speculative theories as to the nature of protoplasm.
But although Bathybias proved delusive, this did not materially affect the
advance and development of the doctrine of protoplasm. Simple forms of
protoplasm were found, although none quite so simple as the hypothetical
Bathybias. The universal presence of protoplasm in the living parts of all animals
and plants and its manifest activities completely demonstrated that it was the
only living substance, and as the result of a few years of experiment and thought
the biologist's conception of life crystallized into something like this: Living
organisms are made of cells, but these cells are simply minute independent bits
of protoplasm. They may contain a nucleus or they may not, but the essence of
the cell is the protoplasm, this alone having the fundamental activities of life.
These bits of living matter aggregate themselves together into groups to form
colonies. Such colonies are animals or plants. The cells divide the work of the
colony among themselves, each cell adopting a form best adapted for the special
work it has to do. The animal or plant is thus simply an aggregate of cells, and its
activities are the sum of the activities of its separate cells; just as the activities of
a city are the sum of the activities of its individual inhabitants. The bit of
protoplasm was the unit, and this was a chemical compound or a simple mixture
of compounds to whose combined physical properties we have given the name
vitality.
The Decline of the Reign of Protoplasm.—Hardly had this extreme chemical
theory of life been clearly conceived before accumulating facts began to show
that it is untenable and that it must at least be vastly modified before it can be
received. The foundation of the chemical theory of life was the conception that
protoplasm is a definite though complex chemical compound. But after a few
years' study it appeared that such a conception of protoplasm was incorrect. It
had long been suspected that protoplasm was more complex than was at first
thought. It was not even at the outset found to be perfectly homogeneous, but
was seen to contain minute granules, together with bodies of larger size.
Although these bodies were seen they were regarded as accidental or secondary,
and were not thought of as forming any serious objection to the conception of
protoplasm as a definite chemical compound. But modern opticians improved
their microscopes, and microscopists greatly improved their methods. With the
new microscopes and new methods there began to appear, about twenty years

ago, new revelations in regard to this protoplasm. Its lack of homogeneity
became more evident, until there has finally been disclosed to us the significant
fact that protoplasm is to be regarded as a substance not only of chemical but
also of high mechanical complexity. The idea of this material as a simple
homogeneous compound or as a mixture of such compounds is absolutely
fallacious. Protoplasm is to-day known to be made up of parts harmoniously
adapted to each other in such a way as to form an extraordinarily intricate
machine; and the microscopist of to-day recognizes clearly that the activities of
this material must be regarded as the result of the machinery which makes up
protoplasm rather than as the simple result of its chemical composition.
Protoplasm is a machine and not a chemical compound.

Structure of Protoplasm.—
The structure of protoplasm
is not yet thoroughly
understood by scientists, but
a few general facts are
known beyond question. It is
thought, in the first place,
that it consists of two quite
different substances. There
is a somewhat solid material
permeating it, usually,
regarded as having a
FIG. 23.—A cell as it appears to the modern reticulate structure. It is
microscope. a, protoplasmic reticulum; variously described,
b, liquid in its meshes; c, nuclear membrane; sometimes as a reticulate
d, nuclear reticulum; e, chromatin network, sometimes as a
reticulum; f, nucleolus; g, centrosome; mass of threads or fibres,
h, centrosphere; i, vacuole; and sometimes as a mass of
j, inert bodies. foam (Fig. 23, a). It is
extremely delicate and only
visible under special conditions and with the best of microscopes. Only
under peculiar conditions can it be seen in protoplasm while alive. There is
no question, however, that all protoplasm is permeated when alive by a
minute delicate mass of material, which may take the form of threads or
fibres or may assume other forms. Within the meshes of this thread or
reticulum there is found a liquid, perfectly clear and transparent, to whose
presence the liquid character of the protoplasm is due (Fig. 23, b). In this
liquid no structure can be determined, and, so far as we know, it is
homogeneous. Still further study discloses other complexities. It appears
that the fibrous material is always marked by the presence of excessively
minute bodies, which have been called by various names, but which we will
speak of as microsomes. Sometimes, indeed, the fibres themselves appear
almost like strings of beads, so that they have been described as made up of
rows of minute elements. It is immaterial for our purpose, however, whether
the fibres are to be regarded as made up of microsomes or not. This much is

sure, that these microsomes —granules of excessive minuteness—occur in
protoplasm and are closely connected with the fibres (Fig. 23, a).
The Nucleus.—(a) Presence of a Nucleus.—If protoplasm has thus become
a new substance in our minds as the result of the discoveries of the last
twenty years, far more marvelous have been the discoveries made in
connection with that body which has been called the nucleus. Even by the
early microscopists the nucleus was recognized, and during the first few
years of the cell doctrine it was frequently looked upon as the most active
part of the cell and as especially connected with its reproduction. The
doctrine of protoplasm, however, so captivated the minds of biologists that
for quite a number of years the nucleus was ignored, at least in all
discussions connected with the nature of life. It was a body in the cell
whose presence was unexplained and which did not fall into accord with the
general view of protoplasm as the physical basis of life. For a while,
therefore, biologists gave little attention to it, and were accustomed to speak
of it simply as a bit of protoplasm a little more dense than the rest. The cell
was a bit of protoplasm with a small piece of more dense protoplasm in its
centre appearing a little different from the rest and perhaps the most active
part of the cell.
As a result of this excessive belief in the efficiency of protoplasm the
question of the presence of a nucleus in the cell was for a while looked
upon as one of comparatively little importance. Many cells were found to
have nucleii while others did not show their presence, and microscopists
therefore believed that the presence of a nucleus was not necessary to
constitute a cell. A German naturalist recognized among lower animals one
group whose distinctive characteristic was that they were made of cells
without nucleii, giving the name Monera to the group. As the method of
studying cells improved microscopists learned better methods of discerning
the presence of the nucleus, and as it was done little by little they began to
find the presence of nucleii in cells in which they had hitherto not been
seen.
As microscopists now studied one after another of these animals and plants
whose cells had been said to contain no nucleus, they began to find nucleii
in them, until the conclusion was finally reached that a nucleus is a
fundamental part of all active cells. Old cells which have lost their activity

may
not
show
nucl
eii,
but,
so
far
as
we
kno
w,
all FIG. 24.—A cell cut into three pieces, each
activ containing a bit of the nucleus. Each
e continues its life indefinitely, soon acquiring the form of the original
cells as at C.
poss
ess these structures, and apparently no cell can carry on its activity without
them. Some cells have several nucleii, and others have the nuclear matter
scattered through the whole cell instead of being aggregated into a mass;
but nuclear matter the cell must have to carry on its life.
Later the experiment was made of depriving cells of their nucleii, and it still
further emphasized the importance of the nucleus. Among unicellular
animals are some which are large enough for direct manipulation, and it is
found that if these cells are cut into pieces the different pieces will behave
very differently in accordance with whether or not they have within them a
piece of the nucleus. All the pieces are capable of carrying on their life
activities for a while.
The pieces of the cell which contain the nucleus of the original cell, or even
a part of it, are capable of carrying on all its life activities perfectly well. In
Fig. 24 is shown such a cell cut into three pieces, each of which contains a
piece of the nucleus. Each carries on its life activities, feeds, grows and
multiplies perfectly well, the life processes seeming to continue as if
nothing had happened. Quite different is it with fragments which contain
none of the nucleus (Fig. 25). These fragments (1 and 3), even though they
may be comparatively large masses of protoplasm, are incapable of carrying

on the functions of their life
continuously. For a while they
continue to move around and
apparently act like the other
fragments, but after a little
their life ceases. They are
incapable of assimilating food
and incapable of reproduction,
and hence their life cannot
continue very long. Facts like
these demonstrate
conclusively the vital
FIG. 25.—A cell cut into three pieces, only importance of the nucleus in
one of which, No. 2, contains any cell activity, and show us that
nucleus. This fragment soon acquires the cell, with its power of
the original form and continues its life continued life, must be
indefinitely, as shown at B. The other regarded as a combination of
two pieces though living for a time, protoplasm with its nucleus,
die without reproducing. and cannot exist without it. It
is not protoplasm, but cell
substance, plus cell nucleus, which forms the simplest basis of life.
As more careful study of protoplasm was made it soon became evident that
there is a very decided difference between the nucleus and the protoplasm.
The old statement that the nucleus is simply a bit of dense protoplasm is not
true. In its chemical and physical composition as well as in its activities the
nucleus shows itself to be entirely different from the protoplasm. It contains
certain definite bodies not found in the cell substance, and it goes through a
series of activities which are entirely unrepresented in the surrounding
protoplasm. It is something entirely distinct, and its relations to the life of
the cell are unique and marvelous. These various facts led to a period in the
discussion of biological topics which may not inappropriately be called the
Reign of the Nucleus. Let us, therefore, see what this structure is which has
demanded so much attention in the last twenty years.
(b) Structure of the Nucleus.—At first the nucleus appears to be very much
like the cell substance. Like the latter, it is made of fibres, which form a
reticulum (Fig. 23), and these fibres, like those of protoplasm, have

microsomes in intimate relation with them and hold a clear liquid in their
meshes. The meshes of the network are usually rather closer than in the
outer cell substance, but their general character appears to be the same. But
a more close study of the nucleus discloses vast differences. In the first
place, the nucleus is usually separated from the cell substance by a
membrane (Fig. 23, c). This membrane is almost always present, but it may
disappear, and usually does disappear, when the nucleus begins to divide.
Within the nucleus we find commonly one or two smaller bodies, the
nucleoli (Fig. 23, f). They appear to be distinct vital parts of the nucleus,
and thus different from certain other solid bodies which are simply excreted
material, and hence lifeless. Further, we find that the reticulum within the
nucleus is made up of two very different parts. One portion is apparently
identical with the reticulum of the cell substance (Fig. 23, d). This forms an
extremely delicate network, whose fibres have chemical relations similar to
those of the cell substance. Indeed, sometimes, the fibres of the nucleus
may be seen to pass directly into those of the network of the cell substance,
and hence they are in all probability identical. This material is called linin,
by which name we shall hereafter refer to it. There is, however, in the
nucleus another material which forms either threads, or a network, or a
mass of granules, which is very different from the linin, and has entirely
different properties. This network has the power of absorbing certain kinds
of stains very actively, and is consequently deeply stained when treated as
the microscopist commonly prepares his specimens. For this reason it has
been named chromatin (Fig, 23, e), although in more recent times other
names have been given to it. Of all parts of the cell this chromatin is the
most remarkable. It appears in great variety in different cells, but it always
has remarkable physiological properties, as will be noticed presently. All
things considered, this chromatin is probably the most remarkable body
connected with organic life.

FIG. 26.—Different forms of nucleii.
The nucleii of different animals and plants all show essentially the
characteristics just described. They all contain a liquid, a linin network, and
a chromatin thread or network, but they differ most remarkably in details,
so that the variety among the nucleii is almost endless (Fig. 26). They differ
first in their size relative to the size of the cell; sometimes—especially in
young cells—the nucleus being very large, while in other cases the nucleus
is very small and the protoplasmic contents of the cell very large; finally, in
cells which have lost their activity the nucleus may almost or entirely
disappear. They differ, secondly, in shape. The typical form appears to be
spherical or nearly so; but from this typical form they may vary, becoming
irregular or elongated. They are sometimes drawn out into long masses
looking like a string of beads (Fig. 24), or, again, resembling minute coiled
worms (Fig. 21), while in still other cells they may be branching like the
twigs of a tree. The form and shape of the chromatin thread differs widely.
Sometimes this appears to be mere reticulum (Fig. 23); at others, a short
thread which is somewhat twisted or coiled (Fig. 26); while in other cells
the chromatin thread is an extremely long, very much twisted convolute
thread so complexly woven into a tangle as to give the appearance of a
minute network. The nucleii differ also in the number of nucleoli they
contain as well as in other less important particulars. Fig. 26 will give a
little notion of the variety to be found among different nucleii; but although
they thus do vary most remarkably in shape in the essential parts of their
structure they are alike.

Centrosome.—Before noticing the activities of the nucleus it will be
necessary to mention a third part of the cell. Within the last few years there
has been found to be present in most cells an organ which has been called
the centrosome. This body is shown at Fig. 23, g. It is found in the cell
substance just outside the nucleus, and commonly appears as an extremely
minute rounded dot, so minute that no internal structure has been discerned.
It may be no larger than the minute granules or microsomes in the cell, and
until recently it entirely escaped the notice of microscopists. It has now,
however, been clearly demonstrated as an active part of the cell and entirely
distinct from the ordinary microsomes. It stains differently, and, as we shall
soon see, it appears to be in most intimate connection with the center of cell
life. In the activities which characterize cell life this centrosome appears to
lead the way. From it radiate the forces which control cell activity, and
hence this centrosome is sometimes called the dynamic center of the cell.
This leads us to the study of cell activity, which discloses to us some of the
most extraordinary phenomena which have come to the knowledge of
science.
Function of the Nucleus.—To understand why it is that the nucleus has
taken such a prominent position in modern biological discussion it will be
only necessary to notice some of the activities of the cell. Of the four
fundamental vital properties of cell life the one which has been most
studied and in regard to which most is known is reproduction. This
knowledge appears chiefly under two heads, viz., cell division and the
fertilization of the egg. Every animal and plant begins its life as a simple
cell, and the growth of the cell into the adult is simply the division of the
original cell into parts accompanied by a differentiation of the parts. The
fundamental phenomena of growth and reproduction is thus cell division,
and if we can comprehend this process in these simple cells we shall
certainly have taken a great step toward the explanation of the mechanics of
life. During the last ten years this cell division has been most thoroughly
studied, and we have a pretty good knowledge of it so far as its
microscopical features are concerned. The following description will
outline the general facts of such cell division, and will apply with
considerable accuracy to all cases of cell division, although the details may
differ not a little.

Cell Division or Karyokinesis.—We will begin with a cell in what is
called the resting stage, shown at Fig. 23. Such a cell has a nucleus, with its
chromatin, its membrane, and linin, as already described. Outside the
nucleus is the centrosome, or, more commonly, two of them lying close
together. If there is only one it soon divides into two, and if it has already
two, this is because a single centrosome which the cell originally possessed
has already divided into two, as we shall presently see. This cell, in short, is
precisely like the typical cell which we have described, except in the
possession of two centrosomes.

FIG. 27 shows the resting stage with the chromatin, cr, in the form of a
network within the nuclear membrane and the centrosome, ce, already
divided into two.
FIG. 28.—The chromatin is broken into threads or chromosomes, cr. The
centrosomes show radiating fibres.
The first indication of the cell division is shown by the chromatin fibres.
During the resting stage this chromatin material may have the form of a
thread, or may form a network of fibres (see Fig. 27). But whatever be its
form during the resting stage, it assumes the form of a thread as the cell
prepares for division. Almost at once this thread breaks into a number of
pieces known as chromosomes (Fig. 28). It is an extremely important fact
that the number of these chromosomes in the ordinary cells of any animal or
plant is always the same. In other words, in all the cells of the body of
animal or plant the chromatin material in the nucleus breaks into the same
number of short threads at the time that the cell is preparing to divide. The
number is the same for all animals of the same species, and is never
departed from. For example, the number in the ox is always sixteen, while

the number in the lily is always twenty-four. During this process of the
formation of the chromosomes the nucleoli disappear, sometimes being
absorbed apparently in the chromosomes, and sometimes being ejected into
the cell body, where they disappear. Whether they have anything to do with
further changes is not yet known.
The next step in the process of division appears in the region of the
centrosomes. Each of the two centrosomes appears to send out from itself
delicate radiating fibres into the surrounding cell substance (Fig. 28).
Whether these actually arise from the centrosome or are simply a
rearrangement of the fibres in the cell substance is not clear, but at all
events the centrosome becomes surrounded by a mass of radiating fibres
which give it a starlike appearance, or, more commonly, the appearance of a
double star, since there are two centrosomes close together (Fig. 28). These
radiating fibres, whether arising from the centrosomes or not, certainly all
centre in these bodies, a fact which indicates that the centrosomes contain
the forces which regulate their appearance. Between the two stars or asters a
set of fibres can be seen running from one to the other (Fig. 29). These two
asters and the centrosomes within them have been spoken of as the dynamic
centre of the cell since they appear to control the forces which lead to cell
division. In all the changes which follow these asters lead the way. The two
asters, with their centrosomes, now move away from each other, always
connected by the spindle fibres, and finally come to lie on opposite sides of
the nucleus (Figs. 29, 30). When they reach this position they are still
surrounded by the radiating fibres, and connected by the spindle fibres.
Meantime the membrane around the nucleus has disappeared, and thus the
spindle fibres readily penetrate into the nuclear substance (Fig. 30).

FIG. 29.—The centrosomes are separating but are connected by fibres.

FIG. 30.—The centrosomes are separate and the equatorial plate of
chromosomes, cr, is between them.
During this time the chromosomes have been changing their position.
Whether this change in position is due to forces within themselves, or
whether they are moved around passively by forces residing in the cell
substances, or whether, which is the most probable, they are pulled or
pushed around by the spindle fibres which are forcing their way into the
nucleus, is not positively known; nor is it, for our purposes, of special
importance. At all events, the result is that when the asters have assumed
their position at opposite poles of the nucleus the chromosomes are
arranged in a plane passing through the middle of the nucleus at equal
distances from each aster. It seems certain that they are pulled or pushed
into this position by forces radiating from the centrosomes. Fig. 30 shows
this central arrangement of the chromosomes, forming what is called the
equatorial plate.
The next step is the most significant of all. It consists in the splitting of each
chromosome into two equal halves. The threads do not divide in their
middle but split lengthwise, so that there are formed two halves identical in
every respect. In this way are produced twice the original number of
chromosomes, but all in pairs. The period at which this splitting of the
chromosomes occurs is not the same in all cells. It may occur, as described,
at about the time the asters have reached the opposite poles of the nucleus,
and an equatorial plate is formed. It is not infrequent, however, for it to
occur at a period considerably earlier, so that the chromosomes are already
divided when they are brought into the equatorial plate.
At some period or other in the cell division this splitting of the
chromosomes takes place. The significance of the splitting is especially
noteworthy. We shall soon find reason for believing that the chromosomes
contain all the hereditary traits which the cell hands down from generation
to generation, and indeed that the chromosomes of the egg contain all the
traits which the parent hands down to the child. Now, if this chromatin
thread consists of a series of units, each representing certain hereditary
characters, then it is plain that the division of the thread by splitting will
give rise to a double series of threads, each of which has identical
characters. Should the division occur across the thread the two halves

would be unlike, but taking place as it does by a longitudinal splitting each
unit in the thread simply divides in half, and thus the resulting half threads
each contain the same number of similar units as the other and the same as
possessed by the original undivided chromosome. This sort of splitting thus
doubles the number of chromosomes, but produces no differentiation of
material.

FIG. 31.—Stage showing the two halves of the chromosomes separated
from each other.
FIG. 32.—Final stage with two nucleii in which the chromosomes have
again assumed the form of a network. The centrosomes have divided
preparatory to the next division, and the cell is beginning to divide.
The next step in the cell division consists in the separation of the two halves
of the chromosomes. Each half of each chromosome separates from its
fellow, and moves to the opposite end of the nucleus toward the two
centrosomes (Fig. 31). Whether they are pulled apart or pushed apart by the
spindle fibres is not certain, although it is apparently sure that these fibres
from the centrosomes are engaged in the matter. Certain it is that some
force exerted from the two centrosomes acts upon the chromosomes, and
forces the two halves of each one to opposite ends of the nucleus, where
they now collect and form two new nucleii, with evidently exactly the same
number of chromosomes as the original, and with characters identical to
each other and to the original (Fig. 32).
The rest of the cell division now follows rapidly. A partition grows in
through the cell body dividing it into two parts (Fig. 32), the division
passing through the middle of the spindle. In this division, in some cases at
least, the spindle fibres bear a part—a fact which again points to the

importance of the centrosomes and the forces which radiate from them.
Now the chromosomes in each daughter nucleus unite to form a single
thread, or may diffuse through the nucleus to form a network, as in Fig. 32.
They now become surrounded by a membrane, so that the new nucleus
appears exactly like the original one. The spindle fibres disappear, and the
astral fibres may either disappear or remain visible. The centrosome may
apparently in some cases disappear, but more commonly remains beside the
daughter nucleii, or it may move into the nucleus. Eventually it divides into
two, the division commonly occurring at once (Fig. 32), but sometimes not
until the next cell division is about to begin. Thus the final result shows two
cells each with a nucleus and two centrosomes, and this is exactly the same
sort of structure with which the process began. (See Frontispiece.)
Viewed as a whole, we may make the following general summary of this
process. The essential object of this complicated phenomena of karyokinesis
is to divide the chromatin into equivalent halves, so that the cells resulting
from the cell division shall contain an exactly equivalent chromatin content.
For this purpose the chromatic elements collect into threads and split
lengthwise. The centrosome, with its fibres, brings about the separation of
these two halves. Plainly, we must conclude that the chromatin material is
something of extraordinary importance to the cell, and the centrosome is a
bit of machinery for controlling its division and thus regulating cell
division.
Fertilization of the Egg.—This description of cell division will certainly
give some idea of the complexity of cell life, but a more marvelous series of
changes still takes place during the time when the egg is preparing for
development. Inasmuch as this process still further illustrates the nature of
the cell, and has further a most intimate bearing upon the fundamental
problem of heredity, it will be necessary for us to consider it here briefly.
The sexual reproduction of the many-celled animals is always essentially
alike. A single one of the body cells is set apart to start the next generation,
and this cell, after separating from the body of the animal or plant which
produced it, begins to divide, as already shown in Fig. 8, and the many cells
which arise from it eventually form the new individual This reproductive
cell is the egg. But before its division can begin there occurs in all cases of
sexual reproduction a process called fertilization, the essential feature of

which is the union of this cell with another commonly from a different
individual. While the phenomenon is subject to considerable difference in
details, it is essentially as follows:
The female reproductive cell is called
the egg, and it is this cell which
divides to form the next generation.
Such a cell is shown in Fig. 33. Like
other cells it has a cell wall, a cell
substance with its linin and fluid
portions, a nucleus surrounded by a
membrane and containing a
reticulum, a nucleolus and chromatic
material, and lastly, a centrosome. FIG. 33—An egg showing the cell
Now such an egg is a complete cell, substance and the nucleus, the latter
but it is not able to begin the process containing chromosomes in large
of division which shall give rise to a number and a nucleolus.
new individual until it has united
with another cell of quite a different sort and commonly derived from a
different individual called the male. Why the egg cell is unable to develop
without such union with male cell does not concern us here, but its purpose
will be evident as the description proceeds. The egg cell as it comes from
the ovary of the female individual is, however, not yet ready for union with
the male cell, but must first go through a series of somewhat remarkable
changes constituting what is called maturation of the egg. This
phenomenon has such an intimate relation to all problems connected with
the cell, that it must be described somewhat in detail. There are
considerable differences in the details of the process as it occurs in various
animals, but they all agree in the fundamental points. The following is a
general description of the process derived from the study of a large variety
of animals and plants.

FIG. 34. This and the following figures represent the process of fertilization
of an egg. In all figures cr is the chromosomes; cs represents the cell
substance (omitted in the following figures); mc is the male reproductive
cell lying in contact with the egg; mn is the male nucleus after entering the
egg.
FIG. 35.—The egg centrosome has divided, and the male cell with its
centrosome has entered the egg.
In the cells of the body of the animal to which this description applies there
are four chromosomes This is true of all the cells of the animal except the
sexual cells. The eggs arise from the other cells of the body, but during their
growth the chromatin splits in such a way that the egg contains double the
number of chromosomes, i.e., eight (Fig. 34). If this egg should now unite
with the other reproductive cell from the male, the resulting fertilized egg
would plainly contain a number of chromosomes larger than that normal for
this species of animal. As a result the next generation would have a larger
number of chromosomes in each cell than the last generation, since the
division of the egg in development is like that already described and always
results in producing new cells with the same number of chromosomes as the
starting cell. Hence, if the number of chromosomes in the next generation is
to be kept equal to that in the last generation, this egg cell must get rid of a
part of its chromatin material.

FIG. 36—The egg centrosomes have changed their position. The male cell
with its centrosome remains inactive until the stage represented in FIG. 42.
FIG. 37—Beginning of the first division for removing superfluous
chromosomes.
This is done by a process shown in Fig. 35. The centrosome divides as in
ordinary cell division (Fig. 35), and after rotating on its axis it approaches
the surface of the egg (Figs. 36 and 37). The egg now divides (Fig. 38), but
the division is of a peculiar kind. Although the chromosomes divide equally
the egg itself divides into two very unequal parts, one part still appearing as
the egg and the other as a minute protuberance called the polar cell (pc' in
Fig. 38). The chromosomes do not split as they do in the cell division
already described, but each of these two cells, the egg and the polar body,
receives four chromosomes (Fig. 38). The result is that the egg has now the
normal number of chromosomes for the ordinary cells of the animal in
question. But this is still too many, for the egg is soon to unite with the male
cell; and this male cell, as we shall see, is to bring in its own quota of
chromosomes. Hence the egg must get rid of still more of its chromatin
material. Consequently, the first division is followed by a second (Fig. 39),
in which there is again produced a large and a small cell. This division, like
the first, occurs without any splitting of the chromosomes, one half of the
remaining chromosomes being ejected in this new cell, the second polar cell
(pc") leaving the larger cell, the egg, with just one half the number of
chromosomes normal for the cells of the animal in question. Meantime the
first pole cell has also divided, so that we have now, as shown in Fig. 40,
four cells, three small and one large, but each containing one half the
normal number of chromosomes. In the example figured, four is the normal
number for the cells of the animal. The egg at the beginning of the process

contained eight, but has now been reduced to two. In the further history of
the egg the smaller cells, called polar cells, take no part, since they soon
disappear and have nothing to do with the animal which is to result from the
further division of the egg. This process of the formation of the polar cells
is thus simply a device for getting rid of some of the chromatin material in
the egg cell, so that it may unite with a second cell without doubling the
normal number of chromosomes.

FIG.38—First division complete and first polar cell formed, pc'.
FIG.39.—Formation of the second polar cell, pc".
FIG.40.—Completion of the process of extrusion of the chromatic material;
fn shows the two chromosomes retained in the egg forming the female
pronucleus. The centrosome has disappeared.
Previously to this process the other sexual cell, the spermatozoon, or male
reproductive cell, has been undergoing a somewhat similar process. This is
also a true cell (Fig. 34, mc), although it is of a decidedly smaller size than
the egg and of a very different shape. It contains cell substance, a nucleus
with chromosomes, and a centrosome, the number of chromosomes, as
shown later, being however only half that normal for the ordinary cells of
the animals. The study of the development of the spermatozoon shows that
it has come from cells which contained the normal number of four, but that
this number has been reduced to one half by a process which is equivalent
to that which we have just noticed in the egg. Thus it comes about that each
of the sexual elements, the egg and the spermatozoon, now contains one
half the normal number of chromosomes.

Now by some mechanical means these two reproductive cells are brought in
contact with each other, shown in Fig. 34, and as soon as they are brought
into each other's vicinity the male cell buries its head in the body of the egg.
The tail by which it has been moving is cast off, and the head containing the
chromosomes and the centrosome enters the egg, forming what is called the
male pronucleus (Fig. 35-38, mn). This entrance of the male cell occurs
either before the formation of the polar cells of the egg or afterward. If,
however, it takes place before, the male pronucleus simply remains dormant
in the egg while the polar cells are being protruded, and not until after that
process is concluded does it begin again to show signs of activity which
result in the cell union.
The further steps in this process appear to be controlled by the centrosome,
although it is not quite certain whence this centrosome is derived.
Originally, as we have seen, the egg contained a centrosome, and the male
cell has also brought a second into the egg (Fig. 35, ce). In some cases, and
this is true for the worm we are describing, it is certain that the egg
centrosome disappears while that of the spermatozoon is retained alone to
direct the further activities (Fig. 41). Possibly this may be the case in all
eggs, but it is not sure. It is a matter of some little interest to have this
settled, for if it should prove true, then it would evidently follow that the
machinery for cell division, in the case of sexual reproduction, is derived
from the father, although the bulk of the cell comes from the mother, while
the chromosomes come from both parents.

FIG. 41.—The chromosomes in the male and female pronucleii have
resolved into a network. The male centrosome begins to show signs of
activity.

FIG. 42.—The centrosome has divided, and the two pronucleii have been
brought together. The network in each nucleus has again resolved itself into
two chromosomes which are now brought together near the centre of the
egg but do not fuse; mcr, represents the chromosomes from the male
nucleus; fcr, the chromosomes from the female nucleus.
In the cases where the process has been most carefully studied, the further
changes are as follows: The head of the spermatozoon, after entrance into
the egg, lies dormant until the egg has thrown off its polar cells, and thus
gotten rid of part of its chromosomes. Close to it lies its centrosomes (Fig.
35, ce), and there is thus formed what is known as the male pronucleus
(Fig. 35-40, mn). The remains of the egg nucleus, after having discharged
the polar cells, form the female nucleus (Fig. 40, fn). The chromatin
material, in both the male and female pronucleus, soon breaks up into a
network in which it is no longer possible to see that each contains two
chromosomes (Fig. 41). Now the centrosome, which is beside the male
pronucleus, shows signs of activity. It becomes surrounded by prominent
rays to form an aster (Fig. 41, ce), and then it begins to move toward the
female pronucleus, apparently dragging the male pronucleus after it. In this
way the centrosome approaches the female pronucleus, and thus finally the
two nucleii are brought into close proximity. Meantime the chromatin
material in each has once more broken up into short threads or
chromosomes, and once more we find that each of the nucleii contains two
of these bodies (Fig. 42). In the subsequent figures the chromosomes of the
male nucleus are lightly shaded, while those of the female are black in order
to distinguish them. As these two nucleii finally come together their
membranes disappear, and the chromatic material comes to lie freely in the
egg, the male and female chromosomes, side by side, but distinct forming
the segmentation nucleus. The egg plainly now contains once more the
number of chromosomes normal for the cells of the animal, but half of them
have been derived from each parent. It is very suggestive to find further that
the chromosomes in this fertilized egg do not fuse with each other, but
remain quite distinct, so that it can be seen that the new nucleus contains
chromosomes derived from each parent (Fig. 42). Nor does there appear to
be, in the future history of this egg, any actual fusion of the chromatic
material, the male and female chromosomes perhaps always remaining
distinct.

FIG. 43.—An equatorial plate is formed and each chromosome has split
into two halves by longitudinal division.
FIG. 44.—The halves of the chromosomes have separated to form two
nucleii, each with male and female chromosomes. The egg has divided into
two cells.
While this mixture of chromosomes has been taking place the centrosome
has divided into two parts, each of which becomes surrounded by an aster
and travels to opposite ends of the nucleus (Fig. 42). There now follows a
division of the nucleus exactly similar to that which occurs in the normal
cell division already described in Figs. 28-34. Each of the chromosomes
splits lengthwise (Fig. 43), and one half of each then travels toward each
centrosome to form a new nucleus (Fig. 44). Since each of the four
chromosomes thus splits, it follows that each of the two daughter nucleii
will, of course, contain four chromosomes; two of which have been derived
from the male and two from the female parent. From now the divisions of
the egg follow rapidly by the normal process of cell division until from this
one egg cell there are eventually derived hundreds of thousands of cells
which are gradually moulded into the adult. All of these cells will, of
course, contain four chromosomes; and, what is more important, half of the
chromosomes will have been derived directly from the male and half from
the female parent. Even into adult life, therefore, the cells of the animal
probably contain chromatin derived by direct descent from each of its
parents.
The Significance of Fertilization.—From this process of fertilization a
number of conclusions, highly important for our purpose, can be drawn. In
the first place, it is evident that the chromosomes form the part of the cell

which contain the hereditary traits handed down from parent to child. This
follows from the fact that the chromosomes are the only part of the cell
which, in the fertilized egg, is derived from both parents. Now the offspring
can certainly inherit from each parent, and hence the hereditary traits must
be associated with some part of the cell which is derived from both. But the
egg substance is derived from the mother alone; the centrosome, at least in
some cases and perhaps in all, is derived only from the father, while the
chromosomes are derived from both parents. Hence it follows that the
hereditary traits must be particularly associated with the chromosomes.
With this understanding we can, at least, in part understand the purpose of
fertilization. As we shall see later, it is very necessary in the building of the
living machine for each individual to inherit characters from more than one
individual. This is necessary to produce the numerous variations which
contribute to the construction of the machine. For this purpose there has
been developed the process of sexual union of reproductive cells, which
introduces into the offspring chromatic material from two parents. But if the
two reproductive cells should unite at once the number of chromosomes
would be doubled in each generation, and hence be constantly increasing.
To prevent this the polar cells are cast out, which reduces the amount of
chromatic material. The union of the two pronucleii is plainly to produce a
nucleus which shall contain chromosomes, and hence hereditary traits from
each parent and the subsequent splitting of these chromosomes and the
separation of the two halves into daughter nucleii insures that all the nucleii,
and hence all cells of the adult, shall possess hereditary traits derived from
both parents. Thus it comes that, even in the adult, every body cell is made
up of chromosomes from each parent, and may hence inherit characters
from each.
The cell of an animal thus consists of three somewhat distinct but active
parts—the cell substance, the chromosomes, and the centrosome. Of these
the cell substance appears to be handed down from the mother; the
centrosome comes, at least in some cases, from the father, and the
chromosomes from both parents. It is not yet certain, however, whether the
centrosome is a constant part of the cell. In some cells it cannot yet be
found, and there are some reasons for believing that it may be formed out of
other parts of the cell. The nucleus is always a direct descendant from the
nucleus of pre-existing cells, so that there is an absolute continuity of

descent between the nucleii of the cells of an individual and those of its
antecedents back for numberless generations. It is not certain that there is
any such continuity of descent in the case of the centrosomes; for, while in
the process of fertilization the centrosome is handed down from parent to
child, there are some reasons for believing that it may disappear in
subsequent cells, and later be redeveloped out of other parts. The only part
of the cell in which complete continuity from parent to child is
demonstrated, is the nucleus and particularly the chromosomes. All of these
facts simply emphasize the importance of the chromosomes, and tell us that
these bodies must be regarded as containing the most important features of
the cell which constitute its individuality.
What is Protoplasm?—Enough has now been given of disclosures of the
modern microscope to show that our old friend Protoplasm has assumed an
entirely new guise, if indeed it has not disappeared altogether. These
simplest life processes are so marvelous and involve the action of such an
intricate mass of machinery that we can no longer retain our earlier notion
of protoplasm as the physical basis of life. There can be no life without the
properties of assimilation, growth, and reproduction; and, so far as we
know, these properties are found only in that combination of bodies which
we call the cell, with its mixture of harmoniously acting parts. Life, at least
the life of a cell, is then not the property of a chemical compound
protoplasm, but is the result of the activities of a machine. Indeed, we are
now at a loss to know how we can retain the term protoplasm. As originally
used it meant the contents of the cell, and the significance in the term was
in the conception of protoplasm as a somewhat homogeneous chemical
compound uniform in all types of life. But we now see that this cell
contains not a single substance, but a large number, including solids, jelly
masses, and liquids, each of which has its own chemical composition. The
number of chemical compounds existing in the material formerly called
protoplasm no one knows, but we do know that they are many, and that the
different substances are combined to form a physical structure. Which of
these various bodies shall we continue to call protoplasm? Shall it be the
linin, or the liquids, or the microsomes, or the chromatin threads, or the
centrosomes? Which of these is the actual physical basis of life? From the
description of cell life which we have given, it will be evident that no one
of them is a material upon which our chemical biologists can longer found a

chemical theory of life. That chemical theory of life, as we have seen, was
founded upon the conception that the primitive life substance is a definite
chemical compound. No such compound has been discovered, and these
disclosures of the microscope of the last few years have been such as to
lead us to abandon hope of ever discovering such a compound. It is
apparently impossible to reduce life to any simpler basis than this
combination of bodies which make up what was formerly called
protoplasm. The term protoplasm is still in use with different meanings as
used by different writers. Sometimes it is used to refer to the entire contents
of the cell; sometimes to the cell substance only outside the nucleus.
Plainly, it is not the protoplasm of earlier years.
With this conclusion one of our fundamental questions has been answered.
We found in our first chapter that the general activities of animals and
plants are easily reduced to the action of a machine, provided we had the
fundamental vital powers residing in the parts of that machine. We then
asked whether these fundamental properties were themselves those of a
chemical compound or whether they were to be reduced to the action of still
smaller machines. The first answer which biologists gave to this question
was that assimilation, growth, and reproduction were the simple properties
of a complex chemical compound. This answer was certainly incorrect. Life
activities are exhibited by no chemical compound, but, so far as we know,
only by the machine called the cell. Thus it is that we are again reduced to
the problem of understanding the action of a machine. It may be well to
pause here a moment to notice that this position very greatly increases the
difficulties in the way of a solution of the life problem. If the physical basis
of life had proved to be a chemical compound, the problem of its origin
would have been a chemical one. Chemical forces exist in nature, and these
forces are sufficient to explain the formation of any kind of chemical
compound. The problem of the origin of the life substance would then have
been simply to account for certain conditions which resulted in such
chemical combination as would give rise to this physical basis of life. But
now that the simplest substance manifesting the phenomena of life is found
to be a machine, we can no longer find in chemical forces efficient causes
for its formation. Chemical forces and chemical affinity can explain
chemical compounds of any degree of complexity, but they cannot explain
the formation of machines. Machines are the result of forces of an entirely

different nature. Man can manufacture machines by taking chemical
compounds and putting them together into such relations that their
interaction will give certain results. Bits of iron and steel, for instance, are
put together to form a locomotive, but the action of the locomotive depends,
not upon the chemical forces which made the steel, but upon the relation of
the bits of steel to each other in the machine. So far as we have had any
experience, machines have been built under the guidance of intelligence
which adapts the parts to each other. When therefore we find that the
simplest life substance is a machine, we are forced to ask what forces exist
in nature which can in a similar way build machines by the adjustment of
parts to each other. But this topic belongs to the second part of our subject,
and must be for the present postponed.
Reaction against the Cell Doctrine.—As the knowledge of cells which we
have outlined was slowly acquired, the conception of the cell passed
through various modifications. At first the cell wall was looked upon as the
fundamental part, but this idea soon gave place to the belief that it was the
protoplasm that was alive. Under the influence of this thought the cell
doctrine developed into something like the following: The cell is simply a
bit of protoplasm and is the unit of living matter. The bodies of all larger
animals and plants are made up of great numbers of these units acting
together, and the activities of the entire organism are simply the sum of the
activities of its cells. The organism is thus simply the sum of the cells which
compose it, and its activities the sum of the activities of the individual cells.
As more facts were disclosed the idea changed slightly. The importance of
the nucleus became more and more forcibly impressed upon microscopists,
and this body came after a little into such prominence as to hide from view
the more familiar protoplasm. The marvellous activities of the nucleus soon
caused it to be regarded as the important part of the cell, while all the rest
was secondary. The cell was now thought of as a bit of nuclear matter
surrounded by secondary parts. The marvellous activities of the nucleus,
and above all, the fact that the nucleus alone is handed down from one
generation to the next in reproduction, all attested to its great importance
and to the secondary importance of the rest of the cell.
This was the most extreme position of the cell doctrine. The cell was the
unit of living action, and the higher animal or plant simply a colony of such
units. An animal was simply an association together for mutual advantage

of independent units, just as a city is an association of independent
individuals. The organization of the animals was simply the result of the
combination of many independent units. There was no activity of the
organism as a whole, but only of its independent parts. Cell life was
superior to organized life. Just as, in a city, the city government is a name
given to the combined action of the individuals, so are the actions of
organisms simply the combined action of their individual cells.
Against such an extreme position there has been in recent years a decided
reaction, and to-day it is becoming more and more evident that such a
position cannot be maintained. In the first place, it is becoming evident that
the cell substance is not to be entirely obliterated by the importance of the
nucleus. That the nucleus is a most important vital centre is clear enough,
but it is equally clear that nucleus and cell substance must be together to
constitute the life substance. The complicated structure of the cell
substance, the decided activity shown by its fibres in the process of cell
division, clearly enough indicate that it is a part of the cell which can not be
neglected in the study of the life substance. Again the discovery of the
centrosome as a distinct morphological element has still further added to the
complexity of the life substance, and proved that neither nucleus nor cell
substance can be regarded as the cell or as constituting life. It is true that we
may not yet know the source of this centrosome. We do not know whether it
is handed down from generation to generation like the nucleus, or whether
it can be made anew out of the cell substance in the life of an ordinary cell.
But this is not material to its recognition as an organ of importance in the
cell activity. Thus the cell proves itself not to; be a bit of nuclear matter
surrounded by secondary parts, but a community of several perhaps equally
important interrelated members.
Another series of observations weakened the cell doctrine in an entirely
different direction. It had been assumed that the body of the multicellular
animal or plant was made of independent units. Microscopists of a few
years ago began to suggest that the cells are in reality not separated from
each other, but are all connected by protoplasmic fibres. In quite a number
of different kinds of tissue it has been determined that fine threads of
protoplasmic material lead from one cell to another in such a way that the
cells are in vital connection. The claim has been made that there is thus a
protoplasmic connection between all the cells of the body of the animal, and

that thus the animal or plant, instead of consisting of a large number of
separate independent cells, consists of one great mass of living matter
which is aggregated into little centres, each commonly holding a nucleus.
Such a conclusion is not yet demonstrated, nor is its significance very clear
should it prove to be a fact; but it is plain that such suggestions quite
decidedly modify the conception of the body as a community of
independent cells.
There is yet another line of thought which is weakening this early
conception of the cell doctrine. There is a growing conviction that the view
of the organism, simply as the sum of the activities of the individual cells, is
not a correct understanding of it. According to this extreme position, a
living thing can have no organization until it appears as the result of cell
multiplication. To take a concrete case, the egg of a starfish can not possess
any organization corresponding to the starfish. The egg is a single cell, and
the starfish a community of cells. The egg can, therefore, no more contain
the organization of a starfish than a hunter in the backwoods can contain
within himself the organization of a great metropolis. The descendants of
individuals like the hunter may unite to form a city, and the descendants of
the egg cell may, by combining, give rise to the starfish. But neither can the
man contain within himself the organization of the city, nor the egg that of
the starfish. It is, perhaps, true that such an extreme position of the cell
doctrine has not been held by any one, but thoughts very closely
approximating to this view have been held by the leading advocates of the
cell doctrine, and have beyond question been the inspiration of the
development of that doctrine.
But certainly no such conception of the significance of cell structure would
longer be held. In spite of the fact that the egg is a single cell, it is
impossible to avoid the belief that in some way it contains the starfish. We
need not, of course, think of it as containing the structure of a starfish, but
we are forced to conclude that in some way its structure is such that it
contains the starfish potentially. The relation of its parts and the forces
therein are such that, when placed under proper conditions, it develops into
a starfish. Another egg placed under identical conditions will develop into a
sea urchin, and another into an oyster. If these three eggs have the power of
developing into three different animals under identical conditions, it is
evident that they must have corresponding differences in spite of the fact

that each is a single cell. Each must in some way contain its corresponding
adult. In other words, the organization must be within the cells, and hence
not simply produced by the associations of cells.
Over this subject there has been a deal of puzzling and not a little
experimentation. The presence of some sort of organization in the egg is
clear—but what is meant by this statement is not quite so clear. Is this adult
organization in the whole egg or only in its nucleus, and especially in the
chromosomes which, as we have seen, contain the hereditary traits? When
the egg begins to divide does each of the first two cells still contain
potentially the organization of the whole adult, or only one half of it? Is the
development of the egg simply the unfolding of some structure already
present; or is the structure constantly developing into more and more
complicated conditions owing to the bringing of its parts into new relations?
To answer these questions experimenters have been engaged in dividing
developing eggs into pieces to determine what powers are still possessed by
the fragments. The results of such experiments are as yet rather conflicting,
but it is evident enough from them that we can no longer look upon the egg
cell as a simple undifferentiated cell. In some way it already contains the
characters of the adult, and when we remember that the characters of the
adult which are to be developed from the egg are already determined, even
to many minute details—such, for instance, as the inheritance of a
congenital mark—it becomes evident that the egg is a body of extraordinary
complexity. And yet the egg is nothing more than a single cell agreeing with
other cells in all its general characters. It is clear, then, that we must look
upon organization as something superior to cells and something existing
within them, or at least within the egg cell, and controlling its development.
We are forced to believe, further, that there may be as important differences
between two cells as there are between two adult animals or plants. In some
way there must be concealed within the two cells which constitute the egg
of the starfish and the man differences which correspond to the differences
between the starfish and the man. Organization, in other words, is superior
to cell structure, and the cell itself is an organization of smaller units.
As the result of these various considerations there has been, in recent years,
something of a reaction against the cell doctrine as formerly held. While the
study of cells is still regarded as the key to the interpretation of life
phenomena, biologists are seeing more and more clearly that they must look

deeper than simple cell structure for their explanation of the life processes.
While the study of cells has thrown an immense amount of light upon life,
we seem hardly nearer the centre of the problem than we were before the
beginning of the series of discoveries inaugurated by the formulation of the
doctrine of protoplasm.
Fundamental Vital Activities as Located in Cells.—We are now in
position to ask whether our knowledge of cells has aided us in finding an
explanation of the fundamental vital actions to which, as we have seen, life
processes are to be reduced. The four properties of irritability,
contractibility, assimilation, and reproduction, belong to these vital units—
the cells, and it is these properties which we are trying to trace to their
source as a foundation of vital activity.
We may first ask whether we have any facts which indicate that any special
parts of the cell are associated with any of these fundamental activities. The
first fact that stands out clearly is that the nucleus is connected most
intimately with the process of reproduction and especially with heredity.
This has long been believed, but has now been clearly demonstrated by the
experiments of cutting into fragments the cell bodies of unicellular animals.
As already noticed, those pieces which possess a nucleus are able to
continue their life and reproduce themselves, while those without a nucleus
are incapable of reproduction. With greater force still is the fact shown by
the process of fertilization of the egg. The egg is very large and the male
reproductive cell is very small, and the amount of material which the
offspring derives from its mother is very great compared with that which it
derives from its father. But the child inherits equally from father and
mother, and hence we must find the hereditary traits handed down in some
element which the offspring obtains equally from father and mother. As we
have seen (Figs. 34-44), the only element which answers this demand is the
nucleus, and more particularly the chromosomes of the nucleus. Clearly
enough, then, we must look upon the nucleus as the special agent in
reproduction of cells.
Again, we have apparently conclusive evidence that the nucleus controls
that part of the assimilative process which we have spoken of as the
constructive processes. The metabolic processes of life are both
constructive and destructive. By the former, the material taken into the cell

in the form of food is built up into cell tissue, such as linin, microsomes,
etc., and, by the latter, these products are to a greater or less extent broken
to pieces again to liberate their energy, and thus give rise to the activities of
the cell. If the destructive processes were to go on alone the organism might
continue to manifest its life activities for a time until it had exhausted the
products stored up in its body for such purposes, but it would die from the
lack of more material for destruction. Life is not complete without both
processes. Now, in the life of the cell we may apparently attribute the
destructive processes to the cell substance and the constructive processes to
the nucleus. In a cell which has been cut into fragments those pieces
without a nucleus continue to show the ordinary activities of life for a time,
but they do not live very long (Fig. 25). The fragment is unable to
assimilate its food sufficiently to build up more material. So long as it still
retains within itself a sufficiency of already formed tissue for its destructive
metabolism, it can continue to move around actively and behave like a
complete cell, but eventually it dies from starvation. On the other hand,
those fragments which retain a piece of the nucleus, even though they have
only a small portion of the cell substance, feed, assimilate, and grow; in
other words, they carry on not only the destructive but also the constructive
changes. Plainly, this means that the nucleus controls the constructive
processes, although it does not necessarily mean that the cell substance has
no share in these constructive processes. Without the nucleus the cell is
unable to perform those processes, while it is able to carry on the
destructive processes readily enough. The nucleus controls, though it may
not entirely carry on, the constructive metabolism.
It is equally clear that the cell substance is the seat of most of the
destructive processes which constitute vital action. The cell substance is
irritable, and is endowed with the power of contractility. Cell fragments
without nucleii are sensitive enough, and can move around as readily as
normal cells. Moreover, the various fibres which surround the centrosomes
in cell division and whose contractions and expansions, as we have seen,
pull the chromosomes apart in cell division, are parts of the cell substance.
All of these are the results of destructive metabolism, and we must,
therefore, conclude that destructive processes are seated in the cell
substance.

The centrosome is too problematical as yet for much comment. It appears to
be a piece of the machinery for bringing about cell division, but beyond this
it is not safe to make any statements.
In brief, then, the cell body is a machine for carrying on destructive
chemical changes, and liberating from the compounds thus broken to pieces
their inclosed energy, which is at once converted into motion or heat or
some other form of active energy. This chemical destruction is, however,
possible only after the chemical compounds have become a part of the cell.
The cell, therefore, possesses a nucleus which has the power of enabling it
to assimilate its food—that is, to convert it into its own substance. The
nucleus further contains a marvellous material—chromatin—which in
someway exercises a controlling influence in its life and is handed down
from one generation to another by continuous descent. Lastly, the cell has
the centrosome, which brings about cell division in such a manner that this
chromatin material is divided equally among the subsequent descendants,
and thus insures that the daughter cells shall all be equivalent to each other
and to the mother cell.
We must therefore look upon the organic cell as a little engine with
admirably adapted parts. Within this engine chemical activity is excited.
The fuel supplied to the engine is combined by chemical forces with the
oxygen of the air. The vigour of the oxidation is partly dependent upon
temperature, just as it is in any other oxidation process, and is of course
dependent upon the presence of fuel to be oxidized, and air to furnish the
oxygen. Unless the fuel is supplied and the air has free access to it, the
machine stops, the cell dies. The energy liberated in this machine is
converted into motion or some other form. We do not indeed understand the
construction of the machine well enough to explain the exact mechanism by
which this conversion takes place, but that there is such a mechanism can
not be doubted, and the structure of the cell is certainly complex enough to
give plenty of room for it. The irritability of the cell is easily understood;
for, since it is made of very unstable chemical compounds, any slight
disturbance or stimulation on one part will tend to upset its chemical
stability and produce reaction; and this is what is meant by irritability.
Or, again, we may look upon the cell as a little chemical laboratory, where
chemical changes are constantly occurring. These changes we do not indeed

understand, but they are undoubtedly chemical changes. The result is that
some compounds are pulled to pieces and part of the fragments liberated or
excreted, while other parts are retained and built into other more complex
compounds. The compounds thus manufactured are retained in the cell
body, and it grows in bulk. This continues until the cell becomes too big,
and then it divides.
If a machine is broken it ceases to carry on its proper duties, and if the parts
are badly broken it is ruined. So with the cell. If it is broken by any means,
mechanical, thermal, or otherwise, it ceases to run—we say it dies. It has
within itself great power of repairing injury, and therefore it does not cease
to act until the injury is so great as to be beyond repair. Thus it only stops its
motion when the machinery has become so badly injured as to be beyond
hope of repair, and hence the cell, after once ceasing its action, can never
resume it again.
There are, of course, other functions of living things besides the few simple
ones which we have considered. But these are the fundamental ones; and if
we can reduce them to an intelligible explanation, we may feel that we have
really grasped the essence of life. If we understand how the cell can move
and grow and reproduce itself, we may rest assured that the other
phenomena of life follow as a natural consequence. If, therefore, we have
obtained an understanding of these fundamental vital phenomena, we have
accomplished our object of comprehending the life phenomena in our
chemical and mechanical laws.
But have we thus reduced these fundamental phenomena to an intelligible
explanation? It must be acknowledged that we have not. We have reduced
them to the action of chemical forces acting in a machine. But the machine
itself is unintelligible. The organic cell is no more intelligible to us than is
the body as a whole. The chemical understanding which we thought we had
a few years ago in protoplasm has failed us, and nothing has taken its place
We have no conception of what may be the primitive life substance. All we
can say is that this most marvellous of all natural phenomena occurs only
within that peculiar piece of machinery which we call the cell, and that it is
the result of the action of physical forces in that machine. How the machine
acts, or even the structure of the machine, we are as far from understanding

as we were fifty years ago. The solution has retreated before us even faster
than we have advanced toward it.
Summary.—We may now notice in a brief summary the position which we
have reached. In our attempt to explain the living organism on the principle
of the machine, we are very successful so far as secondary problems are
concerned. Digestion, circulation, respiration, and motion are readily solved
upon chemical and mechanical principles. Even the phenomena of the
nervous system are, in a measure, capable of comprehension within a
mechanical formula, leaving out of account the purely mental phenomena
which certainly have not been touched by the investigation. All of these
phenomena are reducible to a few simple fundamental activities, and these
fundamental activities we find manifested by simple bits of living matter
unincumbered by the complicated machinery of organisms. With the few
fundamental properties of these bits of organic matter we can construct the
complicated life of the higher organism. When we come, however, to study
these simple bits of matter, they prove to be anything but simple bits of
matter. They, too, are pieces of complicated mechanism whose action we do
not even hope to understand. That their action is dependent upon their
machinery is evident enough from the simple description of cell activity
which we have noticed. That these fundamental vital properties are to be
explained as the result of chemical and mechanical forces acting through
this machinery, can not be doubted. But how this occurs or what constitutes
the guiding force which corresponds to the engineer of the machine, we do
not know.
Thus our mechanical explanation of the living machine lacks a foundation.
We can understand tolerably well the building of the superstructure, but the
foundation stones upon which that structure is built are unintelligible to us.
The running of the living machine is thus only in part understood. The
living organism is a machine or, it is better to say, it is a series of machines
one within the other. As a whole it is a machine, and its parts are separate
machines. Each part is further made up of still smaller machines until we
reach the realm of the microscope. Here still we find the same story. Even
the parts formerly called units, prove to be machines, and when we
recognize the complexity of these cells and their marvellous activities, we
are ready to believe that we may find still further machines within. And thus

vital activity is reduced to a complex of machines, all acting in harmony
with each other to produce together the one result—life.

PART II.
THE BUILDING OF THE LIVING MACHINE.

CHAPTER III.

THE FACTORS CONCERNED IN THE BUILDING OF THE
LIVING MACHINE.

Having now outlined the results of our study into the mechanism of the
living machine, we turn our attention next to the more difficult problem of
the method by which this machine was built. From the facts which we have
been considering in the last two chapters it is evident that the problem we
have before us is a mechanical rather than a chemical one. Of course,
chemical forces lie at the bottom of vital activity, and we must look upon
the force of chemical affinity as the fundamental power to which the
problems must be referred. But a chemical explanation will evidently not
suffice for our purpose; for we have absolutely no reason for believing that
the phenomena of life can occur as the results of the chemical properties of
any compound, however complex. The simplest known form of matter
which manifests life is a machine, and the problem of the origin of life must
be of the origin of that machine. Are there any forces in nature which are of
a sort as to enable us to use them to explain the building of machines?
Plants and animals are the only machines which nature has produced. They
are the only instances in nature of a structure built with its parts
harmoniously adjusted to each other to the performance of certain ends. All
other machines with which we are acquainted were made by man, and in
making them intelligence came in to adapt the parts to each other. But in the
living organism is a similarly adapted machine made by natural means
rather than artificial. How were they built? Does nature, apart from human
intelligence, possess forces which can achieve such results?

Here again we must attack the problem from what seems to be the wrong
end. Apparently it would be simpler to discover the method of the
manufacture of the simplest machine rather than the more complex ones.
But this has proved contrary to the fact. Perhaps the chief reason is that the
simplest living machine is the cell whose study must always involve the use
of the microscope, and for this reason is more difficult. Perhaps it is
because the problem is really a more difficult one than to explain the
building of the more complex machines out of the simpler ones. At all
events, the last fifty years have told us much of the method of the building
of the complex machines out of the simpler ones, while we have as yet not
even a hint as to the solution of the building of the simplest machine from
the inanimate world. Our attention must, therefore, be first directed to the
method by which nature has constructed the complex machines which we
find filling the world to-day in the form of animals and plants.
History of the Living Machine.—In the first place, we must notice that
these machines have not been fashioned suddenly or rapidly, but have been
the result of a very slow growth. They have had a history extending very far
back into the past for a period of years which we can only indefinitely
estimate, but certainly reaching into the millions. As we look over this past
history in the light of our present knowledge we see that whatever have
been the forces which have been concerned in the construction of these
machines they have acted very slowly. It has taken centuries, and, indeed,
thousands of years, to take the successive steps which have been necessary
in this construction. Secondly, we notice that the machines have been built
up step by step, one feature being added to another with the slowly
progressing ages. Thirdly, we notice that in one respect this construction of
the living machine by nature's processes has been different from our
ordinary method of building machines. Our method of building puts the
parts gradually into place in such a way that until the machine is finished it
is incapable of performing its functions. The half-built engine is as useless
and as powerless as so much crude iron. Its power of action only appears
after the last part is fitted into place and the machine finished. But nature's
process in machine building is different. Every step in the process, so far as
we can trace it at least, has produced a complete machine. So far back as we
can follow this history we find that at every point the machine was so
complete as to be always endowed with motion and life activity. Nature's

method has been to take simpler types of machines and slowly change them
into more complicated ones without at any moment impairing their vigour.
It is something as if the steam engine of Watt should be slowly changed by
adding piece after piece until there was finally produced the modern
quadruple expansion engine, but all this change being made upon the
original engine without once stopping its motion.
This gradual construction of the living machines
has been called Organic Evolution, or the Theory of
Descent. It will be necessary for us, in order to
comprehend the problem which we have before us,
to briefly outline the course of this evolution. Our
starting point in this history must be the cell, for
such is the earliest and simplest form of living thing
of which we have any trace. This cell is, of course, FIG. 45.
already a machine, and we must presently return to A group of cells
the problem of its origin. At present we will assume resulting from
this cell as a starting point endowed with its division, representing
fundamental vital powers. It was sensitive, it could the first step in
feel, grow, and reproduce itself. From such a simple machine making.
machine, thus endowed, the history has been
something as follows: In reproducing itself this machine, as we have
already seen, simply divided itself into two halves, each like the other. At
first all the parts thus arising separated from each other and remained
independent. But so long as this habit continued there could be little
advance. After a time some of the cells failed to separate after division, but
remained clinging together (Fig. 45). The cells of such a mass must have
been at first all alike; but, after a little, differences began to appear among
them. Those on the outside of the mass were differently affected by their
surroundings from those in the interior, and soon the cells began to share
among themselves the different duties of life. The cells on the outside were
better situated for protection and capturing food, while those on the inside
could not readily seize food for themselves, and took upon themselves the
duty of digesting the food which was handed to them by the outer cells.
Each of these sets of cells could now carry on its own special duties to
better advantage, since it was freed from other duties, and thus the whole
mass of cells was better served than when each cell tried to do everything

for itself. This was the first step in the building of the machine out of the
active cells (Fig. 46). From such a starting point the subsequent history has
been ever based upon the same principle. There has been a constant
separation of the different functions of life among groups of cells, and as
the history went on this division of labor among the different parts became
greater and greater. Group after group of cells were set apart for one special
duty after another, and the result was a larger and ever more complicated
mass of cells, with a greater and greater differentiation among them. In this
building of the machine there was no time when the machine was not
active. At all points the machine was alive and functional, but each step
made the total function of the machine a little more accurately performed,
and hence raised somewhat the totality of life powers. This parcelling out of
the different duties of life to groups of cells continued age after age, each
step being a little advance over the last, until the result has been the living
machine as we know it in its highest form, with its numerous organs, all
interrelated in such a way as to form a harmoniously acting whole.
But a second principle in this
growth of the machine was
needed to produce the variety
which is found in nature. As the
different cells in the
multicellular mass became
associated into groups for
different duties, the method of
such division of labor was not
FIG. 46. alike in all machines. A city in
A later step in machinebuilding in which China and one in America are
the outer cells have acquired different alike made up of individuals,
form and function from the inner cells: and the fundamental needs of the
ec, the outer cells, whose duties are Chinaman and the American are
protective; en, the inner cells engaged alike. But differences in
in digesting food. industrial and political
conditions have produced
different combinations and
associations, so that Pekin is wonderfully unlike New York. So in these
early developing machines, quite a variety of method of organization was

adopted by the different groups. Now as soon as any special type of
organization was adopted by any animal or plant, the principle of heredity
transmitted the same kind of organization to its descendants, and there thus
arose lines of descent differing from each other, each line having its own
method of organization. As we follow the history of each line the same
thing is repeated. We find that the representatives of each line again
separate into groups, each of which has acquired some new type of
organization, and there has thus been a constant divergence of these lines of
descent in an indefinite number of directions. The members of the different
lines of descent all show a fundamental likeness with each other since they
retain the fundamental characters of their common ancestor, but they show
also the differences which they have themselves acquired. And thus the
process is repeated over and over again. This history of the growth of these
different machines has thus been one of divergence from common centres,
and is to be diagrammatically expressed after the fashion of a branching
tree. The end of each branch represents the highest state of perfection to
which each line has been carried.
One other point in this history must be noted. As the development of the
complication of the machine progressed the possibility of further progress
has been constantly narrowed. When the history of these machines began as
a simple mass of cells, there was a possibility of an almost endless variety
of methods of organization. But as a distinct type of organization was
adopted by one and another line of descendants all subsequent productions
were limited through the law of heredity to the general line of organization
adopted by their ancestors. With each age the further growth of such
machines must consist in the further development in the perfection of its
parts, and not in the adoption of any new system of organization. Hence it is
that the history of the living machine has shown a tendency toward
development along a few well-marked lines, and although this complication
becomes greater, we still see the same fundamental scheme of organization
running through the whole. As the ages have progressed the machines have
become more perfect in the adjustment of their parts, i.e., they have become
more perfect machines, but the history has been simply that of perfecting
the early machines rather than the production of new types.
Evidence for this History.—As just outlined, we see that the living
machines have been gradually brought into their present condition by a

process which has been called organic evolution. But we must pause for a
moment to ask what is our evidence that such has been the history of the
living machine. The whole possibility of understanding living nature
depends upon our accepting this history and finding an explanation of it. At
the outset we have the question of fact, and we must notice the grounds
upon which we stand in assuming this history to be as outlined.
This problem is the one which has occupied such a prominent place in the
scientific world during the last forty years, and which has contributed so
largely toward making modern biology such a different subject from the
earlier studies of natural history. It is simply the evidence for organic
evolution, or the theory of descent. The subject has for forty years been
thoroughly sifted and tested by every conceivable sort of test. As a result of
the interest in the question there has been disclosed an immense mass of
evidence, relevant and irrelevant. As the evidence has accumulated it has
become more and more evident that the evolution theory must be
recognized as the only one which is in accord with the facts, and the
outcome has been a practical unanimity among thinkers that the theory of
descent must be the foundation of our further study. The evidence which
has forced this conclusion upon scientists we must stop for a moment to
consider, since it bears very directly upon the subject we are studying.
Historical.—The first source of evidence is naturally a historical one. This
long history of the construction of the living machine has left its record in
the rocks which form the earth's surface. During this long period the rocks
of the earth's crust have been deposited, and in these rocks have been left
samples of many of the steps in this history of machine building. The
history can be traced by the study of these samples just as the history of any
machine might be traced from a study of the models in a patent office. One
might very easily trace, with most strict accuracy and minute detail, the
history of the printing machine from the models which are preserved in the
patent offices and elsewhere. So is it with the history of the living machine.
To be sure, the history is rather incomplete and at times difficult to read.
Many a period in the development has left no samples for our inspection
and must be interpreted in our history between what went before and what
comes after. Many of the machines, especially the early ones, were made of
such fragile material that they could not be preserved in the rocks. In many
a case, too, the rocks in which the specimens were deposited have been

subjected to such a variety of heatings and pressures, that they have been
twisted out of shape and even crushed out of recognizable form. But in spite
of this the record is showing itself more complete each year. Our
paleontologists are opening layer after layer of these rocks, and thus
examining each year new pages in nature's history. The more recent epochs
in the history have been already read with almost historic accuracy. From
them we have learned in great detail how the finishing touches were given
to these machines, and are able to trace with accuracy how the somewhat
more generalized forms of earlier days were changed to produce our
modern animals.
This fossil record has given us our best knowledge of the course by which
the present living world has been brought into its existing condition. But its
accuracy is largely confined to the recent periods. Of the very early history
fossils tell us little or nothing. All the early rocks, which we may believe
were formed during the period when the first steps in this machine building
were taken, have been so changed by heat and pressure that whatever
specimens they may have originally contained have been crushed out of
shape. Furthermore, the earliest organisms had no hard skeletons, and it was
not until living beings had developed far enough to have hard parts that it
was possible for them to leave traces of themselves in the rocks. Hence, so
far as concerns this earliest history, we can get no record of it in the rocks.
Embryological.—But here comes in another source of evidence which
helps to fill up the gap. In its development every animal to-day begins as an
egg. This is a simple cell, and the animal goes through a series of changes
which eventually lead to the adult. Now these changes appear for the most
part to be parallel to the changes through which the earlier forms of life
passed in their development from the simple to the more complicated
forms. Where it is possible to follow the history of the groups of animals
from their fossil remains and compare it with the history of the individual
animal as it progresses from the egg to the adult, there is found a very
decided parallelism. This parallelism between embryology and past history
has been of great service in helping us toward the history of the past. At one
time it was believed that it was the key which would unlock all doors, and
for a decade biologists eagerly pursued embryology with the expectation
that it would solve all problems in connection with the history of animals.
The result has been somewhat disappointing. Embryology has, it is true,

been of the utmost service in showing relationships of forms to each other,
and in thus revealing past history. But while this record is a valuable one, it
is a record which has unfortunately been subject to such modifying
conditions that in many cases its original meaning has been entirely
obliterated and it has become worthless as a historical record. These
imperfections in regard to the record were early seen after the attention of
biologists was seriously turned to the study of embryology, but it was
expected that it would be possible to correct them and discover the true
meaning underlying the more apparent one. Indeed, in many cases this has
been found possible. But many of the modifications are so profound as to
render it impossible to untangle them and discover the true meaning. As a
result the biologist to-day is showing less confidence in embryology, and is
turning his attention in different directions as more promising of results in
the line desired.
But although the teachings of embryology have failed to realize the great
hopes that were placed upon them, their assistance in the formulation of this
history of the machine has been of extreme value. Many a bit of obscurity
has been cleared up when the embryology of puzzling animals has been
studied. Many a relationship has been made clear, and this is simply another
way of saying that a portion of this history of life has been read. This aid of
embryology has been particularly valuable in just that part of the history
where the evidence from the study of fossils is wanting. The study of
fossils, as we have seen, gives little or no data concerning the early history
of living machines; and it is just here that embryology has proved to be of
the most value. It is a source of evidence that has told us of most of the
steps in the progress from the single-celled animal to the multicellular
organisms, and gives us the clearest idea of the fundamental principles
which have been concerned in the evolution of life and the construction of
the complicated machine out of the simple bit of protoplasm. In spite of its
limits, therefore, embryology has contributed a large quota of the evidence
which we have of the evolution of life.
Anatomical.—A third source of this history is obtained from the facts of
comparative anatomy. The essential feature of this subject is the fact that
animals and plants show relationships. This fact is one of the most patent
and yet one of the most suggestive facts of biology. It has been recognized
from the very beginning of the study of animals and plants. One cannot be

even the most superficial observer without seeing that certain forms show
great likeness to each other while others are much more unlike. The
grouping of animals and plants into orders, genera, and species is dependent
upon this relationship. If two forms are alike in everything except some
slight detail, they are commonly placed in the same genus but in different
species, while if they show a greater unlikeness they may be placed in
separate genera. By thus grouping together forms according to their
resemblance the animal and vegetable kingdoms are classified into groups
subordinate to groups. The principle of relationship, i.e., fundamental
similarity of structure, runs through the whole animal and vegetable
kingdom. Even the animals most unlike each other show certain points of
similarity which indicates a relationship, although of course a distant one.
The fact of such a relationship is too patent to demand more words, but its
significance needs to be pointed out. When we speak of relationship among
men we always mean historical connection. Two brothers are closely related
because they have sprung from common parents, while two cousins are less
closely related because their common point of origin was farther back in
time. More widely we speak of the relationship of the Indo-European races,
meaning thereby that back in the history of man these races had a common
point of origin. We never speak of any real relation of objects unless
thereby we mean to imply historical connection. We are therefore justified
in interpreting the manifest relationships of organisms as pointing to
history. Particularly are we justified in this conclusion when we find that the
relationships which we draw between the types of life now in existence run
parallel to the history of these types as revealed to us by fossils and at the
same time disclosed by the study of embryology.
This subject of comparative anatomy includes a consideration of what is
called homology, and perhaps a concrete example may be instructive both
in illustration and as suggesting the course which nature adopts in
constructing her machines. We speak of a monkey's arm and a bird's wing
as homologous, although they are wonderfully different in appearance and
adapted to different duties. They are called homologous because they have
similar parts in similar relations. This can be seen in Figs. 47 and 48, where
it will be seen that each has the same bones, although in the bird's wing
some of the bones have been fused together and others lost. Their similarity
points to a relationship, but their dissimilarity tells us that the relationship is

a distant one, and that their common point of origin must have been quite
far back in history. Now if we follow back the history of these two kinds of
appendages, as shown to us by fossils, we find them approaching a common
point. The arm can readily be traced to a walking appendage, while the
bird's wing, by means of some interesting connecting links, can in a similar
way be traced to an appendage with its five fingers all free and used for
walking. Fig. 49 shows one of these connecting links representing the
earliest type of bird, where the fingers and bones of the arm were still
distinct, and yet the whole formed a true wing. Thus we see that the
common point of origin which is suggested by the likenesses between an
arm and a wing is no mere imaginary one, for the fossil record has shown
us the path leading to that point of origin. The whole tells us further that
nature's method of producing a grasping or flying organ was here, not to
build a new organ, but to take one that had hitherto been used for other
purposes, and by slow changes modify its form and function until it was
adapted to new duties.

FIG. 47.—The arm of a monkey, a prehensile appendage.
FIG. 48.—The arm of a bird, a flying appendage. In life covered with
feathers.
FIG. 49.—The arm of an ancient half-bird half-reptile animal. In life
covered with feathers and serving as a wing.
Significance of these Sources of History.—The real force of these sources
of evidence comes to us only when we compare them with each other. They
agree in a most remarkable fashion. The history as disclosed by fossils and

that told by embryology agree with each other, and these are in close
harmony with the history as it can be read from comparative anatomy. If
archæologists were to find, in different countries and entirely unconnected
with each other two or more different records of a lost nation, the belief in
the actual existence of that nation would be irresistible. When researches at
Nineveh, for example, unearth tablets which give the history of ancient
nations, and when it proves that among the nations thus mentioned are some
with the same names and having the same facts of history as those
mentioned in the Bible, it is absolutely impossible to avoid the conclusion
that such a nation with such a history did actually exist. Two independent
sources of record could not be false in regard to such a matter as this.
Now, our sources of evidence for this history of the living machine prove to
be of exactly this kind. We have three independent sources of evidence
which are so entirely different from each other that there is almost no
likeness between them. One is written in the rocks, one in bone and muscle,
while the third is recorded in the evanescent and changing pages of
embryology and metamorphosis. Yet each tells the same story. Each tells of
a history of this machine from simple forms to more complex. Each tells of
its greater and greater differentiation of labour and structure as the periods
of time passed. Each tells of a growing complexity and an increasing
perfection of the organisms as successive periods pass. Each tells us of
common points of origin and divergence from these points. Each tells us
how the more complicated forms have arisen as the results of changes in
and modifications of the simpler forms. Each shows us how the individual
parts of the organisms have been enlarged or diminished or changed in
shape to adapt them to new duties. Each, in short, tells the same story of the
gradual construction of the living machine by slow steps and through long
ages of time. When these three sources of history so accurately agree with
each other, it is as impossible to disbelieve in the existence of such history
as it is to disbelieve in the existence of the ancient Hittite nation, after its
history has been told to us by two different sources of record.
Now all this is very germane to our subject. We are trying to learn how this
living machine, with its wonderful capabilities, was built. The history which
we have outlined is undoubtedly the history of the building of this machine,
and the knowledge that these complicated machines have been produced as
the result of slow growth is of the utmost importance to us. This knowledge

gives us at the very start some idea of the nature of the forces which have
been at work. It tells us that in searching for these forces we must look for
those which have been acting constantly. We must look for forces which
produce their effects not by sudden additions to the complication of the
machine. They must be constant forces whose effect at any one time is
comparatively slight, but whose total effect is to increase the complexity of
the machine. They must be forces which produce new types through the
modification of the old ones. We must look for forces which do not adapt
the machine for its future, but only for its present need. Each step in the
history has been a complete animal with its own fully developed powers.
We are not to expect to find forces which planned the perfect machine from
the start, nor forces which were engaged in constructing parts for future use.
Each step in the building of the machine was taken for the good of the
machine at the particular moment, and the forces which we are to look for
must therefore be only such as can adapt the organisms for its present
needs. In other words, nothing has been produced in this machine for the
purpose of being developed later into something of value, but all parts that
have been produced are of value at the time of their appearance. We must,
in short, look for forces constantly in action and always tending in the same
direction of greater complexity of structure.
Is it possible to discover these forces and comprehend their action? Before
the modern development of evolution this question would unhesitatingly
have been answered in the negative. To-day, under the influence of the
descent theory, stimulated, in the first place, by Darwin, the question will be
answered by many with equal promptness in the affirmative. At all events,
we have learned in the last forty years to recognize some of the factors
which have been at work in the construction of this machine. We must turn,
therefore, to the consideration of these factors.
Forces at Work in the Building of the Living Machine.—There are three
primary factors which lie at the bottom of the whole process. They are—
1. Reproduction, which preserves type from generation to generation.
2. Variation, which modifies type from generation to generation.
3. Heredity, which transmits characters from generation to generation.

Each must be considered by itself.
Reproduction.—Reproduction is the primary factor in this process of
machine building, heredity and variation being simply phases of
reproduction. The living machine has developed by natural processes, all
other machines by artificial methods. Reproduction is the one essential
point of difference between the living machine and the others which has
made their construction by natural processes a possibility. What, then, is
reproduction? Reproduction is in all cases at the bottom simple division.
Whether we consider the plant that multiplies by buds or the unicellular
animal that simply divides into two equal parts, or the larger animal that
multiplies by eggs, we find that in all cases the fundamental feature of the
process is division. In all cases the organism divides into two or more parts,
each of which becomes in time like the original. Moreover, when we trace
this division further we find that in all cases it is to be referred back to the
division of the cell, such as we have described in a previous chapter. The
egg is a single cell which has come from the parent by the division of one
of the cells in the body of the parent. A bud is simply a mass of cells which
have all arisen from the parent cells by division. The foundation of
reproduction is thus in all cases cell division. Now, this process of division
is dependent upon the properties of the cell. Firstly, it is a result of the
assimilative powers of the cell, for only through assimilation can the cell
increase in size, and only as it increases in size can it gain sustenance for
cell division. Secondly, it is dependent, as we have seen, upon the
mechanism of the cell body, and especially the nucleus and centrosome.
These structures regulate the cell division, and hence the reproduction of all
animals and plants. We can not, therefore, find any explanation of
reproduction until we have explained the mechanism of the cell. The
fundamental feature, of nature's machine building is thus based upon the
machinery of the nucleus and centrosome of the organic cell.
Aside from the simple fact that it preserves the race, the most important
feature connected with this reproduction is its wonderful fruitfulness. Since
it results from division, it always tends to increase the offspring in
geometrical ratio. In the simplest case, that of the unicellular animals, the
cell divides, giving rise to two animals, each of which divides again,
producing four, and these again, giving eight, etc. The rapidity of this
multiplication is sometimes inconceivable. It depends, of course, upon the

interval of time between the successive divisions, but among the lower
organisms this interval is sometimes not more than half an hour, the result
of which is that a single individual could give rise in the course of twenty-
four hours to sixteen million offspring. This is doubtless an extreme case,
but among all the lower animals the rate is very great. Among larger
animals the process is more complicated; but here, too, there is the same
tendency to geometrical progression, although the intervals between the
successive reproductions may be quite long and irregular. But it is always
so great that if allowed to progress unhindered at its normal rate the
offspring would, in a few years, become so numerous as to crowd other life
out of existence. Even the slow-breeding elephant would, if allowed to
breed unhindered for seven hundred and fifty years, produce nineteen
million offspring—a rate of increase plainly incompatible with the
continued existence of other animals.
Here, then, we have the foundation of nature's method of building animals
and plants of the higher classes. In the machinery of the cell she has a
power of reproduction which produces an increase in geometrical ratio far
beyond the possibility for the surface of the earth to maintain.
Heredity.—The offspring which arise by these processes of division are
like each other, and like the parent from which they sprung. This is the
essence of what is called heredity. Its significance in the process of machine
building is evident at once. It is the conserving force which preserves the
forms already produced and makes it possible for each generation to build
upon the structures of the earlier ones. Without it each generation would
have to begin anew at the beginning, and nothing could be accomplished.
But since this principle brings each individual to the same place where its
parents stand, and thus always builds the offspring into a machine like the
parent, it makes it possible for the successive generations to advance.
Heredity is thus like the power of memory, or better still, like the invention
of printing in the development of civilization. It is a record of past
achievements. By means of printing each age is enabled to benefit by the
discoveries of the previous age, and without it the development of
civilization would be impossible. In the same way heredity enables each
generation to benefit by the achievements of its ancestors in the process of
machine building, and thus to devote its own energies to advancement.

The fact of heredity is patent enough. It has been always clearly recognized
that the child has the characters of its parents, and this belief is so well
attested as to need no proof. It is still a question as to just what characters
may be inherited, and what influences may affect the inheritance. There are
plenty of puzzling problems connected with heredity, but the fact of
heredity is one of the foundation stones of biological science. Upon it must
be built all theories which look toward the explanation of the origin of the
living machine.
This factor of heredity again we must trace back to the machinery of the
cell. We have seen in the previous pages evidence for the wonderful nature
of the chromosomes of the cells. We can not pretend to understand them,
but they must be extraordinarily complex. We have seen proof that these
chromosomes are probably the physical basis of heredity, since they are the
only parts of each parent which are handed down to subsequent generations.
With these various facts of cell division and cell fertilization in mind, we
can reach a very simple explanation of fundamental features of heredity.
The following is an outline of the most widely accepted view of the
hereditary process.
Recognizing that the chromosomes are the physical basis of hereditary
transmission, we can picture to ourselves the transmission of hereditary
characters something as follows: As we have seen, the fertilized egg
contains an equal number of chromosomes from each parent (Fig. 42). Now
when this fertilized cell divides, each of the rods splits lengthwise, half of
each entering each of the two cells arising from the cell division. From this
method of division of the chromosomes it follows that the daughter cells
would be equivalent to each other and equivalent also to the undivided egg.
If the original chromosomes contained potentially all the hereditary traits
handed down from parent to child, the chromosomes of each daughter cell
will contain similar hereditary traits. If, therefore, the original fertilized egg
possessed the power of developing into an adult like the parent, each of the
daughter cells should likewise possess the power of developing into a
similar adult. And thus each cell which arises as the result of such division
should possess similar characters so long as this method of division
continues. But after a little in the development of the egg a differentiation
among the daughter cells arises. They begin to acquire different shapes and
different functions. This we can only believe to be the result of a

differentiation in their chromatin material. In the cell division the
chromosomes no longer split into equivalent halves, but some characters are
portioned off to some cells and others to other cells. Those cells which are
to carry on digestive functions when they are formed receive chromatin
material which especially controls them in the performance of this digestive
function, while those which are to produce sensory organs receive a
different portion of the chromatin material. Thus the adult individual is built
up as the cells receive different portions of this hereditary substance
contained in the original chromosomes. The original chromosomes
contained all hereditary characters, but as development proceeds these are
gradually portioned out among the daughter cells until the adult is formed.
From this method of division it will be seen that each cell of the adult does
not contain all the characters concealed in the original chromosomes of the
egg, although each contains a part which may have been derived from each
parent. It is thought, however, that a part of the original chromatin material
does not thus become differentiated, but remains entirely unchanged as the
individual is developing. This chromatin material may increase in amount
by assimilation, but it remains unchanged during the entire growth of the
individual. It thus follows that the adult will contain, along with its
differentiated material, a certain amount of the original physical basis of
heredity which still retains its original powers. This undifferentiated
chromatin material originally possessed powers of producing a new
individual, and of course it still possesses these powers, since it has
remained dormant without alteration. Further, it will follow that if this
dormant undifferentiated chromatin should start into activity and produce a
new individual, the new individual thus produced would be identical in all
characters with the one which actually did develop from the egg, since both
individuals would have come from a bit of the same chromatin. The child
would be like the parent. This would be true no matter how much this
undifferentiated material should increase in amount by assimilation, so long
as it remained unaltered in character, and it hence follows that every
individual carries around a certain amount of undifferentiated chromatin
material in all respects identical with that from which he developed.
Now whether this undifferentiated germ plasm, as we will now call it, is
distributed all over the body, or is collected at certain points, is immaterial
to our purpose. It is certain that portions of it find their way into the

reproductive organs of the animal or plant. Thus we see that part of the
chromatin material in the egg of the first generation develops into the
second generation, while another part of it remains dormant in that second
generation, eventually becoming the chromatin of its eggs and spermatozoa.
Thus each egg of the second generation receives chromosomes which have
come directly from the first generation, and thus it will follow that each of
these eggs will have identical properties with the egg of the first generation.
Hence if one of these new eggs develops into an adult it will produce an
adult exactly like the second generation, since it contains chromosomes
which are absolutely identical with those from which the second generation
sprung. There is thus no difficulty in understanding why the second
generation will be like the first, and since the process is simply repeated
again in the next reproduction, the third generation will be like the second,
and so on, generation after generation. A study of the accompanying
diagram will make this clear.
In other words, we have here a simple understanding of at least some of the
features of heredity. This explanation is that some of the chromatin material
or germ plasm is handed down from one generation to another, and is stored
temporarily in the nucleii of the reproductive cells. During the life of the
individual this germ plasm is capable of increasing in amount without
changing its nature, and it thus continues to grow and is handed down from
generation to generation, always endowed with the power of developing
into a new individual under proper conditions, and of course when it does
thus give rise to new individuals they will all be alike. We can thus easily
understand why a child is like its parent. It is not because the child can
inherit directly from its parent, but rather because both child and parent
have come from the unfolding of two bits of the same germ plasm. This fact
of the transmission of the hereditary substance from generation to
generation is known as the theory of the continuity of germ plasm.
Such appears to be, at least in part, the machinery of heredity. This
understanding makes the germ substance perpetual and continuous, and
explains why successive generations are alike. It does not explain, indeed,
why an individual inherits from its parents, but why it is like its parents.
While biologists are still in dispute over many problems connected with
heredity, all are agreed to-day that this principle of the continuity of the
heredity substance must be the basis of all attempts to understand the

machinery of heredity. But plainly this whole process is a function of the
cell machinery. While, therefore, the idea of the continuity of germ
substance greatly simplifies our problem, we must acknowledge that once
more we are thrown back upon the mysteries of the cell. Until we can more
fully explain the cell machine we must recognize our inability to solve the
fundamental question of why an individual is like its parents.
A represents an egg of a starfish. From
one half, the unshaded portion, develops
the starfish of the next generation, B. The
other is distributed without change in the
ovaries, ov, of the individual, B. From
these ovaries arises the next egg, A', with
its germ plasm. This germ plasm is
evidently identical with that in A, since it
is merely a bit of the same handed down
through the individual, B. In the
development of the next generation the
process is repeated, and hence B' will be
like B, and the third generation of eggs
identical with the first and second. The
undifferentiated part of the germ plasm is
thus simply handed on from one
generation to the next.]
But plainly reproduction and heredity, as
FIG. 50.—Diagram illustrating we have thus far considered them, will be
the principle of heredity. unable to account for the slow
modification of the machine; for in
accordance with the facts thus far outlined, each generation would be
precisely like the last, and there would be no chance for development and
change from generation to generation. If the individual is simply the
unfolding of the powers possessed by a bit of germ plasm, and if this germ
plasm is simply handed on from generation to generation, the successive
generations must of necessity be identical. But the living machine has been
built by changes in the successive generation, and hence plainly some other
factor is needed. This factor is variation.

Variation.—Variation is the principle that produces modification of type.
Heredity, as just explained, would make all generations alike. But nothing is
more certain than that they are not alike. The fact of variation is patent on
every side, for no two individuals are alike. Successive generations differ
from each other in one respect or another. Birds vary in the length of their
bills or toes; butterflies, in their colours; dogs, in their size and shape and
markings; and so on through an endless category. Plants and animals alike
throughout nature show variations in the greatest profusion. It is these
variations which must furnish us with the foundation of the changes which
have gradually built up the living machine.
Of the fact of these variations there is no question, and the matter need not
detain us. Every one has had too many experiences to ask for proof. Of the
nature of the variations, however, there are some points to be considered
which are very germane to our subject. In the first place, we must notice
that these variations are of two kinds. There is one class which is born with
the individual, so that they are present from the time of birth. In saying that
these variations are born with the individual we do not necessarily mean
that they are externally apparent at birth. A child may inherit from its
parents characters which do not appear till adult life. For example, a child
may inherit the colour of its father's hair, but this colour is not apparent at
birth. It appears only in later life, but it is none the less an inborn character.
In the same way, we may have many inborn variations among individuals
which do not make themselves seen until adult life, but which are none the
less innate. The offspring of the same parents may show decided
differences, although they are put under similar conditions, and such
differences are of course inherent in the nature of the individual. Such
variations are called congenital variations.
There is, however, a second class of variations which are not born in the
individual, but which arise as the result of some conditions affecting its
after-life. The most extreme instances of this kind are mutilations. Some
men have only one leg because the other has been lost by accident. Here is
a variation acquired as the result of circumstances. A blacksmith differs
from other members of his race in having exceptionally large arm muscles;
but here, again, the large muscles have been produced by use. A European
who has lived under a tropical sun has a darkened skin, but this skin has
evidently been darkened by the action of the sun, and is quite a different

thing from the dark skin of the dark races of men. In such instances we have
variations produced in individuals as the result of outside influences acting
upon them. They are not inborn, but are secondarily acquired by each
individual. We call them acquired variations.
It is not always possible to distinguish between these two types of variation.
Frequently a character will be found in regard to which it is impossible to
determine whether it is congenital or acquired. If a child is born under the
tropical sun, how can we tell whether its dark skin was the result of direct
action of the sun on its own skin, or was an inheritance from its dark-
skinned parents? We might suppose that this could be answered by taking a
similar child, bringing it up away from the tropical sun, and seeing whether
his skin remained dark. This would not suffice, however; for if such a child
did then develop a white skin, we could not tell but that this lighter-
coloured skin had been produced by the direct bleaching effect of the
northern climate upon a skin which otherwise would have been dark. In
other words, a conclusive answer can not here be given. It is not our
purpose, however, to attempt to distinguish between these two kinds of
variations, but simply to recognize that they occur.
Our next problem must be to search for an explanation of these variations.
With the acquired variations we have no particular trouble, for they are
easily explained as due to the direct action of the environment upon
animals. One of the fundamental characters of the living protoplasm (using
the word now in its widest sense) is its extreme instability. So unstable is it
that any disturbing influence will affect it. If two similar unicellular
organisms are placed under different conditions they become unlike, since
their unstable protoplasm is directly affected by the surrounding conditions.
With higher animals the process is naturally a little more complicated; but
here, too, they are easily understood as part of the function of the machine.
One of the adjustments of the machine is such that when any organ is used
more than usual the whole machine reacts in such a way as to send more
blood to this special organ. The result is a change in the nutrition of the
organ and a corresponding variation in the individual. Thus acquired
variations are simply functions of the action of the machine.
Congenital variations, however, can not receive such an explanation. Being
born with the individual, they can not be produced by conditions affecting

him, but rather to something affecting the germ plasm from which he
sprung. The nature of the germ plasm controls the nature of the individual,
and congenital variations must consequently be due to its variations. But it
is not so easy to see how this germ plasm can undergo variation. The
conditions which surround the individual would affect its body, but it is not
easy to believe that they would affect the germinal substance. Indeed, it is
not easy to see how any external conditions can have influence upon this
germinal material if it is not an active part of the body, but is simply stored
within it for future use in reproduction. How could any changes in the
environment of the individual have any effect upon this dormant material
stored within it? But if we are correct in regarding this germ material in the
reproductive bodies as the basis of heredity and the guiding force in
development, then it follows that the only way in which congenital
variations can occur is by some variations in the germ plasm. If a child
developed from germ plasm identical with that from which its parents
developed, it would inherit identical characters; and if there are any
congenital variations from its parents, they must be due to some variations
in the germ plasm. In other words, in order to explain congenital variations
we must account for variations in the germ plasm.
Now, there are two methods by which we may suppose that these variations
in the germ may arise. The first is by the direct influence upon the germ
plasm of certain unknown external conditions. The life substance of
organisms is always very unstable, and, as we have seen, acquired
variations are caused by external influences directly affecting it. Now, the
hereditary material is also life substance, and it is plainly a possibility for us
to imagine that this germ material is also subject to influences from the
conditions surrounding it. That such variations do occur appears to be
hardly doubtful, although we do not know what sort of influences can
produce them. If the germ plasm is wholly stored within the reproductive
gland, it is certainly in a position to be only slightly affected by surrounding
conditions which affect the animal. We can readily understand that the use
of an organ like the arm will affect it in such a way as to produce changes in
its protoplasm, but we can hardly imagine that such use of the arm would
produce any change in the hereditary substance which is stored in the
reproductive organs. External conditions may thus readily affect the body,
but not so readily the germ material. Even if such material is distributed

more or less over the body instead of being confined to the reproductive
glands, as some believe, the difficulty is hardly lessened. This difficulty of
understanding how the germ plasm can be affected by external conditions
has led one school of biologists to deny that it is subject to any variation by
external conditions, and hence that all modification of the germ plasm must
come from some other source. Probably no one, however, holds this
position to-day, and it is the general belief that the germ plasm may be to
some slight extent modified by external conditions. Of course, if such
variations do occur in the germ plasm they will become congenital
variations of the next generation, since the next generation is the unfolding
of the germ plasm.
The second method by which the variations of germ plasm may arise is
apparently of more importance. It is based upon the fact that, with all higher
animals and plants at least, each individual has two parents instead of one.
In our study of cells we have seen that the machinery of the cell is such that
it requires in the ordinary process of reproduction the union of germinal
material from two different individuals to produce a cell which can develop
into a new individual. As we have seen, the egg gets rid of half its
chromosomes in order to receive an equal number from a male parent; and
thus the fertilized egg contains chromosomes, and hence hereditary
material, from two different individuals. Now, this sexual reproduction
occurs very widely in the organic world. Among some of the lowest forms
of unicellular organisms it is not known, but in most others some form of
such union is universal. Now, here is plainly an abundant opportunity for
congenital variations; for it is seen that each individual does not come from
germ material identical with that from which either parent came, but from
some of this material mixed with a similar amount from a different parent.
Now, the two parents are never exactly alike, and hence the germ plasm
which each contributes to the offspring will not be exactly alike. The
offspring will thus be the result of the unfolding of a bit of germ plasm
which will be different from that from which either of its parents developed,
and these differences will result in congenital variations. Sexual
reproduction thus results in congenital variations; and if congenital
variations are necessary for the evolution of the living machine—and we
shall soon see reason for believing that they are—we find that sexual
reproduction is a device adopted for bringing out such congenital variations.

Inheritance of Variations.—The reason why congenital variations are
needed for the evolution of the living machine is clear enough. Evanescent
variations can have no effect upon this machine, for they would disappear
with the individual in which they appeared. In order that they should have
any influence in the process of machine building they must be permanent
ones; or, in other words, they must be inherited from generation to
generation. Only as such variations are transmitted by heredity can they be
added to the structure of the developing machine. Therefore we must ask
whether the variations are inherited.
In regard to the congenital variations there can be no difficulty. The very
fact that they are congenital shows us that they have been produced by
variations in the germ plasm, and as such they must be transmitted, not only
to the next generation, but to all following generations, until the germ plasm
becomes again modified. This germ plasm is handed on from generation to
generation with all its variations, and hence the variations will be added
permanently to the machine. Congenital variations are thus a means for
permanently modifying the organism, and by their agency must we in large
measure believe that evolution through the ages has taken place.
With the acquired variations the matter stands quite differently. We can
readily understand how influences surrounding an animal may affect its
organs. The increase in the size of the muscles of the blacksmith's arm by
use we understand readily enough. But with our understanding of the
machinery of heredity we can not see how such an effect can extend to the
next generation. It is only the organ directly affected that is modified by
external conditions. Acquired variations will appear in the part of the body
influenced by the changed conditions. But the germ plasm within the
reproductive glands is not, so far as we can see, subject to the influence of
an increased use, for example, in the arm muscles. The germ material is
derived from the parents, and, if it is simply stored in the individual, how
could an acquired variation affect it? If an individual lose a limb his
offspring will not be without a corresponding limb, for the hereditary
material is in the reproductive organs, and it is impossible to believe that the
loss of the limb can remove from the hereditary material in the reproductive
glands just that part of the germ plasm which was designed for the
production of the limb. So, too, if the germ plasm is simply stored in the
individual, it is impossible to conceive any way that it can be affected by

the conditions around the individual in such a way as to explain the
inheritance of acquired variations. If acquired variations do not affect the
germ plasm they cannot be inherited, and if the germ plasm is only a bit of
protoplasmic substance handed down from generation to generation, we can
not believe that acquired variations can influence it.
From such considerations as these have arisen two quite different views
among biologists; and, while it is not our purpose to deal with disputed
points, these views are so essential to our subject that they must be briefly
referred to. One class of biologists adhere closely to the view already
outlined, and insist for this reason that acquired variations can not under
any conditions be inherited. They insist that all inherited variations are
congenital, and due therefore to direct variations in the germ plasm, and
that all instances of seeming inheritance of acquired variations are capable
of other explanation. The other school is equally insistent that there are
abundant instances of the inheritance of acquired characters, claiming that
these proofs are so strong as to demand their acceptance. Hence this class of
biologists insist that the explanation of heredity given as a simple handing
down from generation to generation of a germ plasm is not complete, and
that while it is doubtless the foundation of heredity, it must be modified in
some way so as to admit of the inheritance of acquired characters. There is
no question that has excited such a wide interest in the biological world
during the last fifteen years as this one of the inheritance of acquired
characters. Until about 1884 the question was not seriously raised. Heredity
was known to be a fact, and it was believed that while congenital characters
are more commonly inherited, acquired characters may also frequently be
handed down from generation to generation. The facts which we have noted
of the continuity of germ plasm have during the last fifteen years led many
biologists to deny the possibility of the latter. The debate which arose has
continued vigorously, and can not be regarded as settled at the present time.
One result of this debate is clear. It has been shown beyond question that
while the inheritance of congenital characters is the rule, the inheritance of
acquired characters is at all events unusual. At the present time many
naturalists would be inclined to think that the balance of evidence indicates
that under certain conditions certain kinds of acquired characters may be
inherited, although this is still disputed by others. Into this discussion we
cannot enter here. The reason for referring to it at all is, however, evident.

We are searching for nature's method of building machines. It is perfectly
clear that variations among animals and plants are the foundations of the
successive steps in advance made in this machine building, but of course
only such variations as can be transmitted to posterity can serve any
purpose in this development. If therefore it should prove that acquired
characters can not be inherited, then we should no longer be able to look
upon the direct influence of the surroundings as a factor in the machine
building. We should then have nothing left except the congenital variations
produced by sexual union, or the direct variation of the germ plasm as a
factor for advance. If, however, it shall prove that acquired characters may
even occasionally be inherited, then the direct effect of the environment
upon the individual will serve as a decided assistance in our problem.
Here, then, we have before us the factors which have been concerned in the
building of the living machine under nature's hands. Reproduction keeps in
existence a constantly active, unstable, readily modified organism as a basis
upon which to build. Variation offers constantly new modifications of the
type, while heredity insures that the modifications produced in the machine
by the influences which give rise to the variations shall be permanently
fixed.
Method of Machine Building.—Natural Selection. The method by which
these factors have worked together to build up the living machines is easily
understood in its general aspects, although there are many details as yet
unsolved. The general facts connected with the evolution of animals are
matters of common knowledge. We need do no more than outline the
subject, since it is well understood by all. The basis of the method is natural
selection, which acts in this machine building something as follows:
The law of reproduction, as we have seen, produces new individuals with
extraordinary rapidity, and as a result more individuals are born than can
possibly find sustenance in the world. Hence only a few of the offspring of
any animal or plant can live long enough to produce offspring in turn. The
many must die that the few may live; and there is, therefore, a constant
struggle among the individuals that are born for food or for room in the
world. In this struggle for existence of course the weakest will go to the
wall, while those that are best adapted for their place in life will be the ones
to get food, live, and reproduce their kind. This is at all events true among

the lower animals, although with mankind the law hardly applies. Now,
among the individuals that are born there will be no two exactly alike, since
variations are universal, many of which are congenital and thus born with
the individual and transmitted by inheritance. Clearly enough those animals
that have a variation which makes them a little better adapted for the
struggle will be the ones to live and hence to produce offspring, while those
without such advantage will be the ones to die. We may suppose, for
example, that some of the individuals had longer necks than the average. In
time of scarcity of food these individuals would be able to get food that the
short-necked individuals could not reach. Hence in times of famine the
long-necked individuals would be the ones to survive. Now if this
peculiarity were a congenital variation it would be already represented in
the germ plasm, and consequently it would be inherited by the next
generation. The short-necked individuals being largely destroyed in this
struggle for food, it would follow that the next generation would be a little
better off than the last, since all would inherit this tendency toward a long
neck. A few generations would then see the disappearance of all individuals
which did not show either this or some other corresponding advantage, and
in this way the lengthened neck would be added permanently as a part of
the machine. When this time came this peculiarity would no longer give its
possessors any advantage over its rivals, since all would possess it. Now,
therefore, some new variation would in the same way determine which
animals should live and which should die in the struggle, and in time a new
modification would be added to the machine. And thus this process
continues, one variation after another being added, until the machine is
slowly built into a more and more complicated structure, always active but
with a constantly increasing efficiency. The construction is a natural one. A
mixing of germ plasm in sexual reproduction or some other agencies
produce congenital variations; natural selection acting upon the numerous
progeny selects the best of the new variations, and heredity preserves and
hands them down to posterity.
All students of whatever school recognize the force of this principle and
look upon natural selection as an efficient agency in machine building. It is
probably the most fundamental of the external laws that have guided the
process. There are, however, certain other laws which have played a more
or less subordinate part. The chief of these are the influence of migration

and isolation, and the direct influence of the environment. Each of these
laws has its own school of advocates, and each has been given by its
advocates the chief role in the process of machine building.
Migration and Isolation.—The production of the various types of
machines has been undoubtedly facilitated by the migrations of animals and
the isolation of different groups of descendants from each other by various
natural barriers. The variations which occur in organisms are so great that
they would sometimes run into abnormal structures were it not for the fact
that sexual reproduction constantly tends to reduce them. In an open
country where animals and plants interbreed freely, it will commonly
happen that individuals with certain peculiarities will mate with others
without such peculiarities, and the offspring will therefore inherit the
peculiarity not in increased degree but in decreased degree. This constant
interbreeding of individuals will tend to prevent the formation of many
modifications in the machine which become started by variations. Now
plainly if some such individuals, with a peculiar variation, should migrate
into a new territory or become isolated from their relatives which do not
have similar variations, these individuals will be obliged to breed with each
other. The result will be that the next generation, arising thus from two
parents each of which shows the same variation, will show it also in equal
or increased degree. Migrations and isolations will thus tend to fix in the
machine variations which sexual union or other influences inaugurate. Now
in the history of the earth's surface there have been many changes which
tend to bring about such migration and isolations, and this factor has
doubtless played a more or less important part in the building of the
machines. How great a part we cannot say, nor is it necessary for our
purpose to decide; for in all these cases the machine building has only been
the result of the hereditary transmission of congenital variation under
certain peculiar conditions. The fundamental process is the same as already
considered, only the details of its working being in question.
Direct Influence of the Environment.—Under this head we have a subject
of great importance. It is an undoubted fact that the environment has a very
decided effect upon the machine. These direct effects of the environment
are very positive and in great variety. The tropical sun darkens the human
skin; cold climate stunts the growth of plants; lack of food dwarfs all
animals and plants, and hundreds of other similar examples could be

selected. Another class of similar influences are those produced by use and
disuse. Beyond question the use of an organ tends to increase its size, and
disuse to decrease it. Combats of animals with each other tend to increase
their strength, flight from enemies their running powers, etc.
Now all these effects are direct modifications of the machine, and if they
are only transmitted to following generations so as to become permanent
modifications, they will be most important agencies in the machine
building. If, on the other hand, they are not transmitted by heredity, they can
have no permanent effect. We have here thus again the problem of the
inheritance of acquired characters. We have already noticed the uncertainty
surrounding this subject, but the almost universal belief in the inheritance of
such characters requires us to refer to it again. It is uncertain whether such
direct effects have any influence upon the offspring, and therefore whether
they have anything to do with this machine building. Still, there are many
facts which point strongly in this direction. For example, as we study the
history of the horse family we find that an originally five-toed animal began
to walk more and more on its middle toe, in such a way that this toe
received more and more use, while the outer toes were used less and less.
Now that such a habit would produce an effect upon the toes in any
generation is evident; but apparently this influence extended from
generation to generation, for, as the history of the animals is followed, it is
found that the outer toes became smaller and smaller with the lapse of ages,
while the middle one became correspondingly larger, until there was finally
produced the horse with its one toe only on each foot. Now here is a line of
descent or machine building in the direct line of the effects of use and
disuse, and it seems very natural to suppose that the modification has been
produced by the direct effect of the use of the organs. There are many other
similar instances where the line of machine building has been quite parallel
to the effects of use and disuse. If, therefore, acquired characters can be
inherited to any extent, we have, in the direct influences of the environment
an important agency in machine building. This direct effect of the
conditions is apparently so manifest that one school of biologists finds in it
the chief cause of the variations which occur, telling us that the conditions
surrounding the organism produce changes in it, and that these variations,
being handed down to subsequent generations, constitute the basis of the
development of the machine. If this factor is entirely excluded, we are

driven back upon the natural selection of congenital variations as the only
kind of variations which can permanently effect the modification of the
machine.
Consciousness.—It may be well here to refer to one other factor in the
problem, because it has somewhat recently been brought into prominence.
This factor is consciousness on the part of the animal. Among plants and the
lower animals this factor can have no significance, but consciousness
certainly occurs among the higher animals. Just when or how it appeared
are questions which are not answered, and perhaps never will be. But
consciousness, after it had once made its appearance, became a controlling
factor in the development of the machine. It must not be understood by this
that animals have had any consciousness of the development of their body,
or that they have made any conscious endeavours to modify its
development. This has not always been understood. It has been frequently
supposed that the claim that consciousness has an influence upon the
development of an animal means that the animal has made conscious efforts
to develop in certain directions. For example, it has been suggested that the
tiger, conscious of the advantage of being striped, had a desire to possess
stripes, and the desire caused their appearance. This is absurd.
Consciousness has been a factor in the development of the machine, but an
indirect one. Consciousness leads to effort, and effort has a direct influence
in development. For example, an animal is conscious of hunger, and this
leads to efforts on his part to obtain food. His efforts to obtain food may
lead to migration or to the adoption of new kinds of food or to conflicts
with various kinds of rivals, and all of these efforts are potent factors in
determining the direction of development. Consciousness, again, may lead
certain animals to take pleasure in each other's society, or to recognize that
in mutual association they have protection against common enemies. Such a
consciousness will give rise to social habits, and social habits are a very
potent factor in determining the direction in which the inherited variations
will tend; not, perhaps, because it effects the variations themselves, but
rather because it determines which variations among the many shall be
preserved and which rejected by natural selection. Consciousness may lead
the antelope to recognize that he has no chance in a combat with a lion, and
this will induce him to flee. The habit of flight would then develop the
power of flight, not because the antelope desired such power, but because

the animals with variations which gave increased power of flight would be
the ones to escape the lion, while the slower ones would die without
offspring. Thus consciousness would indirectly, though not directly, result
in the lengthening of the legs of the animal and in the strengthening of his
running muscles. Beyond a doubt this factor of consciousness has been a
factor of no little moment in the development of the higher types of organic
machines. We can as yet only dimly understand its action, but it must
hereafter be counted as one of the influences in the evolution of the living
machine.
But, after all, these are only questions of the method of the action of certain
well demonstrated, fundamental factors. Whether by natural selection, or by
the inheritance of acquired characters produced by the environment, or
whether by the effect of isolation of groups of individuals, the machine
building has always been produced in the same way. A machine, either
through the direct influence of the environment, or as a result of sexual
combination of germ plasm, shows a variation from its parents. This
variation proves of value to its possessor, who lives and transmits it
permanently to posterity. Thus step by step, one part is added to another,
until the machine has grown into the intricately adapted structure which we
call the animal or plant. This has been nature's method of building
machines, all based upon the three properties possessed by the living cell—
reproduction, variation, and heredity.
Summary of Nature's Power of Building Machines.—Let us now notice
the position we have reached. Our problem in the present chapter has been
to find out whether nature possesses forces adequate to explain the building
of machines with their parts accurately adapted to each other so as to act
harmoniously for certain ends. Astronomy has shown that she has forces for
the building of worlds; geology, that she has forces for making mountain
and valley; and chemistry, that she has forces for building chemical
compounds. But the organism is neither a world, nor a mass of matter, nor a
chemical compound. It is a machine. Has nature any forces for machine
building? We have found that by the use of the three factors, reproduction,
variation, and heredity, nature is able to produce a machine of ever greater
and greater complexity, with the parts all adapted to each other. Now the
difference between a machine and a mass of matter is simply in the
adaptation of parts to act harmoniously for definite ends. Hence if we are

allowed these three factors, we can say that nature does possess forces
adequate to the manufacture of machines. These forces are not chemical
forces, and the construction of the machine has thus been brought about by
forces entirely different from those which produced the chemical molecule.
But we have plainly not reached the bottom of the matter in our attempt to
explain the machinery of living things. We have based the whole process
upon three factors. Reproduction, variation, and heredity are the properties
of all living matter; but they are not, like gravity and chemism, universal
forces of nature. They occur in living organisms only. Why should they
occur in living organisms, and here alone? These three properties are
perhaps the most marvellous properties of nature; and surely we have not
finished our task if we have based the whole process of machine building
upon these mysterious phenomena, leaving them unintelligible. We must
therefore now ask whether we can proceed any farther and find any
explanation of these fundamental powers of the living machine.
It must be confessed that here we are at present forced to stop. We can
proceed no further with any certainty, or even probability. We may say that
variation and heredity are only phases of reproduction, and reproduction is
a property of the living cell. We may say that this power of reproduction is
dependent upon the power of assimilation and growth, for cell division is a
result of cell growth. We may further say that growth and assimilation are
chemical processes resulting from the oxidation of food, and that thus all of
these processes are to be reduced to chemical forces. In this way we may
seem to have a chemical foundation for life phenomena. But clearly this is
far from satisfactory. In the first place, it utterly fails to explain why the
living cell has these properties, while no other body possesses them, nor
why they are possessed by living protoplasms alone, ceasing instantly with
death. Indeed it does not tell us what death can be. Secondly, it utterly fails
to explain the marvels of cell division with resulting hereditary
transmission. For all this we must fall back upon the structure of
protoplasm, and say that the cell machinery is so adjusted that the machine,
when acting as a whole, is capable of transforming the energy of chemical
composition in certain directions. These fundamental properties are then the
properties of the cell machine just as surely as printing is the property of the
printing press. We can no more account for the life phenomena by chemical
powers than we can for printing by chemical forces manifested in the

burning of the coal in the engine room. To be sure, it is the chemical forces
in the engine room that furnishes the energy, but it is the machinery of the
press that explains the printing. So, while chemical forces supply life
energy, it is the cell machinery that must explain the fundamental living
factors. So long as this machine is intact it can continue to run and perform
its duties. But it is a very delicate machine and is easily broken. When it is
broken its activities cease. A broken machine can not run. It is dead. In
short, we come back once more to the idea of the machinery of protoplasm,
and must base our understanding of its properties upon its structure.
It is proper to state that there are still some biologists who insist that the
ultimate explanation of protoplasm is purely chemical and that life
phenomena may be manifested in mixtures of compounds that are purely
physical mixtures and not machines. It is claimed that much of this cell
structure described above is due to imperfection in microscopic methods
and does not really exist in living protoplasm, while the marvellous
activities described are found only in the highly organized cell, but do not
belong to simple protoplasm. It is claimed that simple protoplasm consists
of a physical mixture of two different compounds which form a foam when
thus mixed, and that much of the described structure of protoplasm is only
the appearance of this foam. This conception is certainly not the prevalent
one to-day; and even if it should be the proper one, it would still leave the
cell as an extremely complicated machine. Under any view the cell is a
mechanism and must be resolved into subordinate parts. It may be uncertain
whether these subordinate parts are to be regarded simply as chemical
compounds physically mixed, or as smaller units each of which is a smaller
mechanism. At all events, at the present time we know of no such simple
protoplasm capable of living activities apart from machinery, and the
problem of explaining life, even in the simplest form known, remains the
problem of explaining a mechanism.
The Origin of the Cell Machine.—We have thus set before us another
problem, which is after all the fundamental one, namely, to ask whether we
can tell anything of nature's method of building the protoplasmic machine.
The building of the higher animal and plant, as we have seen, is the result of
the powers of protoplasm; but protoplasm itself is a machine. What has
been its history?

We must first notice that no notion of chemical evolution helps us out. It has
been a favourite thought with some that the origin of the first living thing
was the result of chemical evolution. As the result of physical forces there
was produced, from the original nebulous mass, a more and more
complicated system until the world was formed. Then chemical phenomena
became more and more complicated until, with the production of more and
more complicated compounds, protoplasm was finally produced. A few
years ago, under the impulse of the idea that protoplasm was a compound,
or at least a simple mixture of compounds, this thought of protoplasm as the
result of chemical evolution was quite significant. Physical forces, chemical
forces, and vital forces, explain successively the origin of worlds,
protoplasm, and organisms. This conception has, however, no longer much
significance. We know of no such living chemical compound apart from
cell machinery. A new conception of protoplasm has arisen which demands
a different explanation of its origin. Since it is a machine rather than a
compound, mechanical rather than chemical forces are required for its
explanation.
Have we then any suggestion as to the method of the origin of this
protoplasmic machine? Our answer must, at the present, be certainly in the
negative. The complexity of the cell tells us plainly that it can not be the
ultimate living substance which may have arisen from chemical evolution.
It is made up of parts delicately adapted to act in harmony with each other,
and its activity depends upon the relation of these parts. Whatever chemical
forces may have accomplished, they never could have combined different
bodies into linin, centrosomes, chromosomes, etc., which, as we have seen,
are the basis of cell life. To account for this machine, therefore, we are
driven to assume either that it was produced by some unknown intelligent
power in its present condition of complex adjustment, or to assume that it
has had a long history of building by successive steps, just as we have seen
to be the case with the higher organisms. The latter assumption is, of
course, in harmony with the general trend of thought. To-day protoplasm is
produced only from other protoplasm; but, plainly, the first protoplasm on
the earth must have had a different origin. We must therefore next look for
facts which will enable us to understand its origin. We have seen that the
animal and plant machines have been built up from the simple cell as the
result of its powers acting under the ordinary conditions of nature. Now, in

accordance with this general line of thought, we shall be compelled to
assume that previous to the period of building machinery which we have
been considering, there was another period of machine building during
which this cell machine was built by certain natural forces.
But here we are forced to stop, for nothing which we yet know gives even a
hint as to the method by which this machine was produced. We have,
however, seen that there are forces in nature efficient in building machines,
as well as those for producing chemical compounds; and this, doubtless,
suggests to us that there may be similar forces at work in building
protoplasm. If we can find natural forces by which the simplest bit of living
matter can be built up into a complicated machine like the ox, with its many
delicately adjusted parts, it is certainly natural to imagine that the same
forces may have built this simpler machine with which we started. But such
a conclusion is for a simple reason impossible. We have seen that the
essential factor in this machine building is reproduction, with the correlated
powers of variation and heredity. Without these forces we could not have
advanced in this machine building at all. But these properties are
themselves the result of the machinery of protoplasm. We have no reason
for thinking that this property of reproduction can occur in any other object
in nature except this protoplasmic machine. Of course, then, if reproduction
is the result of the structure of protoplasm we can not use this factor in
explaining the origin of this protoplasm. The powers of the completed
machine can not be brought forward to account for its origin. Thus the one
fundamental factor for machine building is lacking, and if we are to explain
nature's method of producing protoplasm from simpler structures, we must
either suppose that the parts of the cell are capable of reproduction and
subject to heredity, or we must look for some other method. Such a road has
however not yet been found, nor have we any idea in what direction to look.
But the fact that nature has methods of machine building, as we have seen,
may hold out the possibility that some day we may discover her method of
building this primitive living machine, the cell.
It is useless to try to go further at present. The origin of living matter is
shrouded in as great obscurity as ever. We must admit that the disclosures of
the modern microscope have complicated rather than simplified this
problem. While a few years ago chemists and biologists were eagerly
expecting to discover a method of manufacturing a bit of living matter by

artificial means, that hope has now been practically abandoned. The task is
apparently hopeless. We can manipulate chemical forces and produce an
endless series of chemical compounds. But we can not manipulate the
minute bits of matter which make up the living machine. Since living matter
is made of the adjustment of these microscopic parts of matter, we can not
hope to make a bit of living matter until we find some way of making these
little parts and adjusting them together. Most students of protoplasm have
therefore abandoned all expectation of making even the simplest living
thing. We are apparently as far from the real goal of a natural explanation of
life as we were before the discovery of protoplasm.
General Summary.—It is now desirable to close this discussion of
seemingly somewhat unconnected topics by bringing them together in a
brief summary. This will enable us to see more clearly the position in which
science stands to-day upon this matter of the natural explanation of living
phenomena, and to picture to ourselves more concisely our knowledge of
the living machine.
The problem we have set before us is to find out to what extent it is possible
to account for vital phenomena by the application of ordinary natural laws
and forces, and therefore to find out whether it is necessary to assume that
there are forces needed to explain life which are different from those found
in other realms of nature, or whether vital forces are all correlated with
physical forces. It has been evident at a glance that the living body is a
machine. Like other machines it consists of parts adjusted to each other for
the accomplishment of definite ends, and its action depends upon the
adjustment of its parts. Like other machines, it neither creates nor destroys
energy, but simply converts the potential energy of its foods into some form
of active energy, and, like other machines, its power ceases when the
machine is broken.
With this understanding the problem clearly resolved itself into two
separate ones. The first was to determine to what extent known physical and
chemical laws and forces are adequate to an explanation of the various
phenomena of life. The second was to determine whether there are any
known forces which can furnish a natural explanation of the origin of the
living machine. Manifestly, if the first of these problems is insolvable, the
second is insolvable also.

In the study of the first problem we have reached the general conclusion
that the secondary phenomena of life are readily explained by the
application of physical and chemical forces acting in the living machine.
These secondary phenomena include such processes as the digestion and
absorption of food, circulation, respiration, excretion, bodily motion, etc.
Nervous phenomena also doubtless come under this head, at least so far as
concerns nervous force. We have been obliged, however, to exclude from
this correlation the mental phenomena. Mental phenomena can not as yet be
measured, and have not yet been shown to be correlated with physical
energy. In other words, it has not yet been proved that mental force is
energy at all; and if it is not energy, then of course it can not be included in
the laws which govern the physical energy of the universe. Although a close
relation exists between physical changes in the brain cells and mental
phenomena, no further connection has yet been drawn between mental
power and physical force. All other secondary phenomena, however, are
intelligently explained by the action of natural forces in the machinery of
the living organism.
While we have thus found that the secondary phenomena of life are
intelligible as the result of the structure of the machine, certain other
fundamental phenomena have been constantly forcing themselves upon our
attention as a foundation of these secondary activities. The power of
contraction, the power of causing certain kinds of chemical change to occur
which result in metabolism, the property of sensibility, the property of
reproduction—these are fundamental to all living activity, and are, after all,
the real phenomena which we wish to explain. But these are not peculiar to
the complicated machines. We can discard all the apparent machinery of the
animal or plant and find these properties still developed in the simplest bit
of living matter. To learn their significance, therefore, we have turned to the
study of the simplest form of matter in which these fundamental properties
are manifested. This led us at once to the study of the so-called protoplasm,
for protoplasm is the simplest known form of matter that is alive.
Protoplasm itself at first seemed to be a homogeneous body, and was looked
upon as a chemical compound of high complexity. If this were true its
properties would depend upon its composition and would be explained by
the action of chemical forces. Such a conception would have quickly solved
the problem, for it would reduce living properties to chemical powers. But

the conception proved to be delusive. Protoplasm, at least the simplest form
known to possess the fundamental life properties, soon showed itself to be
no chemical compound, but a machine of wonderful intricacy.
The fundamental phenomena of life and of protoplasm have proved to be
both chemical and mechanical. Metabolism is the result of the oxidation of
food, and motion is an instance of transference of force. Our problem then
resolved itself into finding the power that guides the action of these natural
forces. Food will not undergo such an oxidation except in the presence of
protoplasm, nor will the phenomena of metabolism occur except in the
presence of living protoplasm. Clearly, then, the living protoplasm contains
within itself the power of guiding this play of chemical force in such a way
as to give rise to vital phenomena, and our search must be not for chemical
force but for this guiding principle. Our study of protoplasm has told us
clearly enough that we must find this guiding principle in the interaction of
the machinery within the protoplasm. The microscope has told us plainly
that these fundamental principles are based upon machinery. The cell
division (reproduction) is apparently controlled by the centrosomes; the
heredity by the chromosomes; the constructive metabolism by the nucleus
in general, while the destructive metabolism is also seated in the cell
substance outside the nucleus. Whether these statements are strictly
accurate in detail does not particularly affect the general conclusion. It is
clearly enough demonstrated that the activities of the protoplasmic body are
dependent upon the relation of its different parts. Although we have got rid
of the complicated machinery of the organism in general, we are still
confronted with the machinery of the cell.
But our analysis can not, at present, go further. Our knowledge of this
machine has not as yet enabled us to gain any insight as to its method of
action. We can not yet conceive how this machine controls the chemical and
physical forces at its disposal in such a way as to produce the orderly result
of life. The strict correlation between the forces of the physical universe and
those manifested by this protoplasm tells us that a transformation of energy
occurs within it, but of the method of that transformation we as yet know
nothing. Irritability, movement, metabolism, and reproduction appear to be
not chemical properties of a compound, but mechanical properties of a
machine. Our mechanical analysis of the living machine stops short before
it reaches any foundation in the chemical forces of nature.

It is thus clearly apparent that the phenomena of life are dependent upon the
machinery of living things, and we have therefore the second question of
the origin of this machinery to answer. Chemical forces and mechanical
forces have been laboriously investigated, but neither appear adequate to
the manufacture of machines. They produce only chemical compounds and
worlds with their mountains and seas. The construction of artificial
machines has demanded intelligence. But here is a natural machine—the
organism. It is the only machine produced by natural methods, so far as we
know; and we have therefore next asked whether there are, in nature, simple
forces competent to build machines such as living animals and plants?
In pursuance of this question we have found that the complicated machines
have been built out of the simpler ones by the action of known forces and
laws. The factors in this machine building are simply those of the
fundamental vital properties of the simplest protoplasmic machine.
Reproduction, heredity, and variation, acting under the ever-changing
conditions of the earth's surface, are apparently all that are needed to
explain the building of the complex machines out of the simpler ones.
Nature has forces adequate to the building of machines as well as forces
adequate to the formation of chemical compounds and worlds.
But here again we are unable to base our explanation upon chemical and
physical forces. Reproduction, heredity, and variation are properties of the
cell machine, and we are therefore thrown back upon the necessity of
explaining the origin of this machine. Can we find a mechanical or
chemical explanation of the origin of protoplasm? A chemical explanation
of the cell is impossible, since it is not a chemical compound, but a piece of
mechanism. The explanation given for the origin of animals and plants is
also here apparently impossible. The factors upon which that explanation
depended are factors of this completed machine itself, and can not be used
to explain its origin. We are left at present therefore without any foundation
for further advance. The cells must have had a history of construction, but
we do not as yet conceive any forces which may be looked upon as
contributing to that history. Whether life phenomena can be manifested by
any mixture of compounds simpler than the cell we do not yet know.
The great problems still remaining for solution, which have hardly been
touched by modern biology in all its endeavours to find a mechanical

explanation of the living machine, are, therefore, three. First, the relation of
mentality to the general phenomena of the correlation of force; second, the
intelligible understanding of the mechanism of protoplasm which enables it
to guide the blind chemical and physical forces of nature so as to produce
definite results; third, the kind of forces which may have contributed to the
origin of that simplest living machine upon whose activities all vital
phenomena rest—the living cell.
INDEX.
A.

Absorption of food, 20

Acquired characters, inheritance of, 164, 165, 166, 167, 171.
---- variations, 159, 160.

Amœba 73.

Anatomical evidence for evolution, 142.

Aquacity, 80.

Arm compared with wing, 144.

Aristotle, .

Assimilation, 80, 124, 149, 176.

Asters of dividing cells, 98.

B.

Barry, 63, 64.

Bathybias, 84.

Biology a new science, 1, 5, 15.

Blood, 35, 36, 38, 69, 73.

Blood-vessels, 35, 36.

Body as a machine, 22, 25, 49.

Bone cells, 69.

Building of the living machine, 131, 134, 136, 137, 167, 175, 180.

C.

Cartilage cells, 68.
Cell as a machine, 126, 128.
---- description of, 69.
---- division, 95, 96, 101.
---- discovery of, 58.
---- doctrine, 60.
---- substance, 65, 125.

Cells, 56, 84, 86, 118, 119.

Cellular structure of organisms, 65.

Cell wall, 64, 72.

Centrosome, 94, 96, 97, 101, 103, 105, 110.

Challenger expedition, 83.

Chemical evolution, 179.

Chemical theory of vitality, 14.
--of life, 78, 116.

Chemism or mechanism, 57, 176.

Chemistry of digestion, 27, 28;
---- of protoplasm, 76;
---- of respiration, 38.

Chromatin, 92, 94, 96, 102, 149, 153.

Chromosomes, 97, 98, 101, 105, 108, 110, 113, 152.

Circulation, 34.

Colonies of cells, 85.

Comparison of the body and a machine, 22.

Congenital variations, 158, 160, 163;
inheritance of, 164.

Connective-tissue cells, 70.

Conservation of energy, 7, 17.

Consciousness as a factor in machine building, 173.

Constructive chemical processes, 50, 51, 52, 124.

Continuity of germ plasm, 155.

Correlation of vital and physical forces, 13, 16, 22, 23, 24, 25.

Cytoblastema, 62.

Cytology, 10.

D.

Darwin, 81.

Death of the cell, 127.

Decline of the reign of protoplasm, 85.

Destructive chemical processes, 50, 51, 52, 125.

Dialysis, 29, 30, 31.

Digestion, 27.

E.

Egg, 103, 120, 152.
--division of, 63.

Egg, fertilization of, 102.

Embryological evidence for evolution, 140.

Energy of nervous impulse, 43, 54.

Environment, 171.

Evidence for evolution as a method of machine building, 139, 145.

Evolution, 9, 16, 81, 134.

Experiments with developing eggs, 121.

F.

Fat, absorption of, 32.

Female pronucleus, 110.

Fern cells, section of, 67.

Fertilization of the egg, 95, 102;
---- significance of, 112.

Fibres in protoplasm, 87;
---- in spindle, 98, 101.

Forces at work in machine building, 148, 176, 181.

Formed material, 64.

Free cell formation, 64.

G.

Geological evidence for evolution, 139.

Germ plasm, 154.

H.

Heart as a pump, 35.

Heat, 24, 44, 45.

Heredity, 148, 150, 176;
---- explanation of, 152.

Hereditary traits, 113, 153.

Historical geology, 6.

History of the living machine, 133, 147.

Horses' toes, loss of, 172.

Huxley, 11, 75, 83, 84.

I.

Irritability, 54.

Isolation, theory of, 170.

K.

Karyokinesis, 96, 101.

Kidneys, 41.

L.

Leaf, section of, 66.

Life the result of a mechanism, 115, 177.

Linin, 92, 103.

Linnæus, 1.

Lyell, 6.

Lymph, 36, 37.

M.

Machine defined, 20.

Machines the result of mechanical forces, 116.

Male cell, 104, 107.

---- pronucleus, 109.

Maturation of the egg, 104.

Mechanical nature of living organisms, 12.

Mechanical theory of life, 81, 144.

Membrane of the nucleus, 92, 101.

Mental phenomena, 47, 48.

Metabolism, 54.

Microsomes 87.

Migration, theory of, 170.

Monera, 88.

Movement, 54.

Muscle, 36, 71.

N.

Natural selection, 167.

Nerve-fibre cell, 70.

Nervous energy, 42, 44.

---- system, 41.

New biological problems, 15.

Nucleolus, 65, 92, 94.

Nucleus, 65, 84, 87, 93, 101, 103, 113, 124, 149;
----formation of new, 101.

---- function of, 89, 90, 95.

---- presence of, 87, 88, 89.

---- structure of, 91.

O.

Organic chemistry, 78.

Organic compounds, artificial manufacture of, 78, 82.

Origin of cell machine, 178, 179, 180.

Origin of life, 81, 182.

Osmosis, 29.

Oxidation, 80, 176.

---- as a vital process, 39, 56.

P.

Philosophical biology, 4.

Physical basis of life, 75.

Polar cells, 107.

Potato, section of cells, 67.

Properties of chemical compounds, 79.

Protoplasm, 14, 74, 82, 83, 84, 114, 115, 179.

---- artificial manufacture of, 82.

---- as a machine, 86, 178.

---- discovery of, 74.

---- nature of, 76.

---- structure of, 86, 87.

Purpose vs. cause, 11, 12.

R.

Reaction against the cell doctrine, 117.

Reign of law, 4.

---- of the nucleus, 91.

---- of protoplasm, 81, 85.

Relationship, significance of, 143.

Removal of waste, 39, 40.

Reproduction, 54, 80, 124, 148, 176;
---- rapidity of, 149.

Respiration, 37.

Reticulum of cell, 87;
---- of nucleus, 92.

Root tip, section of, 66.

S.

Schultze, 74, 75.

Schwann, 61, 62, 72.

Secretion, 39, 40.

Segmentation nucleus, 110.

Sensations, 46.

Separation of chromosomes, 100.

Sexual reproduction, 102.

Spermatozoan, 107, 109, 154.

Splitting of chromosomes, 99.

Spindle fibres, 101.

Struggle for existence, 168.

Summary of Part I, 128.

---- general, 182.

U.

Undifferentiated protoplasm, 83.

Unicellular animals, 71.

Units of vital activity, 53.

Use and disuse, 171, 172.

V.

Variation, 148, 157, 160, 176.

Variation from sexual union, 162.

Variation in germ plasm, 161.

Vegetative functions, 41.

Villi, 31.

Vital force, vitality, 13, 15, 34, 37, 52, 80, 85.

Vital properties, 54;
---- located in cells, 123.

W.

Wing compared with arm, 144.

Wood cells, 68.
THE END.

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D. APPLETON AND COMPANY'S PUBLICATIONS.
Pioneers of Evolution, from Thales to Huxley By Edward Clodd, President
of the Folk-Lore Society; Author of "The Story of Creation," "The Story of
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of fresh achievements, and, in due course, fresh theories. His book furnishes
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"We are always glad to meet Mr. Clodd. He is never dull; he is always well
informed, and he says what he has to say with clearness and precision....
The interest intensifies as Mr. Clodd attempts to show the part really played
in the growth of the doctrine of evolution by men like Wallace, Darwin,
Huxley, and Spencer.... We commend the book to those who want to know
what evolution really means."—London Times.
"This is a book which was needed.... Altogether, the book could hardly be
better done. It is luminous, lucid, orderly, and temperate. Above all, it is
entirely free from personal partisanship. Each chief actor is sympathetically
treated, and friendship is seldom or never allowed to overweight sound
judgment"—London Academy.
"We can assure the reader that he will find in this work a very useful guide
to the lives and labors of leading evolutionists of the past and present.
Especially serviceable is the account of Mr. Herbert Spencer and his share
in rediscovering evolution, and illustrating its relations to the whole field of
human knowledge. His forcible style and wealth of metaphor make all that
Mr. Clodd writes arrestive and interesting."—London Literary World.
"Can not but prove welcome to fair-minded men.... To read it is to have an
object-lesson in the meaning of evolution.... There is no better book on the
subject for the general reader.... No one could go through the book without
being both refreshed and newly instructed by its masterly survey of the
growth of the most powerful idea of modern times."—The Scotsman.
D. APPLETON AND COMPANY, NEW YORK.
BOOKS ON SOCIAL SCIENCE.
Socialism New and Old.
By Prof. William Graham, 12mo. Cloth, $1.75.
"Professor Graham's book may be confidently recommended to all who are
interested in the study of socialism, and not so intoxicated with its promises
of a new heaven and a new earth as to be impatient of temperate and
reasoned criticism."—London Times.

"Professor Graham presents an outline of the successive schemes of three
writers who have chiefly influenced the development of socialism, and
dwells at length upon the system of Rousseau, that of St. Simon, and on that
of Karl Marx, the founder of the new socialism, 'which has gained favor
with the working classes in all civilized countries,' which agrees with
Rousseau's plan in being democratic, and with St. Simon's in aiming at
collective ownership.... The professor is an independent thinker, whose
endeavor to be clear has resulted in the statement of definite conclusions.
The book is a remarkably fair digest of the subject under consideration."—
Philadelphia Ledger.
Dynamic Sociology:
Or, Applied Social Science, as based upon Statical Sociology and the less
Complex Sciences. By Lester F. Ward, A.M. In 2 vols. 12mo. Cloth, $4.00.
"A book that will amply repay perusal.... Recognizing the danger in which
sociology is, of falling into the class of dead sciences or polite amusements,
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can be breathed into its nostrils.'"—Rochester Post-Express.
"Mr. Ward has evidently put great labor and thought into his two volumes,
and has produced a work of interest and importance. He does not limit his
effort to a contribution to the science of sociology.... He believes that
sociology has already reached the point at which it can be and ought to be
applied, treated as an art, and he urges that 'the State' or Government now
has a new, legitimate, and peculiar field for the exercise of intelligence to
promote the welfare of men."—New York Times.
Criminal Sociology.
By Prof. E. Ferri. A new volume in the Criminology Series, edited by W.
Douglas Morrison, 12mo. Cloth, $1.50.
In this volume Professor Ferri, a distinguished member of the Italian
Parliament, deals with the conditions which produce the criminal
population, and with the methods by which this anti-social section of the
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physical and mental defects; the social causes consist of social

disadvantages of every description. His view is that the true remedy against
crime is to remove individual defects and social disadvantages where it is
possible to remove them. He shows that punishment has comparatively little
effect in this direction, and is apt to divert attention from the true remedy—
the individual and social amelioration of the population as a whole.
D. APPLETON AND COMPANY, NEW YORK.

BOOKS FOR NATURE LOVERS.
Insect Life. (New Edition in Colors.)
By John Henry Comstock, Professor of Entomology in Cornell University.
With 12 full-page plates reproducing butterflies and various insects in their
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Comstock, Member of the Society of American Wood Engravers, 12mo.
Cloth, $1.75 net; postage, 20 cents additional.
"The volume is admirably written, and the simple and lucid style
is a constant delight.... It is sure to serve an excellent purpose in
the direction of popular culture, and the love of natural science
which it will develop in youthful minds can hardly fail to bear
rich fruit."—Boston Beacon.
Familiar Fish: Their Habits and Capture.
A Practical Book on Fresh-Water Game Fish. By Eugene McCarthy. With
an Introduction by Dr. David Starr Jordan, President of Leland Stanford
Junior University, and numerous Illustrations, 12mo. Cloth, $1.50.
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that we know. The author treats his subject with scientific
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reading.... The book should be the companion of all who go a-
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The Art of Taxidermy.
By John Rowley, Chief of the Department of Taxidermy in the American
Museum of Natural History. Illustrated, 12mo. Cloth, $2.00.
"Mr. Rowley will long be gratefully remembered by
taxidermists, amateurs, and others, for the care he has used in
thus meeting a long-felt want."—Bangor, Me., Sportsman.

"The book is not an elaborate treatise upon the abstract
principles which lie at the foundation of artistic taxidermy, but is
rather a compendium full of practical hints and suggestions,
recipes, and formulas for the working taxidermist."—The Dial.
Plants. (Plant Relations and Plant Structures in one volume.)
By John M. Coulter, A.M., Ph.D., Head of Department of Botany,
University of Chicago, 12mo. Cloth, $1.80 net. (One of the Twentieth
Century Text-Books.)
D. APPLETON AND COMPANY, NEW YORK.
EVOLUTION OF MAN AND CHRISTIANITY.
New edition. By the Rev. Howard MacQueary. With a new Preface, in
which the Author answers his Critics, and with some important Additions,
12mo. Cloth, $1.75.
"This is a revised and enlarged edition of a book published last
year. The author reviews criticisms upon the first edition, denies
that he rejects the doctrine of the incarnation, admits his doubts
of the physical resurrection of Christ, and his belief in evolution.
The volume is to be marked as one of the most profound
expressions of the modern movement toward broader
theological positions."—Brooklyn Times.
HISTORY OF THE CONFLICT BETWEEN RELIGION AND SCIENCE. By
Dr. John William Draper. 12mo. Cloth, $1.75.
"The keynote to this volume is found in the antagonism between
the progressive tendencies of the human mind and the
pretensions of ecclesiastical authority, as developed in the
history of modern science. No previous writer has treated the
subject from this point of view, and the present monograph will
be found to possess no less originality of conception than vigor
of reasoning and wealth of erudition."—New York Tribune.
A CRITICAL HISTORY OF FREE THOUGHT IN REFERENCE TO THE
CHRISTIAN RELIGION. By Rev. Canon Adam Storey Farrar, D.D.,

F.R.S., etc. 12mo. Cloth, $1.50.
"A conflict might naturally be anticipated between the reasoning
faculties of man and a religion which claims the right, on
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their exercise. It is the chief of the movements of free thought
which it is my purpose to describe, in their historic succession
and their connection with intellectual causes. We must ascertain
the facts, discover the causes, and read the moral."—The Author.
CREATION OR EVOLUTION? A Philosophical Inquiry. By George Ticnor
Curtis. 12mo. Cloth, $2.00.
"A treatise on the great question of Creation or Evolution by one
who is neither a naturalist nor theologian, and who does not
profess to bring to the discussion a special equipment in either
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reader, before many pages have been turned, that he has a
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is one to be treated with respect. His part is to apply to the
reasonings of the men of science the rigid scrutiny with which
the lawyer is accustomed to test the value and pertinency of
testimony, and the legitimacy of inferences from established
facts."—New York Tribune.
D. APPLETON AND COMPANY, NEW YORK.

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