Title: 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
Author: Conn, H. W. (Herbert William)
Language: English
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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.


                                                               PAGE

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                                                         PAGE

_Amoeba Polypodia_ in six successive stages of division      _Frontispiece_

1. Figure illustrating osmosis                                  30

2. Figure illustrating osmosis                                  31

3. Diagram of the intestinal walls                              32

4. Diagram of a single villus                                   33

5. Enlarged figure of four cells in the villus membrane         33

6. A bit of muscle showing blood-vessels                        36

7. A bit of bark showing cellular structure                     61

8. Successive stages in the division of the developing
   egg                                                          63

9. A typical cell                                               65

10. Cells at a root tip                                         66

11. Section of a leaf showing cells of different shapes         66

12. Plant cells with thick walls, from a fern                   67

13. Section of potato                                           67

14. Various shaped wood cells from plant tissue                 68

15. A bit of cartilage                                          68

16. Frogs' blood                                                69

17. A bit of bone                                               69

18. Connective tissue                                           70

19. A piece of nerve fibre                                      70

20. A muscle fibre                                              71

21. A complex cell, vorticella                                  71

22. An amoeba                                                   73

23. A cell as it appears to the modern microscope               86

24. A cell cut into pieces, each containing a bit of
    nucleus                                                     89

25. A cell cut in pieces, only one of which contains any
    nucleus                                                     90

26. Different forms of nucleii                                  93

27 and 28. Two stages in cell division                          96

29 and 30. Stages in cell division                              98

31 and 32. Latest stages in cell division                      100

33. An egg                                                     103

34 and 35. Stages in the process of fertilization of the
           egg                                                 104

36 and 37. Stages in the process of fertilization of the
           egg                                                 105

38, 39, and 40. Stages in fertilization of the egg             106

41 and 42. Latest stages in the fertilization of the egg       109

43 and 44. Two stages in the division of the egg               111

45. A group of cells resulting from division, the first step
    in machine building                                       135

46. A later step in machine building, the gastrula            135

47. The arm of a monkey                                       144

48. The arm of a bird                                         144

49. The arm of an ancient half-bird, half-reptile animal      144

50. Diagram to illustrate the principle of heredity           156



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
it begins to pass through its walls into the blood. In this absorption
we find engaged another set of forces, the chief of which appears to be
the physical force of _osmosis_. The force of osmosis has no 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.

[Illustration: FIG. 1.--To illustrate osmosis. In the vessel _A_ is a
solution of sugar; in _B_, is pure water. The two are separated by the
membrane _C_. The sugar passes through the membrane into _B_.]

[Illustration: FIG. 2.--In the bladder _A_ is a sugar solution. In the
vessel _B_ is pure water. Sugar passes out and water into the bladder
until it rises in the tube to a.]

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
liquids in the stomach and intestine, and is in proper condition for
dialysis. Furthermore, the structure of the intestine is such as to
produce conditions adapted for dialysis. This can be understood from
Fig. 3, which represents diagrammatically a cross section through the
intestinal wall. Within the intestinal wall, at _A_, is the food mass in
solution. At _B_ are shown little projections of the intestinal wall,
called _villi_ extending 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.

[Illustration: FIG. 3--Diagram of the intestinal walls. _A_, lumen of
intestine filled with digested food. _B_, villi, containing blood
vessels. _C_, larger blood vessel, which carries blood with absorbed
food away from the intestine.]


Secondly, we notice that there is one phase of absorption which is still
obscure. Part of the food is composed of fat, and this fat, as the
result of digestion, is mechanically broken up into extremely minute
droplets. Although these droplets are of microscopic size they are not
actually in solution, and 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.

[Illustration: FIG. 4.--Diagram of a single villus enlarged. _B_
represents the membranous surface covering the villus; _C_, the
blood-vessels within the villus.]

[Illustration: FIG. 5.--An enlarged figure of four cells of the membrane
_B_ in Fig. 4. The free surface is at _a_; _f_ shows fat droplets in
process of passage through the cells.]

_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, after
passing over the body in this closed set of tubes, is finally brought
back again to be forced once more over the same path. As this blood is
carried around the body it conveys from one part of the 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.

[Illustration: FIG. 6.--A bit of muscle with its blood-vessels: _a_, the
muscle fibres; _b_, the minute blood-vessels. The fibres and vessels are
bathed in lymph (not shown in the figure), and food material passes
through the walls of the blood-vessels into this lymph.]


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 intelligible. As we have seen, the
blood coming from the intestine 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 (CO_{2}), water (H_{2}O), 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 CO_{2} and H_{2}O) 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.

[Illustration: FIG. 7.--A bit of bark showing cellular structure.]

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 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.

[Illustration: 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 membrane of varying thickness and shape. This
is in reality lifeless material, and is 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.

[Illustration: FIG. 9.--A cell; _cw_ is the cell wall; _pr_, the cell
substance; _n_, the nucleus.]

==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.

[Illustration: FIG. 10.--Cells at a root tip.]

[Illustration: FIG. 11.--Section of a leaf showing cells of different
shapes.]

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 cell
walls. Each was in its earlier history 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 blood its red colour. The liquid 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. 18, for example, is a bit of connective tissue, made up
of 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 clear at first, become less and less
evident, until the adult tissue seems sometimes to be composed mostly of
what we have called formed material.

[Illustration: FIG. 12.--Plant cells with thick walls, from a fern.]

[Illustration: FIG. 13.--Section of a potato showing different shaped
cells, the inner and larger ones being filled with grains of starch.]

[Illustration: FIG. 14.--Various shaped wood cells from plant tissue.]

[Illustration: FIG. 15.--A bit of cartilage.]

[Illustration: FIG. 16.--Frog's blood: _a_ and _b_ are the cells; _c_ is
the liquid.]

[Illustration: FIG. 17.--A bit of bone, showing the cells imbedded in
the bony matter.]

It must not be imagined, however, that a very rigid line can 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.

[Illustration: FIG. 18.--Connective tissue. The cells of the tissue are
shown at _c_, and the fibres or formed matter at _f_.]

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 those which are to form muscle, and those which are to
form the blood are quite unlike those which are to produce the hairs. By
means of such a differentiation there arises a very complex mass of
cells, with great variety in shape and function.

[Illustration: FIG. 19. A piece of nerve fibre, showing the cell with
its nucleus at _n_.]

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 functions.
The different organs appear within the cell, and the cell is more
complex than the 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.

[Illustration: FIG. 20.--A muscle fibre. The nucleii are shown at _n_.]

[Illustration: FIG. 21.--A complex cell. It is an entire animal, but
composed of only one 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 the activities of the fully formed animal or 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, that this should at first have been looked upon as the
primary part. The idea was that the 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.

[Illustration: FIG. 22.--An amoeba. A single cell without cell wall. _n_
is the nucleus; _f_, a bit of food which the cell has absorbed.]

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 entirely destitute of cell
wall. This is especially true of animal 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 amoeba, 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 (CO_{2}), water (H_{2}O), and
ammonia (NH_{3}), 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 CO_{2} and H_{2}O
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.

[Illustration: FIG. 23.--A cell as it appears to the modern microscope.
_a_, protoplasmic reticulum; _b_, liquid in its meshes; _c_, nuclear
membrane; _d_, nuclear reticulum; _e_, chromatin reticulum; _f_,
nucleolus; _g_, centrosome; _h_, centrosphere; _i_, vacuole; _j_, inert
bodies.]

==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 reticulate structure. It is
variously described, sometimes as a reticulate network, sometimes as a
mass of threads or fibres, and sometimes as a mass of 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 nucleii,
but, so far as we know, all active cells possess 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.

[Illustration: FIG. 24.--A cell cut into three pieces, each containing a
bit of the nucleus. Each continues its life indefinitely, soon acquiring
the form of the original as at _C_.]

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 importance of the nucleus in
cell activity, and show us that the cell, with its power of continued
life, must be regarded as a combination of protoplasm with its nucleus,
and cannot exist without it. It is not protoplasm, but cell substance,
plus cell nucleus, which forms the simplest basis of life.

[Illustration: FIG. 25.--A cell cut into three pieces, only one of
which, No. 2, contains any nucleus. This fragment soon acquires the
original form and continues its life indefinitely, as shown at _B_. The
other two pieces though living for a time, die without reproducing.]

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.

[Illustration: 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.

[Illustration: FIG. 27.--This and the following figures show
stages in cell division. 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.]

[Illustration: FIG. 28.--The chromatin is broken into threads or
chromosomes, _cr._ The centrosomes show radiating fibres.]

==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.
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).

[Illustration: FIG. 29.--The centrosomes are separating but are
connected by fibres.]

[Illustration: 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.

[Illustration: FIG. 31.--Stage showing the two halves of the
chromosomes separated from each other.]

[Illustration: 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:

[Illustration: FIG. 33--An egg showing the cell substance and
the nucleus, the latter containing chromosomes in large number and a
nucleolus]

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.
Now such an egg is a complete cell, but it is not able to begin the
process of division which shall give rise to a 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.

[Illustration 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.]

[Illustration: 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. 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.

[Illustration: FIG. 38--First division complete and first polar cell
formed, _pc'_.]

[Illustration: FIG. 39.--Formation of the second polar cell, _pc"_.]

[Illustration: 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.

[Illustration: FIG. 36--The egg centrosomes have changed their position.
The male cell with its centrosome remains inactive until the stage
represented in Fig. 42.]

[Illustration: FIG. 37--Beginning of the first division for removing
superfluous 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 (Figs. 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.

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.


[Illustration: FIG. 41.--The chromosomes in the male and female
pronucleii have resolved into a network. The male centrosome begins to
show signs of activity.]

[Illustration: 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.]

[Illustration: FIG. 43.--An equatorial plate is formed and each
chromosome has split into two halves by longitudinal division.]

[Illustration: 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.

[Illustration: FIG. 45. A group of cells resulting from division,
representing the first step in machine making.]

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,
already a machine, and we must presently return to the problem of its
origin. At present we will assume this cell as a starting point endowed
with its fundamental vital powers. It was sensitive, it could feel,
grow, and reproduce itself. From such a simple 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.

[Illustration: FIG. 46. A later step in machine building in which the
outer cells have acquired different form and function from the inner
cells: _ec_, the outer cells, whose duties are protective; _en_, the
inner cells engaged in digesting food.]

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 alike in all
machines. A city in China and one in America are alike made up of
individuals, and the fundamental needs of the Chinaman and the American
are alike. But differences in 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.

[Illustration: FIG. 47.--The arm of a monkey, a prehensile appendage.]

[Illustration: FIG. 48.--The arm of a bird, a flying appendage. In life
covered with feathers.]

[Illustration: 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.

[Illustration: FIG. 50.--Diagram illustrating the principle of
heredity.

_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 we have thus far considered
them, will be 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.

Amoeba, 73.

Anatomical evidence for evolution, 142.

Aquacity, 80.

Arm compared with wing, 144.

Aristotle, 1.

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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