47 files changed, 18638 insertions, 0 deletions
diff --git a/.gitattributes b/.gitattributes
new file mode 100644
index 0000000..6833f05
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1,3 @@
+* text=auto
+*.txt text
+*.md text
diff --git a/16487-8.txt b/16487-8.txt
new file mode 100644
index 0000000..ff9b406
--- /dev/null
+++ b/16487-8.txt
@@ -0,0 +1,6115 @@
+The Project Gutenberg EBook of The Story of the Living Machine, by H. W. Conn
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+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: H. W. Conn
+
+Release Date: August 8, 2005 [EBook #16487]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE STORY OF THE LIVING MACHINE ***
+
+
+
+
+Produced by Juliet Sutherland, Janet Blenkinship and the
+Online Distributed Proofreading Team at https://www.pgdp.net
+
+
+
+
+
+
+
+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. Linn�us 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
+h�moglobin. 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 h�moglobin is a normal constituent of the blood, and, being red in
+colour, gives the red colour to the blood. This h�moglobin 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 h�moglobin
+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 h�moglobin 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 h�moglobin, and since the oxygen pressure in the air is
+quite high, this h�moglobin 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 h�moglobin reaches them
+the association of the h�moglobin 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 animalcul� 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 arch�ologists 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.
+
+Linn�us, 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.
+
+
+
+
+==THE LIBRARY OF USEFUL STORIES.==
+
+Illustrated. 16mo. Cloth, 35 cents net per volume; postage, 4 cents per
+volume additional.
+
+The Story of a Grain of Wheat. By W.C. EDGAR.
+The Story of Alchemy. By M.M. PATTISON MUIR.
+The Story of Animal Life. By B. LINDSAY.
+The Story of the Art of Music. By F.J. CROWEST.
+The Story of the Art of Building. By P.L. WATERHOUSE.
+The Story of King Alfred. By Sir WALTER BESANT.
+The Story of Books. By GERTRUDE B. RAWLINGS.
+The Story of the Alphabet. By EDWARD CLODD.
+The Story of Eclipses. By G.F. CHAMBERS, F.R.A.S.
+The Story of the Living Machine. By H.W. CONN.
+The Story of the British Race. By JOHN MUNRO, C.E.
+The Story of Geographical Discovery. By JOSEPH JACOBS.
+The Story of the Cotton Plant. By F. WILKINSON, F.G.S.
+The Story of the Mind. By Prof. J. MARK BALDWIN.
+The Story of Photography. By ALFRED T. STORY.
+The Story of Life in the Seas. By SYDNEY J. HICKSON.
+The Story of Germ Life. By Prof. H.W. CONN.
+The Story of the Earth's Atmosphere. By DOUGLAS ARCHIBALD.
+The Story of Extinct Civilizations of the East.
+By ROBERT ANDERSON, M.A., F.A.S.
+The Story of Electricity. By JOHN MUNRO, C.E.
+The Story of a Piece of Coal. By E.A. MARTIN, F.G.S.
+The Story of the Solar System. By G.F. CHAMBERS, F.R.A.S.
+The Story of the Earth. By H.G. SEELEY, F.R.S.
+The Story of the Plants. By GRANT ALLEN.
+The Story of "Primitive" Man. By EDWARD CLODD.
+The Story of the Stars. By G.F. CHAMBERS, F.R.A.S.
+
+OTHERS IN PREPARATION.
+
+D. APPLETON AND COMPANY, NEW YORK.
+
+
+
+
+NEW EDITION OF HUXLEY'S ESSAYS.
+
+==Collected Essays.==
+
+By THOMAS H. HUXLEY. New complete edition, with revisions, the
+Essays being grouped according to general subject. In nine volumes, a
+new Introduction accompanying each volume. 12mo. Cloth, $1.25 per
+volume.
+
+VOLUME.
+
+==I. Methods and Results.
+  II. Darwiniana.
+ III. Science and Education.
+  IV. Science and Hebrew Tradition.
+   V. Science and Christian Tradition.
+  VI. Hume.
+ VII. Man's Place in Nature.
+VIII. Discourses, Biological and Geological.
+  IX. Evolution and Ethics, and Other Essays.==
+
+"Mr. Huxley has covered a vast variety of topics during the last quarter
+of a century. It gives one an agreeable surprise to look over the tables
+of contents and note the immense territory which he has explored. To
+read these books carefully and studiously is to become thoroughly
+acquainted with the most advanced thought on a large number of
+topics."--_New York Herald._
+
+D. APPLETON AND COMPANY, NEW YORK.
+
+D. APPLETON AND COMPANY'S PUBLICATIONS.
+
+_PIONEERS OF EVOLUTION, from Thales to Huxley_ By EDWARD
+CLODD, President of the Folk-Lore Society; Author of "The Story of
+Creation," "The Story of 'Primitive' Man," etc. With Portraits, 12mo.
+Cloth, $1.50.
+
+"The mass of interesting material which Mr. Clodd has got together and
+woven into a symmetrical story of the progress from ignorance and theory
+to knowledge and the intelligent recording of fact is prodigious.... The
+'goal' to which Mr. Clodd leads us in so masterly a fashion is but the
+starting point of fresh achievements, and, in due course, fresh
+theories. His book furnishes an important contribution to a liberal
+education."--_London Daily Chronicle._
+
+"We are always glad to meet Mr. Clodd. He is never dull; he is always
+well informed, and he says what he has to say with clearness and
+precision.... The interest intensifies as Mr. Clodd attempts to show the
+part really played in the growth of the doctrine of evolution by men
+like Wallace, Darwin, Huxley, and Spencer.... We commend the book to
+those who want to know what evolution really means."--_London Times._
+
+"This is a book which was needed.... Altogether, the book could hardly
+be better done. It is luminous, lucid, orderly, and temperate. Above
+all, it is entirely free from personal partisanship. Each chief actor is
+sympathetically treated, and friendship is seldom or never allowed to
+overweight sound judgment"--_London Academy._
+
+"We can assure the reader that he will find in this work a very useful
+guide to the lives and labors of leading evolutionists of the past and
+present. Especially serviceable is the account of Mr. Herbert Spencer
+and his share in rediscovering evolution, and illustrating its relations
+to the whole field of human knowledge. His forcible style and wealth of
+metaphor make all that Mr. Clodd writes arrestive and
+interesting."--_London Literary World._
+
+"Can not but prove welcome to fair-minded men.... To read it is to have
+an object-lesson in the meaning of evolution.... There is no better book
+on the subject for the general reader.... No one could go through the
+book without being both refreshed and newly instructed by its masterly
+survey of the growth of the most powerful idea of modern times."--_The
+Scotsman._
+
+D. APPLETON AND COMPANY, NEW YORK.
+
+==BOOKS ON SOCIAL SCIENCE.==
+
+==Socialism New and Old.==
+
+By Prof. WILLIAM GRAHAM, 12mo. Cloth, $1.75.
+
+"Professor Graham's book may be confidently recommended to all who are
+interested in the study of socialism, and not so intoxicated with its
+promises of a new heaven and a new earth as to be impatient of temperate
+and reasoned criticism."_--London Times._
+
+"Professor Graham presents an outline of the successive schemes of three
+writers who have chiefly influenced the development of socialism, and
+dwells at length upon the system of Rousseau, that of St. Simon, and on
+that of Karl Marx, the founder of the new socialism, 'which has gained
+favor with the working classes in all civilized countries,' which agrees
+with Rousseau's plan in being democratic, and with St. Simon's in aiming
+at collective ownership.... The professor is an independent thinker,
+whose endeavor to be clear has resulted in the statement of definite
+conclusions. The book is a remarkably fair digest of the subject under
+consideration."_--Philadelphia Ledger._
+
+==Dynamic Sociology:==
+
+_Or, Applied Social Science, as based upon Statical Sociology and the
+less Complex Sciences._ By LESTER F. WARD, A.M. In 2 vols.
+12mo. Cloth, $4.00.
+
+"A book that will amply repay perusal.... Recognizing the danger in
+which sociology is, of falling into the class of dead sciences or polite
+amusements, Mr. Ward has undertaken to 'point out a method by which the
+breath of life can be breathed into its nostrils.'"--_Rochester
+Post-Express._
+
+"Mr. Ward has evidently put great labor and thought into his two
+volumes, and has produced a work of interest and importance. He does not
+limit his effort to a contribution to the science of sociology.... He
+believes that sociology has already reached the point at which it can be
+and ought to be applied, treated as an art, and he urges that 'the
+State' or Government now has a new, legitimate, and peculiar field for
+the exercise of intelligence to promote the welfare of men."--_New York
+Times._
+
+==Criminal Sociology.==
+
+By Prof. E. FERRI. A new volume in the Criminology Series,
+edited by W. Douglas Morrison, 12mo. Cloth, $1.50.
+
+In this volume Professor Ferri, a distinguished member of the Italian
+Parliament, deals with the conditions which produce the criminal
+population, and with the methods by which this anti-social section of
+the community may be diminished. He divides the causes of crime into two
+great classes, individual and social. The individual causes consist of
+physical and mental defects; the social causes consist of social
+disadvantages of every description. His view is that the true remedy
+against crime is to remove individual defects and social disadvantages
+where it is possible to remove them. He shows that punishment has
+comparatively little effect in this direction, and is apt to divert
+attention from the true remedy--the individual and social amelioration
+of the population as a whole.
+
+D. APPLETON AND COMPANY, NEW YORK.
+
+
+
+
+BOOKS FOR NATURE LOVERS.
+
+
+==Insect Life. (New Edition in Colors.)==
+
+By JOHN HENRY COMSTOCK, Professor of Entomology in Cornell
+University. With 12 full-page plates reproducing butterflies and various
+insects in their natural colors, and with many wood engravings by Anna
+Botsford Comstock, Member of the Society of American Wood Engravers,
+12mo. Cloth, $1.75 net; postage, 20 cents additional.
+
+     "The volume is admirably written, and the simple and lucid style is
+     a constant delight.... It is sure to serve an excellent purpose in
+     the direction of popular culture, and the love of natural science
+     which it will develop in youthful minds can hardly fail to bear
+     rich fruit."--_Boston Beacon._
+
+==Familiar Fish: Their Habits and Capture.==
+
+A Practical Book on Fresh-Water Game Fish. By EUGENE MCCARTHY.
+With an Introduction by Dr. David Starr Jordan, President of Leland
+Stanford Junior University, and numerous Illustrations, 12mo. Cloth,
+$1.50.
+
+     "One of the handsomest, most practical, most informing books that
+     we know. The author treats his subject with scientific
+     thoroughness, but with a light touch that makes the book easy
+     reading.... The book should be the companion of all who go
+     a-fishing."--_New York Mail and Express._
+
+==The Art of Taxidermy.==
+
+By JOHN ROWLEY, Chief of the Department of Taxidermy in the
+American Museum of Natural History. Illustrated, 12mo. Cloth, $2.00.
+
+     "Mr. Rowley will long be gratefully remembered by taxidermists,
+     amateurs, and others, for the care he has used in thus meeting a
+     long-felt want."--_Bangor, Me., Sportsman._
+
+     "The book is not an elaborate treatise upon the abstract principles
+     which lie at the foundation of artistic taxidermy, but is rather a
+     compendium full of practical hints and suggestions, recipes, and
+     formulas for the working taxidermist."--_The Dial._
+
+==Plants. (Plant Relations and Plant Structures in one volume.)==
+
+By JOHN M. COULTER, A.M., Ph.D., Head of Department of Botany,
+University of Chicago, 12mo. Cloth, $1.80 net. (One of the Twentieth
+Century Text-Books.)
+
+D. APPLETON AND COMPANY, NEW YORK.
+
+
+_EVOLUTION OF MAN AND CHRISTIANITY._
+
+New edition. By the Rev. HOWARD MACQUEARY. With a new Preface,
+in which the Author answers his Critics, and with some important
+Additions, 12mo. Cloth, $1.75.
+
+     "This is a revised and enlarged edition of a book published last
+     year. The author reviews criticisms upon the first edition, denies
+     that he rejects the doctrine of the incarnation, admits his doubts
+     of the physical resurrection of Christ, and his belief in
+     evolution. The volume is to be marked as one of the most profound
+     expressions of the modern movement toward broader theological
+     positions."--_Brooklyn Times._
+
+
+_HISTORY OF THE CONFLICT BETWEEN RELIGION AND SCIENCE._ By Dr. JOHN
+WILLIAM DRAPER. 12mo. Cloth, $1.75.
+
+     "The keynote to this volume is found in the antagonism between the
+     progressive tendencies of the human mind and the pretensions of
+     ecclesiastical authority, as developed in the history of modern
+     science. No previous writer has treated the subject from this point
+     of view, and the present monograph will be found to possess no less
+     originality of conception than vigor of reasoning and wealth of
+     erudition."--_New York Tribune._
+
+
+_A CRITICAL HISTORY OF FREE THOUGHT IN REFERENCE TO THE CHRISTIAN
+RELIGION._ By Rev. Canon ADAM STOREY FARRAR, D.D., F.R.S., etc.
+12mo. Cloth, $1.50.
+
+     "A conflict might naturally be anticipated between the reasoning
+     faculties of man and a religion which claims the right, on
+     superhuman authority, to impose limits on the field or manner of
+     their exercise. It is the chief of the movements of free thought
+     which it is my purpose to describe, in their historic succession
+     and their connection with intellectual causes. We must ascertain
+     the facts, discover the causes, and read the moral."--_The Author._
+
+
+_CREATION OR EVOLUTION? A Philosophical Inquiry._ By GEORGE TICNOR
+CURTIS. 12mo. Cloth, $2.00.
+
+     "A treatise on the great question of Creation or Evolution by one
+     who is neither a naturalist nor theologian, and who does not
+     profess to bring to the discussion a special equipment in either of
+     the sciences which the controversy arrays against each other, may
+     seem strange at first sight; but Mr. Curtis will satisfy the
+     reader, before many pages have been turned, that he has a
+     substantial contribution to make to the debate, and that his book
+     is one to be treated with respect. His part is to apply to the
+     reasonings of the men of science the rigid scrutiny with which the
+     lawyer is accustomed to test the value and pertinency of testimony,
+     and the legitimacy of inferences from established facts."--_New
+     York Tribune._
+
+D. APPLETON AND COMPANY, NEW YORK.
+
+
+
+
+
+End of Project Gutenberg's The Story of the Living Machine, by H. W. Conn
+
+*** END OF THIS PROJECT GUTENBERG EBOOK THE STORY OF THE LIVING MACHINE ***
+
+***** This file should be named 16487-8.txt or 16487-8.zip *****
+This and all associated files of various formats will be found in:
+        https://www.gutenberg.org/1/6/4/8/16487/
+
+Produced by Juliet Sutherland, Janet Blenkinship and the
+Online Distributed Proofreading Team at https://www.pgdp.net
+
+
+Updated editions will replace the previous one--the old editions
+will be renamed.
+
+Creating the works from public domain print editions means that no
+one owns a United States copyright in these works, so the Foundation
+(and you!) can copy and distribute it in the United States without
+permission and without paying copyright royalties.  Special rules,
+set forth in the General Terms of Use part of this license, apply to
+copying and distributing Project Gutenberg-tm electronic works to
+protect the PROJECT GUTENBERG-tm concept and trademark.  Project
+Gutenberg is a registered trademark, and may not be used if you
+charge for the eBooks, unless you receive specific permission.  If you
+do not charge anything for copies of this eBook, complying with the
+rules is very easy.  You may use this eBook for nearly any purpose
+such as creation of derivative works, reports, performances and
+research.  They may be modified and printed and given away--you may do
+practically ANYTHING with public domain eBooks.  Redistribution is
+subject to the trademark license, especially commercial
+redistribution.
+
+
+
+*** START: FULL LICENSE ***
+
+THE FULL PROJECT GUTENBERG LICENSE
+PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK
+
+To protect the Project Gutenberg-tm mission of promoting the free
+distribution of electronic works, by using or distributing this work
+(or any other work associated in any way with the phrase "Project
+Gutenberg"), you agree to comply with all the terms of the Full Project
+Gutenberg-tm License (available with this file or online at
+https://gutenberg.org/license).
+
+
+Section 1.  General Terms of Use and Redistributing Project Gutenberg-tm
+electronic works
+
+1.A.  By reading or using any part of this Project Gutenberg-tm
+electronic work, you indicate that you have read, understand, agree to
+and accept all the terms of this license and intellectual property
+(trademark/copyright) agreement.  If you do not agree to abide by all
+the terms of this agreement, you must cease using and return or destroy
+all copies of Project Gutenberg-tm electronic works in your possession.
+If you paid a fee for obtaining a copy of or access to a Project
+Gutenberg-tm electronic work and you do not agree to be bound by the
+terms of this agreement, you may obtain a refund from the person or
+entity to whom you paid the fee as set forth in paragraph 1.E.8.
+
+1.B.  "Project Gutenberg" is a registered trademark.  It may only be
+used on or associated in any way with an electronic work by people who
+agree to be bound by the terms of this agreement.  There are a few
+things that you can do with most Project Gutenberg-tm electronic works
+even without complying with the full terms of this agreement.  See
+paragraph 1.C below.  There are a lot of things you can do with Project
+Gutenberg-tm electronic works if you follow the terms of this agreement
+and help preserve free future access to Project Gutenberg-tm electronic
+works.  See paragraph 1.E below.
+
+1.C.  The Project Gutenberg Literary Archive Foundation ("the Foundation"
+or PGLAF), owns a compilation copyright in the collection of Project
+Gutenberg-tm electronic works.  Nearly all the individual works in the
+collection are in the public domain in the United States.  If an
+individual work is in the public domain in the United States and you are
+located in the United States, we do not claim a right to prevent you from
+copying, distributing, performing, displaying or creating derivative
+works based on the work as long as all references to Project Gutenberg
+are removed.  Of course, we hope that you will support the Project
+Gutenberg-tm mission of promoting free access to electronic works by
+freely sharing Project Gutenberg-tm works in compliance with the terms of
+this agreement for keeping the Project Gutenberg-tm name associated with
+the work.  You can easily comply with the terms of this agreement by
+keeping this work in the same format with its attached full Project
+Gutenberg-tm License when you share it without charge with others.
+
+1.D.  The copyright laws of the place where you are located also govern
+what you can do with this work.  Copyright laws in most countries are in
+a constant state of change.  If you are outside the United States, check
+the laws of your country in addition to the terms of this agreement
+before downloading, copying, displaying, performing, distributing or
+creating derivative works based on this work or any other Project
+Gutenberg-tm work.  The Foundation makes no representations concerning
+the copyright status of any work in any country outside the United
+States.
+
+1.E.  Unless you have removed all references to Project Gutenberg:
+
+1.E.1.  The following sentence, with active links to, or other immediate
+access to, the full Project Gutenberg-tm License must appear prominently
+whenever any copy of a Project Gutenberg-tm work (any work on which the
+phrase "Project Gutenberg" appears, or with which the phrase "Project
+Gutenberg" is associated) is accessed, displayed, performed, viewed,
+copied or distributed:
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+1.E.2.  If an individual Project Gutenberg-tm electronic work is derived
+from the public domain (does not contain a notice indicating that it is
+posted with permission of the copyright holder), the work can be copied
+and distributed to anyone in the United States without paying any fees
+or charges.  If you are redistributing or providing access to a work
+with the phrase "Project Gutenberg" associated with or appearing on the
+work, you must comply either with the requirements of paragraphs 1.E.1
+through 1.E.7 or obtain permission for the use of the work and the
+Project Gutenberg-tm trademark as set forth in paragraphs 1.E.8 or
+1.E.9.
+
+1.E.3.  If an individual Project Gutenberg-tm electronic work is posted
+with the permission of the copyright holder, your use and distribution
+must comply with both paragraphs 1.E.1 through 1.E.7 and any additional
+terms imposed by the copyright holder.  Additional terms will be linked
+to the Project Gutenberg-tm License for all works posted with the
+permission of the copyright holder found at the beginning of this work.
+
+1.E.4.  Do not unlink or detach or remove the full Project Gutenberg-tm
+License terms from this work, or any files containing a part of this
+work or any other work associated with Project Gutenberg-tm.
+
+1.E.5.  Do not copy, display, perform, distribute or redistribute this
+electronic work, or any part of this electronic work, without
+prominently displaying the sentence set forth in paragraph 1.E.1 with
+active links or immediate access to the full terms of the Project
+Gutenberg-tm License.
+
+1.E.6.  You may convert to and distribute this work in any binary,
+compressed, marked up, nonproprietary or proprietary form, including any
+word processing or hypertext form.  However, if you provide access to or
+distribute copies of a Project Gutenberg-tm work in a format other than
+"Plain Vanilla ASCII" or other format used in the official version
+posted on the official Project Gutenberg-tm web site (www.gutenberg.org),
+you must, at no additional cost, fee or expense to the user, provide a
+copy, a means of exporting a copy, or a means of obtaining a copy upon
+request, of the work in its original "Plain Vanilla ASCII" or other
+form.  Any alternate format must include the full Project Gutenberg-tm
+License as specified in paragraph 1.E.1.
+
+1.E.7.  Do not charge a fee for access to, viewing, displaying,
+performing, copying or distributing any Project Gutenberg-tm works
+unless you comply with paragraph 1.E.8 or 1.E.9.
+
+1.E.8.  You may charge a reasonable fee for copies of or providing
+access to or distributing Project Gutenberg-tm electronic works provided
+that
+
+- You pay a royalty fee of 20% of the gross profits you derive from
+     the use of Project Gutenberg-tm works calculated using the method
+     you already use to calculate your applicable taxes.  The fee is
+     owed to the owner of the Project Gutenberg-tm trademark, but he
+     has agreed to donate royalties under this paragraph to the
+     Project Gutenberg Literary Archive Foundation.  Royalty payments
+     must be paid within 60 days following each date on which you
+     prepare (or are legally required to prepare) your periodic tax
+     returns.  Royalty payments should be clearly marked as such and
+     sent to the Project Gutenberg Literary Archive Foundation at the
+     address specified in Section 4, "Information about donations to
+     the Project Gutenberg Literary Archive Foundation."
+
+- You provide a full refund of any money paid by a user who notifies
+     you in writing (or by e-mail) within 30 days of receipt that s/he
+     does not agree to the terms of the full Project Gutenberg-tm
+     License.  You must require such a user to return or
+     destroy all copies of the works possessed in a physical medium
+     and discontinue all use of and all access to other copies of
+     Project Gutenberg-tm works.
+
+- You provide, in accordance with paragraph 1.F.3, a full refund of any
+     money paid for a work or a replacement copy, if a defect in the
+     electronic work is discovered and reported to you within 90 days
+     of receipt of the work.
+
+- You comply with all other terms of this agreement for free
+     distribution of Project Gutenberg-tm works.
+
+1.E.9.  If you wish to charge a fee or distribute a Project Gutenberg-tm
+electronic work or group of works on different terms than are set
+forth in this agreement, you must obtain permission in writing from
+both the Project Gutenberg Literary Archive Foundation and Michael
+Hart, the owner of the Project Gutenberg-tm trademark.  Contact the
+Foundation as set forth in Section 3 below.
+
+1.F.
+
+1.F.1.  Project Gutenberg volunteers and employees expend considerable
+effort to identify, do copyright research on, transcribe and proofread
+public domain works in creating the Project Gutenberg-tm
+collection.  Despite these efforts, Project Gutenberg-tm electronic
+works, and the medium on which they may be stored, may contain
+"Defects," such as, but not limited to, incomplete, inaccurate or
+corrupt data, transcription errors, a copyright or other intellectual
+property infringement, a defective or damaged disk or other medium, a
+computer virus, or computer codes that damage or cannot be read by
+your equipment.
+
+1.F.2.  LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right
+of Replacement or Refund" described in paragraph 1.F.3, the Project
+Gutenberg Literary Archive Foundation, the owner of the Project
+Gutenberg-tm trademark, and any other party distributing a Project
+Gutenberg-tm electronic work under this agreement, disclaim all
+liability to you for damages, costs and expenses, including legal
+fees.  YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT
+LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE
+PROVIDED IN PARAGRAPH F3.  YOU AGREE THAT THE FOUNDATION, THE
+TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE
+LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR
+INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH
+DAMAGE.
+
+1.F.3.  LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a
+defect in this electronic work within 90 days of receiving it, you can
+receive a refund of the money (if any) you paid for it by sending a
+written explanation to the person you received the work from.  If you
+received the work on a physical medium, you must return the medium with
+your written explanation.  The person or entity that provided you with
+the defective work may elect to provide a replacement copy in lieu of a
+refund.  If you received the work electronically, the person or entity
+providing it to you may choose to give you a second opportunity to
+receive the work electronically in lieu of a refund.  If the second copy
+is also defective, you may demand a refund in writing without further
+opportunities to fix the problem.
+
+1.F.4.  Except for the limited right of replacement or refund set forth
+in paragraph 1.F.3, this work is provided to you 'AS-IS', WITH NO OTHER
+WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO
+WARRANTIES OF MERCHANTIBILITY OR FITNESS FOR ANY PURPOSE.
+
+1.F.5.  Some states do not allow disclaimers of certain implied
+warranties or the exclusion or limitation of certain types of damages.
+If any disclaimer or limitation set forth in this agreement violates the
+law of the state applicable to this agreement, the agreement shall be
+interpreted to make the maximum disclaimer or limitation permitted by
+the applicable state law.  The invalidity or unenforceability of any
+provision of this agreement shall not void the remaining provisions.
+
+1.F.6.  INDEMNITY - You agree to indemnify and hold the Foundation, the
+trademark owner, any agent or employee of the Foundation, anyone
+providing copies of Project Gutenberg-tm electronic works in accordance
+with this agreement, and any volunteers associated with the production,
+promotion and distribution of Project Gutenberg-tm electronic works,
+harmless from all liability, costs and expenses, including legal fees,
+that arise directly or indirectly from any of the following which you do
+or cause to occur: (a) distribution of this or any Project Gutenberg-tm
+work, (b) alteration, modification, or additions or deletions to any
+Project Gutenberg-tm work, and (c) any Defect you cause.
+
+
+Section  2.  Information about the Mission of Project Gutenberg-tm
+
+Project Gutenberg-tm is synonymous with the free distribution of
+electronic works in formats readable by the widest variety of computers
+including obsolete, old, middle-aged and new computers.  It exists
+because of the efforts of hundreds of volunteers and donations from
+people in all walks of life.
+
+Volunteers and financial support to provide volunteers with the
+assistance they need, is critical to reaching Project Gutenberg-tm's
+goals and ensuring that the Project Gutenberg-tm collection will
+remain freely available for generations to come.  In 2001, the Project
+Gutenberg Literary Archive Foundation was created to provide a secure
+and permanent future for Project Gutenberg-tm and future generations.
+To learn more about the Project Gutenberg Literary Archive Foundation
+and how your efforts and donations can help, see Sections 3 and 4
+and the Foundation web page at https://www.pglaf.org.
+
+
+Section 3.  Information about the Project Gutenberg Literary Archive
+Foundation
+
+The Project Gutenberg Literary Archive Foundation is a non profit
+501(c)(3) educational corporation organized under the laws of the
+state of Mississippi and granted tax exempt status by the Internal
+Revenue Service.  The Foundation's EIN or federal tax identification
+number is 64-6221541.  Its 501(c)(3) letter is posted at
+https://pglaf.org/fundraising.  Contributions to the Project Gutenberg
+Literary Archive Foundation are tax deductible to the full extent
+permitted by U.S. federal laws and your state's laws.
+
+The Foundation's principal office is located at 4557 Melan Dr. S.
+Fairbanks, AK, 99712., but its volunteers and employees are scattered
+throughout numerous locations.  Its business office is located at
+809 North 1500 West, Salt Lake City, UT 84116, (801) 596-1887, email
+business@pglaf.org.  Email contact links and up to date contact
+information can be found at the Foundation's web site and official
+page at https://pglaf.org
+
+For additional contact information:
+     Dr. Gregory B. Newby
+     Chief Executive and Director
+     gbnewby@pglaf.org
+
+Section 4.  Information about Donations to the Project Gutenberg
+Literary Archive Foundation
+
+Project Gutenberg-tm depends upon and cannot survive without wide
+spread public support and donations to carry out its mission of
+increasing the number of public domain and licensed works that can be
+freely distributed in machine readable form accessible by the widest
+array of equipment including outdated equipment.  Many small donations
+($1 to $5,000) are particularly important to maintaining tax exempt
+status with the IRS.
+
+The Foundation is committed to complying with the laws regulating
+charities and charitable donations in all 50 states of the United
+States.  Compliance requirements are not uniform and it takes a
+considerable effort, much paperwork and many fees to meet and keep up
+with these requirements.  We do not solicit donations in locations
+where we have not received written confirmation of compliance.  To
+SEND DONATIONS or determine the status of compliance for any
+particular state visit https://pglaf.org
+
+While we cannot and do not solicit contributions from states where we
+have not met the solicitation requirements, we know of no prohibition
+against accepting unsolicited donations from donors in such states who
+approach us with offers to donate.
+
+International donations are gratefully accepted, but we cannot make
+any statements concerning tax treatment of donations received from
+outside the United States.  U.S. laws alone swamp our small staff.
+
+Please check the Project Gutenberg Web pages for current donation
+methods and addresses.  Donations are accepted in a number of other
+ways including including checks, online payments and credit card
+donations.  To donate, please visit: https://pglaf.org/donate
+
+
+Section 5.  General Information About Project Gutenberg-tm electronic
+works.
+
+Professor Michael S. Hart was the originator of the Project Gutenberg-tm
+concept of a library of electronic works that could be freely shared
+with anyone.  For thirty years, he produced and distributed Project
+Gutenberg-tm eBooks with only a loose network of volunteer support.
+
+Project Gutenberg-tm eBooks are often created from several printed
+editions, all of which are confirmed as Public Domain in the U.S.
+unless a copyright notice is included.  Thus, we do not necessarily
+keep eBooks in compliance with any particular paper edition.
+
+Most people start at our Web site which has the main PG search facility:
+
+     https://www.gutenberg.org
+
+This Web site includes information about Project Gutenberg-tm,
+including how to make donations to the Project Gutenberg Literary
+Archive Foundation, how to help produce our new eBooks, and how to
+subscribe to our email newsletter to hear about new eBooks.
+
+*** END: FULL LICENSE ***
+
diff --git a/16487-8.zip b/16487-8.zip
new file mode 100644
index 0000000..8ef8289
--- /dev/null
+++ b/16487-8.zip
diff --git a/16487-h.zip b/16487-h.zip
new file mode 100644
index 0000000..421608d
--- /dev/null
+++ b/16487-h.zip
diff --git a/16487-h/16487-h.htm b/16487-h/16487-h.htm
new file mode 100644
index 0000000..eeecced
--- /dev/null
+++ b/16487-h/16487-h.htm
@@ -0,0 +1,6392 @@
+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
+    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
+
+<html xmlns="http://www.w3.org/1999/xhtml">
+  <head>
+    <meta http-equiv="Content-Type" content="text/html;charset=iso-8859-1" />
+    <title>
+      The Project Gutenberg eBook of The Story Of The Living Machine, by H.W. Conn.
+    </title>
+    <style type="text/css">
+/*<![CDATA[  XML blockout */
+<!--
+    p {  margin-top: .75em;
+         text-align: justify;
+         margin-bottom: .75em;
+         }
+    img {border:0;}
+    h1,h2,h3,h4,h5,h6 {
+         text-align: center; /* all headings centered */
+         clear: both;
+         }
+    hr { width: 33%;
+	 margin-top: 2em;
+	 margin-bottom: 2em;
+         margin-left: auto;
+         margin-right: auto;
+         clear: both;
+       }
+
+    table {margin-left: auto; margin-right: auto;}
+
+    body{margin-left: 10%;
+         margin-right: 10%;
+        }
+
+    .linenum  {position: absolute; top: auto; left: 4%;} /* poetry number */
+    .blockquot{margin-left: 5%; margin-right: 10%;}
+    .pagenum  {position: absolute; left: 92%; font-size: smaller; text-align: right;} /* page numbers */
+    .sidenote {width: 20%; padding-bottom: .5em; padding-top: .5em;
+               padding-left: .5em; padding-right: .5em; margin-left: 1em;
+               float: right; clear: right; margin-top: 1em;
+               font-size: smaller; background: #eeeeee; border: dashed 1px;}
+
+    .bb       {border-bottom: solid 2px;}
+    .bl       {border-left: solid 2px;}
+    .bt       {border-top: solid 2px;}
+    .br       {border-right: solid 2px;}
+    .bbox     {border: solid 2px;}
+
+    .center   {text-align: center;}
+    .smcap    {font-variant: small-caps;}
+    .u        {text-decoration: underline;}
+
+    .caption  {font-weight: bold;}
+
+    .figcenter   {margin: auto; text-align: center;}
+
+    .figleft     {float: left; clear: left; margin-left: 0; margin-bottom: 1em; margin-top:
+                 1em; margin-right: 1em; padding: 0; text-align: center;}
+
+    .figright    {float: right; clear: right; margin-left: 1em; margin-bottom: 1em;
+                 margin-top: 1em; margin-right: 0; padding: 0; text-align: center;}
+
+    .footnotes        {border: dashed 1px;}
+    .footnote         {margin-left: 10%; margin-right: 10%; font-size: 0.9em;}
+    .footnote .label  {position: absolute; right: 84%; text-align: right;}
+    .fnanchor         {vertical-align: super; font-size: .8em; text-decoration: none;}
+
+    .poem             {margin-left:10%; margin-right:10%; text-align: left;}
+    .poem br          {display: none;}
+    .poem .stanza     {margin: 1em 0em 1em 0em;}
+    .poem span.i0     {display: block; margin-left: 0em;}
+    .poem span.i2     {display: block; margin-left: 2em;}
+    .poem span.i4     {display: block; margin-left: 4em;}
+    // -->
+    /* XML end  ]]>*/
+    </style>
+  </head>
+<body>
+
+
+<pre>
+
+The Project Gutenberg EBook of The Story of the Living Machine, by H. W. Conn
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+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: H. W. Conn
+
+Release Date: August 8, 2005 [EBook #16487]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE STORY OF THE LIVING MACHINE ***
+
+
+
+
+Produced by Juliet Sutherland, Janet Blenkinship and the
+Online Distributed Proofreading Team at https://www.pgdp.net
+
+
+
+
+
+
+</pre>
+
+<p><a name="Page_-7" id="Page_-7"></a></p>
+
+<h1>THE STORY OF THE LIVING MACHINE</h1>
+
+<h3>A REVIEW OF THE CONCLUSIONS OF MODERN BIOLOGY IN REGARD TO THE MECHANISM
+WHICH CONTROLS THE PHENOMENA OF LIVING ACTIVITY</h3>
+
+<h3>BY</h3>
+
+<h2>H.W. CONN</h2>
+
+<h4>PROFESSOR OF BIOLOGY IN WESLEYAN UNIVERSITY</h4>
+
+<h4>AUTHOR OF THE STORY OF GERM
+LIFE, EVOLUTION OF TO-DAY, THE LIVING WORLD, ETC.</h4>
+
+<p class='center'><i>WITH FIFTY ILLUSTRATIONS</i></p>
+
+<p class='center'>NEW YORK D. APPLETON AND COMPANY 1903</p>
+
+<p><a name="Page_-6" id="Page_-6"></a></p>
+
+<p class='center'><span class="smcap">Copyright</span>, 1899,<br />
+By D. APPLETON AND COMPANY.<br />
+</p>
+
+
+
+
+<hr style="width: 65%;" /><p><a name="Page_-5" id="Page_-5"></a></p>
+<h2><a name="PREFACE" id="PREFACE"></a>PREFACE.</h2>
+
+
+<p>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 <a name="Page_-4" id="Page_-4"></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.</p>
+
+<p>
+<span class="smcap">Middletown, Conn.</span>, U.S.A.<br />
+<br />
+<i>October 1, 1898</i>.<br />
+</p>
+
+
+
+<hr style="width: 65%;" /><p><a name="Page_-3" id="Page_-3"></a></p>
+<h2><a name="CONTENTS" id="CONTENTS"></a>CONTENTS.<br /><br /></h2>
+
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'><a href="#PREFACE"><b>PREFACE.</b></a></td></tr>
+<tr><td align='left'><a href="#LIST_OF_ILLUSTRATIONS"><b>LIST OF ILLUSTRATIONS.</b></a></td></tr>
+<tr><td align='left'><a href="#THE_STORY_OF_THE_LIVING_MACHINE"><b>THE STORY OF THE LIVING MACHINE.</b></a></td></tr>
+<tr><td align='left'><span style="margin-left: 2em;"><a href="#PART_I"><b>PART I.</b></a></span></td></tr>
+<tr><td align='left'><span style="margin-left: 3em;"><a href="#CHAPTER_I"><b>CHAPTER I.</b></a></span></td></tr>
+<tr><td align='left'><span style="margin-left: 3em;"><a href="#CHAPTER_II"><b>CHAPTER II.</b></a></span></td></tr>
+<tr><td align='left'><span style="margin-left: 2em;"><a href="#PART_II"><b>PART II.</b></a></span></td></tr>
+<tr><td align='left'><span style="margin-left: 3em;"><a href="#CHAPTER_III"><b>CHAPTER III.</b></a></span></td></tr>
+<tr><td align='left'><a href="#THE_LIBRARY_OF_USEFUL_STORIES"><b>THE LIBRARY OF USEFUL STORIES.</b></a></td></tr>
+<tr><td align='left'><a href="#NEW_EDITION_OF_HUXLEYS_ESSAYS"><b>NEW EDITION OF HUXLEY'S ESSAYS.</b></a></td></tr>
+<tr><td align='left'><a href="#BOOKS_FOR_NATURE_LOVERS"><b>BOOKS FOR NATURE LOVERS.</b></a><br /><br /><br /></td></tr>
+</table></div>
+
+
+
+<p class='center'>
+<span class="smcap">introduction</span>&mdash;Biology a new science&mdash;Historical<br />
+biology&mdash;Conservation of energy&mdash;Evolution&mdash;Cytology&mdash;New<br />
+aspects of biology&mdash;The mechanical<br />
+nature of living organisms&mdash;Significance of the new<br />
+biological problems&mdash;Outline of the subject&nbsp;&nbsp;&nbsp;<a href='#Page_1'><b>1</b></a>
+<br />
+<br />
+PART I.<br />
+<br />
+<i>THE RUNNING OF THE LIVING MACHINE.</i><br />
+<br />
+<br />
+CHAPTER I.<br />
+<br />
+IS THE BODY A MACHINE?<br />
+<br />
+What is a machine?&mdash;A general comparison of a body and<br />
+a machine&mdash;Details of the action of the machine&mdash;Physical<br />
+explanation of the chief vital functions&mdash;The<br />
+living body is a machine&mdash;The living machine<br />
+constructive as well as destructive&mdash;The vital factor&nbsp;&nbsp;&nbsp;<a href='#Page_19'><b>19</b></a><br />
+<br />
+CHAPTER II.<br />
+<br />
+THE CELL AND PROTOPLASM.<br />
+<br />
+Vital properties&mdash;The discovery of cells&mdash;The cell doctrine&mdash;The<br />
+cell&mdash;The cellular structure of organisms&mdash;The<br />
+cell wall&mdash;Protoplasm&mdash;The reign of protoplasm&mdash;The<br />
+decline of the reign of protoplasm&mdash;The<br />
+structure of protoplasm&mdash;The nucleus&mdash;Centrosome&mdash;Function<br />
+of the nucleus&mdash;Cell division or karyokinesis&mdash;Fertilization<br />
+of the egg&mdash;The significance of<br />
+fertilization&mdash;What is protoplasm?&mdash;Reaction against<br /><a name="Page_-2" id="Page_-2"></a>
+the cell doctrine&mdash;Fundamental vital activities as<br />
+located in cells&mdash;Summary&nbsp;&nbsp;&nbsp;<a href='#Page_54'><b>54</b></a><br />
+<br />
+<br />
+PART II.<br />
+<br />
+<i>THE BUILDING OF THE LIVING MACHINE</i>.<br />
+<br />
+CHAPTER III.<br />
+<br />
+THE FACTORS CONCERNED IN THE BUILDING OF THE LIVING<br />
+MACHINE.<br />
+<br />
+History of the living machine&mdash;Evidence for this<br />
+history&mdash;Historical&mdash;Embryological&mdash;Anatomical&mdash;Significance<br />
+of these sources of history&mdash;Forces at work in<br />
+the building of the living machine&mdash;Reproduction&mdash;Heredity&mdash;Variation&mdash;<br />
+Inheritance of variations&mdash;Method of machine building&mdash;Migration and<br />
+isolation&mdash;Direct influence of environment&mdash;Consciousness&mdash;Summary<br />
+of Nature's power of building machines&mdash;The origin of the cell<br />
+machine&mdash;General summary&nbsp;&nbsp;&nbsp;<a href='#Page_131'><b>131</b></a><br />
+</p>
+
+
+
+<hr style="width: 65%;" /><p><a name="Page_-1" id="Page_-1"></a></p>
+<h2><a name="LIST_OF_ILLUSTRATIONS" id="LIST_OF_ILLUSTRATIONS"></a>LIST OF ILLUSTRATIONS.</h2>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="List of Illustrations">
+<tr><td align='left'><a href="#Figure_illustrating_osmosis">Figure_illustrating_osmosis</a></td></tr>
+<tr><td align='left'><a href="#bFigure_illustrating_osmosis">Figure_illustrating_osmosis</a></td></tr>
+<tr><td align='left'><a href="#Diagram_of_the_intestinal_walls">Diagram_of_the_intestinal_walls</a></td></tr>
+<tr><td align='left'><a href="#Diagram_of_a_single_villus">Diagram_of_a_single_villus</a></td></tr>
+<tr><td align='left'><a href="#Enlarged_figure_of_four_cells_in_the_villus_membrane">Enlarged_figure_of_four_cells_in_the_villus_membrane</a></td></tr>
+<tr><td align='left'><a href="#A_bit_of_muscle_showing_blood-vessels">A_bit_of_muscle_showing blood-vessels</a></td></tr>
+<tr><td align='left'><a href="#A_bit_of_bark_showing_cellular_structure">A_bit_of_bark_showing_cellular_structure</a></td></tr>
+<tr><td align='left'><a href="#Successive_stages_in_the_division_of_the_developing_egg">Successive_stages_in_the_division_of_the_developing_egg</a></td></tr>
+<tr><td align='left'><a href="#A_typical_cell">A_typical_cell</a></td></tr>
+<tr><td align='left'><a href="#Cells_at_a_root_tip">Cells_at_a_root_tip</a></td></tr>
+<tr><td align='left'><a href="#Section_of_a_leaf_showing_cells_of_different_shapes">Section_of_a_leaf_showing_cells_of_different_shapes</a></td></tr>
+<tr><td align='left'><a href="#Plant_cells_with_thick_walls_from_a_fern">Plant_cells_with_thick_walls_from_a_fern</a></td></tr>
+<tr><td align='left'><a href="#Section_of_potato">Section_of_potato</a></td></tr>
+<tr><td align='left'><a href="#Various_shaped_wood_cells_from_plant_tissue">Various_shaped_wood_cells_from_plant_tissue</a></td></tr>
+<tr><td align='left'><a href="#A_bit_of_cartilage">A_bit_of_cartilage</a></td></tr>
+<tr><td align='left'><a href="#Frogs_blood">Frogs_blood</a></td></tr>
+<tr><td align='left'><a href="#A_bit_of_bone">A_bit_of_bone</a></td></tr>
+<tr><td align='left'><a href="#Connective_tissue">Connective_tissue</a></td></tr>
+<tr><td align='left'><a href="#A_piece_of_nerve_fibre">A_piece_of_nerve_fibre</a></td></tr>
+<tr><td align='left'><a href="#A_muscle_fibre">A_muscle_fibre</a></td></tr>
+<tr><td align='left'><a href="#A_complex_cell_vorticella">A_complex_cell_vorticella</a></td></tr>
+<tr><td align='left'><a href="#An_amoeba">An_am&oelig;ba</a></td></tr>
+<tr><td align='left'><a href="#A_cell_as_it_appears_to_the_modern_microscope">A_cell_as_it_appears_to_the_modern_microscope</a></td></tr>
+<tr><td align='left'><a href="#A_cell_cut_into_pieces_each_containing_a_bit_of_nucleus">A_cell_cut_into_pieces_each_containing_a_bit_of_nucleus</a></td></tr>
+<tr><td align='left'><a href="#A_cell_cut_in_pieces_only_one_of_which_contains_any_nucleus">A_cell_cut_in_pieces_only_one_of_which_contains_any_nucleus</a></td></tr>
+<tr><td align='left'><a href="#Different_forms_of_nucleii">Different_forms_of_nucleii</a></td></tr>
+<tr><td align='left'><a href="#Two_stages_in_cell_division">Two_stages_in_cell_division</a></td></tr>
+<tr><td align='left'><a href="#Stages_in_cell_division">Stages_in_cell_division</a></td></tr>
+<tr><td align='left'><a href="#Latest_stages_in_cell_division">Latest_stages_in_cell_division</a></td></tr>
+<tr><td align='left'><a href="#An_egg">An_egg</a></td></tr>
+<tr><td align='left'><a href="#Stages_in_the_process_of_fertilization_of_the_egg_1">Stages_in_the_process_of_fertilization_of_the_egg_1</a></td></tr>
+<tr><td align='left'><a href="#Stages_in_the_process_of_fertilization_of_the_egg_2">Stages_in_the_process_of_fertilization_of_the_egg_2</a></td></tr>
+<tr><td align='left'><a href="#Stages_in_fertilization_of_the_egg">Stages_in_fertilization_of_the_egg</a></td></tr>
+<tr><td align='left'><a href="#Latest_stages_in_the_fertilization_of_the_egg">Latest_stages_in_the_fertilization_of_the_egg</a></td></tr>
+<tr><td align='left'><a href="#Two_stages_in_the_division_of_the_egg">Two_stages_in_the_division_of_the_egg</a></td></tr>
+<tr><td align='left'><a href="#A_group_of_cells_resulting_from_division_the_first_step_in_machine_building">A_group_of_cells_resulting_from_division_the_first_step_in_machine_building</a></td></tr>
+<tr><td align='left'><a href="#A_later_step_in_machine_building_the_gastrula">A_later_step_in_machine_building_the_gastrula</a></td></tr>
+<tr><td align='left'><a href="#The_arm_of_a_monkey">The_arm_of_a_monkey</a></td></tr>
+<tr><td align='left'><a href="#The_arm_of_a_bird">The_arm_of_a_bird</a></td></tr>
+<tr><td align='left'><a href="#The_arm_of_an_ancient_half-bird_half-reptile_animal">The_arm_of_an_ancient_half-bird_half-reptile_animal</a></td></tr>
+<tr><td align='left'><a href="#Diagram_to_illustrate_the_principle_of_heredity">Diagram_to_illustrate_the_principle_of_heredity</a></td></tr>
+</table></div>
+
+
+
+<hr style="width: 65%;" /><p><a name="Page_1" id="Page_1"></a></p>
+<h2><a name="THE_STORY_OF_THE_LIVING_MACHINE" id="THE_STORY_OF_THE_LIVING_MACHINE"></a>THE STORY OF THE LIVING MACHINE.</h2>
+
+
+<h3>INTRODUCTION.</h3>
+
+<p><b>Biology a New Science</b>.&mdash;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. Linn&aelig;us 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?</p>
+
+<p>There must be some reason why this, the oldest of all, has been recently
+called a <i>new</i> science, and some explanation of the fact that it has
+only re<a name="Page_2" id="Page_2"></a>cently 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 <i>at rest</i>, while to-day it is studied as <i>in motion</i>. 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 <i>classified</i>; to the biologist of to-day, they
+are objects to be <i>explained</i>.<br /></p>
+
+<p>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 <a name="Page_3" id="Page_3"></a>forces of nature. The wind was the breath of one god,
+and the lightning a bolt thrown from the hands of another.</p>
+
+<p>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.</p>
+
+<p><a name="Page_4" id="Page_4"></a>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.</p>
+
+<p>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, <a name="Page_5" id="Page_5"></a>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 <i>living machine</i>. 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.</p>
+
+<p>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 <a name="Page_6" id="Page_6"></a>for, then at once was created a new department of
+thought and a new science inaugurated.</p>
+
+<p><b>Historical Geology</b>.&mdash;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 <a name="Page_7" id="Page_7"></a>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.</p>
+
+<p>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.</p>
+
+<p><b>Conservation of Energy</b>.&mdash;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&mdash;heat, light, electricity, chemism,
+etc.&mdash;are simply forms of motion, and thus forms of this energy. These
+various types of energy, being only expressions of the universal energy,
+<a name="Page_8" id="Page_8"></a>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.</p>
+
+<p>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 <a name="Page_9" id="Page_9"></a>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.</p>
+
+<p>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.</p>
+
+<p><b>Evolution</b>&mdash;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 <a name="Page_10" id="Page_10"></a>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.</p>
+
+<p><b>Cytology.</b>&mdash;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
+<a name="Page_11" id="Page_11"></a>extraordinary stimulus to the students who were trying to find some way
+of understanding life.</p>
+
+<p><b>New Aspects of Biology</b>.&mdash;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.</p>
+
+<p>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 <i>purpose</i> 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 <i>cause.</i>
+Natural law is blind and can have no purpose. To the scientist, filled
+with the thought of the <a name="Page_12" id="Page_12"></a>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 <i>teleological</i>, the latter <i>scientific;</i> 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.</p>
+
+<p><b>The Mechanical Nature of Living Organisms.</b>&mdash;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&mdash;light, heat, electricity,
+etc.&mdash;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
+na<a name="Page_13" id="Page_13"></a>ture. <i>Vital force</i>, or <i>vitality</i>, 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.</p>
+
+<p><a name="Page_14" id="Page_14"></a>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, <i>protoplasm.</i> 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.</p>
+
+<p>These ideas arose shortly after the middle of the century, and have
+dominated the development of biological science up to the present time.
+It <a name="Page_15" id="Page_15"></a>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
+<i>vital energy</i>, 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.</p>
+
+<p><b>Significance of the New Biological Problems</b>&mdash;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 <a name="Page_16" id="Page_16"></a>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.</p>
+
+<p>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 adop<a name="Page_17" id="Page_17"></a>tion 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.</p>
+
+<p><b>Outline of the Subject</b>.&mdash;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 <a name="Page_18" id="Page_18"></a>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.</p>
+
+<p>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:</p>
+
+<p class='center'>I. <i>The Running of the Living Machine</i>.</p>
+
+<p class='center'>II. <i>The Origin of the Living Machine</i>.</p>
+
+
+
+<hr style="width: 65%;" /><p><a name="Page_19" id="Page_19"></a></p>
+<h2><a name="PART_I" id="PART_I"></a>PART I.</h2>
+
+<h3><i>THE RUNNING OF THE LIVING MACHINE.</i></h3>
+
+<hr style='width: 45%;' />
+
+<h3><a name="CHAPTER_I" id="CHAPTER_I"></a>CHAPTER I.</h3>
+
+<h4>IS THE BODY A MACHINE?</h4>
+
+
+<p>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.</p>
+
+<p>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 <a name="Page_20" id="Page_20"></a>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.</p>
+
+<p><b>What is a Machine?</b>&mdash;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: <i>A machine is a piece of apparatus so designed that it can
+change one kind of energy into another for a definite purpose</i>.<a name="Page_21" id="Page_21"></a> 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, <a name="Page_22" id="Page_22"></a>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.</p>
+
+<p><b>A General Comparison of a Body and a Machine</b>.--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&mdash;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.</p>
+
+<p>The first question which presents itself is whether the only power the
+body possesses is, as in the case with other machines, to <i>transform</i>
+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?</p>
+
+<p>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 ex<a name="Page_23" id="Page_23"></a>perimental 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 <a name="Page_24" id="Page_24"></a>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.</p>
+
+<p>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 <a name="Page_25" id="Page_25"></a>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.</p>
+
+<p>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, <a name="Page_26" id="Page_26"></a>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.</p>
+
+<p>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, <i>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</i>.</p>
+
+<p><b>Details of the Action of the Machine.</b>&mdash;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 <a name="Page_27" id="Page_27"></a>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?</p>
+
+<p><b>Physical Explanation of the Chief Vital Functions.</b>&mdash;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.</p>
+
+<p><i>Digestion.</i>&mdash;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 intel<a name="Page_28" id="Page_28"></a>ligible 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 <a name="Page_29" id="Page_29"></a>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.</p>
+
+<p>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.</p>
+
+<p><a name="Figure_illustrating_osmosis" id="Figure_illustrating_osmosis"></a></p>
+
+<div class="figleft">
+             <img   src="images/038fig1.png"
+             alt="FIG 1." /><br />
+         FIG. 1.&mdash;To illustrate osmosis. In the<br />vessel <i>A</i> is a solution
+of sugar; in <i>B</i><br /> is pure water. The two are separated<br />
+by the mebrane <i>C</i>. The<br /> sugar passes through the membrane<br />
+into <i>B</i>.
+</div>
+
+<p><i>Absorption of food</i>.&mdash;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 <i>osmosis</i>. 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
+<a name="Page_30" id="Page_30"></a>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 <i>a</i>(Fig. 2). The force here concerned is a force
+known as <i>osmosis</i> or <i>dialysis</i>, 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.</p>
+
+<p><a name="bFigure_illustrating_osmosis" id="bFigure_illustrating_osmosis"></a></p>
+
+<div class="figright">
+             <img src="images/039fig2.png"
+             alt="FIG. 2." /><br />
+         FIG. 2.&mdash;In the bladder<br /> <i>A</i> is a sugar solution<br />In the
+vessel <i>B</i><br /> is pure water.<br /> Sugar passes out<br /> and water into<br /> the bladder
+until it<br /> rises in the tube<br /> to a.
+    </div>
+
+<p>This process of osmosis lies at the basis of the absorption of food from
+the alimentary canal.<a name="Page_31" id="Page_31"></a> 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 <i>dialyzable</i>&mdash;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 <i>A</i>, is the food mass in
+solution. At <i>B</i> are shown little projections of the intestinal wall,
+called <i>villi</i> extending into this food and covered by a membrane. One
+of these <i>villi</i> is shown more highly magnified in Fig. 4, in which <i>B</i>
+shows this membrane. Inside of these villi are blood-vessels, <i>C</i>, and
+it will be thus seen that the membrane, <i>B</i>, 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
+<a name="Page_32" id="Page_32"></a>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.</p>
+
+<p><a name="Diagram_of_the_intestinal_walls" id="Diagram_of_the_intestinal_walls"></a></p>
+
+<div class="figleft">
+             <img src="images/040fig3.png"
+             alt="FIG. 3" /><br />
+         FIG. 3&mdash;Diagram of the intestinal<br /> walls. <i>A</i>, lumen of
+intestine<br /> filled with digested food. <i>B</i>,<br /> villi, containing blood
+vessels.<br /> <i>C</i>, larger blood vessel, which<br /> carries blood with absorbed
+food<br /> away from the intestine.
+    </div>
+
+<p>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.</p>
+
+<p>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 <a name="Page_33" id="Page_33"></a>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<a name="Page_34" id="Page_34"></a> <i>vital</i> process, and not one simply
+controlled by physical forces like osmosis. Here our explanation runs
+against what we call <i>vital power</i> 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.</p>
+
+<p><a name="Diagram_of_a_single_villus" id="Diagram_of_a_single_villus"></a></p>
+<p><a name="Enlarged_figure_of_four_cells_in_the_villus_membrane" id="Enlarged_figure_of_four_cells_in_the_villus_membrane"></a></p>
+<div class="figright">
+             <img src="images/041fig4.png"
+             alt="FIG. 4." /><br />
+         FIG. 4.&mdash;Diagram of a single villus enlarged.<br /> <i>B</i>
+represents the membranous<br /> surface covering the villus; <i>C</i>, the
+blood-vessels<br /> within the villus.<br />
+FIG. 5.&mdash;An enlarged figure of four cells of<br /> the membrane
+<i>B</i> in Fig. 4. The free<br /> surface is at <i>a</i>; <i>f</i> shows fat droplets in<br />
+process of passage through the cells.
+    </div>
+
+<p><i>Circulation</i>.&mdash;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 <a name="Page_35" id="Page_35"></a>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.</p>
+
+<p>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&mdash;leaving out of consideration for a
+moment its muscular power&mdash;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&mdash;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 <a name="Page_36" id="Page_36"></a>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.</p>
+
+<p>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.</p>
+
+<p><a name="A_bit_of_muscle_showing_blood-vessels" id="A_bit_of_muscle_showing_blood-vessels"></a></p>
+<div class="figleft">
+             <img   src="images/044fig6.png"
+             alt="FIG. 6." /><br />
+        FIG. 6.&mdash;A bit of muscle with its blood-vessels:<br /> <i>a</i>, the
+muscle fibres; <i>b</i>, the minute blood-vessels.<br /> The fibres and vessels are
+bathed in lymph<br /> (not shown in the figure), and food material passes
+through<br /> the walls of the blood-vessels into this lymph.
+</div>
+
+<p>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&mdash;for instance, the muscles&mdash;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.<a name="Page_37" id="Page_37"></a> 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 <i>vital</i> processes which we defer for a moment.</p>
+
+<p><i>Respiration</i>.&mdash;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&mdash;so thin that it forms no
+<a name="Page_38" id="Page_38"></a>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.</p>
+
+<p>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
+h&aelig;moglobin. 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 h&aelig;moglobin is a normal constituent of the blood, and, being red in
+colour, gives the red colour to the blood. This h&aelig;moglobin 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 h&aelig;moglobin
+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 h&aelig;moglobin 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
+<a name="Page_39" id="Page_39"></a>what occurs as the foundation of respiration. The blood coming to the
+lungs contains h&aelig;moglobin, and since the oxygen pressure in the air is
+quite high, this h&aelig;moglobin 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 h&aelig;moglobin reaches them
+the association of the h&aelig;moglobin 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.</p>
+
+<p><i>Removal of Waste</i>.&mdash;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.</p>
+
+<p>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 <a name="Page_40" id="Page_40"></a>up that fuel into
+simple compounds&mdash;which may be chiefly considered as three&mdash;carbonic
+dioxide (CO<sub>2</sub>), water (H<sub>2</sub>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<sub>2</sub> and H<sub>2</sub>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.</p>
+
+<p>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 <a name="Page_41" id="Page_41"></a>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.</p>
+
+<p>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.</p>
+
+<p>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?</p>
+
+<p><i>Nervous System</i>.&mdash;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 <a name="Page_42" id="Page_42"></a>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.</p>
+
+<p>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.</p>
+
+<p>That there is a correlation between nervous energy and physical energy
+is, however, pretty <a name="Page_43" id="Page_43"></a>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.</p>
+
+<p>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 <a name="Page_44" id="Page_44"></a>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.</p>
+
+<p>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.</p>
+
+<p>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 <a name="Page_45" id="Page_45"></a>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.</p>
+
+<p>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 name="Page_46" id="Page_46"></a>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.</p>
+
+<p><i>Sensations</i>.&mdash;Pursuing this subject further, we next notice that it is
+possible to trace a connection between physical energy and <i>sensations</i>.
+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
+<a name="Page_47" id="Page_47"></a>broken, the external force has no longer any power of acting upon it,
+and the individual becomes deaf or blind.</p>
+
+<p><i>Mental Phenomena</i>.&mdash;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.</p>
+
+<p>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&mdash;a fact which,
+of course, <a name="Page_48" id="Page_48"></a>proves that mental power is dependent upon brain structure.
+Further, it is found that certain visible changes occur in certain parts
+of the brain&mdash;the brain cells&mdash;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.</p>
+
+<p><a name="Page_49" id="Page_49"></a><b>The Living Body is a Machine.</b>&mdash;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&mdash;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.</p>
+
+<p>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 <a name="Page_50" id="Page_50"></a>other than those
+which are at our command in the scientific laboratory.</p>
+
+<p><b>The Living Machine Constructive as well as Destructive.</b>&mdash;In one
+respect the living machine differs from all others. The action of all
+other machines results in the <i>destruction</i> of organized material, and
+thus in a <i>degradation of matter</i>. For example, a steam engine receives
+coal, a substance of high chemical composition, and breaks it into <i>more
+simple</i> 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 <i>destruction</i> 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.</p>
+
+<p>As a result of the action of the living machine, however, we have the
+opposite process of <i>construction</i> 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 <a name="Page_51" id="Page_51"></a>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. <i>Living machines alone increase the
+amount of chemical compounds of high complexity.</i></p>
+
+<p>We must notice, however, that this power of construction in distinction
+from destruction, is possessed only by one special class of living
+machines. <i>Green plants</i> 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 <a name="Page_52" id="Page_52"></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.</p>
+
+<p><b>The Vital Factor.</b>&mdash;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.<a name="Page_53" id="Page_53"></a> That a muscle is a
+machine in the sense of our definition is beyond question. But the
+problem of <i>why</i> 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.</p>
+
+<p>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?</p>
+
+<p>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 com<a name="Page_54" id="Page_54"></a>plicated 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.</p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="CHAPTER_II" id="CHAPTER_II"></a>CHAPTER II.</h2>
+
+<h3>THE CELL AND PROTOPLASM.</h3>
+
+
+<p><b>Vital Properties.</b>&mdash;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.</p>
+
+<p>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) <i>Irritability</i>, or the property possessed by living
+matter of reacting when stimulated. (2) <i>Movement</i>, or the power of
+contracting when stimulated. (3) <i>Metabolism</i>, 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) <i>Repro<a name="Page_55" id="Page_55"></a>duction</i>, 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.</p>
+
+<p>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.</p>
+
+<p><a name="Page_56" id="Page_56"></a>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 <i>cell</i>, and if we analyze the
+tissues of higher animals we find the ultimate parts to be <i>cells</i>.
+Thus, in either direction, the study of the cell is forced upon us.</p>
+
+<p>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 <i>in the living
+machine</i>. 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 <a name="Page_57" id="Page_57"></a>occurs in the chemist's
+laboratory, or even in a bit of dead protoplasm, it simply gives rise to
+heat?</p>
+
+<p>One of the primary questions to demand attention in this search is
+whether we are to find the explanation, at the bottom, a <i>chemical</i> or a
+<i>mechanical</i> 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:</p>
+
+<p>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 <a name="Page_58" id="Page_58"></a>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.</p>
+
+<p>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.</p>
+
+<p><b>The Discovery of Cells.</b>&mdash;It is difficult for us to-day to have any
+adequate idea of the wonderful flood of light that was thrown upon
+scientific <a name="Page_59" id="Page_59"></a>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.</p>
+
+<p>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, <a name="Page_60" id="Page_60"></a>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.</p>
+
+<p>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.</p>
+
+<p><b>The Cell Doctrine</b>.&mdash;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 <a name="Page_61" id="Page_61"></a>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 <a name="Page_62" id="Page_62"></a>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.</p>
+
+<p><a name="A_bit_of_bark_showing_cellular_structure" id="A_bit_of_bark_showing_cellular_structure"></a></p>
+<div class="figright">
+             <img src="images/069fig7.png"
+             alt="FIG. 7." /><br />
+         FIG. 7.&mdash;A bit of bark showing cellular structure.
+    </div>
+
+<p>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 <i>cytoblastema</i>, 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.</p>
+
+<p>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 <a name="Page_63" id="Page_63"></a>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.</p>
+
+<p>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, <i>a</i> and
+<i>b</i>). 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, <i>b</i> to <i>f</i>,) 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 <a name="Page_64" id="Page_64"></a>cell. In such a history each cell comes
+from pre-existing cells and a cytoblastema plays no part.</p>
+
+<p><a name="Successive_stages_in_the_division_of_the_developing_egg" id="Successive_stages_in_the_division_of_the_developing_egg"></a></p>
+<div class="figcenter">
+             <img   src="images/071fig8.png"
+             alt="FIG. 8." /><br />
+          FIG. 8.&mdash;Successive stages in the division of the
+developing egg.
+    </div>
+
+<p>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.</p>
+
+<p><a name="A_typical_cell" id="A_typical_cell"></a></p>
+<div class="figright">
+             <img   src="images/073fig9.png"
+             alt="FIG. 9." /><br />
+         FIG. 9.&mdash;A cell; <i>cw</i> is the cell wall;<br /> <i>pr</i>, the cell
+substance; <i>n</i>, the<br /> nucleus.
+    </div>
+
+<p><b>The Cell</b>.&mdash;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 <i>cell wall
+(cw)</i> 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 <i>formed</i><a name="Page_65" id="Page_65"></a> material in distinction from the rest, which is
+<i>active</i> material. Inside this vesicle is contained a somewhat
+transparent semifluid material which has received various names, but
+which for the present we will call <i>cell substance</i> (Fig. 9, <i>pr</i>). 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 <i>nucleus</i> (Fig. 9, <i>n</i>). 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 <i>nucleoli</i>. 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.</p>
+
+<p><b>Cellular Structure of Organisms</b>.&mdash;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.</p>
+
+<p><a name="Page_66" id="Page_66"></a></p>
+<p><a name="Cells_at_a_root_tip" id="Cells_at_a_root_tip"></a></p>
+<div class="figcenter">
+             <img src="images/074fig10.png"
+             alt="FIG. 10." /><br />
+         FIG. 10.&mdash;Cells at a root tip.
+    </div>
+
+<p>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.</p>
+
+<p><a name="Section_of_a_leaf_showing_cells_of_different_shapes" id="Section_of_a_leaf_showing_cells_of_different_shapes"></a></p>
+<div class="figleft">
+             <img src="images/074fig11.png"
+             alt="FIG. 11." /><br />
+         FIG. 11.&mdash;Section of a leaf showing<br />cells of
+different shapes.
+    </div>
+
+<p>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 <a name="Page_67" id="Page_67"></a>tissue would
+readily show itself to be made of similar cells.</p>
+
+<p>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.</p>
+
+<p><a name="Plant_cells_with_thick_walls_from_a_fern" id="Plant_cells_with_thick_walls_from_a_fern"></a></p>
+<div class="figleft">
+             <img   src="images/075fig12.png"
+             alt="FIG. 12." /><br />
+         FIG. 12.&mdash;Plant cells with thick walls, from<br /> a fern.
+    </div>
+
+<p>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 <a name="Page_68"
+id="Page_68"></a>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.</p>
+
+<p><a name="Section_of_potato" id="Section_of_potato"></a></p>
+<div class="figright">
+             <img   src="images/075fig13.png"
+             alt="FIG. 13." /><br />
+         FIG. 13.&mdash;Section of a potato showing different<br /> shaped
+cells, the inner and larger ones being<br /> filled with grains of starch.
+    </div>
+
+<p>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 (<i>c</i>) 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 (<i>n</i>), but if
+the muscle be studied in a young condition its cellular <a
+name="Page_69" id="Page_69"></a>structure is more evident.</p>
+
+<p><a name="Various_shaped_wood_cells_from_plant_tissue" id="Various_shaped_wood_cells_from_plant_tissue"></a></p>
+<div class="figleft">
+             <img   src="images/076fig14.png"
+             alt="FIG. 14." /><br />
+         FIG. 14.&mdash;Various shaped wood cells<br /> from plant tissue.
+    </div>
+
+<p><a name="A_bit_of_cartilage" id="A_bit_of_cartilage"></a></p>
+<div class="figright">
+             <img   src="images/076fig15.png"
+             alt="FIG. 15." /><br />
+         FIG. 15.&mdash;A bit of cartilage.
+    </div>
+
+<p>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.</p>
+
+<p><a name="Frogs_blood" id="Frogs_blood"></a></p>
+<div class="figright">
+             <img   src="images/077fig16.png"
+             alt="FIG. 16." /><br />
+         FIG. 16.&mdash;Frog's blood: <i>a</i> and<br /> <i>b</i> are the cells; <i>c</i> is
+the liquid.
+    </div>
+
+<p><a name="A_bit_of_bone" id="A_bit_of_bone"></a></p>
+<div class="figleft">
+             <img   src="images/077fig17.png"
+             alt="FIG. 17." /><br />
+         FIG. 17.&mdash;A bit of bone, showing<br /> the cells imbedded in
+the bony matter.
+    </div>
+
+<p>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 <a name="Page_70" id="Page_70"></a>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.</p>
+
+<p><a name="Connective_tissue" id="Connective_tissue"></a></p>
+<div class="figleft">
+             <img   src="images/078fig18.png"
+             alt="FIG. 18." /><br />
+         FIG. 18.&mdash;Connective<br /> tissue. The cells<br /> of the tissue are<br />
+shown at <i>c</i>, and the<br /> fibres or formed<br /> matter at <i>f</i>.
+    </div>
+
+<p>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.</p>
+
+<p><a name="A_piece_of_nerve_fibre" id="A_piece_of_nerve_fibre"></a></p>
+<div class="figright">
+             <img   src="images/078fig19.png"
+             alt="FIG. 19." /><br />
+         FIG. 19. A piece of nerve fibre,<br /> showing the cell with
+its<br /> nucleus at <i>n</i>.
+    </div>
+
+<p>It should be noticed further that there are some animals and plants in
+which the whole <a name="Page_71" id="Page_71"></a>animal is composed of a single cell. These organisms
+are usually of extremely minute size, and they comprise most of the
+so-called animalcul&aelig; 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.</p>
+
+<p><a name="A_muscle_fibre" id="A_muscle_fibre"></a></p>
+<div class="figright">
+             <img   src="images/079fig20.png"
+             alt="FIG. 20." /><br />
+         FIG. 20.&mdash;A muscle fibre. The<br /> nucleii are shown at <i>n</i>.
+    </div>
+
+<p>Such, in brief, is the cell to whose activities <a name="Page_72" id="Page_72"></a>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.</p>
+<p><a name="A_complex_cell_vorticella" id="A_complex_cell_vorticella"></a></p>
+<div class="figleft">
+             <img   src="images/079fig21.png"
+             alt="FIG. 21." /><br />
+         FIG. 21.&mdash;A complex cell. It is<br /> an entire animal, but
+composed<br /> of only one cell.
+    </div>
+
+<p><b>The Cell Wall.</b>&mdash;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 re<a name="Page_73" id="Page_73"></a>garded the other parts of the
+cell as of importance.</p>
+
+<p><a name="An_amoeba" id="An_amoeba"></a></p>
+<div class="figright">
+             <img   src="images/081fig22.png"
+             alt="FIG. 22." /><br />
+         FIG. 22.&mdash;An am&#339;ba. A single<br /> cell without cell wall. <i>n</i>
+is the nucleus; <i>f</i>, a bit<br /> of food which the cell has absorbed.
+    </div>
+
+<p>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&mdash;as, for example,
+the white blood-cells. Fig. 22 shows an am&#339;ba, 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 <a name="Page_74" id="Page_74"></a>made. This was the formulation of the doctrine of the nature
+of <i>protoplasm</i>.</p>
+
+<p>Protoplasm.&mdash;(a) <i>Discovery</i>. 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 <i>protoplasm</i> 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 proto<a name="Page_75" id="Page_75"></a>plasm, 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 <i>all</i> cells essentially <i>identical.</i> 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.</p>
+
+<p>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&mdash;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&mdash;irritability, contractibility,
+assimilation, and reproduction. It was a compound which seemingly
+deserved the name of "<i>physical basis of life</i>", which was soon given to
+it by Huxley. With this conception of protoplasm as the physical basis
+of life <a name="Page_76" id="Page_76"></a>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 <i>one</i> 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.</p>
+
+<p>(b) <i>Nature of Protoplasm</i>.&mdash;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 <a name="Page_77" id="Page_77"></a>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.</p>
+
+<p>(c) <i>Significance of Protoplasm</i>.&mdash;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 com<a name="Page_78" id="Page_78"></a>pound 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
+<i>chemical theory of life</i>, may be outlined somewhat as follows:</p>
+
+<p>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<sub>2</sub>), water (H<sub>2</sub>O), and
+ammonia (NH<sub>3</sub>), 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 meth<a name="Page_79" id="Page_79"></a>ods. 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.</p>
+
+<p>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 oxi<a name="Page_80" id="Page_80"></a>dation 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.</p>
+
+<p>In other words, the formulation of the doctrine of protoplasm made it
+possible to assume that <i>life</i> 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 <i>aquacity</i> to the
+properties possessed by water, so we have actually given the name
+<i>vitality</i> 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
+<a name="Page_81" id="Page_81"></a>mechanics, we have here in this mechanical theory of life the complete
+reduction of the body to a machine.</p>
+
+<p><b>The Reign of Protoplasm.</b>&mdash;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 <i>origin</i> of life
+by natural means appeared now to be a simple matter.</p>
+
+<p>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 remark<a name="Page_82" id="Page_82"></a>able 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<sub>2</sub> and H<sub>2</sub>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.</p>
+
+<p><a name="Page_83" id="Page_83"></a>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 <i>undifferentiated
+protoplasm</i>. 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 <a name="Page_84" id="Page_84"></a>reached. Huxley named
+his substance <i>Bathybias</i>, 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.</p>
+
+<p>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 <a name="Page_85" id="Page_85"></a>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.</p>
+
+<p><b>The Decline of the Reign of Protoplasm.</b>&mdash;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 reve<a name="Page_86" id="Page_86"></a>lations 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.</p>
+
+<p><a name="A_cell_as_it_appears_to_the_modern_microscope" id="A_cell_as_it_appears_to_the_modern_microscope"></a></p>
+<div class="figleft">
+             <img   src="images/094fig23.png"
+             alt="FIG. 23." /><br />
+         FIG. 23.&mdash;A cell as it appears to the modern<br /> microscope.
+<i>a</i>, protoplasmic reticulum;<br /> <i>b</i>, liquid in its meshes; <i>c</i>, nuclear
+membrane;<br /> <i>d</i>, nuclear reticulum; <i>e</i>, chromatin<br /> reticulum; <i>f</i>,
+nucleolus; <i>g</i>, centrosome;<br /> <i>h</i>, centrosphere; <i>i</i>, vacuole;<br /> <i>j</i>, inert
+bodies.
+    </div>
+
+
+<p><b>Structure of Protoplasm</b>.&mdash;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
+<a name="Page_87" id="Page_87"></a>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,
+<i>a</i>). 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, <i>b</i>). 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 <i>microsomes</i>. 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
+&mdash;granules of excessive minuteness&mdash;occur in protoplasm and are closely
+connected with the fibres (Fig. 23, <i>a</i>).</p>
+
+<p><b>The Nucleus.</b>&mdash;(a) <i>Presence of a Nucleus</i>.&mdash;If protoplasm has thus
+become a new substance in our minds as the result of the discoveries of
+the last <a name="Page_88" id="Page_88"></a>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.</p>
+
+<p>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 <i>Monera</i> to the group. As the
+method of studying cells improved microscopists learned better methods
+of discerning the presence of the <a name="Page_89" id="Page_89"></a>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.</p>
+
+<p><a name="A_cell_cut_into_pieces_each_containing_a_bit_of_nucleus" id="A_cell_cut_into_pieces_each_containing_a_bit_of_nucleus"></a></p>
+<div class="figright">
+             <img   src="images/097fig24.png"
+             alt="FIG. 24." /><br />
+         FIG. 24.&mdash;A cell cut into three pieces, each<br /> containing a
+bit of the nucleus. Each<br /> continues its life indefinitely, soon acquiring
+the form of the original<br /> as at <i>C</i>.
+    </div>
+<p>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.</p>
+
+<p>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
+<a name="Page_90" id="Page_90"></a>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.</p>
+<p><a name="A_cell_cut_in_pieces_only_one_of_which_contains_any_nucleus" id="A_cell_cut_in_pieces_only_one_of_which_contains_any_nucleus"></a></p>
+<div class="figleft">
+             <img   src="images/098fig25.png"
+             alt="FIG. 25." /><br />
+          FIG. 25.&mdash;A cell cut into three pieces, only<br /> one of
+which, No. 2, contains any<br /> nucleus. This fragment soon acquires<br /> the
+original form and continues its life<br /> indefinitely, as shown at <i>B</i>. The
+other<br /> two pieces though living for a time,<br /> die without reproducing.
+    </div>
+<p>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 <a name="Page_91" id="Page_91"></a>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.</p>
+
+<p>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.</p>
+
+<p>(b) <i>Structure of the Nucleus</i>.&mdash;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
+<a name="Page_92" id="Page_92"></a>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, <i>c</i>). 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, <i>f</i>).
+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,
+<i>d</i>). 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 <i>linin</i>, 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 <i>chromatin</i> (Fig, 23, <i>e</i>), although in more
+recent times other names have been given to it. Of all parts of the cell
+this chromatin is the most remarkable.<a name="Page_93" id="Page_93"></a> 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.</p>
+
+<p><a name="Different_forms_of_nucleii" id="Different_forms_of_nucleii"></a></p>
+<div class="figcenter">
+             <img   src="images/101fig26.png"
+             alt=" FIG. 26." /><br />
+          FIG. 26.&mdash;Different forms of nucleii.
+    </div>
+
+<p>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&mdash;especially in young cells&mdash;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; <a name="Page_94" id="Page_94"></a>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.</p>
+
+<p><b>Centrosome.</b>&mdash;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 <i>centrosome.</i> This body is shown at Fig. 23, <i>g</i>. 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
+<a name="Page_95" id="Page_95"></a>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.</p>
+
+<p><b>Function of the Nucleus.</b>&mdash;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., <i>cell division</i> and the
+<i>fertilization of the egg</i>. 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.</p>
+
+<p><a name="Page_96" id="Page_96"></a></p>
+
+<p><b>Cell Division or Karyokinesis.</b>&mdash;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.</p>
+<p><a name="Two_stages_in_cell_division" id="Two_stages_in_cell_division"></a></p>
+<div class="figcenter">
+             <img   src="images/104fig27-28.png"
+             alt="FIG. 27-28." /><br />
+             FIG. 27 shows the resting stage with the
+chromatin, <i>cr</i>, in the form of a network within the nuclear membrane
+and the centrosome, <i>ce</i>, already divided into two.<br />
+FIG. 28.&mdash;The chromatin is broken into threads or chromosomes,
+<i>cr.</i> The centrosomes show radiating fibres.
+    </div>
+
+
+<p>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 <a name="Page_97" id="Page_97"></a>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 <i>chromosomes</i> (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.</p>
+
+
+
+<p>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 <a name="Page_98" id="Page_98"></a>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).</p>
+
+<p><a name="Stages_in_cell_division" id="Stages_in_cell_division"></a></p>
+<div class="figcenter">
+             <img   src="images/106fig29-30.png"
+             alt="FIG. 29-30." /><br />
+         FIG. 29.&mdash;The centrosomes are separating but are
+connected by fibres.<br />FIG. 30.&mdash;The centrosomes are separate and the
+equatorial plate of chromosomes, <i>cr</i>, is between them.
+    </div>
+
+<p>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, <a name="Page_99" id="Page_99"></a>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 <i>equatorial plate</i>.</p>
+
+<p>The next step is the most significant of all. It consists in the
+splitting of each chromosome into two equal halves. The threads <i>do not
+divide in their middle but split lengthwise</i>, 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.</p>
+
+<p>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 <a name="Page_100" id="Page_100"></a>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 <i>across</i>
+the thread the two halves would be unlike, but taking place as it does
+by a <i>longitudinal splitting</i> 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.</p>
+
+<p><a name="Latest_stages_in_cell_division" id="Latest_stages_in_cell_division"></a></p>
+<div class="figcenter">
+             <img   src="images/108fig31-32.png"
+             alt="FIG. 31-32." /><br />
+         FIG. 31.&mdash;Stage showing the two halves of the
+chromosomes separated from each other.<br />
+FIG. 32.&mdash;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.
+    </div>
+
+<p>The next step in the cell division consists in the separation of the two
+halves of the chromosomes. Each half of each chromosome separates <a name="Page_101" id="Page_101"></a>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 <i>new nucleii</i>, with
+evidently exactly the same number of chromosomes as the original, and
+with characters identical to each other and to the original (Fig. 32).</p>
+
+<p>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&mdash;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 <a name="Page_102" id="Page_102"></a>nucleus and
+two centrosomes, and this is exactly the same sort of structure with
+which the process began. (<i>See Frontispiece</i>.)</p>
+
+<p>Viewed as a whole, we may make the following general summary of this
+process. The essential object of this complicated phenomena of
+<i>karyokinesis</i> 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.</p>
+
+<p><b>Fertilization of the Egg.</b>&mdash;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.</p>
+
+<p>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 <a name="Page_103"
+id="Page_103"></a>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:</p>
+
+<p><a name="An_egg" id="An_egg"></a></p>
+<div class="figright">
+             <img   src="images/111fig33.png"
+             alt="FIG. 33." /><br />
+         FIG. 33&mdash;An egg showing the cell<br /> substance and
+the nucleus, the latter<br /> containing chromosomes in large<br /> number and a
+nucleolus.
+    </div>
+
+<p>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 <a name="Page_104" id="Page_104"></a>for union
+with the male cell, but must first go through a series of somewhat
+remarkable changes constituting what is called <i>maturation</i> 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.</p>
+
+<p><a name="Stages_in_the_process_of_fertilization_of_the_egg_1" id="Stages_in_the_process_of_fertilization_of_the_egg_1"></a></p>
+<div class="figcenter">
+             <img   src="images/112fig34-35.png"
+             alt="FIG. 35-35" /><br />
+         FIG. 34. This and the following figures
+represent the process of fertilization of an egg. In all figures <i>cr</i> is
+the chromosomes; <i>cs</i> represents the cell substance (omitted in the
+following figures); <i>mc</i> is the male reproductive cell lying in contact
+with the egg; <i>mn</i> is the male nucleus after entering the egg.<br />
+FIG. 35.&mdash;The egg centrosome has divided, and
+the male cell with its centrosome has entered the egg.
+    </div>
+
+<p>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 <a name="Page_105" id="Page_105"></a>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.</p>
+
+<p><a name="Stages_in_the_process_of_fertilization_of_the_egg_2" id="Stages_in_the_process_of_fertilization_of_the_egg_2"></a></p>
+<div class="figcenter">
+             <img   src="images/113fig36-37.png"
+             alt="FIG. 36-37" /><br />
+         FIG. 36&mdash;The egg centrosomes have changed their position.
+The male cell with its centrosome remains inactive until the stage
+represented in FIG. 42.<br />FIG. 37&mdash;Beginning of the first division for removing
+superfluous chromosomes.
+    </div>
+
+<p>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 <a name="Page_106" id="Page_106"></a>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 (<i>pc'</i> 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 <a
+name="Page_107" id="Page_107"></a>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 (<i>pc"</i>) 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 <i>polar cells</i>, 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.</p>
+
+<p><a name="Stages_in_fertilization_of_the_egg" id="Stages_in_fertilization_of_the_egg"></a></p>
+<div class="figcenter">
+             <img   src="images/114fig38-39-40.png"
+             alt="FIG. 38-39-40" /><br />
+         FIG.38&mdash;First division complete and first polar cell
+formed, <i>pc'</i>.<br />FIG.39.&mdash;Formation of the second polar cell, <i>pc"</i>.<br />
+FIG.40.&mdash;Completion of the process of extrusion of the
+chromatic material; <i>fn</i> shows the two chromosomes retained in the egg
+forming the female pronucleus. The centrosome has disappeared.
+    </div>
+
+<p>Previously to this process the other sexual cell, the <i>spermatozoon</i>, or
+male reproductive cell, has been undergoing a somewhat similar process.
+This is also a true cell (Fig. 34, <i>mc</i>), 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
+<a name="Page_108" id="Page_108"></a>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.</p>
+
+<p>Now by some mechanical means these two reproductive cells are brought in
+contact with each other, shown in Fig. 34, and as soon as they are
+brought into each other's vicinity the male cell buries its head in the
+body of the egg. The tail by which it has been moving is cast off, and
+the head containing the chromosomes and the centrosome enters the egg,
+forming what is called the male pronucleus (Fig. 35-38, <i>mn</i>). 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.</p>
+
+<p>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,
+<i>ce</i>). 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 re<a name="Page_109" id="Page_109"></a>tained 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.</p>
+
+<p><a name="Latest_stages_in_the_fertilization_of_the_egg" id="Latest_stages_in_the_fertilization_of_the_egg"></a></p>
+<div class="figcenter">
+             <img   src="images/117fig41-42.png"
+             alt="FIG. 41-42" /><br />
+          FIG. 41.&mdash;The chromosomes in the male and female
+pronucleii have resolved into a network. The male centrosome begins to
+show signs of activity.<br />FIG. 42.&mdash;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; <i>mcr</i>, represents
+the chromosomes from the male nucleus; <i>fcr</i>, the chromosomes from the
+female nucleus.
+    </div>
+
+<p>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, <i>ce</i>), and there is thus formed what is
+known as the <i>male pronucleus</i> (Fig. 35-40, <a name="Page_110" id="Page_110"></a><i>mn</i>). The remains of the
+egg nucleus, after having discharged the polar cells, form the <i>female
+nucleus</i> (Fig. 40, <i>fn</i>). 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, <i>ce</i>), 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 <i>segmentation nucleus</i>. 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 <i>fertilized egg</i>
+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, <a name="Page_111" id="Page_111"></a>the male and
+female chromosomes perhaps always remaining distinct.</p>
+
+<p><a name="Two_stages_in_the_division_of_the_egg" id="Two_stages_in_the_division_of_the_egg"></a></p>
+<div class="figcenter">
+             <img   src="images/119fig43-44.png"
+             alt="FIG. 43-44" /><br />
+          FIG. 43.&mdash;An equatorial plate is formed and each
+chromosome has split into two halves by longitudinal division.<br />
+FIG. 44.&mdash;The halves of the chromosomes have separated to
+form two nucleii, each with male and female chromosomes. The egg has
+divided into two cells.
+    </div>
+
+<p>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 <a name="Page_112" id="Page_112"></a>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.</p>
+
+<p><b>The Significance of Fertilization.</b>&mdash;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
+<i>both</i> parents. Hence it follows that the hereditary traits must be
+particularly associated with the chromosomes.</p>
+
+<p>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 <a name="Page_113" id="Page_113"></a>cells, which introduces into the offspring chromatic
+material from <i>two</i> 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.</p>
+
+<p>The cell of an animal thus consists of three somewhat distinct but
+active parts&mdash;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, <a name="Page_114" id="Page_114"></a>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.</p>
+
+<p><b>What is Protoplasm?</b>&mdash;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. <i>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.</i> 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, <a name="Page_115" id="Page_115"></a>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.</p>
+
+<p>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
+<a name="Page_116" id="Page_116"></a>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.<a name="Page_117" id="Page_117"></a> 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.</p>
+
+<p><b>Reaction against the Cell Doctrine.</b>&mdash;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 <a name="Page_118" id="Page_118"></a>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.</p>
+
+<p>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.</p>
+
+<p>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 <a name="Page_119" id="Page_119"></a>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.</p>
+
+<p>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
+<a name="Page_120" id="Page_120"></a>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.</p>
+
+<p>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.</p>
+
+<p><a name="Page_121" id="Page_121"></a>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.</p>
+
+<p>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&mdash;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 condi<a name="Page_122" id="Page_122"></a>tions 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&mdash;such, for instance, as
+the inheritance of a congenital mark&mdash;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.</p>
+
+<p>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 <a name="Page_123" id="Page_123"></a>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.</p>
+
+<p><b>Fundamental Vital Activities as Located in Cells.</b>&mdash;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&mdash;the cells, and it is these properties which we are
+trying to trace to their source as a foundation of vital activity.</p>
+
+<p>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 <a name="Page_124" id="Page_124"></a>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.</p>
+
+<p>Again, we have apparently conclusive evidence that the <i>nucleus</i>
+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 destruc<a name="Page_125" id="Page_125"></a>tive 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.</p>
+
+<p>It is equally clear that the <i>cell substance</i> 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.</p>
+
+<p>The <i>centrosome</i> 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.</p>
+
+<p>In brief, then, the cell body is a machine for <a name="Page_126" id="Page_126"></a>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&mdash;that is, to convert it into
+its own substance. The nucleus further contains a marvellous
+material&mdash;chromatin&mdash;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.</p>
+
+<p>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 <i>dies</i>. 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, <a name="Page_127" id="Page_127"></a>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.</p>
+
+<p>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.</p>
+
+<p>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&mdash;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.</p>
+
+<p>There are, of course, other functions of living things besides the few
+simple ones which we have considered. But these are the fundamental
+ones; <a name="Page_128" id="Page_128"></a>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.</p>
+
+<p>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.</p>
+
+<p><b>Summary.</b>&mdash;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 <a name="Page_129" id="Page_129"></a>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.</p>
+
+<p>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 <a name="Page_130" id="Page_130"></a>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&mdash;life.</p>
+
+
+
+<hr style="width: 65%;" /><p><a name="Page_131" id="Page_131"></a></p>
+<h2><a name="PART_II" id="PART_II"></a>PART II.</h2>
+
+<h3><i>THE BUILDING OF THE LIVING MACHINE</i>.</h3>
+
+<hr style='width: 45%;' />
+
+<h3><a name="CHAPTER_III" id="CHAPTER_III"></a>CHAPTER III.</h3>
+
+<h4>THE FACTORS CONCERNED IN THE BUILDING OF THE LIVING MACHINE.</h4>
+
+
+<p>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 <a name="Page_132" id="Page_132"></a>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?</p>
+
+<p>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.</p>
+
+<p><a name="Page_133" id="Page_133"></a><b>History of the Living Machine.</b>&mdash;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 <a name="Page_134" id="Page_134"></a>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.</p>
+
+<p><a name="A_group_of_cells_resulting_from_division_the_first_step_in_machine_building" id="A_group_of_cells_resulting_from_division_the_first_step_in_machine_building"></a></p>
+<div class="figright">
+             <img   src="images/143fig45.png"
+             alt="FIG. 45" /><br />
+          FIG. 45.<br /> A group of cells<br /> resulting from<br /> division,
+representing<br /> the first step in<br /> machine making.
+    </div>
+<p>This gradual construction of the living machines has been called
+<i>Organic Evolution</i>, or the <i>Theory of Descent</i>. 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 <a name="Page_135" id="Page_135"></a>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 <a name="Page_136" id="Page_136"></a>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.</p>
+
+<p><a name="A_later_step_in_machine_building_the_gastrula" id="A_later_step_in_machine_building_the_gastrula"></a></p>
+<div class="figleft">
+             <img   src="images/143fig46.png"
+             alt="FIG. 46" /><br />
+          FIG. 46.<br /> A later step in machinebuilding in which<br /> the
+outer cells have acquired different<br /> form and function from the inner
+cells:<br /> <i>ec</i>, the outer cells, whose duties are<br />protective; <i>en</i>, the
+inner cells engaged<br /> in digesting food.
+    </div>
+
+<p>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 <a name="Page_137" id="Page_137"></a>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.</p>
+
+<p>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 his<a name="Page_138" id="Page_138"></a>tory has been simply that of perfecting
+the early machines rather than the production of new types.</p>
+
+<p><b>Evidence for this History.</b>&mdash;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.</p>
+
+<p>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.</p>
+
+<p><a name="Page_139" id="Page_139"></a><b>Historical.</b>&mdash;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 <a name="Page_140" id="Page_140"></a>able to trace
+with accuracy how the somewhat more generalized forms of earlier days
+were changed to produce our modern animals.</p>
+
+<p>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.</p>
+
+<p><b>Embryological.</b>&mdash;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 <a name="Page_141" id="Page_141"></a>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.</p>
+
+<p>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 <a name="Page_142" id="Page_142"></a>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.</p>
+
+<p><b>Anatomical.</b>&mdash;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 <a name="Page_143" id="Page_143"></a>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.</p>
+
+<p>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.</p>
+
+<p>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.<a name="Page_144" id="Page_144"></a> 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 <a name="Page_145" id="Page_145"></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.</p>
+
+<p><a name="The_arm_of_a_monkey" id="The_arm_of_a_monkey"></a></p>
+<p><a name="The_arm_of_a_bird" id="The_arm_of_a_bird"></a></p>
+<p><a name="The_arm_of_an_ancient_half-bird_half-reptile_animal" id="The_arm_of_an_ancient_half-bird_half-reptile_animal"></a></p>
+
+<div class="figcenter">
+             <img   src="images/152fig47-48-49.png"
+             alt="FIG. 47-48-49" /><br />
+          FIG. 47.&mdash;The arm of a monkey, a prehensile appendage.<br />
+FIG. 48.&mdash;The arm of a bird, a flying appendage. In life
+covered with feathers.<br />FIG. 49.&mdash;The arm of an ancient half-bird half-reptile
+animal. In life covered with feathers and serving as a wing.
+    </div>
+
+<p><b>Significance of these Sources of History.</b>&mdash;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 arch&aelig;ologists 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
+<a name="Page_146" id="Page_146"></a>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.</p>
+
+<p>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.</p>
+
+<p>Now all this is very germane to our subject.<a name="Page_147" id="Page_147"></a> 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 <a name="Page_148" id="Page_148"></a>action and always tending in the same direction of
+greater complexity of structure.</p>
+
+<p>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.</p>
+
+<p><b>Forces at Work in the Building of the Living Machine.</b>&mdash;There are three
+primary factors which lie at the bottom of the whole process. They are&mdash;</p>
+
+<p>1. <i>Reproduction</i>, which preserves type from generation to generation.</p>
+
+<p>2. <i>Variation</i>, which modifies type from generation to generation.</p>
+
+<p>3. <i>Heredity</i>, which transmits characters from generation to generation.</p>
+
+<p>Each must be considered by itself.</p>
+
+<p><b>Reproduction.</b>&mdash;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 <a name="Page_149" id="Page_149"></a>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.</p>
+
+<p>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 <a name="Page_150" id="Page_150"></a>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&mdash;a rate of increase plainly incompatible with
+the continued existence of other animals.</p>
+
+<p>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.</p>
+
+<p><b>Heredity.</b>&mdash;The offspring which arise by these processes of division
+are like each other, and like <a name="Page_151" id="Page_151"></a>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.</p>
+
+<p>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.</p>
+
+<p><a name="Page_152" id="Page_152"></a>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.</p>
+
+<p>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 <a name="Page_153" id="Page_153"></a>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 <i>all</i> hereditary characters, but as development
+proceeds these are gradually portioned out among the daughter cells
+until the adult is formed.</p>
+
+<p>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 <a name="Page_154" id="Page_154"></a>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, <i>so long as it remained unaltered in character</i>, 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.</p>
+
+<p>Now whether this undifferentiated <i>germ plasm</i>, 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 <a name="Page_155" id="Page_155"></a>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.</p>
+
+<p>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 <i>continuity of germ plasm</i>.</p>
+
+<p>Such appears to be, at least in part, the ma<a name="Page_156" id="Page_156"></a>chinery 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 in<a name="Page_157" id="Page_157"></a>herits 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.</p>
+
+<p><a name="Diagram_to_illustrate_the_principle_of_heredity" id="Diagram_to_illustrate_the_principle_of_heredity"></a></p>
+<div class="figleft">
+             <img   src="images/164fig50.png"
+             alt="FIG. 50" /><br />
+          FIG. 50.&mdash;Diagram illustrating<br /> the principle of
+heredity.
+    </div>
+
+<p><i>A</i> represents an egg of a starfish. From one half, the unshaded
+portion, develops the starfish of the next generation, <i>B</i>. The other is
+distributed without change in the ovaries, <i>ov</i>, of the individual, <i>B</i>.
+From these ovaries arises the next egg, <i>A'</i>, with its germ plasm. This
+germ plasm is evidently identical with that in <i>A</i>, since it is merely a
+bit of the same handed down through the individual, <i>B</i>. In the
+development of the next generation the process is repeated, and hence
+<i>B'</i> will be like <i>B</i>, 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.]</p>
+
+<p>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 <i>precisely like the last</i>, 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 <i>variation</i>.</p>
+
+<p><b>Variation.</b>&mdash;Variation is the principle that produces <i>modification of
+type</i>. 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 <a name="Page_158" id="Page_158"></a>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.</p>
+
+<p>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 <i>congenital variations</i>.</p>
+
+<p><a name="Page_159" id="Page_159"></a>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 <i>acquired variations</i>.</p>
+
+<p>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 <a name="Page_160" id="Page_160"></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.</p>
+
+<p>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.</p>
+
+<p>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 <a name="Page_161" id="Page_161"></a>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 <i>identical</i> 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.</p>
+
+<p>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
+<a name="Page_162" id="Page_162"></a>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 <i>arm</i> 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.</p>
+
+<p>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
+<a name="Page_163" id="Page_163"></a>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 <i>identical with that from which either parent came, but from
+some of this material mixed with a similar amount from a different
+parent</i>. 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 <i>congenital
+variations</i>. Sexual reproduction thus results in congenital variations;
+and if congenital variations are necessary for the evolution of the
+living machine&mdash;and we shall soon see reason for believing that they
+are&mdash;we find that sexual reproduction is a device adopted for bringing
+out such congenital variations.</p>
+
+<p><b>Inheritance of Variations.</b>&mdash;The reason why congenital variations are
+needed for the evolution of the living machine is clear enough.
+Evanescent <a name="Page_164" id="Page_164"></a>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.</p>
+
+<p>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.</p>
+
+<p>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 <a name="Page_165" id="Page_165"></a>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.</p>
+
+<p>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
+<i>can not</i> 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 <a name="Page_166" id="Page_166"></a>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 <a name="Page_167" id="Page_167"></a>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.</p>
+
+<p>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.</p>
+
+<p><b>Method of Machine Building.</b>&mdash;<i>Natural Selection.</i> 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.<a name="Page_168" id="Page_168"></a> We need do no more than
+outline the subject, since it is well understood by all. The basis of
+the method is <i>natural selection</i>, which acts in this machine building
+something as follows:</p>
+
+<p>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 <i>struggle for existence</i> 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 repre<a name="Page_169" id="Page_169"></a>sented 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 <i>part of the machine</i>.
+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.</p>
+
+<p>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 <a name="Page_170" id="Page_170"></a>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.</p>
+
+<p><b>Migration and Isolation.</b>&mdash;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 <i>fix</i> in the machine
+variations which sexual union or other influences inaugurate. Now in the
+history of the earth's surface there have been many <a name="Page_171" id="Page_171"></a>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.</p>
+
+<p><b>Direct Influence of the Environment.</b>&mdash;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 <i>use</i> and <i>disuse</i>. 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.</p>
+
+<p>Now all these effects are direct modifications of the machine, and if
+they are only transmitted to following generations so as to become
+<i>permanent</i> 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 <a name="Page_172" id="Page_172"></a>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
+<i>any</i> 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 <a name="Page_173" id="Page_173"></a>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.</p>
+
+<p><b>Consciousness.</b>&mdash;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 <i>indirect</i> one.
+Consciousness leads to effort, and effort has a direct influence in
+development. For example, an animal is <a name="Page_174" id="Page_174"></a>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 <i>habit</i> of flight would then develop
+the <i>power</i> 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.</p>
+
+<p>But, after all, these are only questions of the <a name="Page_175" id="Page_175"></a>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&mdash;reproduction, variation, and
+heredity.</p>
+
+<p><b>Summary of Nature's Power of Building Machines.</b>&mdash;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 <a name="Page_176" id="Page_176"></a>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 <i>does possess forces adequate to the manufacture of
+machines</i>. 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.</p>
+
+<p>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.</p>
+
+<p>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 <a name="Page_177" id="Page_177"></a>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 <i>alone</i>, 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
+<i>machine</i> 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 pro<a name="Page_178" id="Page_178"></a>toplasm, and must base our understanding of its
+properties upon its structure.</p>
+
+<p>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.</p>
+
+<p><b>The Origin of the Cell Machine.</b>&mdash;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 build<a name="Page_179" id="Page_179"></a>ing 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?</p>
+
+<p>We must first notice that no notion of <i>chemical evolution</i> 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. <i>Physical forces</i>, <i>chemical forces</i>, and <i>vital forces</i>,
+explain successively the origin of <i>worlds</i>, <i>protoplasm</i>, and
+<i>organisms</i>. 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.</p>
+
+<p>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
+evo<a name="Page_180" id="Page_180"></a>lution. 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.</p>
+
+<p>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,
+<a name="Page_181" id="Page_181"></a>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 <i>parts</i> 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.</p>
+
+<p><a name="Page_182" id="Page_182"></a>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.</p>
+
+<p><b>General Summary.</b>&mdash;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.</p>
+
+<p>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 <a name="Page_183" id="Page_183"></a>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.</p>
+
+<p>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.</p>
+
+<p>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 <a name="Page_184" id="Page_184"></a>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.</p>
+
+<p>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 <i>foundation</i> 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&mdash;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 <a name="Page_185" id="Page_185"></a>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.</p>
+
+<p>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 <i>living</i> 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,
+<a name="Page_186" id="Page_186"></a>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.</p>
+
+<p>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.</p>
+
+<p>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 <i>origin</i> 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
+con<a name="Page_187" id="Page_187"></a>struction of artificial machines has demanded intelligence. But here
+is a natural machine&mdash;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?</p>
+
+<p>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 <i>has</i> forces adequate to the building of machines
+as well as forces adequate to the formation of chemical compounds and
+worlds.</p>
+
+<p>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 there<a name="Page_188" id="Page_188"></a>fore
+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.</p>
+
+<p>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&mdash;the living cell.</p>
+
+
+
+<p>
+INDEX.<br /><a name="Page_189" id="Page_189"></a>
+
+
+
+
+A. <br />
+<br />
+Absorption of food, <a href='#Page_20'>20</a><br />
+<br />
+Acquired characters, inheritance of, <a href='#Page_164'>164</a>, <a href='#Page_165'>165</a>, <a href='#Page_166'>166</a>, <a href='#Page_167'>167</a>, <a href='#Page_171'>171</a>.<br />
+---- variations, <a href='#Page_159'>159</a>, <a href='#Page_160'>160</a>.<br />
+<br />
+Am&#339;ba <a href='#Page_73'>73</a>.<br />
+<br />
+Anatomical evidence for evolution, <a href='#Page_142'>142</a>.<br />
+<br />
+Aquacity, <a href='#Page_80'>80</a>.<br />
+<br />
+Arm compared with wing, <a href='#Page_144'>144</a>.<br />
+<br />
+Aristotle, .<br />
+<br />
+Assimilation, <a href='#Page_80'>80</a>, <a href='#Page_124'>124</a>, <a href='#Page_149'>149</a>, <a href='#Page_176'>176</a>.<br />
+<br />
+Asters of dividing cells, <a href='#Page_98'>98</a>.<br />
+<br />
+B. <br />
+<br />
+Barry, <a href='#Page_63'>63</a>, <a href='#Page_64'>64</a>.<br />
+<br />
+Bathybias, <a href='#Page_84'>84</a>.<br />
+<br />
+Biology a new science, <a href='#Page_1'>1</a>, <a href='#Page_5'>5</a>, <a href='#Page_15'>15</a>.<br />
+<br />
+Blood, <a href='#Page_35'>35</a>, <a href='#Page_36'>36</a>, <a href='#Page_38'>38</a>, <a href='#Page_69'>69</a>, <a href='#Page_73'>73</a>.<br />
+<br />
+Blood-vessels, <a href='#Page_35'>35</a>, <a href='#Page_36'>36</a>.<br />
+<br />
+Body as a machine, <a href='#Page_22'>22</a>, <a href='#Page_25'>25</a>, <a href='#Page_49'>49</a>.<br />
+<br />
+Bone cells, <a href='#Page_69'>69</a>.<br />
+<br />
+Building of the living machine, <a href='#Page_131'>131</a>, <a href='#Page_134'>134</a>, <a href='#Page_136'>136</a>, <a href='#Page_137'>137</a>, <a href='#Page_167'>167</a>, <a href='#Page_175'>175</a>, <a href='#Page_180'>180</a>.<br />
+<br />
+<br />
+C. <br />
+<br />
+Cartilage cells, <a href='#Page_68'>68</a>.<br />
+Cell as a machine, <a href='#Page_126'>126</a>, <a href='#Page_128'>128</a>.<br />
+---- description of, <a href='#Page_69'>69</a>.<br />
+---- division, <a href='#Page_95'>95</a>, <a href='#Page_96'>96</a>, <a href='#Page_101'>101</a>.<br />
+---- discovery of, <a href='#Page_58'>58</a>.<br />
+---- doctrine, <a href='#Page_60'>60</a>.<br />
+---- substance, <a href='#Page_65'>65</a>, <a href='#Page_125'>125</a>.<br />
+<br />
+Cells, <a href='#Page_56'>56</a>, <a href='#Page_84'>84</a>, <a href='#Page_86'>86</a>, <a href='#Page_118'>118</a>, <a href='#Page_119'>119</a>.<br />
+<br />
+Cellular structure of organisms, <a href='#Page_65'>65</a>.<br />
+<br />
+Cell wall, <a href='#Page_64'>64</a>, <a href='#Page_72'>72</a>.<br />
+<br />
+Centrosome, <a href='#Page_94'>94</a>, <a href='#Page_96'>96</a>, <a href='#Page_97'>97</a>, <a href='#Page_101'>101</a>, <a href='#Page_103'>103</a>, <a href='#Page_105'>105</a>, <a href='#Page_110'>110</a>.<br />
+<br />
+Challenger expedition, <a href='#Page_83'>83</a>.<br />
+<br />
+Chemical evolution, <a href='#Page_179'>179</a>.<br />
+<br />
+Chemical theory of vitality, <a href='#Page_14'>14</a>.<br />
+--of life, <a href='#Page_78'>78</a>, <a href='#Page_116'>116</a>.<br />
+<br />
+Chemism or mechanism, <a href='#Page_57'>57</a>, <a href='#Page_176'>176</a>.<br />
+<br />
+Chemistry of digestion, <a href='#Page_27'>27</a>, <a href='#Page_28'>28</a>;<br />
+---- of protoplasm, <a href='#Page_76'>76</a>;<br />
+---- of respiration, <a href='#Page_38'>38</a>.<br />
+<br />
+Chromatin, <a href='#Page_92'>92</a>, <a href='#Page_94'>94</a>, <a href='#Page_96'>96</a>, <a href='#Page_102'>102</a>, <a href='#Page_149'>149</a>, <a href='#Page_153'>153</a>.<br />
+<br />
+Chromosomes, <a href='#Page_97'>97</a>, <a href='#Page_98'>98</a>, <a href='#Page_101'>101</a>, <a href='#Page_105'>105</a>, <a href='#Page_108'>108</a>, <a href='#Page_110'>110</a>, <a href='#Page_113'>113</a>, <a href='#Page_152'>152</a>.<br />
+<br />
+Circulation, <a href='#Page_34'>34</a>.<br />
+<br />
+Colonies of cells, <a href='#Page_85'>85</a>.<br />
+<br />
+Comparison of the body and a machine, <a href='#Page_22'>22</a>.<br />
+<br />
+Congenital variations, <a href='#Page_158'>158</a>, <a href='#Page_160'>160</a>, <a href='#Page_163'>163</a>;<br />
+<span style="margin-left: <a href='#Page_1'>1</a>em;">inheritance of, <a href='#Page_164'>164</a>.<br />
+<br />
+Connective-tissue cells, <a href='#Page_70'>70</a>.<br />
+<br />
+Conservation of energy, <a href='#Page_7'>7</a>, <a href='#Page_17'>17</a>.<br />
+<br />
+Consciousness as a factor in machine building, <a href='#Page_173'>173</a>.<br />
+<br />
+Constructive chemical processes, <a href='#Page_50'>50</a>, <a href='#Page_51'>51</a>, <a href='#Page_52'>52</a>, <a href='#Page_124'>124</a>.<br />
+<br />
+Continuity of germ plasm, <a href='#Page_155'>155</a>.<br />
+<br />
+Correlation of vital and physical forces, <a href='#Page_13'>13</a>, <a href='#Page_16'>16</a>, <a href='#Page_22'>22</a>, <a href='#Page_23'>23</a>, <a href='#Page_24'>24</a>, <a href='#Page_25'>25</a>.<br />
+<br />
+Cytoblastema, <a href='#Page_62'>62</a>.<br />
+<br />
+Cytology, <a href='#Page_10'>10</a>.<br />
+<br />
+<br />
+D. <br />
+<br />
+Darwin, <a href='#Page_81'>81</a>.<br />
+<br />
+Death of the cell, <a href='#Page_127'>127</a>.<br />
+<br />
+Decline of the reign of protoplasm, <a href='#Page_85'>85</a>.<br />
+<br />
+Destructive chemical processes, <a href='#Page_50'>50</a>, <a href='#Page_51'>51</a>, <a href='#Page_52'>52</a>, <a href='#Page_125'>125</a>.<br />
+<br />
+Dialysis, <a href='#Page_29'>29</a>, <a href='#Page_30'>30</a>, <a href='#Page_31'>31</a>.<br />
+<br />
+Digestion, <a href='#Page_27'>27</a>.<br />
+<br />
+<br />
+E. <br />
+<br />
+Egg, <a href='#Page_103'>103</a>, <a href='#Page_120'>120</a>, <a href='#Page_152'>152</a>.<br />
+--division of, <a href='#Page_63'>63</a>.<br /><a name="Page_190" id="Page_190"></a>
+<br />
+Egg, fertilization of, <a href='#Page_102'>102</a>.<br />
+<br />
+Embryological evidence for evolution, <a href='#Page_140'>140</a>.<br />
+<br />
+Energy of nervous impulse, <a href='#Page_43'>43</a>, <a href='#Page_54'>54</a>.<br />
+<br />
+Environment, <a href='#Page_171'>171</a>.<br />
+<br />
+Evidence for evolution as a method of machine building, <a href='#Page_139'>139</a>, <a href='#Page_145'>145</a>.<br />
+<br />
+Evolution, <a href='#Page_9'>9</a>, <a href='#Page_16'>16</a>, <a href='#Page_81'>81</a>, <a href='#Page_134'>134</a>.<br />
+<br />
+Experiments with developing eggs, <a href='#Page_121'>121</a>.<br />
+<br />
+<br />
+F. <br />
+<br />
+Fat, absorption of, <a href='#Page_32'>32</a>.<br />
+<br />
+Female pronucleus, <a href='#Page_110'>110</a>.<br />
+<br />
+Fern cells, section of, <a href='#Page_67'>67</a>.<br />
+<br />
+Fertilization of the egg, <a href='#Page_95'>95</a>, <a href='#Page_102'>102</a>;<br />
+---- significance of, <a href='#Page_112'>112</a>.<br />
+<br />
+Fibres in protoplasm, <a href='#Page_87'>87</a>;<br />
+---- in spindle, <a href='#Page_98'>98</a>, <a href='#Page_101'>101</a>.<br />
+<br />
+Forces at work in machine building, <a href='#Page_148'>148</a>, <a href='#Page_176'>176</a>, <a href='#Page_181'>181</a>.<br />
+<br />
+Formed material, <a href='#Page_64'>64</a>.<br />
+<br />
+Free cell formation, <a href='#Page_64'>64</a>.<br />
+<br />
+<br />
+G. <br />
+<br />
+Geological evidence for evolution, <a href='#Page_139'>139</a>.<br />
+<br />
+Germ plasm, <a href='#Page_154'>154</a>.<br />
+<br />
+<br />
+H. <br />
+<br />
+Heart as a pump, <a href='#Page_35'>35</a>.<br />
+<br />
+Heat, <a href='#Page_24'>24</a>, <a href='#Page_44'>44</a>, <a href='#Page_45'>45</a>.<br />
+<br />
+Heredity, <a href='#Page_148'>148</a>, <a href='#Page_150'>150</a>, <a href='#Page_176'>176</a>;<br />
+---- explanation of, <a href='#Page_152'>152</a>.</span><br />
+<br />
+Hereditary traits, <a href='#Page_113'>113</a>, <a href='#Page_153'>153</a>.<br />
+<br />
+Historical geology, <a href='#Page_6'>6</a>.<br />
+<br />
+History of the living machine, <a href='#Page_133'>133</a>, <a href='#Page_147'>147</a>.<br />
+<br />
+Horses' toes, loss of, <a href='#Page_172'>172</a>.<br />
+<br />
+Huxley, <a href='#Page_11'>11</a>, <a href='#Page_75'>75</a>, <a href='#Page_83'>83</a>, <a href='#Page_84'>84</a>.<br />
+<br />
+<br />
+I. <br />
+<br />
+Irritability, <a href='#Page_54'>54</a>.<br />
+<br />
+Isolation, theory of, <a href='#Page_170'>170</a>.<br />
+<br />
+<br />
+K. <br />
+<br />
+Karyokinesis, <a href='#Page_96'>96</a>, <a href='#Page_101'>101</a>.<br />
+<br />
+Kidneys, <a href='#Page_41'>41</a>.<br />
+<br />
+<br />
+L. <br />
+<br />
+Leaf, section of, <a href='#Page_66'>66</a>.<br />
+<br />
+Life the result of a mechanism, <a href='#Page_115'>115</a>, <a href='#Page_177'>177</a>.<br />
+<br />
+Linin, <a href='#Page_92'>92</a>, <a href='#Page_103'>103</a>.<br />
+<br />
+Linn&aelig;us, <a href='#Page_1'>1</a>.<br />
+<br />
+Lyell, <a href='#Page_6'>6</a>.<br />
+<br />
+Lymph, <a href='#Page_36'>36</a>, <a href='#Page_37'>37</a>.<br />
+<br />
+<br />
+M. <br />
+<br />
+Machine defined, <a href='#Page_20'>20</a>.<br />
+<br />
+Machines the result of mechanical forces, <a href='#Page_116'>116</a>.<br />
+<br />
+Male cell, <a href='#Page_104'>104</a>, <a href='#Page_107'>107</a>.<br />
+<br />
+---- pronucleus, <a href='#Page_109'>109</a>.<br />
+<br />
+Maturation of the egg, <a href='#Page_104'>104</a>.<br />
+<br />
+Mechanical nature of living organisms, <a href='#Page_12'>12</a>.<br />
+<br />
+Mechanical theory of life, <a href='#Page_81'>81</a>, <a href='#Page_144'>144</a>.<br />
+<br />
+Membrane of the nucleus, <a href='#Page_92'>92</a>, <a href='#Page_101'>101</a>.<br />
+<br />
+Mental phenomena, <a href='#Page_47'>47</a>, <a href='#Page_48'>48</a>.<br />
+<br />
+Metabolism, <a href='#Page_54'>54</a>.<br />
+<br />
+Microsomes <a href='#Page_87'>87</a>.<br />
+<br />
+Migration, theory of, <a href='#Page_170'>170</a>.<br />
+<br />
+Monera, <a href='#Page_88'>88</a>.<br />
+<br />
+Movement, <a href='#Page_54'>54</a>.<br />
+<br />
+Muscle, <a href='#Page_36'>36</a>, <a href='#Page_71'>71</a>.<br />
+<br />
+<br />
+N. <br />
+<br />
+Natural selection, <a href='#Page_167'>167</a>.<br />
+<br />
+Nerve-fibre cell, <a href='#Page_70'>70</a>.<br />
+<br />
+Nervous energy, <a href='#Page_42'>42</a>, <a href='#Page_44'>44</a>.<br />
+<br />
+---- system, <a href='#Page_41'>41</a>.<br />
+<br />
+New biological problems, <a href='#Page_15'>15</a>.<br />
+<br />
+Nucleolus, <a href='#Page_65'>65</a>, <a href='#Page_92'>92</a>, <a href='#Page_94'>94</a>.<br />
+<br />
+Nucleus, <a href='#Page_65'>65</a>, <a href='#Page_84'>84</a>, <a href='#Page_87'>87</a>, <a href='#Page_93'>93</a>, <a href='#Page_101'>101</a>, <a href='#Page_103'>103</a>, <a href='#Page_113'>113</a>, <a href='#Page_124'>124</a>, <a href='#Page_149'>149</a>;<br />
+----formation of new, <a href='#Page_101'>101</a>.<br />
+<br />
+---- function of, <a href='#Page_89'>89</a>, <a href='#Page_90'>90</a>, <a href='#Page_95'>95</a>.<br />
+<br />
+---- presence of, <a href='#Page_87'>87</a>, <a href='#Page_88'>88</a>, <a href='#Page_89'>89</a>.<br />
+<br />
+---- structure of, <a href='#Page_91'>91</a>.<br />
+<br />
+<br />
+O. <br />
+<br />
+Organic chemistry, <a href='#Page_78'>78</a>.<br />
+<br />
+Organic compounds, artificial manufacture of, <a href='#Page_78'>78</a>, <a href='#Page_82'>82</a>.<br />
+<br />
+Origin of cell machine, <a href='#Page_178'>178</a>, <a href='#Page_179'>179</a>, <a href='#Page_180'>180</a>.<br />
+<br />
+Origin of life, <a href='#Page_81'>81</a>, <a href='#Page_182'>182</a>.<br />
+<br />
+Osmosis, <a href='#Page_29'>29</a>.<br />
+<br />
+Oxidation, <a href='#Page_80'>80</a>, <a href='#Page_176'>176</a>.<br />
+<br />
+---- as a vital process, <a href='#Page_39'>39</a>, <a href='#Page_56'>56</a>.<br />
+<br />
+<br />
+P. <br />
+<br />
+Philosophical biology, <a href='#Page_4'>4</a>.<br />
+<br />
+Physical basis of life, <a href='#Page_75'>75</a>.<br />
+<br />
+Polar cells, <a href='#Page_107'>107</a>.<br />
+<br />
+Potato, section of cells, <a href='#Page_67'>67</a>.<br />
+<br />
+Properties of chemical compounds, <a href='#Page_79'>79</a>.<br />
+<br /><a name="Page_191" id="Page_191"></a>
+Protoplasm, <a href='#Page_14'>14</a>, <a href='#Page_74'>74</a>, <a href='#Page_82'>82</a>, <a href='#Page_83'>83</a>, <a href='#Page_84'>84</a>, <a href='#Page_114'>114</a>, <a href='#Page_115'>115</a>, <a href='#Page_179'>179</a>.<br />
+<br />
+---- artificial manufacture of, <a href='#Page_82'>82</a>.<br />
+<br />
+---- as a machine, <a href='#Page_86'>86</a>, <a href='#Page_178'>178</a>.<br />
+<br />
+---- discovery of, <a href='#Page_74'>74</a>.<br />
+<br />
+---- nature of, <a href='#Page_76'>76</a>.<br />
+<br />
+---- structure of, <a href='#Page_86'>86</a>, <a href='#Page_87'>87</a>.<br />
+<br />
+Purpose <i>vs.</i> cause, <a href='#Page_11'>11</a>, <a href='#Page_12'>12</a>.<br />
+<br />
+<br />
+R. <br />
+<br />
+Reaction against the cell doctrine, <a href='#Page_117'>117</a>.<br />
+<br />
+Reign of law, <a href='#Page_4'>4</a>.<br />
+<br />
+---- of the nucleus, <a href='#Page_91'>91</a>.<br />
+<br />
+---- of protoplasm, <a href='#Page_81'>81</a>, <a href='#Page_85'>85</a>.<br />
+<br />
+Relationship, significance of, <a href='#Page_143'>143</a>.<br />
+<br />
+Removal of waste, <a href='#Page_39'>39</a>, <a href='#Page_40'>40</a>.<br />
+<br />
+Reproduction, <a href='#Page_54'>54</a>, <a href='#Page_80'>80</a>, <a href='#Page_124'>124</a>, <a href='#Page_148'>148</a>, <a href='#Page_176'>176</a>;<br />
+---- rapidity of, <a href='#Page_149'>149</a>.<br />
+<br />
+Respiration, <a href='#Page_37'>37</a>.<br />
+<br />
+Reticulum of cell, <a href='#Page_87'>87</a>;<br />
+---- of nucleus, <a href='#Page_92'>92</a>.<br />
+<br />
+Root tip, section of, <a href='#Page_66'>66</a>.<br />
+<br />
+<br />
+S.<br />
+<br />
+<br />
+Schultze, <a href='#Page_74'>74</a>, <a href='#Page_75'>75</a>.<br />
+<br />
+Schwann, <a href='#Page_61'>61</a>, <a href='#Page_62'>62</a>, <a href='#Page_72'>72</a>.<br />
+<br />
+Secretion, <a href='#Page_39'>39</a>, <a href='#Page_40'>40</a>.<br />
+<br />
+Segmentation nucleus, <a href='#Page_110'>110</a>.<br />
+<br />
+Sensations, <a href='#Page_46'>46</a>.<br />
+<br />
+Separation of chromosomes, <a href='#Page_100'>100</a>.<br />
+<br />
+Sexual reproduction, <a href='#Page_102'>102</a>.<br />
+<br />
+Spermatozoan, <a href='#Page_107'>107</a>, <a href='#Page_109'>109</a>, <a href='#Page_154'>154</a>.<br />
+<br />
+Splitting of chromosomes, <a href='#Page_99'>99</a>.<br />
+<br />
+Spindle fibres, <a href='#Page_101'>101</a>.<br />
+<br />
+Struggle for existence, <a href='#Page_168'>168</a>.<br />
+<br />
+Summary of Part I, <a href='#Page_128'>128</a>.<br />
+<br />
+---- general, <a href='#Page_182'>182</a>.<br />
+<br />
+<br />
+U.<br />
+<br />
+Undifferentiated protoplasm, <a href='#Page_83'>83</a>.<br />
+<br />
+Unicellular animals, <a href='#Page_71'>71</a>.<br />
+<br />
+Units of vital activity, <a href='#Page_53'>53</a>.<br />
+<br />
+Use and disuse, <a href='#Page_171'>171</a>, <a href='#Page_172'>172</a>.<br />
+<br />
+<br />
+V.<br />
+<br />
+Variation, <a href='#Page_148'>148</a>, <a href='#Page_157'>157</a>, <a href='#Page_160'>160</a>, <a href='#Page_176'>176</a>.<br />
+<br />
+Variation from sexual union, <a href='#Page_162'>162</a>.<br />
+<br />
+Variation in germ plasm, <a href='#Page_161'>161</a>.<br />
+<br />
+Vegetative functions, <a href='#Page_41'>41</a>.<br />
+<br />
+Villi, <a href='#Page_31'>31</a>.<br />
+<br />
+Vital force, vitality, <a href='#Page_13'>13</a>, <a href='#Page_15'>15</a>, <a href='#Page_34'>34</a>, <a href='#Page_37'>37</a>, <a href='#Page_52'>52</a>, <a href='#Page_80'>80</a>, <a href='#Page_85'>85</a>.<br />
+<br />
+Vital properties, <a href='#Page_54'>54</a>;<br />
+---- located in cells, <a href='#Page_123'>123</a>.<br />
+<br />
+<br />
+W.<br />
+<br />
+Wing compared with arm, <a href='#Page_144'>144</a>.<br />
+<br />
+Wood cells, <a href='#Page_68'>68</a>.<br />
+</p>
+
+
+
+<p class='center'>THE END.</p><p><a name="Page_192" id="Page_192"></a></p><p><a name="Page_193" id="Page_193"></a></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="THE_LIBRARY_OF_USEFUL_STORIES" id="THE_LIBRARY_OF_USEFUL_STORIES"></a><b>THE LIBRARY OF USEFUL STORIES.</b></h2>
+
+<p>Illustrated. 16mo. Cloth, 35 cents net per volume; postage, 4 cents per
+volume additional.</p>
+
+<p>
+The Story of a Grain of Wheat. By <span class="smcap">W.C. Edgar.</span><br />
+The Story of Alchemy. By <span class="smcap">M.M. Pattison Muir.</span><br />
+The Story of Animal Life. By <span class="smcap">B. Lindsay.</span><br />
+The Story of the Art of Music. By <span class="smcap">F.J. Crowest.</span><br />
+The Story of the Art of Building. By <span class="smcap">P.L. Waterhouse.</span><br />
+The Story of King Alfred. By Sir <span class="smcap">Walter Besant.</span><br />
+The Story of Books. By <span class="smcap">Gertrude B. Rawlings.</span><br />
+The Story of the Alphabet. By <span class="smcap">Edward Clodd.</span><br />
+The Story of Eclipses. By <span class="smcap">G.F. Chambers, F.R.A.S.</span><br />
+The Story of the Living Machine. By <span class="smcap">H.W. Conn.</span><br />
+The Story of the British Race. By <span class="smcap">John Munro, C.E.</span><br />
+The Story of Geographical Discovery. By <span class="smcap">Joseph Jacobs.</span><br />
+The Story of the Cotton Plant. By <span class="smcap">F. Wilkinson, F.G.S.</span><br />
+The Story of the Mind. By Prof. <span class="smcap">J. Mark Baldwin.</span><br />
+The Story of Photography. By <span class="smcap">Alfred T. Story.</span><br />
+The Story of Life in the Seas. By <span class="smcap">Sydney J. Hickson.</span><br />
+The Story of Germ Life. By Prof. <span class="smcap">H.W. Conn.</span><br />
+The Story of the Earth's Atmosphere. By <span class="smcap">Douglas Archibald.</span><br />
+The Story of Extinct Civilizations of the East. By <span class="smcap">Robert Anderson, M.A., F.A.S.</span><br />
+The Story of Electricity. By <span class="smcap">John Munro, C.E.</span><br />
+The Story of a Piece of Coal. By <span class="smcap">E.A. Martin, F.G.S.</span><br />
+The Story of the Solar System. By <span class="smcap">G.F. Chambers, F.R.A.S.</span><br />
+The Story of the Earth. By <span class="smcap">H.G. Seeley, F.R.S.</span><br />
+The Story of the Plants. By <span class="smcap">Grant Allen.</span><br />
+The Story of "Primitive" Man. By <span class="smcap">Edward Clodd.</span><br />
+The Story of the Stars. By <span class="smcap">G.F. Chambers, F.R.A.S.</span><br />
+</p>
+
+<p>OTHERS IN PREPARATION.</p>
+
+<p>D. APPLETON AND COMPANY, NEW YORK.</p><p><a name="Page_194" id="Page_194"></a></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="NEW_EDITION_OF_HUXLEYS_ESSAYS" id="NEW_EDITION_OF_HUXLEYS_ESSAYS"></a><span class="smcap">New Edition of Huxley's Essays</span>.</h2>
+
+<p><b>Collected Essays.</b></p>
+
+<p>By <span class="smcap">Thomas H. Huxley</span>. New complete edition, with revisions, the
+Essays being grouped according to general subject. In nine volumes, a
+new Introduction accompanying each volume. 12mo. Cloth, $1.25 per
+volume.</p>
+
+<p><span class="smcap">Volume</span>.</p>
+
+<p>
+I. Methods and Results.
+II. Darwiniana.
+III. Science and Education.
+IV. Science and Hebrew Tradition.
+V. Science and Christian Tradition.
+VI. Hume.
+VII. Man's Place in Nature.
+VIII. Discourses, Biological and Geological.
+IX. Evolution and Ethics, and Other Essays.
+</p>
+
+<p>"Mr. Huxley has covered a vast variety of topics during the last quarter
+of a century. It gives one an agreeable surprise to look over the tables
+of contents and note the immense territory which he has explored. To
+read these books carefully and studiously is to become thoroughly
+acquainted with the most advanced thought on a large number of
+topics."&mdash;<i>New York Herald</i>.</p>
+
+<p>D. APPLETON AND COMPANY, NEW YORK.</p>
+
+<p><a name="Page_195" id="Page_195"></a>D. APPLETON AND COMPANY'S PUBLICATIONS.</p>
+
+<p><i><span class="smcap">Pioneers of Evolution</span>, from Thales to Huxley</i> By <span class="smcap">Edward
+Clodd</span>, President of the Folk-Lore Society; Author of "The Story of
+Creation," "The Story of 'Primitive' Man," etc. With Portraits, 12mo.
+Cloth, $1.50.</p>
+
+<p>"The mass of interesting material which Mr. Clodd has got together and
+woven into a symmetrical story of the progress from ignorance and theory
+to knowledge and the intelligent recording of fact is prodigious.... The
+'goal' to which Mr. Clodd leads us in so masterly a fashion is but the
+starting point of fresh achievements, and, in due course, fresh
+theories. His book furnishes an important contribution to a liberal
+education."&mdash;<i>London Daily Chronicle.</i></p>
+
+<p>"We are always glad to meet Mr. Clodd. He is never dull; he is always
+well informed, and he says what he has to say with clearness and
+precision.... The interest intensifies as Mr. Clodd attempts to show the
+part really played in the growth of the doctrine of evolution by men
+like Wallace, Darwin, Huxley, and Spencer.... We commend the book to
+those who want to know what evolution really means."&mdash;<i>London Times.</i></p>
+
+<p>"This is a book which was needed.... Altogether, the book could hardly
+be better done. It is luminous, lucid, orderly, and temperate. Above
+all, it is entirely free from personal partisanship. Each chief actor is
+sympathetically treated, and friendship is seldom or never allowed to
+overweight sound judgment"&mdash;<i>London Academy.</i></p>
+
+<p>"We can assure the reader that he will find in this work a very useful
+guide to the lives and labors of leading evolutionists of the past and
+present. Especially serviceable is the account of Mr. Herbert Spencer
+and his share in rediscovering evolution, and illustrating its relations
+to the whole field of human knowledge. His forcible style and wealth of
+metaphor make all that Mr. Clodd writes arrestive and
+interesting."&mdash;<i>London Literary World.</i></p>
+
+<p>"Can not but prove welcome to fair-minded men.... To read it is to have
+an object-lesson in the meaning of evolution.... There is no better book
+on the subject for the general reader.... No one could go through the
+book without being both refreshed and newly instructed by its masterly
+survey of the growth of the most powerful idea of modern times."&mdash;<i>The
+Scotsman.</i></p>
+
+<p>D. APPLETON AND COMPANY, NEW YORK.</p>
+
+<p><a name="Page_196" id="Page_196"></a><b>BOOKS ON SOCIAL SCIENCE.</b></p>
+
+<p><b>Socialism New and Old.</b></p>
+
+<p>By Prof. <span class="smcap">William Graham</span>, 12mo. Cloth, $1.75.</p>
+
+<p>"Professor Graham's book may be confidently recommended to all who are
+interested in the study of socialism, and not so intoxicated with its
+promises of a new heaven and a new earth as to be impatient of temperate
+and reasoned criticism."<i>&mdash;London Times.</i></p>
+
+<p>"Professor Graham presents an outline of the successive schemes of three
+writers who have chiefly influenced the development of socialism, and
+dwells at length upon the system of Rousseau, that of St. Simon, and on
+that of Karl Marx, the founder of the new socialism, 'which has gained
+favor with the working classes in all civilized countries,' which agrees
+with Rousseau's plan in being democratic, and with St. Simon's in aiming
+at collective ownership.... The professor is an independent thinker,
+whose endeavor to be clear has resulted in the statement of definite
+conclusions. The book is a remarkably fair digest of the subject under
+consideration."&mdash;<i>Philadelphia Ledger.</i></p>
+
+<p><b>Dynamic Sociology:</b></p>
+
+<p><i>Or, Applied Social Science, as based upon Statical Sociology and the
+less Complex Sciences.</i> By <span class="smcap">Lester F. Ward</span>, A.M. In 2 vols.
+12mo. Cloth, $4.00.</p>
+
+<p>"A book that will amply repay perusal.... Recognizing the danger in
+which sociology is, of falling into the class of dead sciences or polite
+amusements, Mr. Ward has undertaken to 'point out a method by which the
+breath of life can be breathed into its nostrils.'"&mdash;<i>Rochester
+Post-Express.</i></p>
+
+<p>"Mr. Ward has evidently put great labor and thought into his two
+volumes, and has produced a work of interest and importance. He does not
+limit his effort to a contribution to the science of sociology.... He
+believes that sociology has already reached the point at which it can be
+and ought to be applied, treated as an art, and he urges that 'the
+State' or Government now has a new, legitimate, and peculiar field for
+the exercise of intelligence to promote the welfare of men."&mdash;<i>New York
+Times.</i></p>
+
+<p><b>Criminal Sociology.</b></p>
+
+<p>By Prof. <span class="smcap">E. Ferri.</span> A new volume in the Criminology Series,
+edited by W. Douglas Morrison, 12mo. Cloth, $1.50.</p>
+
+<p>In this volume Professor Ferri, a distinguished member of the Italian
+Parliament, deals with the conditions which produce the criminal
+population, and with the methods by which this anti-social section of
+the community may be diminished. He divides the causes of crime into two
+great classes, individual and social. The individual causes consist of
+physical and mental defects; the social causes consist of social
+disadvantages of every description. His view is that the true remedy
+against crime is to remove individual defects and social disadvantages
+where it is possible to remove them. He shows that punishment has
+comparatively little effect in this direction, and is apt to divert
+attention from the true remedy&mdash;the individual and social amelioration
+of the population as a whole.</p>
+
+<p>D. APPLETON AND COMPANY, NEW YORK.</p>
+
+
+
+<hr style="width: 65%;" /><p><a name="Page_197" id="Page_197"></a></p>
+<h2><a name="BOOKS_FOR_NATURE_LOVERS" id="BOOKS_FOR_NATURE_LOVERS"></a>BOOKS FOR NATURE LOVERS.</h2>
+
+
+<p><b>Insect Life. (New Edition in Colors.)</b></p>
+
+<p>By <span class="smcap">John Henry Comstock</span>, Professor of Entomology in Cornell
+University. With 12 full-page plates reproducing butterflies and various
+insects in their natural colors, and with many wood engravings by Anna
+Botsford Comstock, Member of the Society of American Wood Engravers,
+12mo. Cloth, $1.75 net; postage, 20 cents additional.</p>
+
+<div class="blockquot"><p>"The volume is admirably written, and the simple and lucid style is
+a constant delight.... It is sure to serve an excellent purpose in
+the direction of popular culture, and the love of natural science
+which it will develop in youthful minds can hardly fail to bear
+rich fruit."&mdash;<i>Boston Beacon.</i></p></div>
+
+<p><b>Familiar Fish: Their Habits and Capture.</b></p>
+
+<p>A Practical Book on Fresh-Water Game Fish. By <span class="smcap">Eugene McCarthy</span>.
+With an Introduction by Dr. David Starr Jordan, President of Leland
+Stanford Junior University, and numerous Illustrations, 12mo. Cloth,
+$1.50.</p>
+
+<div class="blockquot"><p>"One of the handsomest, most practical, most informing books that
+we know. The author treats his subject with scientific
+thoroughness, but with a light touch that makes the book easy
+reading.... The book should be the companion of all who go
+a-fishing."&mdash;<i>New York Mail and Express.</i></p></div>
+
+<p><b>The Art of Taxidermy.</b></p>
+
+<p>By <span class="smcap">John Rowley</span>, Chief of the Department of Taxidermy in the
+American Museum of Natural History. Illustrated, 12mo. Cloth, $2.00.</p>
+
+<div class="blockquot"><p>"Mr. Rowley will long be gratefully remembered by taxidermists,
+amateurs, and others, for the care he has used in thus meeting a
+long-felt want."&mdash;<i>Bangor, Me., Sportsman.</i></p>
+
+<p>"The book is not an elaborate treatise upon the abstract principles
+which lie at the foundation of artistic taxidermy, but is rather a
+compendium full of practical hints and suggestions, recipes, and
+formulas for the working taxidermist."&mdash;<i>The Dial.</i></p></div>
+
+<p><b>Plants. (Plant Relations and Plant Structures in one volume.)</b></p>
+
+<p>By <span class="smcap">John M. Coulter</span>, A.M., Ph.D., Head of Department of Botany,
+University of Chicago, 12mo. Cloth, $1.80 net. (One of the Twentieth
+Century Text-Books.)</p>
+
+<p>D. APPLETON AND COMPANY, NEW YORK.</p><p><a name="Page_198" id="Page_198"></a></p>
+
+
+<p><i>EVOLUTION OF MAN AND CHRISTIANITY.</i></p>
+
+<p>New edition. By the Rev. <span class="smcap">Howard MacQueary</span>. With a new Preface,
+in which the Author answers his Critics, and with some important
+Additions, 12mo. Cloth, $1.75.</p>
+
+<div class="blockquot"><p>"This is a revised and enlarged edition of a book published last
+year. The author reviews criticisms upon the first edition, denies
+that he rejects the doctrine of the incarnation, admits his doubts
+of the physical resurrection of Christ, and his belief in
+evolution. The volume is to be marked as one of the most profound
+expressions of the modern movement toward broader theological
+positions."&mdash;<i>Brooklyn Times.</i></p></div>
+
+
+<p><i>HISTORY OF THE CONFLICT BETWEEN RELIGION AND SCIENCE.</i> By Dr. <span class="smcap">John
+William Draper</span>. 12mo. Cloth, $1.75.</p>
+
+<div class="blockquot"><p>"The keynote to this volume is found in the antagonism between the
+progressive tendencies of the human mind and the pretensions of
+ecclesiastical authority, as developed in the history of modern
+science. No previous writer has treated the subject from this point
+of view, and the present monograph will be found to possess no less
+originality of conception than vigor of reasoning and wealth of
+erudition."&mdash;<i>New York Tribune.</i></p></div>
+
+
+<p><i>A CRITICAL HISTORY OF FREE THOUGHT IN REFERENCE TO THE CHRISTIAN
+RELIGION.</i> By Rev. Canon <span class="smcap">Adam Storey Farrar</span>, D.D., F.R.S., etc.
+12mo. Cloth, $1.50.</p>
+
+<div class="blockquot"><p>"A conflict might naturally be anticipated between the reasoning
+faculties of man and a religion which claims the right, on
+superhuman authority, to impose limits on the field or manner of
+their exercise. It is the chief of the movements of free thought
+which it is my purpose to describe, in their historic succession
+and their connection with intellectual causes. We must ascertain
+the facts, discover the causes, and read the moral."&mdash;<i>The Author.</i></p></div>
+
+
+<p><i>CREATION OR EVOLUTION? A Philosophical Inquiry.</i> By <span class="smcap">George Ticnor
+Curtis</span>. 12mo. Cloth, $2.00.</p>
+
+<div class="blockquot"><p>"A treatise on the great question of Creation or Evolution by one
+who is neither a naturalist nor theologian, and who does not
+profess to bring to the discussion a special equipment in either of
+the sciences which the controversy arrays against each other, may
+seem strange at first sight; but Mr. Curtis will satisfy the
+reader, before many pages have been turned, that he has a
+substantial contribution to make to the debate, and that his book
+is one to be treated with respect. His part is to apply to the
+reasonings of the men of science the rigid scrutiny with which the
+lawyer is accustomed to test the value and pertinency of testimony,
+and the legitimacy of inferences from established facts."&mdash;<i>New
+York Tribune.</i></p></div>
+
+<p>D. APPLETON AND COMPANY, NEW YORK.</p>
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of Project Gutenberg's The Story of the Living Machine, by H. W. Conn
+
+*** END OF THIS PROJECT GUTENBERG EBOOK THE STORY OF THE LIVING MACHINE ***
+
+***** This file should be named 16487-h.htm or 16487-h.zip *****
+This and all associated files of various formats will be found in:
+        https://www.gutenberg.org/1/6/4/8/16487/
+
+Produced by Juliet Sutherland, Janet Blenkinship and the
+Online Distributed Proofreading Team at https://www.pgdp.net
+
+
+Updated editions will replace the previous one--the old editions
+will be renamed.
+
+Creating the works from public domain print editions means that no
+one owns a United States copyright in these works, so the Foundation
+(and you!) can copy and distribute it in the United States without
+permission and without paying copyright royalties.  Special rules,
+set forth in the General Terms of Use part of this license, apply to
+copying and distributing Project Gutenberg-tm electronic works to
+protect the PROJECT GUTENBERG-tm concept and trademark.  Project
+Gutenberg is a registered trademark, and may not be used if you
+charge for the eBooks, unless you receive specific permission.  If you
+do not charge anything for copies of this eBook, complying with the
+rules is very easy.  You may use this eBook for nearly any purpose
+such as creation of derivative works, reports, performances and
+research.  They may be modified and printed and given away--you may do
+practically ANYTHING with public domain eBooks.  Redistribution is
+subject to the trademark license, especially commercial
+redistribution.
+
+
+
+*** START: FULL LICENSE ***
+
+THE FULL PROJECT GUTENBERG LICENSE
+PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK
+
+To protect the Project Gutenberg-tm mission of promoting the free
+distribution of electronic works, by using or distributing this work
+(or any other work associated in any way with the phrase "Project
+Gutenberg"), you agree to comply with all the terms of the Full Project
+Gutenberg-tm License (available with this file or online at
+https://gutenberg.org/license).
+
+
+Section 1.  General Terms of Use and Redistributing Project Gutenberg-tm
+electronic works
+
+1.A.  By reading or using any part of this Project Gutenberg-tm
+electronic work, you indicate that you have read, understand, agree to
+and accept all the terms of this license and intellectual property
+(trademark/copyright) agreement.  If you do not agree to abide by all
+the terms of this agreement, you must cease using and return or destroy
+all copies of Project Gutenberg-tm electronic works in your possession.
+If you paid a fee for obtaining a copy of or access to a Project
+Gutenberg-tm electronic work and you do not agree to be bound by the
+terms of this agreement, you may obtain a refund from the person or
+entity to whom you paid the fee as set forth in paragraph 1.E.8.
+
+1.B.  "Project Gutenberg" is a registered trademark.  It may only be
+used on or associated in any way with an electronic work by people who
+agree to be bound by the terms of this agreement.  There are a few
+things that you can do with most Project Gutenberg-tm electronic works
+even without complying with the full terms of this agreement.  See
+paragraph 1.C below.  There are a lot of things you can do with Project
+Gutenberg-tm electronic works if you follow the terms of this agreement
+and help preserve free future access to Project Gutenberg-tm electronic
+works.  See paragraph 1.E below.
+
+1.C.  The Project Gutenberg Literary Archive Foundation ("the Foundation"
+or PGLAF), owns a compilation copyright in the collection of Project
+Gutenberg-tm electronic works.  Nearly all the individual works in the
+collection are in the public domain in the United States.  If an
+individual work is in the public domain in the United States and you are
+located in the United States, we do not claim a right to prevent you from
+copying, distributing, performing, displaying or creating derivative
+works based on the work as long as all references to Project Gutenberg
+are removed.  Of course, we hope that you will support the Project
+Gutenberg-tm mission of promoting free access to electronic works by
+freely sharing Project Gutenberg-tm works in compliance with the terms of
+this agreement for keeping the Project Gutenberg-tm name associated with
+the work.  You can easily comply with the terms of this agreement by
+keeping this work in the same format with its attached full Project
+Gutenberg-tm License when you share it without charge with others.
+
+1.D.  The copyright laws of the place where you are located also govern
+what you can do with this work.  Copyright laws in most countries are in
+a constant state of change.  If you are outside the United States, check
+the laws of your country in addition to the terms of this agreement
+before downloading, copying, displaying, performing, distributing or
+creating derivative works based on this work or any other Project
+Gutenberg-tm work.  The Foundation makes no representations concerning
+the copyright status of any work in any country outside the United
+States.
+
+1.E.  Unless you have removed all references to Project Gutenberg:
+
+1.E.1.  The following sentence, with active links to, or other immediate
+access to, the full Project Gutenberg-tm License must appear prominently
+whenever any copy of a Project Gutenberg-tm work (any work on which the
+phrase "Project Gutenberg" appears, or with which the phrase "Project
+Gutenberg" is associated) is accessed, displayed, performed, viewed,
+copied or distributed:
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+1.E.2.  If an individual Project Gutenberg-tm electronic work is derived
+from the public domain (does not contain a notice indicating that it is
+posted with permission of the copyright holder), the work can be copied
+and distributed to anyone in the United States without paying any fees
+or charges.  If you are redistributing or providing access to a work
+with the phrase "Project Gutenberg" associated with or appearing on the
+work, you must comply either with the requirements of paragraphs 1.E.1
+through 1.E.7 or obtain permission for the use of the work and the
+Project Gutenberg-tm trademark as set forth in paragraphs 1.E.8 or
+1.E.9.
+
+1.E.3.  If an individual Project Gutenberg-tm electronic work is posted
+with the permission of the copyright holder, your use and distribution
+must comply with both paragraphs 1.E.1 through 1.E.7 and any additional
+terms imposed by the copyright holder.  Additional terms will be linked
+to the Project Gutenberg-tm License for all works posted with the
+permission of the copyright holder found at the beginning of this work.
+
+1.E.4.  Do not unlink or detach or remove the full Project Gutenberg-tm
+License terms from this work, or any files containing a part of this
+work or any other work associated with Project Gutenberg-tm.
+
+1.E.5.  Do not copy, display, perform, distribute or redistribute this
+electronic work, or any part of this electronic work, without
+prominently displaying the sentence set forth in paragraph 1.E.1 with
+active links or immediate access to the full terms of the Project
+Gutenberg-tm License.
+
+1.E.6.  You may convert to and distribute this work in any binary,
+compressed, marked up, nonproprietary or proprietary form, including any
+word processing or hypertext form.  However, if you provide access to or
+distribute copies of a Project Gutenberg-tm work in a format other than
+"Plain Vanilla ASCII" or other format used in the official version
+posted on the official Project Gutenberg-tm web site (www.gutenberg.org),
+you must, at no additional cost, fee or expense to the user, provide a
+copy, a means of exporting a copy, or a means of obtaining a copy upon
+request, of the work in its original "Plain Vanilla ASCII" or other
+form.  Any alternate format must include the full Project Gutenberg-tm
+License as specified in paragraph 1.E.1.
+
+1.E.7.  Do not charge a fee for access to, viewing, displaying,
+performing, copying or distributing any Project Gutenberg-tm works
+unless you comply with paragraph 1.E.8 or 1.E.9.
+
+1.E.8.  You may charge a reasonable fee for copies of or providing
+access to or distributing Project Gutenberg-tm electronic works provided
+that
+
+- You pay a royalty fee of 20% of the gross profits you derive from
+     the use of Project Gutenberg-tm works calculated using the method
+     you already use to calculate your applicable taxes.  The fee is
+     owed to the owner of the Project Gutenberg-tm trademark, but he
+     has agreed to donate royalties under this paragraph to the
+     Project Gutenberg Literary Archive Foundation.  Royalty payments
+     must be paid within 60 days following each date on which you
+     prepare (or are legally required to prepare) your periodic tax
+     returns.  Royalty payments should be clearly marked as such and
+     sent to the Project Gutenberg Literary Archive Foundation at the
+     address specified in Section 4, "Information about donations to
+     the Project Gutenberg Literary Archive Foundation."
+
+- You provide a full refund of any money paid by a user who notifies
+     you in writing (or by e-mail) within 30 days of receipt that s/he
+     does not agree to the terms of the full Project Gutenberg-tm
+     License.  You must require such a user to return or
+     destroy all copies of the works possessed in a physical medium
+     and discontinue all use of and all access to other copies of
+     Project Gutenberg-tm works.
+
+- You provide, in accordance with paragraph 1.F.3, a full refund of any
+     money paid for a work or a replacement copy, if a defect in the
+     electronic work is discovered and reported to you within 90 days
+     of receipt of the work.
+
+- You comply with all other terms of this agreement for free
+     distribution of Project Gutenberg-tm works.
+
+1.E.9.  If you wish to charge a fee or distribute a Project Gutenberg-tm
+electronic work or group of works on different terms than are set
+forth in this agreement, you must obtain permission in writing from
+both the Project Gutenberg Literary Archive Foundation and Michael
+Hart, the owner of the Project Gutenberg-tm trademark.  Contact the
+Foundation as set forth in Section 3 below.
+
+1.F.
+
+1.F.1.  Project Gutenberg volunteers and employees expend considerable
+effort to identify, do copyright research on, transcribe and proofread
+public domain works in creating the Project Gutenberg-tm
+collection.  Despite these efforts, Project Gutenberg-tm electronic
+works, and the medium on which they may be stored, may contain
+"Defects," such as, but not limited to, incomplete, inaccurate or
+corrupt data, transcription errors, a copyright or other intellectual
+property infringement, a defective or damaged disk or other medium, a
+computer virus, or computer codes that damage or cannot be read by
+your equipment.
+
+1.F.2.  LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right
+of Replacement or Refund" described in paragraph 1.F.3, the Project
+Gutenberg Literary Archive Foundation, the owner of the Project
+Gutenberg-tm trademark, and any other party distributing a Project
+Gutenberg-tm electronic work under this agreement, disclaim all
+liability to you for damages, costs and expenses, including legal
+fees.  YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT
+LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE
+PROVIDED IN PARAGRAPH F3.  YOU AGREE THAT THE FOUNDATION, THE
+TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE
+LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR
+INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH
+DAMAGE.
+
+1.F.3.  LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a
+defect in this electronic work within 90 days of receiving it, you can
+receive a refund of the money (if any) you paid for it by sending a
+written explanation to the person you received the work from.  If you
+received the work on a physical medium, you must return the medium with
+your written explanation.  The person or entity that provided you with
+the defective work may elect to provide a replacement copy in lieu of a
+refund.  If you received the work electronically, the person or entity
+providing it to you may choose to give you a second opportunity to
+receive the work electronically in lieu of a refund.  If the second copy
+is also defective, you may demand a refund in writing without further
+opportunities to fix the problem.
+
+1.F.4.  Except for the limited right of replacement or refund set forth
+in paragraph 1.F.3, this work is provided to you 'AS-IS', WITH NO OTHER
+WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO
+WARRANTIES OF MERCHANTIBILITY OR FITNESS FOR ANY PURPOSE.
+
+1.F.5.  Some states do not allow disclaimers of certain implied
+warranties or the exclusion or limitation of certain types of damages.
+If any disclaimer or limitation set forth in this agreement violates the
+law of the state applicable to this agreement, the agreement shall be
+interpreted to make the maximum disclaimer or limitation permitted by
+the applicable state law.  The invalidity or unenforceability of any
+provision of this agreement shall not void the remaining provisions.
+
+1.F.6.  INDEMNITY - You agree to indemnify and hold the Foundation, the
+trademark owner, any agent or employee of the Foundation, anyone
+providing copies of Project Gutenberg-tm electronic works in accordance
+with this agreement, and any volunteers associated with the production,
+promotion and distribution of Project Gutenberg-tm electronic works,
+harmless from all liability, costs and expenses, including legal fees,
+that arise directly or indirectly from any of the following which you do
+or cause to occur: (a) distribution of this or any Project Gutenberg-tm
+work, (b) alteration, modification, or additions or deletions to any
+Project Gutenberg-tm work, and (c) any Defect you cause.
+
+
+Section  2.  Information about the Mission of Project Gutenberg-tm
+
+Project Gutenberg-tm is synonymous with the free distribution of
+electronic works in formats readable by the widest variety of computers
+including obsolete, old, middle-aged and new computers.  It exists
+because of the efforts of hundreds of volunteers and donations from
+people in all walks of life.
+
+Volunteers and financial support to provide volunteers with the
+assistance they need, is critical to reaching Project Gutenberg-tm's
+goals and ensuring that the Project Gutenberg-tm collection will
+remain freely available for generations to come.  In 2001, the Project
+Gutenberg Literary Archive Foundation was created to provide a secure
+and permanent future for Project Gutenberg-tm and future generations.
+To learn more about the Project Gutenberg Literary Archive Foundation
+and how your efforts and donations can help, see Sections 3 and 4
+and the Foundation web page at https://www.pglaf.org.
+
+
+Section 3.  Information about the Project Gutenberg Literary Archive
+Foundation
+
+The Project Gutenberg Literary Archive Foundation is a non profit
+501(c)(3) educational corporation organized under the laws of the
+state of Mississippi and granted tax exempt status by the Internal
+Revenue Service.  The Foundation's EIN or federal tax identification
+number is 64-6221541.  Its 501(c)(3) letter is posted at
+https://pglaf.org/fundraising.  Contributions to the Project Gutenberg
+Literary Archive Foundation are tax deductible to the full extent
+permitted by U.S. federal laws and your state's laws.
+
+The Foundation's principal office is located at 4557 Melan Dr. S.
+Fairbanks, AK, 99712., but its volunteers and employees are scattered
+throughout numerous locations.  Its business office is located at
+809 North 1500 West, Salt Lake City, UT 84116, (801) 596-1887, email
+business@pglaf.org.  Email contact links and up to date contact
+information can be found at the Foundation's web site and official
+page at https://pglaf.org
+
+For additional contact information:
+     Dr. Gregory B. Newby
+     Chief Executive and Director
+     gbnewby@pglaf.org
+
+Section 4.  Information about Donations to the Project Gutenberg
+Literary Archive Foundation
+
+Project Gutenberg-tm depends upon and cannot survive without wide
+spread public support and donations to carry out its mission of
+increasing the number of public domain and licensed works that can be
+freely distributed in machine readable form accessible by the widest
+array of equipment including outdated equipment.  Many small donations
+($1 to $5,000) are particularly important to maintaining tax exempt
+status with the IRS.
+
+The Foundation is committed to complying with the laws regulating
+charities and charitable donations in all 50 states of the United
+States.  Compliance requirements are not uniform and it takes a
+considerable effort, much paperwork and many fees to meet and keep up
+with these requirements.  We do not solicit donations in locations
+where we have not received written confirmation of compliance.  To
+SEND DONATIONS or determine the status of compliance for any
+particular state visit https://pglaf.org
+
+While we cannot and do not solicit contributions from states where we
+have not met the solicitation requirements, we know of no prohibition
+against accepting unsolicited donations from donors in such states who
+approach us with offers to donate.
+
+International donations are gratefully accepted, but we cannot make
+any statements concerning tax treatment of donations received from
+outside the United States.  U.S. laws alone swamp our small staff.
+
+Please check the Project Gutenberg Web pages for current donation
+methods and addresses.  Donations are accepted in a number of other
+ways including including checks, online payments and credit card
+donations.  To donate, please visit: https://pglaf.org/donate
+
+
+Section 5.  General Information About Project Gutenberg-tm electronic
+works.
+
+Professor Michael S. Hart was the originator of the Project Gutenberg-tm
+concept of a library of electronic works that could be freely shared
+with anyone.  For thirty years, he produced and distributed Project
+Gutenberg-tm eBooks with only a loose network of volunteer support.
+
+Project Gutenberg-tm eBooks are often created from several printed
+editions, all of which are confirmed as Public Domain in the U.S.
+unless a copyright notice is included.  Thus, we do not necessarily
+keep eBooks in compliance with any particular paper edition.
+
+Most people start at our Web site which has the main PG search facility:
+
+     https://www.gutenberg.org
+
+This Web site includes information about Project Gutenberg-tm,
+including how to make donations to the Project Gutenberg Literary
+Archive Foundation, how to help produce our new eBooks, and how to
+subscribe to our email newsletter to hear about new eBooks.
+
+*** END: FULL LICENSE ***
+
+
+
+</pre>
+
+</body>
+</html>
diff --git a/16487-h/images/038fig1.png b/16487-h/images/038fig1.png
new file mode 100644
index 0000000..4346e2d
--- /dev/null
+++ b/16487-h/images/038fig1.png
diff --git a/16487-h/images/039fig2.png b/16487-h/images/039fig2.png
new file mode 100644
index 0000000..d26c8b3
--- /dev/null
+++ b/16487-h/images/039fig2.png
diff --git a/16487-h/images/040fig3.png b/16487-h/images/040fig3.png
new file mode 100644
index 0000000..9cd265f
--- /dev/null
+++ b/16487-h/images/040fig3.png
diff --git a/16487-h/images/041fig4.png b/16487-h/images/041fig4.png
new file mode 100644
index 0000000..1dda17f
--- /dev/null
+++ b/16487-h/images/041fig4.png
diff --git a/16487-h/images/044fig6.png b/16487-h/images/044fig6.png
new file mode 100644
index 0000000..1bade7a
--- /dev/null
+++ b/16487-h/images/044fig6.png
diff --git a/16487-h/images/069fig7.png b/16487-h/images/069fig7.png
new file mode 100644
index 0000000..758994c
--- /dev/null
+++ b/16487-h/images/069fig7.png
diff --git a/16487-h/images/071fig8.png b/16487-h/images/071fig8.png
new file mode 100644
index 0000000..78f51a0
--- /dev/null
+++ b/16487-h/images/071fig8.png
diff --git a/16487-h/images/073fig9.png b/16487-h/images/073fig9.png
new file mode 100644
index 0000000..c9ae1d5
--- /dev/null
+++ b/16487-h/images/073fig9.png
diff --git a/16487-h/images/074fig10.png b/16487-h/images/074fig10.png
new file mode 100644
index 0000000..cb5a8eb
--- /dev/null
+++ b/16487-h/images/074fig10.png
diff --git a/16487-h/images/074fig11.png b/16487-h/images/074fig11.png
new file mode 100644
index 0000000..dc56cc1
--- /dev/null
+++ b/16487-h/images/074fig11.png
diff --git a/16487-h/images/075fig12.png b/16487-h/images/075fig12.png
new file mode 100644
index 0000000..fa14422
--- /dev/null
+++ b/16487-h/images/075fig12.png
diff --git a/16487-h/images/075fig13.png b/16487-h/images/075fig13.png
new file mode 100644
index 0000000..759edfd
--- /dev/null
+++ b/16487-h/images/075fig13.png
diff --git a/16487-h/images/076fig14.png b/16487-h/images/076fig14.png
new file mode 100644
index 0000000..f005dd0
--- /dev/null
+++ b/16487-h/images/076fig14.png
diff --git a/16487-h/images/076fig15.png b/16487-h/images/076fig15.png
new file mode 100644
index 0000000..7a98608
--- /dev/null
+++ b/16487-h/images/076fig15.png
diff --git a/16487-h/images/077fig16.png b/16487-h/images/077fig16.png
new file mode 100644
index 0000000..db96be6
--- /dev/null
+++ b/16487-h/images/077fig16.png
diff --git a/16487-h/images/077fig17.png b/16487-h/images/077fig17.png
new file mode 100644
index 0000000..34a2865
--- /dev/null
+++ b/16487-h/images/077fig17.png
diff --git a/16487-h/images/078fig18.png b/16487-h/images/078fig18.png
new file mode 100644
index 0000000..0313648
--- /dev/null
+++ b/16487-h/images/078fig18.png
diff --git a/16487-h/images/078fig19.png b/16487-h/images/078fig19.png
new file mode 100644
index 0000000..0a9f39b
--- /dev/null
+++ b/16487-h/images/078fig19.png
diff --git a/16487-h/images/079fig20.png b/16487-h/images/079fig20.png
new file mode 100644
index 0000000..f563ad5
--- /dev/null
+++ b/16487-h/images/079fig20.png
diff --git a/16487-h/images/079fig21.png b/16487-h/images/079fig21.png
new file mode 100644
index 0000000..e575599
--- /dev/null
+++ b/16487-h/images/079fig21.png
diff --git a/16487-h/images/081fig22.png b/16487-h/images/081fig22.png
new file mode 100644
index 0000000..0e31fb1
--- /dev/null
+++ b/16487-h/images/081fig22.png
diff --git a/16487-h/images/094fig23.png b/16487-h/images/094fig23.png
new file mode 100644
index 0000000..e5a44c3
--- /dev/null
+++ b/16487-h/images/094fig23.png
diff --git a/16487-h/images/097fig24.png b/16487-h/images/097fig24.png
new file mode 100644
index 0000000..a72c928
--- /dev/null
+++ b/16487-h/images/097fig24.png
diff --git a/16487-h/images/098fig25.png b/16487-h/images/098fig25.png
new file mode 100644
index 0000000..c779b60
--- /dev/null
+++ b/16487-h/images/098fig25.png
diff --git a/16487-h/images/101fig26.png b/16487-h/images/101fig26.png
new file mode 100644
index 0000000..b804558
--- /dev/null
+++ b/16487-h/images/101fig26.png
diff --git a/16487-h/images/104fig27-28.png b/16487-h/images/104fig27-28.png
new file mode 100644
index 0000000..8108cc3
--- /dev/null
+++ b/16487-h/images/104fig27-28.png
diff --git a/16487-h/images/106fig29-30.png b/16487-h/images/106fig29-30.png
new file mode 100644
index 0000000..5d7b29c
--- /dev/null
+++ b/16487-h/images/106fig29-30.png
diff --git a/16487-h/images/108fig31-32.png b/16487-h/images/108fig31-32.png
new file mode 100644
index 0000000..f3dad46
--- /dev/null
+++ b/16487-h/images/108fig31-32.png
diff --git a/16487-h/images/111fig33.png b/16487-h/images/111fig33.png
new file mode 100644
index 0000000..741068e
--- /dev/null
+++ b/16487-h/images/111fig33.png
diff --git a/16487-h/images/112fig34-35.png b/16487-h/images/112fig34-35.png
new file mode 100644
index 0000000..0f2180c
--- /dev/null
+++ b/16487-h/images/112fig34-35.png
diff --git a/16487-h/images/113fig36-37.png b/16487-h/images/113fig36-37.png
new file mode 100644
index 0000000..989e978
--- /dev/null
+++ b/16487-h/images/113fig36-37.png
diff --git a/16487-h/images/114fig38-39-40.png b/16487-h/images/114fig38-39-40.png
new file mode 100644
index 0000000..e9a95e3
--- /dev/null
+++ b/16487-h/images/114fig38-39-40.png
diff --git a/16487-h/images/117fig41-42.png b/16487-h/images/117fig41-42.png
new file mode 100644
index 0000000..dd2d68f
--- /dev/null
+++ b/16487-h/images/117fig41-42.png
diff --git a/16487-h/images/119fig43-44.png b/16487-h/images/119fig43-44.png
new file mode 100644
index 0000000..1ff5598
--- /dev/null
+++ b/16487-h/images/119fig43-44.png
diff --git a/16487-h/images/143fig45.png b/16487-h/images/143fig45.png
new file mode 100644
index 0000000..8981e3d
--- /dev/null
+++ b/16487-h/images/143fig45.png
diff --git a/16487-h/images/143fig46.png b/16487-h/images/143fig46.png
new file mode 100644
index 0000000..ecef8c5
--- /dev/null
+++ b/16487-h/images/143fig46.png
diff --git a/16487-h/images/152fig47-48-49.png b/16487-h/images/152fig47-48-49.png
new file mode 100644
index 0000000..53b81a0
--- /dev/null
+++ b/16487-h/images/152fig47-48-49.png
diff --git a/16487-h/images/164fig50.png b/16487-h/images/164fig50.png
new file mode 100644
index 0000000..61054a1
--- /dev/null
+++ b/16487-h/images/164fig50.png
diff --git a/16487.txt b/16487.txt
new file mode 100644
index 0000000..0d8bdb9
--- /dev/null
+++ b/16487.txt
@@ -0,0 +1,6115 @@
+The Project Gutenberg EBook of The Story of the Living Machine, by H. W. Conn
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+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: H. W. Conn
+
+Release Date: August 8, 2005 [EBook #16487]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE STORY OF THE LIVING MACHINE ***
+
+
+
+
+Produced by Juliet Sutherland, Janet Blenkinship and the
+Online Distributed Proofreading Team at https://www.pgdp.net
+
+
+
+
+
+
+
+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. Linnaeus 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
+haemoglobin. 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 haemoglobin is a normal constituent of the blood, and, being red in
+colour, gives the red colour to the blood. This haemoglobin 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 haemoglobin
+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 haemoglobin 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 haemoglobin, and since the oxygen pressure in the air is
+quite high, this haemoglobin 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 haemoglobin reaches them
+the association of the haemoglobin 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 animalculae 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 archaeologists 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.
+
+Linnaeus, 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.
+
+
+
+
+==THE LIBRARY OF USEFUL STORIES.==
+
+Illustrated. 16mo. Cloth, 35 cents net per volume; postage, 4 cents per
+volume additional.
+
+The Story of a Grain of Wheat. By W.C. EDGAR.
+The Story of Alchemy. By M.M. PATTISON MUIR.
+The Story of Animal Life. By B. LINDSAY.
+The Story of the Art of Music. By F.J. CROWEST.
+The Story of the Art of Building. By P.L. WATERHOUSE.
+The Story of King Alfred. By Sir WALTER BESANT.
+The Story of Books. By GERTRUDE B. RAWLINGS.
+The Story of the Alphabet. By EDWARD CLODD.
+The Story of Eclipses. By G.F. CHAMBERS, F.R.A.S.
+The Story of the Living Machine. By H.W. CONN.
+The Story of the British Race. By JOHN MUNRO, C.E.
+The Story of Geographical Discovery. By JOSEPH JACOBS.
+The Story of the Cotton Plant. By F. WILKINSON, F.G.S.
+The Story of the Mind. By Prof. J. MARK BALDWIN.
+The Story of Photography. By ALFRED T. STORY.
+The Story of Life in the Seas. By SYDNEY J. HICKSON.
+The Story of Germ Life. By Prof. H.W. CONN.
+The Story of the Earth's Atmosphere. By DOUGLAS ARCHIBALD.
+The Story of Extinct Civilizations of the East.
+By ROBERT ANDERSON, M.A., F.A.S.
+The Story of Electricity. By JOHN MUNRO, C.E.
+The Story of a Piece of Coal. By E.A. MARTIN, F.G.S.
+The Story of the Solar System. By G.F. CHAMBERS, F.R.A.S.
+The Story of the Earth. By H.G. SEELEY, F.R.S.
+The Story of the Plants. By GRANT ALLEN.
+The Story of "Primitive" Man. By EDWARD CLODD.
+The Story of the Stars. By G.F. CHAMBERS, F.R.A.S.
+
+OTHERS IN PREPARATION.
+
+D. APPLETON AND COMPANY, NEW YORK.
+
+
+
+
+NEW EDITION OF HUXLEY'S ESSAYS.
+
+==Collected Essays.==
+
+By THOMAS H. HUXLEY. New complete edition, with revisions, the
+Essays being grouped according to general subject. In nine volumes, a
+new Introduction accompanying each volume. 12mo. Cloth, $1.25 per
+volume.
+
+VOLUME.
+
+==I. Methods and Results.
+  II. Darwiniana.
+ III. Science and Education.
+  IV. Science and Hebrew Tradition.
+   V. Science and Christian Tradition.
+  VI. Hume.
+ VII. Man's Place in Nature.
+VIII. Discourses, Biological and Geological.
+  IX. Evolution and Ethics, and Other Essays.==
+
+"Mr. Huxley has covered a vast variety of topics during the last quarter
+of a century. It gives one an agreeable surprise to look over the tables
+of contents and note the immense territory which he has explored. To
+read these books carefully and studiously is to become thoroughly
+acquainted with the most advanced thought on a large number of
+topics."--_New York Herald._
+
+D. APPLETON AND COMPANY, NEW YORK.
+
+D. APPLETON AND COMPANY'S PUBLICATIONS.
+
+_PIONEERS OF EVOLUTION, from Thales to Huxley_ By EDWARD
+CLODD, President of the Folk-Lore Society; Author of "The Story of
+Creation," "The Story of 'Primitive' Man," etc. With Portraits, 12mo.
+Cloth, $1.50.
+
+"The mass of interesting material which Mr. Clodd has got together and
+woven into a symmetrical story of the progress from ignorance and theory
+to knowledge and the intelligent recording of fact is prodigious.... The
+'goal' to which Mr. Clodd leads us in so masterly a fashion is but the
+starting point of fresh achievements, and, in due course, fresh
+theories. His book furnishes an important contribution to a liberal
+education."--_London Daily Chronicle._
+
+"We are always glad to meet Mr. Clodd. He is never dull; he is always
+well informed, and he says what he has to say with clearness and
+precision.... The interest intensifies as Mr. Clodd attempts to show the
+part really played in the growth of the doctrine of evolution by men
+like Wallace, Darwin, Huxley, and Spencer.... We commend the book to
+those who want to know what evolution really means."--_London Times._
+
+"This is a book which was needed.... Altogether, the book could hardly
+be better done. It is luminous, lucid, orderly, and temperate. Above
+all, it is entirely free from personal partisanship. Each chief actor is
+sympathetically treated, and friendship is seldom or never allowed to
+overweight sound judgment"--_London Academy._
+
+"We can assure the reader that he will find in this work a very useful
+guide to the lives and labors of leading evolutionists of the past and
+present. Especially serviceable is the account of Mr. Herbert Spencer
+and his share in rediscovering evolution, and illustrating its relations
+to the whole field of human knowledge. His forcible style and wealth of
+metaphor make all that Mr. Clodd writes arrestive and
+interesting."--_London Literary World._
+
+"Can not but prove welcome to fair-minded men.... To read it is to have
+an object-lesson in the meaning of evolution.... There is no better book
+on the subject for the general reader.... No one could go through the
+book without being both refreshed and newly instructed by its masterly
+survey of the growth of the most powerful idea of modern times."--_The
+Scotsman._
+
+D. APPLETON AND COMPANY, NEW YORK.
+
+==BOOKS ON SOCIAL SCIENCE.==
+
+==Socialism New and Old.==
+
+By Prof. WILLIAM GRAHAM, 12mo. Cloth, $1.75.
+
+"Professor Graham's book may be confidently recommended to all who are
+interested in the study of socialism, and not so intoxicated with its
+promises of a new heaven and a new earth as to be impatient of temperate
+and reasoned criticism."_--London Times._
+
+"Professor Graham presents an outline of the successive schemes of three
+writers who have chiefly influenced the development of socialism, and
+dwells at length upon the system of Rousseau, that of St. Simon, and on
+that of Karl Marx, the founder of the new socialism, 'which has gained
+favor with the working classes in all civilized countries,' which agrees
+with Rousseau's plan in being democratic, and with St. Simon's in aiming
+at collective ownership.... The professor is an independent thinker,
+whose endeavor to be clear has resulted in the statement of definite
+conclusions. The book is a remarkably fair digest of the subject under
+consideration."_--Philadelphia Ledger._
+
+==Dynamic Sociology:==
+
+_Or, Applied Social Science, as based upon Statical Sociology and the
+less Complex Sciences._ By LESTER F. WARD, A.M. In 2 vols.
+12mo. Cloth, $4.00.
+
+"A book that will amply repay perusal.... Recognizing the danger in
+which sociology is, of falling into the class of dead sciences or polite
+amusements, Mr. Ward has undertaken to 'point out a method by which the
+breath of life can be breathed into its nostrils.'"--_Rochester
+Post-Express._
+
+"Mr. Ward has evidently put great labor and thought into his two
+volumes, and has produced a work of interest and importance. He does not
+limit his effort to a contribution to the science of sociology.... He
+believes that sociology has already reached the point at which it can be
+and ought to be applied, treated as an art, and he urges that 'the
+State' or Government now has a new, legitimate, and peculiar field for
+the exercise of intelligence to promote the welfare of men."--_New York
+Times._
+
+==Criminal Sociology.==
+
+By Prof. E. FERRI. A new volume in the Criminology Series,
+edited by W. Douglas Morrison, 12mo. Cloth, $1.50.
+
+In this volume Professor Ferri, a distinguished member of the Italian
+Parliament, deals with the conditions which produce the criminal
+population, and with the methods by which this anti-social section of
+the community may be diminished. He divides the causes of crime into two
+great classes, individual and social. The individual causes consist of
+physical and mental defects; the social causes consist of social
+disadvantages of every description. His view is that the true remedy
+against crime is to remove individual defects and social disadvantages
+where it is possible to remove them. He shows that punishment has
+comparatively little effect in this direction, and is apt to divert
+attention from the true remedy--the individual and social amelioration
+of the population as a whole.
+
+D. APPLETON AND COMPANY, NEW YORK.
+
+
+
+
+BOOKS FOR NATURE LOVERS.
+
+
+==Insect Life. (New Edition in Colors.)==
+
+By JOHN HENRY COMSTOCK, Professor of Entomology in Cornell
+University. With 12 full-page plates reproducing butterflies and various
+insects in their natural colors, and with many wood engravings by Anna
+Botsford Comstock, Member of the Society of American Wood Engravers,
+12mo. Cloth, $1.75 net; postage, 20 cents additional.
+
+     "The volume is admirably written, and the simple and lucid style is
+     a constant delight.... It is sure to serve an excellent purpose in
+     the direction of popular culture, and the love of natural science
+     which it will develop in youthful minds can hardly fail to bear
+     rich fruit."--_Boston Beacon._
+
+==Familiar Fish: Their Habits and Capture.==
+
+A Practical Book on Fresh-Water Game Fish. By EUGENE MCCARTHY.
+With an Introduction by Dr. David Starr Jordan, President of Leland
+Stanford Junior University, and numerous Illustrations, 12mo. Cloth,
+$1.50.
+
+     "One of the handsomest, most practical, most informing books that
+     we know. The author treats his subject with scientific
+     thoroughness, but with a light touch that makes the book easy
+     reading.... The book should be the companion of all who go
+     a-fishing."--_New York Mail and Express._
+
+==The Art of Taxidermy.==
+
+By JOHN ROWLEY, Chief of the Department of Taxidermy in the
+American Museum of Natural History. Illustrated, 12mo. Cloth, $2.00.
+
+     "Mr. Rowley will long be gratefully remembered by taxidermists,
+     amateurs, and others, for the care he has used in thus meeting a
+     long-felt want."--_Bangor, Me., Sportsman._
+
+     "The book is not an elaborate treatise upon the abstract principles
+     which lie at the foundation of artistic taxidermy, but is rather a
+     compendium full of practical hints and suggestions, recipes, and
+     formulas for the working taxidermist."--_The Dial._
+
+==Plants. (Plant Relations and Plant Structures in one volume.)==
+
+By JOHN M. COULTER, A.M., Ph.D., Head of Department of Botany,
+University of Chicago, 12mo. Cloth, $1.80 net. (One of the Twentieth
+Century Text-Books.)
+
+D. APPLETON AND COMPANY, NEW YORK.
+
+
+_EVOLUTION OF MAN AND CHRISTIANITY._
+
+New edition. By the Rev. HOWARD MACQUEARY. With a new Preface,
+in which the Author answers his Critics, and with some important
+Additions, 12mo. Cloth, $1.75.
+
+     "This is a revised and enlarged edition of a book published last
+     year. The author reviews criticisms upon the first edition, denies
+     that he rejects the doctrine of the incarnation, admits his doubts
+     of the physical resurrection of Christ, and his belief in
+     evolution. The volume is to be marked as one of the most profound
+     expressions of the modern movement toward broader theological
+     positions."--_Brooklyn Times._
+
+
+_HISTORY OF THE CONFLICT BETWEEN RELIGION AND SCIENCE._ By Dr. JOHN
+WILLIAM DRAPER. 12mo. Cloth, $1.75.
+
+     "The keynote to this volume is found in the antagonism between the
+     progressive tendencies of the human mind and the pretensions of
+     ecclesiastical authority, as developed in the history of modern
+     science. No previous writer has treated the subject from this point
+     of view, and the present monograph will be found to possess no less
+     originality of conception than vigor of reasoning and wealth of
+     erudition."--_New York Tribune._
+
+
+_A CRITICAL HISTORY OF FREE THOUGHT IN REFERENCE TO THE CHRISTIAN
+RELIGION._ By Rev. Canon ADAM STOREY FARRAR, D.D., F.R.S., etc.
+12mo. Cloth, $1.50.
+
+     "A conflict might naturally be anticipated between the reasoning
+     faculties of man and a religion which claims the right, on
+     superhuman authority, to impose limits on the field or manner of
+     their exercise. It is the chief of the movements of free thought
+     which it is my purpose to describe, in their historic succession
+     and their connection with intellectual causes. We must ascertain
+     the facts, discover the causes, and read the moral."--_The Author._
+
+
+_CREATION OR EVOLUTION? A Philosophical Inquiry._ By GEORGE TICNOR
+CURTIS. 12mo. Cloth, $2.00.
+
+     "A treatise on the great question of Creation or Evolution by one
+     who is neither a naturalist nor theologian, and who does not
+     profess to bring to the discussion a special equipment in either of
+     the sciences which the controversy arrays against each other, may
+     seem strange at first sight; but Mr. Curtis will satisfy the
+     reader, before many pages have been turned, that he has a
+     substantial contribution to make to the debate, and that his book
+     is one to be treated with respect. His part is to apply to the
+     reasonings of the men of science the rigid scrutiny with which the
+     lawyer is accustomed to test the value and pertinency of testimony,
+     and the legitimacy of inferences from established facts."--_New
+     York Tribune._
+
+D. APPLETON AND COMPANY, NEW YORK.
+
+
+
+
+
+End of Project Gutenberg's The Story of the Living Machine, by H. W. Conn
+
+*** END OF THIS PROJECT GUTENBERG EBOOK THE STORY OF THE LIVING MACHINE ***
+
+***** This file should be named 16487.txt or 16487.zip *****
+This and all associated files of various formats will be found in:
+        https://www.gutenberg.org/1/6/4/8/16487/
+
+Produced by Juliet Sutherland, Janet Blenkinship and the
+Online Distributed Proofreading Team at https://www.pgdp.net
+
+
+Updated editions will replace the previous one--the old editions
+will be renamed.
+
+Creating the works from public domain print editions means that no
+one owns a United States copyright in these works, so the Foundation
+(and you!) can copy and distribute it in the United States without
+permission and without paying copyright royalties.  Special rules,
+set forth in the General Terms of Use part of this license, apply to
+copying and distributing Project Gutenberg-tm electronic works to
+protect the PROJECT GUTENBERG-tm concept and trademark.  Project
+Gutenberg is a registered trademark, and may not be used if you
+charge for the eBooks, unless you receive specific permission.  If you
+do not charge anything for copies of this eBook, complying with the
+rules is very easy.  You may use this eBook for nearly any purpose
+such as creation of derivative works, reports, performances and
+research.  They may be modified and printed and given away--you may do
+practically ANYTHING with public domain eBooks.  Redistribution is
+subject to the trademark license, especially commercial
+redistribution.
+
+
+
+*** START: FULL LICENSE ***
+
+THE FULL PROJECT GUTENBERG LICENSE
+PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK
+
+To protect the Project Gutenberg-tm mission of promoting the free
+distribution of electronic works, by using or distributing this work
+(or any other work associated in any way with the phrase "Project
+Gutenberg"), you agree to comply with all the terms of the Full Project
+Gutenberg-tm License (available with this file or online at
+https://gutenberg.org/license).
+
+
+Section 1.  General Terms of Use and Redistributing Project Gutenberg-tm
+electronic works
+
+1.A.  By reading or using any part of this Project Gutenberg-tm
+electronic work, you indicate that you have read, understand, agree to
+and accept all the terms of this license and intellectual property
+(trademark/copyright) agreement.  If you do not agree to abide by all
+the terms of this agreement, you must cease using and return or destroy
+all copies of Project Gutenberg-tm electronic works in your possession.
+If you paid a fee for obtaining a copy of or access to a Project
+Gutenberg-tm electronic work and you do not agree to be bound by the
+terms of this agreement, you may obtain a refund from the person or
+entity to whom you paid the fee as set forth in paragraph 1.E.8.
+
+1.B.  "Project Gutenberg" is a registered trademark.  It may only be
+used on or associated in any way with an electronic work by people who
+agree to be bound by the terms of this agreement.  There are a few
+things that you can do with most Project Gutenberg-tm electronic works
+even without complying with the full terms of this agreement.  See
+paragraph 1.C below.  There are a lot of things you can do with Project
+Gutenberg-tm electronic works if you follow the terms of this agreement
+and help preserve free future access to Project Gutenberg-tm electronic
+works.  See paragraph 1.E below.
+
+1.C.  The Project Gutenberg Literary Archive Foundation ("the Foundation"
+or PGLAF), owns a compilation copyright in the collection of Project
+Gutenberg-tm electronic works.  Nearly all the individual works in the
+collection are in the public domain in the United States.  If an
+individual work is in the public domain in the United States and you are
+located in the United States, we do not claim a right to prevent you from
+copying, distributing, performing, displaying or creating derivative
+works based on the work as long as all references to Project Gutenberg
+are removed.  Of course, we hope that you will support the Project
+Gutenberg-tm mission of promoting free access to electronic works by
+freely sharing Project Gutenberg-tm works in compliance with the terms of
+this agreement for keeping the Project Gutenberg-tm name associated with
+the work.  You can easily comply with the terms of this agreement by
+keeping this work in the same format with its attached full Project
+Gutenberg-tm License when you share it without charge with others.
+
+1.D.  The copyright laws of the place where you are located also govern
+what you can do with this work.  Copyright laws in most countries are in
+a constant state of change.  If you are outside the United States, check
+the laws of your country in addition to the terms of this agreement
+before downloading, copying, displaying, performing, distributing or
+creating derivative works based on this work or any other Project
+Gutenberg-tm work.  The Foundation makes no representations concerning
+the copyright status of any work in any country outside the United
+States.
+
+1.E.  Unless you have removed all references to Project Gutenberg:
+
+1.E.1.  The following sentence, with active links to, or other immediate
+access to, the full Project Gutenberg-tm License must appear prominently
+whenever any copy of a Project Gutenberg-tm work (any work on which the
+phrase "Project Gutenberg" appears, or with which the phrase "Project
+Gutenberg" is associated) is accessed, displayed, performed, viewed,
+copied or distributed:
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+1.E.2.  If an individual Project Gutenberg-tm electronic work is derived
+from the public domain (does not contain a notice indicating that it is
+posted with permission of the copyright holder), the work can be copied
+and distributed to anyone in the United States without paying any fees
+or charges.  If you are redistributing or providing access to a work
+with the phrase "Project Gutenberg" associated with or appearing on the
+work, you must comply either with the requirements of paragraphs 1.E.1
+through 1.E.7 or obtain permission for the use of the work and the
+Project Gutenberg-tm trademark as set forth in paragraphs 1.E.8 or
+1.E.9.
+
+1.E.3.  If an individual Project Gutenberg-tm electronic work is posted
+with the permission of the copyright holder, your use and distribution
+must comply with both paragraphs 1.E.1 through 1.E.7 and any additional
+terms imposed by the copyright holder.  Additional terms will be linked
+to the Project Gutenberg-tm License for all works posted with the
+permission of the copyright holder found at the beginning of this work.
+
+1.E.4.  Do not unlink or detach or remove the full Project Gutenberg-tm
+License terms from this work, or any files containing a part of this
+work or any other work associated with Project Gutenberg-tm.
+
+1.E.5.  Do not copy, display, perform, distribute or redistribute this
+electronic work, or any part of this electronic work, without
+prominently displaying the sentence set forth in paragraph 1.E.1 with
+active links or immediate access to the full terms of the Project
+Gutenberg-tm License.
+
+1.E.6.  You may convert to and distribute this work in any binary,
+compressed, marked up, nonproprietary or proprietary form, including any
+word processing or hypertext form.  However, if you provide access to or
+distribute copies of a Project Gutenberg-tm work in a format other than
+"Plain Vanilla ASCII" or other format used in the official version
+posted on the official Project Gutenberg-tm web site (www.gutenberg.org),
+you must, at no additional cost, fee or expense to the user, provide a
+copy, a means of exporting a copy, or a means of obtaining a copy upon
+request, of the work in its original "Plain Vanilla ASCII" or other
+form.  Any alternate format must include the full Project Gutenberg-tm
+License as specified in paragraph 1.E.1.
+
+1.E.7.  Do not charge a fee for access to, viewing, displaying,
+performing, copying or distributing any Project Gutenberg-tm works
+unless you comply with paragraph 1.E.8 or 1.E.9.
+
+1.E.8.  You may charge a reasonable fee for copies of or providing
+access to or distributing Project Gutenberg-tm electronic works provided
+that
+
+- You pay a royalty fee of 20% of the gross profits you derive from
+     the use of Project Gutenberg-tm works calculated using the method
+     you already use to calculate your applicable taxes.  The fee is
+     owed to the owner of the Project Gutenberg-tm trademark, but he
+     has agreed to donate royalties under this paragraph to the
+     Project Gutenberg Literary Archive Foundation.  Royalty payments
+     must be paid within 60 days following each date on which you
+     prepare (or are legally required to prepare) your periodic tax
+     returns.  Royalty payments should be clearly marked as such and
+     sent to the Project Gutenberg Literary Archive Foundation at the
+     address specified in Section 4, "Information about donations to
+     the Project Gutenberg Literary Archive Foundation."
+
+- You provide a full refund of any money paid by a user who notifies
+     you in writing (or by e-mail) within 30 days of receipt that s/he
+     does not agree to the terms of the full Project Gutenberg-tm
+     License.  You must require such a user to return or
+     destroy all copies of the works possessed in a physical medium
+     and discontinue all use of and all access to other copies of
+     Project Gutenberg-tm works.
+
+- You provide, in accordance with paragraph 1.F.3, a full refund of any
+     money paid for a work or a replacement copy, if a defect in the
+     electronic work is discovered and reported to you within 90 days
+     of receipt of the work.
+
+- You comply with all other terms of this agreement for free
+     distribution of Project Gutenberg-tm works.
+
+1.E.9.  If you wish to charge a fee or distribute a Project Gutenberg-tm
+electronic work or group of works on different terms than are set
+forth in this agreement, you must obtain permission in writing from
+both the Project Gutenberg Literary Archive Foundation and Michael
+Hart, the owner of the Project Gutenberg-tm trademark.  Contact the
+Foundation as set forth in Section 3 below.
+
+1.F.
+
+1.F.1.  Project Gutenberg volunteers and employees expend considerable
+effort to identify, do copyright research on, transcribe and proofread
+public domain works in creating the Project Gutenberg-tm
+collection.  Despite these efforts, Project Gutenberg-tm electronic
+works, and the medium on which they may be stored, may contain
+"Defects," such as, but not limited to, incomplete, inaccurate or
+corrupt data, transcription errors, a copyright or other intellectual
+property infringement, a defective or damaged disk or other medium, a
+computer virus, or computer codes that damage or cannot be read by
+your equipment.
+
+1.F.2.  LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right
+of Replacement or Refund" described in paragraph 1.F.3, the Project
+Gutenberg Literary Archive Foundation, the owner of the Project
+Gutenberg-tm trademark, and any other party distributing a Project
+Gutenberg-tm electronic work under this agreement, disclaim all
+liability to you for damages, costs and expenses, including legal
+fees.  YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT
+LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE
+PROVIDED IN PARAGRAPH F3.  YOU AGREE THAT THE FOUNDATION, THE
+TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE
+LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR
+INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH
+DAMAGE.
+
+1.F.3.  LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a
+defect in this electronic work within 90 days of receiving it, you can
+receive a refund of the money (if any) you paid for it by sending a
+written explanation to the person you received the work from.  If you
+received the work on a physical medium, you must return the medium with
+your written explanation.  The person or entity that provided you with
+the defective work may elect to provide a replacement copy in lieu of a
+refund.  If you received the work electronically, the person or entity
+providing it to you may choose to give you a second opportunity to
+receive the work electronically in lieu of a refund.  If the second copy
+is also defective, you may demand a refund in writing without further
+opportunities to fix the problem.
+
+1.F.4.  Except for the limited right of replacement or refund set forth
+in paragraph 1.F.3, this work is provided to you 'AS-IS', WITH NO OTHER
+WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO
+WARRANTIES OF MERCHANTIBILITY OR FITNESS FOR ANY PURPOSE.
+
+1.F.5.  Some states do not allow disclaimers of certain implied
+warranties or the exclusion or limitation of certain types of damages.
+If any disclaimer or limitation set forth in this agreement violates the
+law of the state applicable to this agreement, the agreement shall be
+interpreted to make the maximum disclaimer or limitation permitted by
+the applicable state law.  The invalidity or unenforceability of any
+provision of this agreement shall not void the remaining provisions.
+
+1.F.6.  INDEMNITY - You agree to indemnify and hold the Foundation, the
+trademark owner, any agent or employee of the Foundation, anyone
+providing copies of Project Gutenberg-tm electronic works in accordance
+with this agreement, and any volunteers associated with the production,
+promotion and distribution of Project Gutenberg-tm electronic works,
+harmless from all liability, costs and expenses, including legal fees,
+that arise directly or indirectly from any of the following which you do
+or cause to occur: (a) distribution of this or any Project Gutenberg-tm
+work, (b) alteration, modification, or additions or deletions to any
+Project Gutenberg-tm work, and (c) any Defect you cause.
+
+
+Section  2.  Information about the Mission of Project Gutenberg-tm
+
+Project Gutenberg-tm is synonymous with the free distribution of
+electronic works in formats readable by the widest variety of computers
+including obsolete, old, middle-aged and new computers.  It exists
+because of the efforts of hundreds of volunteers and donations from
+people in all walks of life.
+
+Volunteers and financial support to provide volunteers with the
+assistance they need, is critical to reaching Project Gutenberg-tm's
+goals and ensuring that the Project Gutenberg-tm collection will
+remain freely available for generations to come.  In 2001, the Project
+Gutenberg Literary Archive Foundation was created to provide a secure
+and permanent future for Project Gutenberg-tm and future generations.
+To learn more about the Project Gutenberg Literary Archive Foundation
+and how your efforts and donations can help, see Sections 3 and 4
+and the Foundation web page at https://www.pglaf.org.
+
+
+Section 3.  Information about the Project Gutenberg Literary Archive
+Foundation
+
+The Project Gutenberg Literary Archive Foundation is a non profit
+501(c)(3) educational corporation organized under the laws of the
+state of Mississippi and granted tax exempt status by the Internal
+Revenue Service.  The Foundation's EIN or federal tax identification
+number is 64-6221541.  Its 501(c)(3) letter is posted at
+https://pglaf.org/fundraising.  Contributions to the Project Gutenberg
+Literary Archive Foundation are tax deductible to the full extent
+permitted by U.S. federal laws and your state's laws.
+
+The Foundation's principal office is located at 4557 Melan Dr. S.
+Fairbanks, AK, 99712., but its volunteers and employees are scattered
+throughout numerous locations.  Its business office is located at
+809 North 1500 West, Salt Lake City, UT 84116, (801) 596-1887, email
+business@pglaf.org.  Email contact links and up to date contact
+information can be found at the Foundation's web site and official
+page at https://pglaf.org
+
+For additional contact information:
+     Dr. Gregory B. Newby
+     Chief Executive and Director
+     gbnewby@pglaf.org
+
+Section 4.  Information about Donations to the Project Gutenberg
+Literary Archive Foundation
+
+Project Gutenberg-tm depends upon and cannot survive without wide
+spread public support and donations to carry out its mission of
+increasing the number of public domain and licensed works that can be
+freely distributed in machine readable form accessible by the widest
+array of equipment including outdated equipment.  Many small donations
+($1 to $5,000) are particularly important to maintaining tax exempt
+status with the IRS.
+
+The Foundation is committed to complying with the laws regulating
+charities and charitable donations in all 50 states of the United
+States.  Compliance requirements are not uniform and it takes a
+considerable effort, much paperwork and many fees to meet and keep up
+with these requirements.  We do not solicit donations in locations
+where we have not received written confirmation of compliance.  To
+SEND DONATIONS or determine the status of compliance for any
+particular state visit https://pglaf.org
+
+While we cannot and do not solicit contributions from states where we
+have not met the solicitation requirements, we know of no prohibition
+against accepting unsolicited donations from donors in such states who
+approach us with offers to donate.
+
+International donations are gratefully accepted, but we cannot make
+any statements concerning tax treatment of donations received from
+outside the United States.  U.S. laws alone swamp our small staff.
+
+Please check the Project Gutenberg Web pages for current donation
+methods and addresses.  Donations are accepted in a number of other
+ways including including checks, online payments and credit card
+donations.  To donate, please visit: https://pglaf.org/donate
+
+
+Section 5.  General Information About Project Gutenberg-tm electronic
+works.
+
+Professor Michael S. Hart was the originator of the Project Gutenberg-tm
+concept of a library of electronic works that could be freely shared
+with anyone.  For thirty years, he produced and distributed Project
+Gutenberg-tm eBooks with only a loose network of volunteer support.
+
+Project Gutenberg-tm eBooks are often created from several printed
+editions, all of which are confirmed as Public Domain in the U.S.
+unless a copyright notice is included.  Thus, we do not necessarily
+keep eBooks in compliance with any particular paper edition.
+
+Most people start at our Web site which has the main PG search facility:
+
+     https://www.gutenberg.org
+
+This Web site includes information about Project Gutenberg-tm,
+including how to make donations to the Project Gutenberg Literary
+Archive Foundation, how to help produce our new eBooks, and how to
+subscribe to our email newsletter to hear about new eBooks.
+
+*** END: FULL LICENSE ***
+
diff --git a/16487.zip b/16487.zip
new file mode 100644
index 0000000..96b7c6d
--- /dev/null
+++ b/16487.zip
diff --git a/LICENSE.txt b/LICENSE.txt
new file mode 100644
index 0000000..6312041
--- /dev/null
+++ b/LICENSE.txt
@@ -0,0 +1,11 @@
+This eBook, including all associated images, markup, improvements,
+metadata, and any other content or labor, has been confirmed to be
+in the PUBLIC DOMAIN IN THE UNITED STATES.
+
+Procedures for determining public domain status are described in
+the "Copyright How-To" at https://www.gutenberg.org.
+
+No investigation has been made concerning possible copyrights in
+jurisdictions other than the United States. Anyone seeking to utilize
+this eBook outside of the United States should confirm copyright
+status under the laws that apply to them.
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..69cb675
--- /dev/null
+++ b/README.md
@@ -0,0 +1,2 @@
+Project Gutenberg (https://www.gutenberg.org) public repository for
+eBook #16487 (https://www.gutenberg.org/ebooks/16487)