CHAPTER III.

THE FACTORS CONCERNED IN THE BUILDING OF THE LIVING MACHINE.

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

Here again we must attack the problem from what seems to be the wrong end. Apparently it would be simpler to discover the method of the manufacture of the simplest machine rather than the more complex ones. But this has proved contrary to the fact. Perhaps the chief reason is that the simplest living machine is the cell whose study must always involve the use of the microscope, and for this reason is more difficult. Perhaps it is because the problem is really a more difficult one than to explain the building of the more complex machines out of the simpler ones. At all events, the last fifty years have told us much of the method of the building of the complex machines out of the simpler ones, while we have as yet not even a hint as to the solution of the building of the simplest machine from the inanimate world. Our attention must, therefore, be first directed to the method by which nature has constructed the complex machines which we find filling the world to-day in the form of animals and plants.

==History of the Living Machine.==--In the first place, we must notice that these machines have not been fashioned suddenly or rapidly, but have been the result of a very slow growth. They have had a history extending very far back into the past for a period of years which we can only indefinitely estimate, but certainly reaching into the millions. As we look over this past history in the light of our present knowledge we see that whatever have been the forces which have been concerned in the construction of these machines they have acted very slowly. It has taken centuries, and, indeed, thousands of years, to take the successive steps which have been necessary in this construction. Secondly, we notice that the machines have been built up step by step, one feature being added to another with the slowly progressing ages. Thirdly, we notice that in one respect this construction of the living machine by nature's processes has been different from our ordinary method of building machines. Our method of building puts the parts gradually into place in such a way that until the machine is finished it is incapable of performing its functions. The half-built engine is as useless and as powerless as so much crude iron. Its power of action only appears after the last part is fitted into place and the machine finished. But nature's process in machine building is different. Every step in the process, so far as we can trace it at least, has produced a complete machine. So far back as we can follow this history we find that at every point the machine was so complete as to be always endowed with motion and life activity. Nature's method has been to take simpler types of machines and slowly change them into more complicated ones without at any moment impairing their vigour. It is something as if the steam engine of Watt should be slowly changed by adding piece after piece until there was finally produced the modern quadruple expansion engine, but all this change being made upon the original engine without once stopping its motion.

This gradual construction of the living machines has been called _Organic Evolution_, or the _Theory of Descent_. It will be necessary for us, in order to comprehend the problem which we have before us, to briefly outline the course of this evolution. Our starting point in this history must be the cell, for such is the earliest and simplest form of living thing of which we have any trace. This cell is, of course, 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.

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.

==Significance of these Sources of History.==--The real force of these sources of evidence comes to us only when we compare them with each other. They agree in a most remarkable fashion. The history as disclosed by fossils and that told by embryology agree with each other, and these are in close harmony with the history as it can be read from comparative anatomy. If archæologists were to find, in different countries and entirely unconnected with each other two or more different records of a lost nation, the belief in the actual existence of that nation would be irresistible. When researches at Nineveh, for example, unearth tablets which give the history of ancient nations, and when it proves that among the nations thus mentioned are some with the same names and having the same facts of history as those mentioned in the Bible, it is absolutely impossible to avoid the conclusion that such a nation with such a history did actually exist. Two independent sources of record could not be false in regard to such a matter as this.

Now, our sources of evidence for this history of the living machine prove to be of exactly this kind. We have three independent sources of evidence which are so entirely different from each other that there is almost no likeness between them. One is written in the rocks, one in bone and muscle, while the third is recorded in the evanescent and changing pages of embryology and metamorphosis. Yet each tells the same story. Each tells of a history of this machine from simple forms to more complex. Each tells of its greater and greater differentiation of labour and structure as the periods of time passed. Each tells of a growing complexity and an increasing perfection of the organisms as successive periods pass. Each tells us of common points of origin and divergence from these points. Each tells us how the more complicated forms have arisen as the results of changes in and modifications of the simpler forms. Each shows us how the individual parts of the organisms have been enlarged or diminished or changed in shape to adapt them to new duties. Each, in short, tells the same story of the gradual construction of the living machine by slow steps and through long ages of time. When these three sources of history so accurately agree with each other, it is as impossible to disbelieve in the existence of such history as it is to disbelieve in the existence of the ancient Hittite nation, after its history has been told to us by two different sources of record.

Now all this is very germane to our subject. We are trying to learn how this living machine, with its wonderful capabilities, was built. The history which we have outlined is undoubtedly the history of the building of this machine, and the knowledge that these complicated machines have been produced as the result of slow growth is of the utmost importance to us. This knowledge gives us at the very start some idea of the nature of the forces which have been at work. It tells us that in searching for these forces we must look for those which have been acting constantly. We must look for forces which produce their effects not by sudden additions to the complication of the machine. They must be constant forces whose effect at any one time is comparatively slight, but whose total effect is to increase the complexity of the machine. They must be forces which produce new types through the modification of the old ones. We must look for forces which do not adapt the machine for its future, but only for its present need. Each step in the history has been a complete animal with its own fully developed powers. We are not to expect to find forces which planned the perfect machine from the start, nor forces which were engaged in constructing parts for future use. Each step in the building of the machine was taken for the good of the machine at the particular moment, and the forces which we are to look for must therefore be only such as can adapt the organisms for its present needs. In other words, nothing has been produced in this machine for the purpose of being developed later into something of value, but all parts that have been produced are of value at the time of their appearance. We must, in short, look for forces constantly in action and always tending in the same direction of greater complexity of structure.

Is it possible to discover these forces and comprehend their action? Before the modern development of evolution this question would unhesitatingly have been answered in the negative. To-day, under the influence of the descent theory, stimulated, in the first place, by Darwin, the question will be answered by many with equal promptness in the affirmative. At all events, we have learned in the last forty years to recognize some of the factors which have been at work in the construction of this machine. We must turn, therefore, to the consideration of these factors.

==Forces at Work in the Building of the Living Machine.==--There are three primary factors which lie at the bottom of the whole process. They are--

1. _Reproduction_, which preserves type from generation to generation.

2. _Variation_, which modifies type from generation to generation.

3. _Heredity_, which transmits characters from generation to generation.

Each must be considered by itself.

==Reproduction.==--Reproduction is the primary factor in this process of machine building, heredity and variation being simply phases of reproduction. The living machine has developed by natural processes, all other machines by artificial methods. Reproduction is the one essential point of difference between the living machine and the others which has made their construction by natural processes a possibility. What, then, is reproduction? Reproduction is in all cases at the bottom simple division. Whether we consider the plant that multiplies by buds or the unicellular animal that simply divides into two equal parts, or the larger animal that multiplies by eggs, we find that in all cases the fundamental feature of the process is division. In all cases the organism divides into two or more parts, each of which becomes in time like the original. Moreover, when we trace this division further we find that in all cases it is to be referred back to the division of the cell, such as we have described in a previous chapter. The egg is a single cell which has come from the parent by the division of one of the cells in the body of the parent. A bud is simply a mass of cells which have all arisen from the parent cells by division. The foundation of reproduction is thus in all cases cell division. Now, this process of division is dependent upon the properties of the cell. Firstly, it is a result of the assimilative powers of the cell, for only through assimilation can the cell increase in size, and only as it increases in size can it gain sustenance for cell division. Secondly, it is dependent, as we have seen, upon the mechanism of the cell body, and especially the nucleus and centrosome. These structures regulate the cell division, and hence the reproduction of all animals and plants. We can not, therefore, find any explanation of reproduction until we have explained the mechanism of the cell. The fundamental feature, of nature's machine building is thus based upon the machinery of the nucleus and centrosome of the organic cell.

Aside from the simple fact that it preserves the race, the most important feature connected with this reproduction is its wonderful fruitfulness. Since it results from division, it always tends to increase the offspring in geometrical ratio. In the simplest case, that of the unicellular animals, the cell divides, giving rise to two animals, each of which divides again, producing four, and these again, giving eight, etc. The rapidity of this multiplication is sometimes inconceivable. It depends, of course, upon the interval of time between the successive divisions, but among the lower organisms this interval is sometimes not more than half an hour, the result of which is that a single individual could give rise in the course of twenty-four hours to sixteen million offspring. This is doubtless an extreme case, but among all the lower animals the rate is very great. Among larger animals the process is more complicated; but here, too, there is the same tendency to geometrical progression, although the intervals between the successive reproductions may be quite long and irregular. But it is always so great that if allowed to progress unhindered at its normal rate the offspring would, in a few years, become so numerous as to crowd other life out of existence. Even the slow-breeding elephant would, if allowed to breed unhindered for seven hundred and fifty years, produce nineteen million offspring--a rate of increase plainly incompatible with the continued existence of other animals.

Here, then, we have the foundation of nature's method of building animals and plants of the higher classes. In the machinery of the cell she has a power of reproduction which produces an increase in geometrical ratio far beyond the possibility for the surface of the earth to maintain.

==Heredity.==--The offspring which arise by these processes of division are like each other, and like the parent from which they sprung. This is the essence of what is called heredity. Its significance in the process of machine building is evident at once. It is the conserving force which preserves the forms already produced and makes it possible for each generation to build upon the structures of the earlier ones. Without it each generation would have to begin anew at the beginning, and nothing could be accomplished. But since this principle brings each individual to the same place where its parents stand, and thus always builds the offspring into a machine like the parent, it makes it possible for the successive generations to advance. Heredity is thus like the power of memory, or better still, like the invention of printing in the development of civilization. It is a record of past achievements. By means of printing each age is enabled to benefit by the discoveries of the previous age, and without it the development of civilization would be impossible. In the same way heredity enables each generation to benefit by the achievements of its ancestors in the process of machine building, and thus to devote its own energies to advancement.

The fact of heredity is patent enough. It has been always clearly recognized that the child has the characters of its parents, and this belief is so well attested as to need no proof. It is still a question as to just what characters may be inherited, and what influences may affect the inheritance. There are plenty of puzzling problems connected with heredity, but the fact of heredity is one of the foundation stones of biological science. Upon it must be built all theories which look toward the explanation of the origin of the living machine.

This factor of heredity again we must trace back to the machinery of the cell. We have seen in the previous pages evidence for the wonderful nature of the chromosomes of the cells. We can not pretend to understand them, but they must be extraordinarily complex. We have seen proof that these chromosomes are probably the physical basis of heredity, since they are the only parts of each parent which are handed down to subsequent generations. With these various facts of cell division and cell fertilization in mind, we can reach a very simple explanation of fundamental features of heredity. The following is an outline of the most widely accepted view of the hereditary process.

Recognizing that the chromosomes are the physical basis of hereditary transmission, we can picture to ourselves the transmission of hereditary characters something as follows: As we have seen, the fertilized egg contains an equal number of chromosomes from each parent (Fig. 42). Now when this fertilized cell divides, each of the rods splits lengthwise, half of each entering each of the two cells arising from the cell division. From this method of division of the chromosomes it follows that the daughter cells would be equivalent to each other and equivalent also to the undivided egg. If the original chromosomes contained potentially all the hereditary traits handed down from parent to child, the chromosomes of each daughter cell will contain similar hereditary traits. If, therefore, the original fertilized egg possessed the power of developing into an adult like the parent, each of the daughter cells should likewise possess the power of developing into a similar adult. And thus each cell which arises as the result of such division should possess similar characters so long as this method of division continues. But after a little in the development of the egg a differentiation among the daughter cells arises. They begin to acquire different shapes and different functions. This we can only believe to be the result of a differentiation in their chromatin material. In the cell division the chromosomes no longer split into equivalent halves, but some characters are portioned off to some cells and others to other cells. Those cells which are to carry on digestive functions when they are formed receive chromatin material which especially controls them in the performance of this digestive function, while those which are to produce sensory organs receive a different portion of the chromatin material. Thus the adult individual is built up as the cells receive different portions of this hereditary substance contained in the original chromosomes. The original chromosomes contained _all_ hereditary characters, but as development proceeds these are gradually portioned out among the daughter cells until the adult is formed.

From this method of division it will be seen that each cell of the adult does not contain all the characters concealed in the original chromosomes of the egg, although each contains a part which may have been derived from each parent. It is thought, however, that a part of the original chromatin material does not thus become differentiated, but remains entirely unchanged as the individual is developing. This chromatin material may increase in amount by assimilation, but it remains unchanged during the entire growth of the individual. It thus follows that the adult will contain, along with its differentiated material, a certain amount of the original physical basis of heredity which still retains its original powers. This undifferentiated chromatin material originally possessed powers of producing a new individual, and of course it still possesses these powers, since it has remained dormant without alteration. Further, it will follow that if this dormant undifferentiated chromatin should start into activity and produce a new individual, the new individual thus produced would be identical in all characters with the one which actually did develop from the egg, since both individuals would have come from a bit of the same chromatin. The child would be like the parent. This would be true no matter how much this undifferentiated material should increase in amount by assimilation, _so long as it remained unaltered in character_, and it hence follows that every individual carries around a certain amount of undifferentiated chromatin material in all respects identical with that from which he developed.

Now whether this undifferentiated _germ plasm_, as we will now call it, is distributed all over the body, or is collected at certain points, is immaterial to our purpose. It is certain that portions of it find their way into the reproductive organs of the animal or plant. Thus we see that part of the chromatin material in the egg of the first generation develops into the second generation, while another part of it remains dormant in that second generation, eventually becoming the chromatin of its eggs and spermatozoa. Thus each egg of the second generation receives chromosomes which have come directly from the first generation, and thus it will follow that each of these eggs will have identical properties with the egg of the first generation. Hence if one of these new eggs develops into an adult it will produce an adult exactly like the second generation, since it contains chromosomes which are absolutely identical with those from which the second generation sprung. There is thus no difficulty in understanding why the second generation will be like the first, and since the process is simply repeated again in the next reproduction, the third generation will be like the second, and so on, generation after generation. A study of the accompanying diagram will make this clear.

In other words, we have here a simple understanding of at least some of the features of heredity. This explanation is that some of the chromatin material or germ plasm is handed down from one generation to another, and is stored temporarily in the nucleii of the reproductive cells. During the life of the individual this germ plasm is capable of increasing in amount without changing its nature, and it thus continues to grow and is handed down from generation to generation, always endowed with the power of developing into a new individual under proper conditions, and of course when it does thus give rise to new individuals they will all be alike. We can thus easily understand why a child is like its parent. It is not because the child can inherit directly from its parent, but rather because both child and parent have come from the unfolding of two bits of the same germ plasm. This fact of the transmission of the hereditary substance from generation to generation is known as the theory of the _continuity of germ plasm_.

Such appears to be, at least in part, the machinery of heredity. This understanding makes the germ substance perpetual and continuous, and explains why successive generations are alike. It does not explain, indeed, why an individual inherits from its parents, but why it is like its parents. While biologists are still in dispute over many problems connected with heredity, all are agreed to-day that this principle of the continuity of the heredity substance must be the basis of all attempts to understand the machinery of heredity. But plainly this whole process is a function of the cell machinery. While, therefore, the idea of the continuity of germ substance greatly simplifies our problem, we must acknowledge that once more we are thrown back upon the mysteries of the cell. Until we can more fully explain the cell machine we must recognize our inability to solve the fundamental question of why an individual is like its parents.

But plainly reproduction and heredity, as we have thus far considered them, will be unable to account for the slow modification of the machine; for in accordance with the facts thus far outlined, each generation would be _precisely like the last_, and there would be no chance for development and change from generation to generation. If the individual is simply the unfolding of the powers possessed by a bit of germ plasm, and if this germ plasm is simply handed on from generation to generation, the successive generations must of necessity be identical. But the living machine has been built by changes in the successive generation, and hence plainly some other factor is needed. This factor is _variation_.

==Variation.==--Variation is the principle that produces _modification of type_. Heredity, as just explained, would make all generations alike. But nothing is more certain than that they are not alike. The fact of variation is patent on every side, for no two individuals are alike. Successive generations differ from each other in one respect or another. Birds vary in the length of their bills or toes; butterflies, in their colours; dogs, in their size and shape and markings; and so on through an endless category. Plants and animals alike throughout nature show variations in the greatest profusion. It is these variations which must furnish us with the foundation of the changes which have gradually built up the living machine.

Of the fact of these variations there is no question, and the matter need not detain us. Every one has had too many experiences to ask for proof. Of the nature of the variations, however, there are some points to be considered which are very germane to our subject. In the first place, we must notice that these variations are of two kinds. There is one class which is born with the individual, so that they are present from the time of birth. In saying that these variations are born with the individual we do not necessarily mean that they are externally apparent at birth. A child may inherit from its parents characters which do not appear till adult life. For example, a child may inherit the colour of its father's hair, but this colour is not apparent at birth. It appears only in later life, but it is none the less an inborn character. In the same way, we may have many inborn variations among individuals which do not make themselves seen until adult life, but which are none the less innate. The offspring of the same parents may show decided differences, although they are put under similar conditions, and such differences are of course inherent in the nature of the individual. Such variations are called _congenital variations_.

There is, however, a second class of variations which are not born in the individual, but which arise as the result of some conditions affecting its after-life. The most extreme instances of this kind are mutilations. Some men have only one leg because the other has been lost by accident. Here is a variation acquired as the result of circumstances. A blacksmith differs from other members of his race in having exceptionally large arm muscles; but here, again, the large muscles have been produced by use. A European who has lived under a tropical sun has a darkened skin, but this skin has evidently been darkened by the action of the sun, and is quite a different thing from the dark skin of the dark races of men. In such instances we have variations produced in individuals as the result of outside influences acting upon them. They are not inborn, but are secondarily acquired by each individual. We call them _acquired variations_.

It is not always possible to distinguish between these two types of variation. Frequently a character will be found in regard to which it is impossible to determine whether it is congenital or acquired. If a child is born under the tropical sun, how can we tell whether its dark skin was the result of direct action of the sun on its own skin, or was an inheritance from its dark-skinned parents? We might suppose that this could be answered by taking a similar child, bringing it up away from the tropical sun, and seeing whether his skin remained dark. This would not suffice, however; for if such a child did then develop a white skin, we could not tell but that this lighter-coloured skin had been produced by the direct bleaching effect of the northern climate upon a skin which otherwise would have been dark. In other words, a conclusive answer can not here be given. It is not our purpose, however, to attempt to distinguish between these two kinds of variations, but simply to recognize that they occur.

Our next problem must be to search for an explanation of these variations. With the acquired variations we have no particular trouble, for they are easily explained as due to the direct action of the environment upon animals. One of the fundamental characters of the living protoplasm (using the word now in its widest sense) is its extreme instability. So unstable is it that any disturbing influence will affect it. If two similar unicellular organisms are placed under different conditions they become unlike, since their unstable protoplasm is directly affected by the surrounding conditions. With higher animals the process is naturally a little more complicated; but here, too, they are easily understood as part of the function of the machine. One of the adjustments of the machine is such that when any organ is used more than usual the whole machine reacts in such a way as to send more blood to this special organ. The result is a change in the nutrition of the organ and a corresponding variation in the individual. Thus acquired variations are simply functions of the action of the machine.

Congenital variations, however, can not receive such an explanation. Being born with the individual, they can not be produced by conditions affecting him, but rather to something affecting the germ plasm from which he sprung. The nature of the germ plasm controls the nature of the individual, and congenital variations must consequently be due to its variations. But it is not so easy to see how this germ plasm can undergo variation. The conditions which surround the individual would affect its body, but it is not easy to believe that they would affect the germinal substance. Indeed, it is not easy to see how any external conditions can have influence upon this germinal material if it is not an active part of the body, but is simply stored within it for future use in reproduction. How could any changes in the environment of the individual have any effect upon this dormant material stored within it? But if we are correct in regarding this germ material in the reproductive bodies as the basis of heredity and the guiding force in development, then it follows that the only way in which congenital variations can occur is by some variations in the germ plasm. If a child developed from germ plasm _identical_ with that from which its parents developed, it would inherit identical characters; and if there are any congenital variations from its parents, they must be due to some variations in the germ plasm. In other words, in order to explain congenital variations we must account for variations in the germ plasm.

Now, there are two methods by which we may suppose that these variations in the germ may arise. The first is by the direct influence upon the germ plasm of certain unknown external conditions. The life substance of organisms is always very unstable, and, as we have seen, acquired variations are caused by external influences directly affecting it. Now, the hereditary material is also life substance, and it is plainly a possibility for us to imagine that this germ material is also subject to influences from the conditions surrounding it. That such variations do occur appears to be hardly doubtful, although we do not know what sort of influences can produce them. If the germ plasm is wholly stored within the reproductive gland, it is certainly in a position to be only slightly affected by surrounding conditions which affect the animal. We can readily understand that the use of an organ like the arm will affect it in such a way as to produce changes in its protoplasm, but we can hardly imagine that such use of the _arm_ would produce any change in the hereditary substance which is stored in the reproductive organs. External conditions may thus readily affect the body, but not so readily the germ material. Even if such material is distributed more or less over the body instead of being confined to the reproductive glands, as some believe, the difficulty is hardly lessened. This difficulty of understanding how the germ plasm can be affected by external conditions has led one school of biologists to deny that it is subject to any variation by external conditions, and hence that all modification of the germ plasm must come from some other source. Probably no one, however, holds this position to-day, and it is the general belief that the germ plasm may be to some slight extent modified by external conditions. Of course, if such variations do occur in the germ plasm they will become congenital variations of the next generation, since the next generation is the unfolding of the germ plasm.

The second method by which the variations of germ plasm may arise is apparently of more importance. It is based upon the fact that, with all higher animals and plants at least, each individual has two parents instead of one. In our study of cells we have seen that the machinery of the cell is such that it requires in the ordinary process of reproduction the union of germinal material from two different individuals to produce a cell which can develop into a new individual. As we have seen, the egg gets rid of half its chromosomes in order to receive an equal number from a male parent; and thus the fertilized egg contains chromosomes, and hence hereditary material, from two different individuals. Now, this sexual reproduction occurs very widely in the organic world. Among some of the lowest forms of unicellular organisms it is not known, but in most others some form of such union is universal. Now, here is plainly an abundant opportunity for congenital variations; for it is seen that each individual does not come from germ material _identical with that from which either parent came, but from some of this material mixed with a similar amount from a different parent_. Now, the two parents are never exactly alike, and hence the germ plasm which each contributes to the offspring will not be exactly alike. The offspring will thus be the result of the unfolding of a bit of germ plasm which will be different from that from which either of its parents developed, and these differences will result in _congenital variations_. Sexual reproduction thus results in congenital variations; and if congenital variations are necessary for the evolution of the living machine--and we shall soon see reason for believing that they are--we find that sexual reproduction is a device adopted for bringing out such congenital variations.

==Inheritance of Variations.==--The reason why congenital variations are needed for the evolution of the living machine is clear enough. Evanescent variations can have no effect upon this machine, for they would disappear with the individual in which they appeared. In order that they should have any influence in the process of machine building they must be permanent ones; or, in other words, they must be inherited from generation to generation. Only as such variations are transmitted by heredity can they be added to the structure of the developing machine. Therefore we must ask whether the variations are inherited.

In regard to the congenital variations there can be no difficulty. The very fact that they are congenital shows us that they have been produced by variations in the germ plasm, and as such they must be transmitted, not only to the next generation, but to all following generations, until the germ plasm becomes again modified. This germ plasm is handed on from generation to generation with all its variations, and hence the variations will be added permanently to the machine. Congenital variations are thus a means for permanently modifying the organism, and by their agency must we in large measure believe that evolution through the ages has taken place.

With the acquired variations the matter stands quite differently. We can readily understand how influences surrounding an animal may affect its organs. The increase in the size of the muscles of the blacksmith's arm by use we understand readily enough. But with our understanding of the machinery of heredity we can not see how such an effect can extend to the next generation. It is only the organ directly affected that is modified by external conditions. Acquired variations will appear in the part of the body influenced by the changed conditions. But the germ plasm within the reproductive glands is not, so far as we can see, subject to the influence of an increased use, for example, in the arm muscles. The germ material is derived from the parents, and, if it is simply stored in the individual, how could an acquired variation affect it? If an individual lose a limb his offspring will not be without a corresponding limb, for the hereditary material is in the reproductive organs, and it is impossible to believe that the loss of the limb can remove from the hereditary material in the reproductive glands just that part of the germ plasm which was designed for the production of the limb. So, too, if the germ plasm is simply stored in the individual, it is impossible to conceive any way that it can be affected by the conditions around the individual in such a way as to explain the inheritance of acquired variations. If acquired variations do not affect the germ plasm they cannot be inherited, and if the germ plasm is only a bit of protoplasmic substance handed down from generation to generation, we can not believe that acquired variations can influence it.

From such considerations as these have arisen two quite different views among biologists; and, while it is not our purpose to deal with disputed points, these views are so essential to our subject that they must be briefly referred to. One class of biologists adhere closely to the view already outlined, and insist for this reason that acquired variations _can not_ under any conditions be inherited. They insist that all inherited variations are congenital, and due therefore to direct variations in the germ plasm, and that all instances of seeming inheritance of acquired variations are capable of other explanation. The other school is equally insistent that there are abundant instances of the inheritance of acquired characters, claiming that these proofs are so strong as to demand their acceptance. Hence this class of biologists insist that the explanation of heredity given as a simple handing down from generation to generation of a germ plasm is not complete, and that while it is doubtless the foundation of heredity, it must be modified in some way so as to admit of the inheritance of acquired characters. There is no question that has excited such a wide interest in the biological world during the last fifteen years as this one of the inheritance of acquired characters. Until about 1884 the question was not seriously raised. Heredity was known to be a fact, and it was believed that while congenital characters are more commonly inherited, acquired characters may also frequently be handed down from generation to generation. The facts which we have noted of the continuity of germ plasm have during the last fifteen years led many biologists to deny the possibility of the latter. The debate which arose has continued vigorously, and can not be regarded as settled at the present time. One result of this debate is clear. It has been shown beyond question that while the inheritance of congenital characters is the rule, the inheritance of acquired characters is at all events unusual. At the present time many naturalists would be inclined to think that the balance of evidence indicates that under certain conditions certain kinds of acquired characters may be inherited, although this is still disputed by others. Into this discussion we cannot enter here. The reason for referring to it at all is, however, evident. We are searching for nature's method of building machines. It is perfectly clear that variations among animals and plants are the foundations of the successive steps in advance made in this machine building, but of course only such variations as can be transmitted to posterity can serve any purpose in this development. If therefore it should prove that acquired characters can not be inherited, then we should no longer be able to look upon the direct influence of the surroundings as a factor in the machine building. We should then have nothing left except the congenital variations produced by sexual union, or the direct variation of the germ plasm as a factor for advance. If, however, it shall prove that acquired characters may even occasionally be inherited, then the direct effect of the environment upon the individual will serve as a decided assistance in our problem.

Here, then, we have before us the factors which have been concerned in the building of the living machine under nature's hands. Reproduction keeps in existence a constantly active, unstable, readily modified organism as a basis upon which to build. Variation offers constantly new modifications of the type, while heredity insures that the modifications produced in the machine by the influences which give rise to the variations shall be permanently fixed.

==Method of Machine Building.==--_Natural Selection._ The method by which these factors have worked together to build up the living machines is easily understood in its general aspects, although there are many details as yet unsolved. The general facts connected with the evolution of animals are matters of common knowledge. We need do no more than outline the subject, since it is well understood by all. The basis of the method is _natural selection_, which acts in this machine building something as follows:

The law of reproduction, as we have seen, produces new individuals with extraordinary rapidity, and as a result more individuals are born than can possibly find sustenance in the world. Hence only a few of the offspring of any animal or plant can live long enough to produce offspring in turn. The many must die that the few may live; and there is, therefore, a constant struggle among the individuals that are born for food or for room in the world. In this _struggle for existence_ of course the weakest will go to the wall, while those that are best adapted for their place in life will be the ones to get food, live, and reproduce their kind. This is at all events true among the lower animals, although with mankind the law hardly applies. Now, among the individuals that are born there will be no two exactly alike, since variations are universal, many of which are congenital and thus born with the individual and transmitted by inheritance. Clearly enough those animals that have a variation which makes them a little better adapted for the struggle will be the ones to live and hence to produce offspring, while those without such advantage will be the ones to die. We may suppose, for example, that some of the individuals had longer necks than the average. In time of scarcity of food these individuals would be able to get food that the short-necked individuals could not reach. Hence in times of famine the long-necked individuals would be the ones to survive. Now if this peculiarity were a congenital variation it would be already represented in the germ plasm, and consequently it would be inherited by the next generation. The short-necked individuals being largely destroyed in this struggle for food, it would follow that the next generation would be a little better off than the last, since all would inherit this tendency toward a long neck. A few generations would then see the disappearance of all individuals which did not show either this or some other corresponding advantage, and in this way the lengthened neck would be added permanently as a _part of the machine_. When this time came this peculiarity would no longer give its possessors any advantage over its rivals, since all would possess it. Now, therefore, some new variation would in the same way determine which animals should live and which should die in the struggle, and in time a new modification would be added to the machine. And thus this process continues, one variation after another being added, until the machine is slowly built into a more and more complicated structure, always active but with a constantly increasing efficiency. The construction is a natural one. A mixing of germ plasm in sexual reproduction or some other agencies produce congenital variations; natural selection acting upon the numerous progeny selects the best of the new variations, and heredity preserves and hands them down to posterity.

All students of whatever school recognize the force of this principle and look upon natural selection as an efficient agency in machine building. It is probably the most fundamental of the external laws that have guided the process. There are, however, certain other laws which have played a more or less subordinate part. The chief of these are the influence of migration and isolation, and the direct influence of the environment. Each of these laws has its own school of advocates, and each has been given by its advocates the chief role in the process of machine building.

==Migration and Isolation.==--The production of the various types of machines has been undoubtedly facilitated by the migrations of animals and the isolation of different groups of descendants from each other by various natural barriers. The variations which occur in organisms are so great that they would sometimes run into abnormal structures were it not for the fact that sexual reproduction constantly tends to reduce them. In an open country where animals and plants interbreed freely, it will commonly happen that individuals with certain peculiarities will mate with others without such peculiarities, and the offspring will therefore inherit the peculiarity not in increased degree but in decreased degree. This constant interbreeding of individuals will tend to prevent the formation of many modifications in the machine which become started by variations. Now plainly if some such individuals, with a peculiar variation, should migrate into a new territory or become isolated from their relatives which do not have similar variations, these individuals will be obliged to breed with each other. The result will be that the next generation, arising thus from two parents each of which shows the same variation, will show it also in equal or increased degree. Migrations and isolations will thus tend to _fix_ in the machine variations which sexual union or other influences inaugurate. Now in the history of the earth's surface there have been many changes which tend to bring about such migration and isolations, and this factor has doubtless played a more or less important part in the building of the machines. How great a part we cannot say, nor is it necessary for our purpose to decide; for in all these cases the machine building has only been the result of the hereditary transmission of congenital variation under certain peculiar conditions. The fundamental process is the same as already considered, only the details of its working being in question.

==Direct Influence of the Environment.==--Under this head we have a subject of great importance. It is an undoubted fact that the environment has a very decided effect upon the machine. These direct effects of the environment are very positive and in great variety. The tropical sun darkens the human skin; cold climate stunts the growth of plants; lack of food dwarfs all animals and plants, and hundreds of other similar examples could be selected. Another class of similar influences are those produced by _use_ and _disuse_. Beyond question the use of an organ tends to increase its size, and disuse to decrease it. Combats of animals with each other tend to increase their strength, flight from enemies their running powers, etc.

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

==Consciousness.==--It may be well here to refer to one other factor in the problem, because it has somewhat recently been brought into prominence. This factor is consciousness on the part of the animal. Among plants and the lower animals this factor can have no significance, but consciousness certainly occurs among the higher animals. Just when or how it appeared are questions which are not answered, and perhaps never will be. But consciousness, after it had once made its appearance, became a controlling factor in the development of the machine. It must not be understood by this that animals have had any consciousness of the development of their body, or that they have made any conscious endeavours to modify its development. This has not always been understood. It has been frequently supposed that the claim that consciousness has an influence upon the development of an animal means that the animal has made conscious efforts to develop in certain directions. For example, it has been suggested that the tiger, conscious of the advantage of being striped, had a desire to possess stripes, and the desire caused their appearance. This is absurd. Consciousness has been a factor in the development of the machine, but an _indirect_ one. Consciousness leads to effort, and effort has a direct influence in development. For example, an animal is conscious of hunger, and this leads to efforts on his part to obtain food. His efforts to obtain food may lead to migration or to the adoption of new kinds of food or to conflicts with various kinds of rivals, and all of these efforts are potent factors in determining the direction of development. Consciousness, again, may lead certain animals to take pleasure in each other's society, or to recognize that in mutual association they have protection against common enemies. Such a consciousness will give rise to social habits, and social habits are a very potent factor in determining the direction in which the inherited variations will tend; not, perhaps, because it effects the variations themselves, but rather because it determines which variations among the many shall be preserved and which rejected by natural selection. Consciousness may lead the antelope to recognize that he has no chance in a combat with a lion, and this will induce him to flee. The _habit_ of flight would then develop the _power_ of flight, not because the antelope desired such power, but because the animals with variations which gave increased power of flight would be the ones to escape the lion, while the slower ones would die without offspring. Thus consciousness would indirectly, though not directly, result in the lengthening of the legs of the animal and in the strengthening of his running muscles. Beyond a doubt this factor of consciousness has been a factor of no little moment in the development of the higher types of organic machines. We can as yet only dimly understand its action, but it must hereafter be counted as one of the influences in the evolution of the living machine.

But, after all, these are only questions of the method of the action of certain well demonstrated, fundamental factors. Whether by natural selection, or by the inheritance of acquired characters produced by the environment, or whether by the effect of isolation of groups of individuals, the machine building has always been produced in the same way. A machine, either through the direct influence of the environment, or as a result of sexual combination of germ plasm, shows a variation from its parents. This variation proves of value to its possessor, who lives and transmits it permanently to posterity. Thus step by step, one part is added to another, until the machine has grown into the intricately adapted structure which we call the animal or plant. This has been nature's method of building machines, all based upon the three properties possessed by the living cell--reproduction, variation, and heredity.

==Summary of Nature's Power of Building Machines.==--Let us now notice the position we have reached. Our problem in the present chapter has been to find out whether nature possesses forces adequate to explain the building of machines with their parts accurately adapted to each other so as to act harmoniously for certain ends. Astronomy has shown that she has forces for the building of worlds; geology, that she has forces for making mountain and valley; and chemistry, that she has forces for building chemical compounds. But the organism is neither a world, nor a mass of matter, nor a chemical compound. It is a machine. Has nature any forces for machine building? We have found that by the use of the three factors, reproduction, variation, and heredity, nature is able to produce a machine of ever greater and greater complexity, with the parts all adapted to each other. Now the difference between a machine and a mass of matter is simply in the adaptation of parts to act harmoniously for definite ends. Hence if we are allowed these three factors, we can say that nature _does possess forces adequate to the manufacture of machines_. These forces are not chemical forces, and the construction of the machine has thus been brought about by forces entirely different from those which produced the chemical molecule.

But we have plainly not reached the bottom of the matter in our attempt to explain the machinery of living things. We have based the whole process upon three factors. Reproduction, variation, and heredity are the properties of all living matter; but they are not, like gravity and chemism, universal forces of nature. They occur in living organisms only. Why should they occur in living organisms, and here alone? These three properties are perhaps the most marvellous properties of nature; and surely we have not finished our task if we have based the whole process of machine building upon these mysterious phenomena, leaving them unintelligible. We must therefore now ask whether we can proceed any farther and find any explanation of these fundamental powers of the living machine.

It must be confessed that here we are at present forced to stop. We can proceed no further with any certainty, or even probability. We may say that variation and heredity are only phases of reproduction, and reproduction is a property of the living cell. We may say that this power of reproduction is dependent upon the power of assimilation and growth, for cell division is a result of cell growth. We may further say that growth and assimilation are chemical processes resulting from the oxidation of food, and that thus all of these processes are to be reduced to chemical forces. In this way we may seem to have a chemical foundation for life phenomena. But clearly this is far from satisfactory. In the first place, it utterly fails to explain why the living cell has these properties, while no other body possesses them, nor why they are possessed by living protoplasms _alone_, ceasing instantly with death. Indeed it does not tell us what death can be. Secondly, it utterly fails to explain the marvels of cell division with resulting hereditary transmission. For all this we must fall back upon the structure of protoplasm, and say that the cell machinery is so adjusted that the machine, when acting as a whole, is capable of transforming the energy of chemical composition in certain directions. These fundamental properties are then the properties of the cell _machine_ just as surely as printing is the property of the printing press. We can no more account for the life phenomena by chemical powers than we can for printing by chemical forces manifested in the burning of the coal in the engine room. To be sure, it is the chemical forces in the engine room that furnishes the energy, but it is the machinery of the press that explains the printing. So, while chemical forces supply life energy, it is the cell machinery that must explain the fundamental living factors. So long as this machine is intact it can continue to run and perform its duties. But it is a very delicate machine and is easily broken. When it is broken its activities cease. A broken machine can not run. It is dead. In short, we come back once more to the idea of the machinery of protoplasm, and must base our understanding of its properties upon its structure.

It is proper to state that there are still some biologists who insist that the ultimate explanation of protoplasm is purely chemical and that life phenomena may be manifested in mixtures of compounds that are purely physical mixtures and not machines. It is claimed that much of this cell structure described above is due to imperfection in microscopic methods and does not really exist in living protoplasm, while the marvellous activities described are found only in the highly organized cell, but do not belong to simple protoplasm. It is claimed that simple protoplasm consists of a physical mixture of two different compounds which form a foam when thus mixed, and that much of the described structure of protoplasm is only the appearance of this foam. This conception is certainly not the prevalent one to-day; and even if it should be the proper one, it would still leave the cell as an extremely complicated machine. Under any view the cell is a mechanism and must be resolved into subordinate parts. It may be uncertain whether these subordinate parts are to be regarded simply as chemical compounds physically mixed, or as smaller units each of which is a smaller mechanism. At all events, at the present time we know of no such simple protoplasm capable of living activities apart from machinery, and the problem of explaining life, even in the simplest form known, remains the problem of explaining a mechanism.

==The Origin of the Cell Machine.==--We have thus set before us another problem, which is after all the fundamental one, namely, to ask whether we can tell anything of nature's method of building the protoplasmic machine. The building of the higher animal and plant, as we have seen, is the result of the powers of protoplasm; but protoplasm itself is a machine. What has been its history?

We must first notice that no notion of _chemical evolution_ helps us out. It has been a favourite thought with some that the origin of the first living thing was the result of chemical evolution. As the result of physical forces there was produced, from the original nebulous mass, a more and more complicated system until the world was formed. Then chemical phenomena became more and more complicated until, with the production of more and more complicated compounds, protoplasm was finally produced. A few years ago, under the impulse of the idea that protoplasm was a compound, or at least a simple mixture of compounds, this thought of protoplasm as the result of chemical evolution was quite significant. _Physical forces_, _chemical forces_, and _vital forces_, explain successively the origin of _worlds_, _protoplasm_, and _organisms_. This conception has, however, no longer much significance. We know of no such living chemical compound apart from cell machinery. A new conception of protoplasm has arisen which demands a different explanation of its origin. Since it is a machine rather than a compound, mechanical rather than chemical forces are required for its explanation.

Have we then any suggestion as to the method of the origin of this protoplasmic machine? Our answer must, at the present, be certainly in the negative. The complexity of the cell tells us plainly that it can not be the ultimate living substance which may have arisen from chemical evolution. It is made up of parts delicately adapted to act in harmony with each other, and its activity depends upon the relation of these parts. Whatever chemical forces may have accomplished, they never could have combined different bodies into linin, centrosomes, chromosomes, etc., which, as we have seen, are the basis of cell life. To account for this machine, therefore, we are driven to assume either that it was produced by some unknown intelligent power in its present condition of complex adjustment, or to assume that it has had a long history of building by successive steps, just as we have seen to be the case with the higher organisms. The latter assumption is, of course, in harmony with the general trend of thought. To-day protoplasm is produced only from other protoplasm; but, plainly, the first protoplasm on the earth must have had a different origin. We must therefore next look for facts which will enable us to understand its origin. We have seen that the animal and plant machines have been built up from the simple cell as the result of its powers acting under the ordinary conditions of nature. Now, in accordance with this general line of thought, we shall be compelled to assume that previous to the period of building machinery which we have been considering, there was another period of machine building during which this cell machine was built by certain natural forces.

But here we are forced to stop, for nothing which we yet know gives even a hint as to the method by which this machine was produced. We have, however, seen that there are forces in nature efficient in building machines, as well as those for producing chemical compounds; and this, doubtless, suggests to us that there may be similar forces at work in building protoplasm. If we can find natural forces by which the simplest bit of living matter can be built up into a complicated machine like the ox, with its many delicately adjusted parts, it is certainly natural to imagine that the same forces may have built this simpler machine with which we started. But such a conclusion is for a simple reason impossible. We have seen that the essential factor in this machine building is reproduction, with the correlated powers of variation and heredity. Without these forces we could not have advanced in this machine building at all. But these properties are themselves the result of the machinery of protoplasm. We have no reason for thinking that this property of reproduction can occur in any other object in nature except this protoplasmic machine. Of course, then, if reproduction is the result of the structure of protoplasm we can not use this factor in explaining the origin of this protoplasm. The powers of the completed machine can not be brought forward to account for its origin. Thus the one fundamental factor for machine building is lacking, and if we are to explain nature's method of producing protoplasm from simpler structures, we must either suppose that the _parts_ of the cell are capable of reproduction and subject to heredity, or we must look for some other method. Such a road has however not yet been found, nor have we any idea in what direction to look. But the fact that nature has methods of machine building, as we have seen, may hold out the possibility that some day we may discover her method of building this primitive living machine, the cell.

It is useless to try to go further at present. The origin of living matter is shrouded in as great obscurity as ever. We must admit that the disclosures of the modern microscope have complicated rather than simplified this problem. While a few years ago chemists and biologists were eagerly expecting to discover a method of manufacturing a bit of living matter by artificial means, that hope has now been practically abandoned. The task is apparently hopeless. We can manipulate chemical forces and produce an endless series of chemical compounds. But we can not manipulate the minute bits of matter which make up the living machine. Since living matter is made of the adjustment of these microscopic parts of matter, we can not hope to make a bit of living matter until we find some way of making these little parts and adjusting them together. Most students of protoplasm have therefore abandoned all expectation of making even the simplest living thing. We are apparently as far from the real goal of a natural explanation of life as we were before the discovery of protoplasm.

==General Summary.==--It is now desirable to close this discussion of seemingly somewhat unconnected topics by bringing them together in a brief summary. This will enable us to see more clearly the position in which science stands to-day upon this matter of the natural explanation of living phenomena, and to picture to ourselves more concisely our knowledge of the living machine.

The problem we have set before us is to find out to what extent it is possible to account for vital phenomena by the application of ordinary natural laws and forces, and therefore to find out whether it is necessary to assume that there are forces needed to explain life which are different from those found in other realms of nature, or whether vital forces are all correlated with physical forces. It has been evident at a glance that the living body is a machine. Like other machines it consists of parts adjusted to each other for the accomplishment of definite ends, and its action depends upon the adjustment of its parts. Like other machines, it neither creates nor destroys energy, but simply converts the potential energy of its foods into some form of active energy, and, like other machines, its power ceases when the machine is broken.

With this understanding the problem clearly resolved itself into two separate ones. The first was to determine to what extent known physical and chemical laws and forces are adequate to an explanation of the various phenomena of life. The second was to determine whether there are any known forces which can furnish a natural explanation of the origin of the living machine. Manifestly, if the first of these problems is insolvable, the second is insolvable also.

In the study of the first problem we have reached the general conclusion that the secondary phenomena of life are readily explained by the application of physical and chemical forces acting in the living machine. These secondary phenomena include such processes as the digestion and absorption of food, circulation, respiration, excretion, bodily motion, etc. Nervous phenomena also doubtless come under this head, at least so far as concerns nervous force. We have been obliged, however, to exclude from this correlation the mental phenomena. Mental phenomena can not as yet be measured, and have not yet been shown to be correlated with physical energy. In other words, it has not yet been proved that mental force is energy at all; and if it is not energy, then of course it can not be included in the laws which govern the physical energy of the universe. Although a close relation exists between physical changes in the brain cells and mental phenomena, no further connection has yet been drawn between mental power and physical force. All other secondary phenomena, however, are intelligently explained by the action of natural forces in the machinery of the living organism.

While we have thus found that the secondary phenomena of life are intelligible as the result of the structure of the machine, certain other fundamental phenomena have been constantly forcing themselves upon our attention as a _foundation_ of these secondary activities. The power of contraction, the power of causing certain kinds of chemical change to occur which result in metabolism, the property of sensibility, the property of reproduction--these are fundamental to all living activity, and are, after all, the real phenomena which we wish to explain. But these are not peculiar to the complicated machines. We can discard all the apparent machinery of the animal or plant and find these properties still developed in the simplest bit of living matter. To learn their significance, therefore, we have turned to the study of the simplest form of matter in which these fundamental properties are manifested. This led us at once to the study of the so-called protoplasm, for protoplasm is the simplest known form of matter that is alive. Protoplasm itself at first seemed to be a homogeneous body, and was looked upon as a chemical compound of high complexity. If this were true its properties would depend upon its composition and would be explained by the action of chemical forces. Such a conception would have quickly solved the problem, for it would reduce living properties to chemical powers. But the conception proved to be delusive. Protoplasm, at least the simplest form known to possess the fundamental life properties, soon showed itself to be no chemical compound, but a machine of wonderful intricacy.

The fundamental phenomena of life and of protoplasm have proved to be both chemical and mechanical. Metabolism is the result of the oxidation of food, and motion is an instance of transference of force. Our problem then resolved itself into finding the power that guides the action of these natural forces. Food will not undergo such an oxidation except in the presence of protoplasm, nor will the phenomena of metabolism occur except in the presence of _living_ protoplasm. Clearly, then, the living protoplasm contains within itself the power of guiding this play of chemical force in such a way as to give rise to vital phenomena, and our search must be not for chemical force but for this guiding principle. Our study of protoplasm has told us clearly enough that we must find this guiding principle in the interaction of the machinery within the protoplasm. The microscope has told us plainly that these fundamental principles are based upon machinery. The cell division (reproduction) is apparently controlled by the centrosomes; the heredity by the chromosomes; the constructive metabolism by the nucleus in general, while the destructive metabolism is also seated in the cell substance outside the nucleus. Whether these statements are strictly accurate in detail does not particularly affect the general conclusion. It is clearly enough demonstrated that the activities of the protoplasmic body are dependent upon the relation of its different parts. Although we have got rid of the complicated machinery of the organism in general, we are still confronted with the machinery of the cell.

But our analysis can not, at present, go further. Our knowledge of this machine has not as yet enabled us to gain any insight as to its method of action. We can not yet conceive how this machine controls the chemical and physical forces at its disposal in such a way as to produce the orderly result of life. The strict correlation between the forces of the physical universe and those manifested by this protoplasm tells us that a transformation of energy occurs within it, but of the method of that transformation we as yet know nothing. Irritability, movement, metabolism, and reproduction appear to be not chemical properties of a compound, but mechanical properties of a machine. Our mechanical analysis of the living machine stops short before it reaches any foundation in the chemical forces of nature.

It is thus clearly apparent that the phenomena of life are dependent upon the machinery of living things, and we have therefore the second question of the _origin_ of this machinery to answer. Chemical forces and mechanical forces have been laboriously investigated, but neither appear adequate to the manufacture of machines. They produce only chemical compounds and worlds with their mountains and seas. The construction of artificial machines has demanded intelligence. But here is a natural machine--the organism. It is the only machine produced by natural methods, so far as we know; and we have therefore next asked whether there are, in nature, simple forces competent to build machines such as living animals and plants?

In pursuance of this question we have found that the complicated machines have been built out of the simpler ones by the action of known forces and laws. The factors in this machine building are simply those of the fundamental vital properties of the simplest protoplasmic machine. Reproduction, heredity, and variation, acting under the ever-changing conditions of the earth's surface, are apparently all that are needed to explain the building of the complex machines out of the simpler ones. Nature _has_ forces adequate to the building of machines as well as forces adequate to the formation of chemical compounds and worlds.

But here again we are unable to base our explanation upon chemical and physical forces. Reproduction, heredity, and variation are properties of the cell machine, and we are therefore thrown back upon the necessity of explaining the origin of this machine. Can we find a mechanical or chemical explanation of the origin of protoplasm? A chemical explanation of the cell is impossible, since it is not a chemical compound, but a piece of mechanism. The explanation given for the origin of animals and plants is also here apparently impossible. The factors upon which that explanation depended are factors of this completed machine itself, and can not be used to explain its origin. We are left at present therefore without any foundation for further advance. The cells must have had a history of construction, but we do not as yet conceive any forces which may be looked upon as contributing to that history. Whether life phenomena can be manifested by any mixture of compounds simpler than the cell we do not yet know.

The great problems still remaining for solution, which have hardly been touched by modern biology in all its endeavours to find a mechanical explanation of the living machine, are, therefore, three. First, the relation of mentality to the general phenomena of the correlation of force; second, the intelligible understanding of the mechanism of protoplasm which enables it to guide the blind chemical and physical forces of nature so as to produce definite results; third, the kind of forces which may have contributed to the origin of that simplest living machine upon whose activities all vital phenomena rest--the living cell.

INDEX.

A.

Absorption of food, 20.

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

Amoeba, 73.

Anatomical evidence for evolution, 142.

Aquacity, 80.

Arm compared with wing, 144.

Aristotle, 1.

Assimilation, 80, 124, 149, 176.

Asters of dividing cells, 98.

B.

Barry, 63, 64.

Bathybias, 84.

Biology a new science, 1, 5, 15.

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

Blood-vessels, 35, 36.

Body as a machine, 22, 25, 49.

Bone cells, 69.

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

C.

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

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

Cellular structure of organisms, 65.

Cell wall, 64, 72.

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

Challenger expedition, 83.

Chemical evolution, 179.

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

Chemism or mechanism, 57, 176.

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

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

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

Circulation, 34.

Colonies of cells, 85.

Comparison of the body and a machine, 22.

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

Connective-tissue cells, 70.

Conservation of energy, 7, 17.

Consciousness as a factor in machine building, 173.

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

Continuity of germ plasm, 155.

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

Cytoblastema. 62.

Cytology, 10.

D.

Darwin, 81.

Death of the cell, 127.

Decline of the reign of protoplasm, 85.

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

Dialysis, 29, 30, 31.

Digestion, 27.

E.

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

Egg, fertilization of, 102.

Embryological evidence for evolution, 140.

Energy of nervous impulse, 43, 54.

Environment, 171.

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

Evolution, 9, 16, 81, 134.

Experiments with developing eggs, 121.

F.

Fat, absorption of, 32.

Female pronucleus, 110.

Fern cells, section of, 67.

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

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

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

Formed material, 64.

Free cell formation, 64.

G.

Geological evidence for evolution, 139.

Germ plasm, 154.

H.

Heart as a pump, 35.

Heat, 24, 44, 45.

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

Hereditary traits, 113, 153.

Historical geology, 6.

History of the living machine, 133, 147.

Horses' toes, loss of, 172.

Huxley, 11, 75, 83, 84.

I.

Irritability, 54.

Isolation, theory of, 170.

K.

Karyokinesis, 96, 101.

Kidneys, 41.

L.

Leaf, section of, 66.

Life the result of a mechanism, 115, 177.

Linin, 92, 103.

Linnæus, 1.

Lyell, 6.

Lymph, 36, 37.

M.

Machine defined, 20.

Machines the result of mechanical forces, 116.

Male cell, 104, 107.

---- pronucleus, 109.

Maturation of the egg, 104.

Mechanical nature of living organisms, 12.

Mechanical theory of life, 81, 144.

Membrane of the nucleus, 92, 101.

Mental phenomena, 47, 48.

Metabolism, 54.

Microsomes, 87.

Migration, theory of, 170.

Monera, 88.

Movement, 54.

Muscle, 36, 71.

N.

Natural selection, 167.

Nerve-fibre cell, 70.

Nervous energy, 42, 44.

---- system, 41.

New biological problems, 15.

Nucleolus, 65, 92, 94.

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

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

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

---- structure of, 91.

O.

Organic chemistry, 78.

Organic compounds, artificial manufacture of, 78, 82.

Origin of cell machine, 178, 179, 180.

Origin of life, 81, 182.

Osmosis, 29.

Oxidation, 80, 176.

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

P.

Philosophical biology, 4.

Physical basis of life, 75.

Polar cells, 107.

Potato, section of cells, 67.

Properties of chemical compounds, 79.

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

---- artificial manufacture of, 82.

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

---- discovery of, 74.

---- nature of, 76.

---- structure of, 86, 87.

Purpose _vs._ cause, 11, 12.

R.

Reaction against the cell doctrine, 117.

Reign of law, 4.

---- of the nucleus, 91.

---- of protoplasm, 81, 85.

Relationship, significance of, 143.

Removal of waste, 39, 40.

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

Respiration, 37.

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

Root tip, section of, 66.

S.

Schultze, 74, 75.

Schwann, 61, 62, 72.

Secretion, 39, 40.

Segmentation nucleus, 110.

Sensations, 46.

Separation of chromosomes, 100.

Sexual reproduction, 102.

Spermatozoan, 107, 109, 154.

Splitting of chromosomes, 99.

Spindle fibres, 101.

Struggle for existence, 168.

Summary of Part I, 128.

---- general, 182.

U.

Undifferentiated protoplasm, 83.

Unicellular animals, 71.

Units of vital activity, 53.

Use and disuse, 171, 172.

V.

Variation, 148, 157, 160, 176.

Variation from sexual union, 162.

Variation in germ plasm, 161.

Vegetative functions, 41.

Villi, 31.

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

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

W.

Wing compared with arm, 144.

Wood cells, 68.

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End of Project Gutenberg's The Story of the Living Machine, by H. W. Conn