Of these the cell substance appears to be handed down from the mother; the centrosome comes, at least in some cases, from the father, and the chromosomes from both parents. It is not yet certain, however, whether the centrosome is a constant part of the cell. In some cells it cannot yet be found, and there are some reasons for believing that it may be formed out of other parts of the cell. The nucleus is always a direct descendant from the nucleus of pre-existing cells, so that there is an absolute continuity of descent between the nucleii of the cells of an individual and those of its antecedents back for numberless generations.

It is not certain that there is any such continuity of descent in the case of the centrosomes; for, while in the process of fertilization the centrosome is handed down from parent to child, there are some reasons for believing that it may disappear in subsequent cells, and later be redeveloped out of other parts. The only part of the cell in which complete continuity from parent to child is demonstrated, is the nucleus and particularly the chromosomes. All of these facts simply emphasize the importance of the chromosomes, and tell us that these bodies must be regarded as containing the most important features of the cell which const.i.tute its individuality.

==What is Protoplasm?==--Enough has now been given of disclosures of the modern microscope to show that our old friend Protoplasm has a.s.sumed an entirely new guise, if indeed it has not disappeared altogether. These simplest life processes are so marvelous and involve the action of such an intricate ma.s.s of machinery that we can no longer retain our earlier notion of protoplasm as the physical basis of life. There can be no life without the properties of a.s.similation, growth, and reproduction; and, so far as we know, these properties are found only in that combination of bodies which we call the cell, with its mixture of harmoniously acting parts. _Life, at least the life of a cell, is then not the property of a chemical compound protoplasm, but is the result of the activities of a machine._ Indeed, we are now at a loss to know how we can retain the term protoplasm. As originally used it meant the contents of the cell, and the significance in the term was in the conception of protoplasm as a somewhat h.o.m.ogeneous chemical compound uniform in all types of life. But we now see that this cell contains not a single substance, but a large number, including solids, jelly ma.s.ses, and liquids, each of which has its own chemical composition. The number of chemical compounds existing in the material formerly called protoplasm no one knows, but we do know that they are many, and that the different substances are combined to form a physical structure. Which of these various bodies shall we continue to call protoplasm? Shall it be the linin, or the liquids, or the microsomes, or the chromatin threads, or the centrosomes? Which of these is the actual physical basis of life?

From the description of cell life which we have given, it will be evident that no one of them is a material upon which our chemical biologists can longer found a chemical theory of life. That chemical theory of life, as we have seen, was founded upon the conception that the primitive life substance is a definite chemical compound. No such compound has been discovered, and these disclosures of the microscope of the last few years have been such as to lead us to abandon hope of ever discovering such a compound. It is apparently impossible to reduce life to any simpler basis than this combination of bodies which make up what was formerly called protoplasm. The term protoplasm is still in use with different meanings as used by different writers. Sometimes it is used to refer to the entire contents of the cell; sometimes to the cell substance only outside the nucleus. Plainly, it is not the protoplasm of earlier years.

With this conclusion one of our fundamental questions has been answered.

We found in our first chapter that the general activities of animals and plants are easily reduced to the action of a machine, provided we had the fundamental vital powers residing in the parts of that machine. We then asked whether these fundamental properties were themselves those of a chemical compound or whether they were to be reduced to the action of still smaller machines. The first answer which biologists gave to this question was that a.s.similation, growth, and reproduction were the simple properties of a complex chemical compound. This answer was certainly incorrect. Life activities are exhibited by no chemical compound, but, so far as we know, only by the machine called the cell.

Thus it is that we are again reduced to the problem of understanding the action of a machine. It may be well to pause here a moment to notice that this position very greatly increases the difficulties in the way of a solution of the life problem. If the physical basis of life had proved to be a chemical compound, the problem of its origin would have been a chemical one. Chemical forces exist in nature, and these forces are sufficient to explain the formation of any kind of chemical compound.

The problem of the origin of the life substance would then have been simply to account for certain conditions which resulted in such chemical combination as would give rise to this physical basis of life. But now that the simplest substance manifesting the phenomena of life is found to be a machine, we can no longer find in chemical forces efficient causes for its formation. Chemical forces and chemical affinity can explain chemical compounds of any degree of complexity, but they cannot explain the formation of machines. Machines are the result of forces of an entirely different nature. Man can manufacture machines by taking chemical compounds and putting them together into such relations that their interaction will give certain results. Bits of iron and steel, for instance, are put together to form a locomotive, but the action of the locomotive depends, not upon the chemical forces which made the steel, but upon the relation of the bits of steel to each other in the machine. So far as we have had any experience, machines have been built under the guidance of intelligence which adapts the parts to each other.

When therefore we find that the simplest life substance is a machine, we are forced to ask what forces exist in nature which can in a similar way build machines by the adjustment of parts to each other. But this topic belongs to the second part of our subject, and must be for the present postponed.

==Reaction against the Cell Doctrine.==--As the knowledge of cells which we have outlined was slowly acquired, the conception of the cell pa.s.sed through various modifications. At first the cell wall was looked upon as the fundamental part, but this idea soon gave place to the belief that it was the protoplasm that was alive. Under the influence of this thought the cell doctrine developed into something like the following: The cell is simply a bit of protoplasm and is the unit of living matter.

The bodies of all larger animals and plants are made up of great numbers of these units acting together, and the activities of the entire organism are simply the sum of the activities of its cells. The organism is thus simply the sum of the cells which compose it, and its activities the sum of the activities of the individual cells. As more facts were disclosed the idea changed slightly. The importance of the nucleus became more and more forcibly impressed upon microscopists, and this body came after a little into such prominence as to hide from view the more familiar protoplasm. The marvellous activities of the nucleus soon caused it to be regarded as the important part of the cell, while all the rest was secondary. The cell was now thought of as a bit of nuclear matter surrounded by secondary parts. The marvellous activities of the nucleus, and above all, the fact that the nucleus alone is handed down from one generation to the next in reproduction, all attested to its great importance and to the secondary importance of the rest of the cell.

This was the most extreme position of the cell doctrine. The cell was the unit of living action, and the higher animal or plant simply a colony of such units. An animal was simply an a.s.sociation together for mutual advantage of independent units, just as a city is an a.s.sociation of independent individuals. The organization of the animals was simply the result of the combination of many independent units. There was no activity of the organism as a whole, but only of its independent parts.

Cell life was superior to organized life. Just as, in a city, the city government is a name given to the combined action of the individuals, so are the actions of organisms simply the combined action of their individual cells.

Against such an extreme position there has been in recent years a decided reaction, and to-day it is becoming more and more evident that such a position cannot be maintained. In the first place, it is becoming evident that the cell substance is not to be entirely obliterated by the importance of the nucleus. That the nucleus is a most important vital centre is clear enough, but it is equally clear that nucleus and cell substance must be together to const.i.tute the life substance. The complicated structure of the cell substance, the decided activity shown by its fibres in the process of cell division, clearly enough indicate that it is a part of the cell which can not be neglected in the study of the life substance. Again the discovery of the centrosome as a distinct morphological element has still further added to the complexity of the life substance, and proved that neither nucleus nor cell substance can be regarded as the cell or as const.i.tuting life. It is true that we may not yet know the source of this centrosome. We do not know whether it is handed down from generation to generation like the nucleus, or whether it can be made anew out of the cell substance in the life of an ordinary cell. But this is not material to its recognition as an organ of importance in the cell activity. Thus the cell proves itself not to; be a bit of nuclear matter surrounded by secondary parts, but a community of several perhaps equally important interrelated members.

Another series of observations weakened the cell doctrine in an entirely different direction. It had been a.s.sumed that the body of the multicellular animal or plant was made of independent units.

Microscopists of a few years ago began to suggest that the cells are in reality not separated from each other, but are all connected by protoplasmic fibres. In quite a number of different kinds of tissue it has been determined that fine threads of protoplasmic material lead from one cell to another in such a way that the cells are in vital connection. The claim has been made that there is thus a protoplasmic connection between all the cells of the body of the animal, and that thus the animal or plant, instead of consisting of a large number of separate independent cells, consists of one great ma.s.s of living matter which is aggregated into little centres, each commonly holding a nucleus. Such a conclusion is not yet demonstrated, nor is its significance very clear should it prove to be a fact; but it is plain that such suggestions quite decidedly modify the conception of the body as a community of independent cells.

There is yet another line of thought which is weakening this early conception of the cell doctrine. There is a growing conviction that the view of the organism, simply as the sum of the activities of the individual cells, is not a correct understanding of it. According to this extreme position, a living thing can have no organization until it appears as the result of cell multiplication. To take a concrete case, the egg of a starfish can not possess any organization corresponding to the starfish. The egg is a single cell, and the starfish a community of cells. The egg can, therefore, no more contain the organization of a starfish than a hunter in the backwoods can contain within himself the organization of a great metropolis. The descendants of individuals like the hunter may unite to form a city, and the descendants of the egg cell may, by combining, give rise to the starfish. But neither can the man contain within himself the organization of the city, nor the egg that of the starfish. It is, perhaps, true that such an extreme position of the cell doctrine has not been held by any one, but thoughts very closely approximating to this view have been held by the leading advocates of the cell doctrine, and have beyond question been the inspiration of the development of that doctrine.

But certainly no such conception of the significance of cell structure would longer be held. In spite of the fact that the egg is a single cell, it is impossible to avoid the belief that in some way it contains the starfish. We need not, of course, think of it as containing the structure of a starfish, but we are forced to conclude that in some way its structure is such that it contains the starfish potentially. The relation of its parts and the forces therein are such that, when placed under proper conditions, it develops into a starfish. Another egg placed under identical conditions will develop into a sea urchin, and another into an oyster. If these three eggs have the power of developing into three different animals under identical conditions, it is evident that they must have corresponding differences in spite of the fact that each is a single cell. Each must in some way contain its corresponding adult.

In other words, the organization must be within the cells, and hence not simply produced by the a.s.sociations of cells.

Over this subject there has been a deal of puzzling and not a little experimentation. The presence of some sort of organization in the egg is clear--but what is meant by this statement is not quite so clear. Is this adult organization in the whole egg or only in its nucleus, and especially in the chromosomes which, as we have seen, contain the hereditary traits? When the egg begins to divide does each of the first two cells still contain potentially the organization of the whole adult, or only one half of it? Is the development of the egg simply the unfolding of some structure already present; or is the structure constantly developing into more and more complicated conditions owing to the bringing of its parts into new relations? To answer these questions experimenters have been engaged in dividing developing eggs into pieces to determine what powers are still possessed by the fragments. The results of such experiments are as yet rather conflicting, but it is evident enough from them that we can no longer look upon the egg cell as a simple undifferentiated cell. In some way it already contains the characters of the adult, and when we remember that the characters of the adult which are to be developed from the egg are already determined, even to many minute details--such, for instance, as the inheritance of a congenital mark--it becomes evident that the egg is a body of extraordinary complexity. And yet the egg is nothing more than a single cell agreeing with other cells in all its general characters.

It is clear, then, that we must look upon organization as something superior to cells and something existing within them, or at least within the egg cell, and controlling its development. We are forced to believe, further, that there may be as important differences between two cells as there are between two adult animals or plants. In some way there must be concealed within the two cells which const.i.tute the egg of the starfish and the man differences which correspond to the differences between the starfish and the man. Organization, in other words, is superior to cell structure, and the cell itself is an organization of smaller units.

As the result of these various considerations there has been, in recent years, something of a reaction against the cell doctrine as formerly held. While the study of cells is still regarded as the key to the interpretation of life phenomena, biologists are seeing more and more clearly that they must look deeper than simple cell structure for their explanation of the life processes. While the study of cells has thrown an immense amount of light upon life, we seem hardly nearer the centre of the problem than we were before the beginning of the series of discoveries inaugurated by the formulation of the doctrine of protoplasm.

==Fundamental Vital Activities as Located in Cells.==--We are now in position to ask whether our knowledge of cells has aided us in finding an explanation of the fundamental vital actions to which, as we have seen, life processes are to be reduced. The four properties of irritability, contractibility, a.s.similation, and reproduction, belong to these vital units--the cells, and it is these properties which we are trying to trace to their source as a foundation of vital activity.

We may first ask whether we have any facts which indicate that any special parts of the cell are a.s.sociated with any of these fundamental activities. The first fact that stands out clearly is that the nucleus is connected most intimately with the process of reproduction and especially with heredity. This has long been believed, but has now been clearly demonstrated by the experiments of cutting into fragments the cell bodies of unicellular animals. As already noticed, those pieces which possess a nucleus are able to continue their life and reproduce themselves, while those without a nucleus are incapable of reproduction.

With greater force still is the fact shown by the process of fertilization of the egg. The egg is very large and the male reproductive cell is very small, and the amount of material which the offspring derives from its mother is very great compared with that which it derives from its father. But the child inherits equally from father and mother, and hence we must find the hereditary traits handed down in some element which the offspring obtains equally from father and mother.

As we have seen (Figs. 34-44), the only element which answers this demand is the nucleus, and more particularly the chromosomes of the nucleus.

Clearly enough, then, we must look upon the nucleus as the special agent in reproduction of cells.

Again, we have apparently conclusive evidence that the _nucleus_ controls that part of the a.s.similative process which we have spoken of as the constructive processes. The metabolic processes of life are both constructive and destructive. By the former, the material taken into the cell in the form of food is built up into cell tissue, such as linin, microsomes, etc., and, by the latter, these products are to a greater or less extent broken to pieces again to liberate their energy, and thus give rise to the activities of the cell. If the destructive processes were to go on alone the organism might continue to manifest its life activities for a time until it had exhausted the products stored up in its body for such purposes, but it would die from the lack of more material for destruction. Life is not complete without both processes.

Now, in the life of the cell we may apparently attribute the destructive processes to the cell substance and the constructive processes to the nucleus. In a cell which has been cut into fragments those pieces without a nucleus continue to show the ordinary activities of life for a time, but they do not live very long (Fig. 25). The fragment is unable to a.s.similate its food sufficiently to build up more material. So long as it still retains within itself a sufficiency of already formed tissue for its destructive metabolism, it can continue to move around actively and behave like a complete cell, but eventually it dies from starvation.

On the other hand, those fragments which retain a piece of the nucleus, even though they have only a small portion of the cell substance, feed, a.s.similate, and grow; in other words, they carry on not only the destructive but also the constructive changes. Plainly, this means that the nucleus controls the constructive processes, although it does not necessarily mean that the cell substance has no share in these constructive processes. Without the nucleus the cell is unable to perform those processes, while it is able to carry on the destructive processes readily enough. The nucleus controls, though it may not entirely carry on, the constructive metabolism.

It is equally clear that the _cell substance_ is the seat of most of the destructive processes which const.i.tute vital action. The cell substance is irritable, and is endowed with the power of contractility. Cell fragments without nucleii are sensitive enough, and can move around as readily as normal cells. Moreover, the various fibres which surround the centrosomes in cell division and whose contractions and expansions, as we have seen, pull the chromosomes apart in cell division, are parts of the cell substance. All of these are the results of destructive metabolism, and we must, therefore, conclude that destructive processes are seated in the cell substance.

The _centrosome_ is too problematical as yet for much comment. It appears to be a piece of the machinery for bringing about cell division, but beyond this it is not safe to make any statements.

In brief, then, the cell body is a machine for carrying on destructive chemical changes, and liberating from the compounds thus broken to pieces their inclosed energy, which is at once converted into motion or heat or some other form of active energy. This chemical destruction is, however, possible only after the chemical compounds have become a part of the cell. The cell, therefore, possesses a nucleus which has the power of enabling it to a.s.similate its food--that is, to convert it into its own substance. The nucleus further contains a marvellous material--chromatin--which in someway exercises a controlling influence in its life and is handed down from one generation to another by continuous descent. Lastly, the cell has the centrosome, which brings about cell division in such a manner that this chromatin material is divided equally among the subsequent descendants, and thus insures that the daughter cells shall all be equivalent to each other and to the mother cell.

We must therefore look upon the organic cell as a little engine with admirably adapted parts. Within this engine chemical activity is excited. The fuel supplied to the engine is combined by chemical forces with the oxygen of the air. The vigour of the oxidation is partly dependent upon temperature, just as it is in any other oxidation process, and is of course dependent upon the presence of fuel to be oxidized, and air to furnish the oxygen. Unless the fuel is supplied and the air has free access to it, the machine stops, the cell _dies_. The energy liberated in this machine is converted into motion or some other form. We do not indeed understand the construction of the machine well enough to explain the exact mechanism by which this conversion takes place, but that there is such a mechanism can not be doubted, and the structure of the cell is certainly complex enough to give plenty of room for it. The irritability of the cell is easily understood; for, since it is made of very unstable chemical compounds, any slight disturbance or stimulation on one part will tend to upset its chemical stability and produce reaction; and this is what is meant by irritability.

Or, again, we may look upon the cell as a little chemical laboratory, where chemical changes are constantly occurring. These changes we do not indeed understand, but they are undoubtedly chemical changes. The result is that some compounds are pulled to pieces and part of the fragments liberated or excreted, while other parts are retained and built into other more complex compounds. The compounds thus manufactured are retained in the cell body, and it grows in bulk. This continues until the cell becomes too big, and then it divides.

If a machine is broken it ceases to carry on its proper duties, and if the parts are badly broken it is ruined. So with the cell. If it is broken by any means, mechanical, thermal, or otherwise, it ceases to run--we say it dies. It has within itself great power of repairing injury, and therefore it does not cease to act until the injury is so great as to be beyond repair. Thus it only stops its motion when the machinery has become so badly injured as to be beyond hope of repair, and hence the cell, after once ceasing its action, can never resume it again.

There are, of course, other functions of living things besides the few simple ones which we have considered. But these are the fundamental ones; and if we can reduce them to an intelligible explanation, we may feel that we have really grasped the essence of life. If we understand how the cell can move and grow and reproduce itself, we may rest a.s.sured that the other phenomena of life follow as a natural consequence. If, therefore, we have obtained an understanding of these fundamental vital phenomena, we have accomplished our object of comprehending the life phenomena in our chemical and mechanical laws.

But have we thus reduced these fundamental phenomena to an intelligible explanation? It must be acknowledged that we have not. We have reduced them to the action of chemical forces acting in a machine. But the machine itself is unintelligible. The organic cell is no more intelligible to us than is the body as a whole. The chemical understanding which we thought we had a few years ago in protoplasm has failed us, and nothing has taken its place We have no conception of what may be the primitive life substance. All we can say is that this most marvellous of all natural phenomena occurs only within that peculiar piece of machinery which we call the cell, and that it is the result of the action of physical forces in that machine. How the machine acts, or even the structure of the machine, we are as far from understanding as we were fifty years ago. The solution has retreated before us even faster than we have advanced toward it.

==Summary.==--We may now notice in a brief summary the position which we have reached. In our attempt to explain the living organism on the principle of the machine, we are very successful so far as secondary problems are concerned. Digestion, circulation, respiration, and motion are readily solved upon chemical and mechanical principles. Even the phenomena of the nervous system are, in a measure, capable of comprehension within a mechanical formula, leaving out of account the purely mental phenomena which certainly have not been touched by the investigation. All of these phenomena are reducible to a few simple fundamental activities, and these fundamental activities we find manifested by simple bits of living matter uninc.u.mbered by the complicated machinery of organisms. With the few fundamental properties of these bits of organic matter we can construct the complicated life of the higher organism. When we come, however, to study these simple bits of matter, they prove to be anything but simple bits of matter. They, too, are pieces of complicated mechanism whose action we do not even hope to understand. That their action is dependent upon their machinery is evident enough from the simple description of cell activity which we have noticed. That these fundamental vital properties are to be explained as the result of chemical and mechanical forces acting through this machinery, can not be doubted. But how this occurs or what const.i.tutes the guiding force which corresponds to the engineer of the machine, we do not know.

Thus our mechanical explanation of the living machine lacks a foundation. We can understand tolerably well the building of the superstructure, but the foundation stones upon which that structure is built are unintelligible to us. The running of the living machine is thus only in part understood. The living organism is a machine or, it is better to say, it is a series of machines one within the other. As a whole it is a machine, and its parts are separate machines. Each part is further made up of still smaller machines until we reach the realm of the microscope. Here still we find the same story. Even the parts formerly called units, prove to be machines, and when we recognize the complexity of these cells and their marvellous activities, we are ready to believe that we may find still further machines within. And thus vital activity is reduced to a complex of machines, all acting in harmony with each other to produce together the one result--life.

PART II.

_THE BUILDING OF THE LIVING MACHINE_.

CHAPTER III.

THE FACTORS CONCERNED IN THE BUILDING OF THE LIVING MACHINE.

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

How were they built? Does nature, apart from human intelligence, possess forces which can achieve such results?

Here again we must attack the problem from what seems to be the wrong end. Apparently it would be simpler to discover the method of the manufacture of the simplest machine rather than the more complex ones.

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

==History of the Living Machine.==--In the first place, we must notice that these machines have not been fashioned suddenly or rapidly, but have been the result of a very slow growth. They have had a history extending very far back into the past for a period of years which we can only indefinitely estimate, but certainly reaching into the millions. As we look over this past history in the light of our present knowledge we see that whatever have been the forces which have been concerned in the construction of these machines they have acted very slowly. It has taken centuries, and, indeed, thousands of years, to take the successive steps which have been necessary in this construction. Secondly, we notice that the machines have been built up step by step, one feature being added to another with the slowly progressing ages. Thirdly, we notice that in one respect this construction of the living machine by nature's processes has been different from our ordinary method of building machines. Our method of building puts the parts gradually into place in such a way that until the machine is finished it is incapable of performing its functions. The half-built engine is as useless and as powerless as so much crude iron. Its power of action only appears after the last part is fitted into place and the machine finished. But nature's process in machine building is different. Every step in the process, so far as we can trace it at least, has produced a complete machine. So far back as we can follow this history we find that at every point the machine was so complete as to be always endowed with motion and life activity.

Nature's method has been to take simpler types of machines and slowly change them into more complicated ones without at any moment impairing their vigour. It is something as if the steam engine of Watt should be slowly changed by adding piece after piece until there was finally produced the modern quadruple expansion engine, but all this change being made upon the original engine without once stopping its motion.

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

This gradual construction of the living machines has been called _Organic Evolution_, or the _Theory of Descent_. It will be necessary for us, in order to comprehend the problem which we have before us, to briefly outline the course of this evolution. Our starting point in this history must be the cell, for such is the earliest and simplest form of living thing of which we have any trace. This cell is, of course, already a machine, and we must presently return to the problem of its origin. At present we will a.s.sume 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 ma.s.s must have been at first all alike; but, after a little, differences began to appear among them. Those on the outside of the ma.s.s 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 ma.s.s 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 ma.s.s 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.

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

But a second principle in this growth of the machine was needed to produce the variety which is found in nature. As the different cells in the multicellular ma.s.s became a.s.sociated 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 a.s.sociations, 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 ma.s.s 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 a.s.suming 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 ma.s.s of evidence, relevant and irrelevant. As the evidence has acc.u.mulated 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.