The Story of the Living Machine / A Review of the Conclusions of Modern Biology in Regard / to the Mechanism Which Controls the Phenomena of Living / Activity

by H. W. Conn

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.