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struggle for life.' As fast as the faculties are multiplied, so fast does it become possible for the several members of a species to have various kinds of superiorities over one another. While one saves its life by higher speed, another does the like by clearer vision, another by keener scent, another by quicker hearing, another by greater strength, another by unusual power of enduring cold or hunger, another by special sagacity, another by special timidity, another by special courage; and others by other bodily and mental attributes. Now it is unquestionably true that, other things equal, each of these attributes, giving its possessor an extra chance of life, is likely to be transmitted to posterity. But there seems no reason to suppose that it will be increased in subsequent generations by natural selection. That it may be thus increased, the individuals not possessing more than average endowments of it must be more frequently killed off than individuals highly endowed with it; and this can happen only when the attribute is one of greater importance, for the time being, than most of the other attributes. If those members of the species which have but ordinary shares of it, nevertheless survive by virtue of other superiorities which they severally possess, then it is not easy to see how this particular attribute can be developed by natural selection in subsequent generations. The probability seems rather to be that, by gamogenesis, this extra endowment will, on the average, be diminished in posterity-just serving in the long run to compensate the deficient endowments of other individuals whose special powers lie in other directions, and so to keep up the normal structure of the species. The working out of the process is here somewhat difficult to follow; but it appears to me that as fast as the number of bodily and mental faculties increases, and as fast as the maintenance of life comes to depend less on the amount of any one, and more on the combined action of all, so fast does the production of specialties of character by natural selection alone become difficult. Particularly does this seem to be so with a species so multitudinous in its powers as mankind, and above all does it seem to be so with such of the human powers as have but minor shares in aiding the struggle for life-the æsthetic faculties for example."-Spencer, "Principles of Biology," 166.

Can thus natural selection, acting upon fortuitous variations, be the sole guiding process concerned in progress? Must there not be some combining power to produce the higher individuals which are prerequisites to the working of natural selection?

We are considering the efficiency of natural selection in enhancing useful variations through a series of generations. Let us return to the distinction between productiveness and prospectiveness of social capital. Applied to variations productiveness means immediate advantage, prospectiveness the greater future and permanent returns. Now all persisting variations must, in animals below man, apparently be somewhat productive, else they would not continue, much less increase. Now the immediate return from prospective variations is often smaller than from productive. It looks at first as if productive variations would always be preserved by natural selection, and that prospective variations would not long advance. Yet in the muscular system variations valuable largely for their future value are neither few nor unimportant. How can the brain in its infancy develop until it gains supremacy over muscle, or muscle have done the same with digestion? Now a partial explanation of this is to be found in the correlation of organs. This is therefore a factor of vast importance in progress through evolution.

Progress in any one line demands correlated changes in many organs. Thus in the advance of annelids to insects the muscular system increases in relative bulk, and absolutely in complexity. But a change or increase in the muscle must be accompanied by corresponding changes in the motor-nerve fibrils; and these

again would be useless unless accompanied by increased complexity and more or less readjustment of the cells and fibrils of the nerve-centres. And all these additions to, and readjustments of, the nervecentres must take place without any disturbance of the other necessary adjustments already attained. This is no simple problem.

We will here neglect the fact that many other changes are going on simultaneously. Legs are being formed or moulded into jaws, the anterior segments are fusing into a head, and their ganglia into a brain; an external skeleton is developing. Furthermore the increase of the muscular and nervous systems must be accompanied by increased powers of digestion, respiration, and excretion. Practically the whole body is being recast. We insist only on the necessity of simultaneous and parallel changes in muscles, nerves, and nerve-centres; though what is true of these is true, in greater or less degree, of all the other organs.

You may answer that this is to be explained by the law of correlation of organs; that when changes in one organ demand corresponding changes in another, these two change similarly and more or less at the same time and rate. But this is evidently not an explanation but a restatement of the fact. The question remains, What makes the organs vary simultaneously so as to always correspond to each other? The whole series of changes must to some extent be effected at once and in the same individual, if it is to be preserved by natural selection. Fortuitous variations here and there along the line of the series are of little or no avail. That the whole series of variations should happen to occur in one animal is altogether

against the law of probabilities; if the favorable variation occurs in only a part of the series it remains useless until the corresponding variation has taken place in the other terms. And while the variation is thus awaiting its completion, so to speak, it is useless, and cannot be fostered by natural selection.

Evolution by means of fortuitous variations, combined and controlled only through natural selection, seems to me at least impossible; and this view is, I think, steadily gaining ground.

Natural selection, while a real and very important factor in evolution, cannot be its sole and exclusive explanation. It presupposes other factors, which we as yet but dimly perceive. And this does not impeach the validity of Mr. Darwin's theory any more than Newton's theory of gravitation is impeached by the fact that it offers no explanation as to why the apple falls or how bodies attract one another.

For natural selection explains the survival, but not the origin, of the fittest. Given a species or other group composed of more and less fit individuals and the fittest will survive. How does it come about that there are any more and less fit individuals? This brings us to the consideration of the subject of variation.

Let us begin with a simple case of change in the adult body. The workman grasps his tools day after day, and his hands become horny. The skin has evidently thickened, somewhat as on the soles of the feet. This is no mere mechanical result of pressure alone. Continuous pressure would produce the opposite result. But under the stimulus of intermittent pressure the capillaries, or smallest blood vessels, furnish

more nutriment to the cells composing the lowest layer of the outer skin or epidermis. These cells, being better nourished, reproduce by division more rapidly, and the epidermis, becoming composed of a greater number of layers of cells, thickens. The outermost layers, being farthest from the blood supply, dry up and are packed together into a horny mass.

If I go out into the sunshine I become tanned. This again is not a direct and purely chemical or physical result of the sun's rays, but these have stimulated the cells of the skin to undergo certain modifications. Any change in the living body under changed conditions is not passive, but an active reaction to a stimulus furnished by the surroundings. The same stimulus may excite very different reactions in different individuals or species.

Early in this century a farmer, Seth Wright, found among his lambs a young ram with short legs and long body. The farmer kept the ram, reasoning that his short legs would prevent him from leading the flock over the farm-walls and fences. From this ram was descended the breed of ancon, or otter, sheep. Now the stimulus which had excited this variation must have been applied early in embryonic life, or perhaps during the formation or maturing of the germcells themselves. Such a variation we call a con

genital variation.

These cases are merely illustrations of the general truth that in every variation there are two factors concerned the living being with its constitution and inherent tendencies and the external stimulus.

The courses of the different balls in a charge of grape-shot, hurled from a cannon, are evidently due

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