Page:Popular Science Monthly Volume 74.djvu/379

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375
FOR DARWIN

tion has no scientific standing. The great law of causation is not abrogated, but the outcome is only the result of a large number of small influences whose effects depend on the nature of the material and on the nature of the conditions. It is so important that this fact be clearly understood that I may be pardoned if I call to mind some familiar illustrations. No two leaves on a tree are identical, yet if many are measured, they give the curve of probability. Men are of different heights, yet they range about a mode. Color appears in various shades, yet if standardized, it is found to follow the same laws of chance variation.

What value have these facts for the theory of evolution? If in every generation we find that the same kinds of individuals recur, the results mean stability, not progress. That this state of affairs actually exists in many species living under the same environment during successive generations there can be little doubt. But change the environment and the results also change. Another factor comes to light that is independent of outside conditions. It is what has been called preferential mating. If within a group the males and females of certain kinds tend more often to pair with each other, the collective group becomes modified in one or more directions. In man this factor assumes a special importance, for, as Pearson has shown, there is measurable evidence that such mating occurs.

It has often been urged, and I think with much justification, that the selection of individual, or fluctuating variations could never produce anything new, since they never transgress the limits of their species, even after the most rigorous selection—at least the best evidence that we have at present seems to point in this direction. But a new situation has arisen. There are variations within the limits of Linnean species that are definite and inherited, and there is more than a suspicion that by their presence the possibility is assured of further definite variation in the same direction which may further and further transcend the limits of the first steps. If this point can be established beyond dispute, we shall have met one of the most serious criticisms of the theory of natural selection.

It is not without interest to note in this connection that Darwin often assumed that fluctuating variations are transmitted to the offspring. The idea that they are not was a later development—the result, it is true, of a better knowledge of the law of fortuitous effects, or of probability. But we have discovered the additional fact that some small variations are inherited. Let us call these definite variations, and if these be the material with which evolution is concerned, Darwin's assumption in regard to the nature of variation will be, in part, justified.

These small, definite variations appear to be closely allied to those larger, more visible definite variations that we now call mutations.