Page:Popular Science Monthly Volume 66.djvu/235

Rh only assignable cause; subsequently I realized that it was not explainable by the Lamarckian hypothesis.

I then attributed it to an unknown law of evolution, and there I believe it rests to-day, namely: as a process of which we do not know the cause. Still more recently, however, comes the discovery that original kinship is partly at least a control-principle. For example, in the descent of independent stocks of hornless animals arising from a common stock, rudimentary horn cores are found to appear independently in exactly the same region of the skull, indicating a kind of predetermination in the stock, or potential of similar evolution. The facts on which this law of mutation, properly called, rests have been misunderstood, totally denied, or explained away by selectionists as survivals of favorable out of indiscriminate variations. Even my colleague, Scott, has identified these phenomena with the saltations of De S Vries. Nevertheless, I regard the genesis of new adaptive characters from almost imperceptible beginnings as a vera causa, and as one of the greatest problems we have to solve.

That a natural solution will be found goes without saying, although this principle, as stated, is undoubtedly of a teleological nature. Its philosophical bearings are of far reaching importance. Just as we demand a continent to transfer land animals from Australia to South America, so we demand a natural law to explain these facts.

The creative factors of fitness cooperating with selection, which, in my judgment, are now well demonstrated, reside either primarily in the environment, in the bodies of animals, or in the germinal cells—they all ultimately find their way into the germinal cells. They may be summarized as follows:

1. Segregation.—Besides the familiar geographical segregation of animals, which reaches its highest expression in insular forms, such as the pygmy fossil elephants of Malta and those recently discovered in Cyprus (Wade), there is the no less effective segregation of habit among animals existing in the same geographical regions and under the same climatic conditions, but seeking different varieties of food on different kinds of soil. These give rise to what I have called local adaptive radiations, a principle which explains the occurrence in the same country, and almost side by side, of very conservative as well as very progressive forms.

2. Adaptive Modification.—This is a plastic principle which tends in the course of life to an increasing fitness of the bodies of individuals to their special environments and habits, well illustrated among men in the influence of various trades and occupations and operating both in active and in passive structures. Consistent with the adaptive modification principle is the fact that every individual requires habit and environment to model it into its parental form; and in every change of environment or habit every individual is carried an infinitesimal