internal law of growth, independently of all questions of natural selection.
There is another class of natural phenomena upon which Mr. Wallace writes much that is deeply interesting, but to which it may possibly be questioned whether the principle of survival by natural selection is applicable—namely, the phenomena of mimicry. Of course it is quite intelligible, to take an instance, that a living creature which is very much like a leaf will escape many enemies, and even have such an advantage in the struggle for life that many other living creatures would be like leaves if they could. But when we endeavor to go back in imagination to the commencement of the mimicking process, we must conceive of a creature not at all like a leaf, but among whose offspring there are certain individuals which have a slightly leaf-like appearance, and that these survive in preference to others not having the appearance in question. The conception involves two difficulties: First, the notion of certain individuals having a slightly leaf-like appearance is eccentric and hard to accept. It is different from that of individuals varying by length of leg, or strength of wing, or what not. It is a variation, so to speak, not of degree but of kind. And, secondly, it is difficult to see why a resemblance to a leaf, admitted to be slight, and therefore one would imagine not easily perceived, should be any substantial protection from enemies, and so an appreciable advantage in the struggle for life.
Similar difficulties occur with regard to other cases of mimicry. My space does not permit me to examine them in detail; but I have come to the conclusion that, while mimicry may probably be always connected with some advantage which it confers on the animal, it is difficult to conceive of the mimicking transformation being originally brought into operation by any process of natural selection.
This failure of the principle of natural selection to explain much that is connected with the evolution both of men and of inferior creatures may lead us to inquire, to what extent the principle satisfies etiological requirements even in those cases in which its application appears most complete. The modification and multiplication of species require three conditions to be postulated: (1) an original species; (2) the power of multiplying that species by reproduction; and (3) the occurrence of variations in the successive generations.
Now (1) the existence of the original living germ or germs must, I suppose, be left by universal consent in mystery. Mr. Darwin treated of the Origin of "Species," not the Origin of "living things" This latter question is not likely ever to come within the reach of human science; certainly it has not done so yet. Given the existence of the material universe, or the existence of