Page:Popular Science Monthly Volume 76.djvu/49

Rh Then Mr. Wallace goes on to speak of the letter from Mr. Darwin containing the quotation given above, and says:

There you have the difference between the two men's minds, at least in their attitude toward survival of the fittest, put in a nut-shell by "Wallace himself. What seemed to him from two days' reflection able "to settle a great deal," seemed to Darwin after twenty years' reflection, not able to "fix or settle anything."

Another highly instructive exemplification of this difference is furnished by a discussion between the two on natural selection and sterility. This is given us in "More Letters of Charles Darwin," letters 209-216, the correspondence having been carried on in 1868. Wallace presents (letter 211) "what appears to me a demonstration on your own principles, that natural selection could produce sterility of hybrids." Darwin's general position on the point is well indicated in letter 213. He says:

Although Darwin was sure there was something wrong in Wallace's argument, and was not budged an inch by it, he seems not to have recognized its logical preposterousness. He did not explicitly point out to Mr. Wallace that since over-productivity, or geometrical increase, is one of the fundamental presuppositions of natural selection, to suppose natural selection could produce sterility would make it capable of negativing one of its own presuppositions. Stating the matter figuratively, Darwin saw no logical difficulty in the supposition that natural selection might commit suicide. With him it was merely a question of whether it does this or not, and he could not make out from the evidence that it does. But he did not see that Wallace's argument carried out consistently would not only enable natural selection to commit suicide, but in reality would make it able to prevent itself from coming into existence.

The scope and balance of Darwin's mind are seen nowhere to better advantage than in his efforts to prevent his own causal hypothesis from going beyond bounds. Nevertheless the true inwardness of the difference between himself and those who later were to produce the socalled neo-Darwinian school, the school founded on the omnipotence of