Here we have a curious example of one writer supporting the statements of another, while appearing to be under the impression that he is controverting those statements. Both Professor Herdman's imaginary case, and its realization in Livingstone's account, go to show "the tendency of the members of a variety to breed with one another." This is what I have called "psychological selection," and, far from "ignoring" it, I have always laid stress upon it as an obviously important form of isolation or _prevention_ of free intercrossing. But it is a form of isolation which can only occur in the higher animals, and, therefore, the whole of Professor Herdman's criticism is merely a restatement of my own views as already published in the paper which he is criticizing. For all that his argument goes to prove is, first, the necessity for _some_ form of isolation if the overwhelming effects of intercrossing are to be obviated; and, secondly, the manifest consequence that where the psychological form is unavailable (as in many of the lower animals and in all plants), some other form must be present if divergent evolution is taking place on a common area. * * * * * Seeing that so much misunderstanding has been shown with reference to my views on "the swamping effects of intercrossing," and seeing also that this misunderstanding extends quite as much to Mr. Gulick's views as to my own, I will here supply brief extracts from both our original papers, for the double purpose of showing our complete agreement, and of leaving it to be judged whether we can fairly be held responsible for the misunderstanding in question. After having supplied these quotations, I will conclude this historical sketch by considering what Mr. Wallace has said in reply to the views therein presented. I will transcribe but a single passage from our papers, beginning with my own. Any theory of the origin of species in the way of descent must be prepared with an answer to the question, Why have species _multiplied_? How is it that, in the course of evolution, species have not simply become transmuted in linear series instead of ramifying into branches? This question Mr. Darwin seeks to answer "from the simple circumstance that the more diversified the descendants from any one species becomes in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the economy of nature, and so be enabled to increase in numbers." And he proceeds to illustrate this principle by means of a diagram, showing the hypothetical divergence of character undergone by the descendants of seven species. Thus, he attributes divergence of character exclusively to the influence of natural selection.