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Show 174 DARWINISM CIIAl'. possibly have produced tho difference that often occurs between reciprocal crosses, one of these being sometimes fertile, while the other is sterile. The extremely different amounts of infertility or sterility between different species of the same genus, the infertility often bearing no proportion to the difference between the species crossed, is also an important objection. But none of these objections wouhl have much weight if it could be clearly shown that natural selection is able to increase the infertility variations of incipient species, as it is certainly able to increase and develop all useful variations of form, structure, instincts, or habits. Ample causes of infertility have been shown to exist, in the nature of the organism and the laws of correlation; the agency of natural selection is only needed to accumulate the effects produced by these causes, and to render their final results more uniform and more in accordance with the facts that exist. About twenty years ago I had much correspondence and discussion with Mr. Darwin on this question. I then believed that I was able to demonstrate the action of natural selection in accumulating infertility ; but I could not convince him, owing to the extreme complexity of the process under the conditions which he thought most probable. I have recently returned to the question; and, with the fuller knowledge of the facts of variation we now possess, I think it may be shown that natural selection is, in some probable cases at all events, able to accumulate variations in infertility between incipient species. The simplest case to consider, will be that in which t"\vo forms or varieties of a species, occupying an extensive area, are in process of adaptation to somewhat different modes of life within the same area. If these two forms freely intercross with each other, and produce mongrel off.'3pring which are quite fertile inte1' se, then the fnrther diffcrenti ~Ltion of the forms into two distinct species will be retarded, or perhaps entirely prevented; for the offspring of the crossed unions will be, perhaps, more vigorous on acconn t of the cross, although less perfectly adapted to the conditions of existence than either of the pure breeds; and this would ccrta.inly estaJ>lish a powerful antagonistic influence to the fnrth er differentiation of the two forms. VII ON 'rilE INll'ERTILI'l'Y OF CROSSES 175 Now, let us suppose that a partinJ sterility of the hybrids between the two forms arises, in correlation with the different modes of life and the slight external or internal peculiarities that exist between them, both of which we have seen to be real can. cs of infertility. The result will be that, even if the hybrids between the two forms are still freely produced, these hybrids will not themsel vcs increase so rapidly as the two pure forms ; and as these latter are, by the terms of the problem, better suited to their conditions of life than are the hybrids between them, they will not o11ly increase more rapidly, but will also tend to supplant the hybrids altogether whenever the struggle for existence becomes cxccptionnJly severe. Thus, the more complete the sterility of the hybrids the more rapidly will they die out and leave the two parent forms pure. Hence it will follow that, if there is greater infertility between the two forms in one part of the area than the other, these forms will be kept more pure w hcrever this greater infertility prevails, will thercfol'C have an advantage at each recurring period of severe strnggle for existence, and will thus ultimately supplant the los~ infertile or completely fertile forms that may exist in other portions of .the area. It thus appears that, in such a case as here supposed, natural selection would pres rve those portions of the two breeds which were mo 't infertile with each other or whose hybrid offspring were most infertile; and wo~ld, therefore, if variations in fertility continued to arise, tend to increase that infertility. It must particularly be noted that this effect would result, not by the preservation of the infertile variations on account of their infertility, but by the inferiority of the hybrid offspring, both as being fewer in numbers, less able to continue their race, and less adapted to the conditions of existence than either of the pure forms. It is this inferiority of the hybrid offspring that is the essential point; anu as the number of these hybrids will Lc permanently less where the infertility is greatest, therefore those portions of the two forms in which infertility is greatest will have the advantage, and will ultimately survive in the struggle for existence. The differentiation of the two forms into distinct species, with the increase of infertility between them, woulu be |