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Show 178 DARWINISM CUAl'. physiological peculiarity. Thus, incipient species which were infertile together would have an . advantage over other incipient species which were fcrt1le, and, whenever the struO'gle for existence became severe, would prevail over them and ~ake their place. Such infertility, being correlated with constitutional or structural differences, would prohably, as already suggested, go on increasing as these differences increased; and thus, by the time the new species bcca.me fully differentiated from its parent form (or brother variety) the infertHity might have become as well marked as we usually find it to be between distinct species. This discussion has led us to some conclusions of the o-rcatc~t importance as bearing on the difficult problem of the cause of the sterility of the hybrids between distinct species. Accepting, as highly probable, the fact of variations in fertility occurring in correlation with variations in habits, colour, or structure, we sec, that so long as such variations occurred only spomdically, and aff'ected bnt a small proportion of the individuals in any area, the infertility could not be in creased by natural selection, but would tend to die ont almost as fast as it was produced. If, however, it was so closely correlated with physical variations or diverse modes of life as to affect, even in a small degree, a considemblc proportion of the indiviclu:ds of the two forms in definite areas, it wonl<l be preserved by natnml selection, and the portion of the varying species thus affected would increase at the expense of those portions which were more fertile when crossed. Ea<.:h further variation towards infertility between ' the two forms would be again preserved, and thus the incipient inf01-tiliLy of the hybrid offspring might be increased till it became so great as almost to amount to sterility. Y ct furth er, we h:we seen that if several competing species in the . ame are:t WPm being simultaneously modified, those hctween whose vari eties infertility arose would have an advantage over those whose varieties remained fertile inter se, and would ultimately supplant them. Tho preceding argument, it will be seen, depends entirely upon the assumption that some amount of infertility characterises the distinct varieties which arc in procc:s of differentiation into species; and it may be objected that of VII ON THE INFI~R1'ILITY Oli' CROSSES 179 such infertility there is no proof. This is admitted; but it is urged th;tt f<tcts have been adduced which render such infertility probable, at least in some cases, and this is all th;tt is required. It is by no means necessary that all varieties should exhibit incipient infertility, lmt only some varieties; for we know that, of the innumerable varieties that occur but few become developed into di stinct :pecies, and it may bo that the absence of infertility, to obviate the effects of intercrossing, is one of the usual causes of their failure. All I have attempted to show is, that ~vhen incipient infertility does occur in correlation with other varietal differences, that infertility can be, and in fact mnst be, increased by natural selection ; and this, it appears to me, is a decided stop in ad vance in the solution of the problem.' 1 As this argument is a rather difficult one to follow, while its theoretical importance is very great, I add her the following briefer exposition of it, in a series of propositions ; being, with a few verbal alterations, a copy or what I wrote 011 the subject about twenty years back. Some readers may find this easier to follow than the full er discussio11 in the tex1 :-- Can Sterility of Hybrids have been Pr·oduced by Natural Sellction 1 ' 1. Let there be a species which has varied into two for-ms each adapted to certain existing conditions better than the parent form, which they soon supplant. 2. If these two forms, which are supposed to coexist in the same district, do not in torero ·s, natural selection will accumulate all favourable variations till they become well suited to their conditions of life, and form two slightly difl'ering species. 3. But if these two fo7"1118 freely intercross with each other, and produce hybrids, which are also quite fertile ?·nter se, then the f?rmation of the two distinct races or species will be retarded, or perha]JS entirely prevented; for the o(fspring ot' the cro.·sed unions will be 1n01 .. e vigm·ous owing to the cross, alt11ongh less adapted to their conditions of life than either of the pure breeds. 4. Now, let a partial sterility of the hybrids of some considerable proportion of these two forms arise; and, a this woulcl probably he due to son1c special conditions of life, we may fairly suppose it to arise iu some definite portion of the area occupied by the two forn1s. 5. The result will be that, in that area, the hyhricls (altl10ugh continually produced by first crosse: almost as freely a before) will not themsehes increase so rapidly as the two pure forms; and as the two pure forms are, hy the terms of the problem, better su ited to their several cond itions of life thnn the hybrids, tl1ey will inevitably increase more rapidly, and will continual.ly ten<l to supplant the ltybrius altogether at every recurrent severo struggle for existence. 6. We may fairly suppose, also, that as soon ns any sterility appears some disinclination to cross 1mions will appear, and this will further tend to t.he diminution of the production of l1ybrids, |