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Show 170 DARWINISM CHAP, grc:..-..tly assisted by two other important fnctors in the problem. It has already . been showr~ that,_ with ?ach modification of form awl habits, and especmlly w1th mochficn.tions of colour, there arises a disinclination of the two forms to pair together; and this wo~ld produce. a~ ~mount of isolation which would greatly assist the spccmh at10n of the forms in adaptation to their different conditions of life. A~rain evidence has been adduced that change of conditions ot~ of' mode of life is a potent cause of d istnrh~-..n cc of 1 he I'Cproductivc system, and, consequently, of i11f 'rtllity. "\\r c may therefore assume thi-..t, as the two forms adopted more and more different motles of life, and pcrhap acquired also decided pcculin.rities of form and coloratio11, the infcrtil it.y between them would i ncrcasc or become more general ; all(l n.. we have seen thn.t every such increase of infertillty would give that portion of the species in which it arose an advan tag ' over the remaining portions in which the two vn.rictie. wcr ' more fertile together, all this induced infertility would maintain itself, and still fmther increase the general infertility between the two forms of the species. It follows, then, that specialisation to separate conditions of life, differentiation of external character·, tlisincliJtation to cross-unions, and the infertility of the hybrid produce of thrst' unions, would all proceed pari passu, and would ultimn.tt-1y lead to the production of two distinct forms having all t lw characteristics, physiological as well as structural, of true species. In the case now discussed it has been supposed, that some amount of general infertility might arise in correlation wit.h the different modes of life of two varieties or incipi c11L species. A con. idcrable body of facts already a.<ldn<·<·<l renders it probable that this is the mode in whi ch :1nj widespread infertility would arise; and, if so, it has hc<'n shown that, by the influence of naturn.l selection :md the known laws which affect varieties, the i11fertility would l>c gradually increased. But, if we suppose the infertility to arise sporadically within the two form·, a.nd to afTect only a small proportion of the individuals in any area, it "\rill l>c difficult, if not impossible, to show that such infertility wonl <l have any tendency to increase, or would produce any bnt a VII ON THE INFERTILITY OF CROSSES '177 prejudicial effect. If, for example, five per cent of each form thus varied so as to be infertile with the other form, the result would be hardly perceptible, because the individuals which formed cross-unions and produced hybrids would constitute a very small portion of the whole species ; and the hybrid offs~)ring, being at a disadvantage in the :trugglc for existence and being themselves infertile, would soon die out, whib the much more numerous fertile portion of the two forms would increase rapidly, and furnish a sufficient number of pure-bred offspring of each form to take the place of the somewhat iufcrior hybrids between them whenever the struggle for existence became severe. 'V e must suppo ·c that the norm;-..l fertile forms would transmit thch' fertility to their progeny, ancl the few infertile forms their infertility ; hut the latter would necessarily lose half their proper increase by the stel'ility of their hybrid offspring whenever they crossed with the other form, and when they bred with their own form the tendency to sterility would die out except in the very minute proportion of the five per cent (one-twentieth) that chance . would lead to p<-..i r together. U ndcr these circumstances the incipient sterility between the two forms would rapidly be elimilln.tcd, and could never rise much above the numbers which were produced by sporadic variation each year. It was, probably, by a consideration of some such case as this that Mr. Darwin came to the conclusion that infertility arising between incipient species could not be increased by natural selection ; and this is the more likely, u.s he was always disposed to minimise both the frequency and the amount even of structural variations. We have yet to notice another mode of action of natural selection in favouring and perpetuating any infertility that may arise between two incipient species. If several distinct species are 'lilndergoing modification at the same time and in the same area, to adapt them to some new conditions that have arisen there, then a.ny species in which the structural or colour differences that have arisen between it and its varieties or close allies were correlated with infertility of the crosses between them, would have an advantage over the corresponding varieties of other species in which there wa.s no such N |
