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Show 186 HYBRIDISnf. CHAP. XIX. some one compatriot, sterility with other species would follow as a necessary consequence. In the second place, it is as much opposed to the th eory of natural selection, as to the theory of special creation, that in reciprocal crosses the male element of one · form should have been rendered utterly impotent on a second form whilst at the same time the male element of this second form' is enabled freely to fertilise the first form; for this peculiar state of the reproductive system could not possibly be advantageous to either species. In considering the probability of natural selection having come into action in rendering species mutually sterile, one great difficulty will be found to lie in the existence of many graduated steps from slightly lessened fertility to absolute sterility. It may be admitted, on the principle above explained, that it would profit an incipient species if it were rendered in some slight degree sterile when crossed with its parent-form or with some other variety ; for thus fewer bastardised and deteriorated offspring would be produced to commingle their blood with the new species in process of formation. But he who will take the trouble to reflect on the steps by which this first degree of sterility could be increased through natural selection to that higher degree which is common to so many species, and which is universal with species which have been differentiated to a generic or family rank, will find the subject extraordinarily complex. After mature reflection it seems to me that this could not have been effected through natural selection; for it could have been of no direct advantage to an individual animal to breed badly with another individual of a different variety, and thus leave few offspring; consequently such individuals could not have been preserved or selected. Or take the case of two species which in . their presen~ state, when crossed, produce few and sterile offspring; now, what is there which could favour the survival of those individuals which happened to be endowed in a slightly higher degree with mutual infertility and which thus approached by one small step towards absolute sterility? yet an advance of this kind, if the theory of natural ~election be brought to bear, must have incessantly occurred w1th many species, for a multitude are mutually quite barren. vVith sterile neuter insects we have reason to CllAP. XIX. HYBRIDISM. 187 believe that modifications in their strudure have been slowly accumulated by natural selection, from an advantage having been thus indireetly given to the community to whieh they belonged over other communities of the same species; but an individual animal, if rendered slightly sterile when crossed with some other variety, would not thus in itself gain any advantage, or indirectly give any advantage to its nearest relatives or to other individuals of the same variety, leading to their preservation. I infer from these considerations that, as far as animals are concerned, the various degrees of lessened fertility which occur with species when crossed cannot have been slowly accumulated by means of natural selection. vVith plants, it is possible that the case may be somewhat different. vVith many kinds, insects constantly carry pollen from neighbouring plants to the stigmas of each flower ; and with some species this is effected by the wind. Now, if the pollen of a variety, when deposited on the stigma of the same variety, should become by spontaneous variation in ever so slight a degree prepotent over the pollen of other varieties, this would certainly be an advantage to the variety; for its own pollen would thus obliterate the effects of the pollen of other varieties, and prevent deterioration of character. And the more prepotent the variety's own pollen cou1d be rendered through natural selection, the greater the advantage WQuld be. \iV e know from the researches of Gi:irtner that, with speeies which are mutually sterile, the pollen of each is always prepotent on its own stigma over that of the other species ; but we do not know whether this prepotency is a consequence of the mutual sterility, or the sterility a consequence of the prepotency. If the latter view be correct, as the prepotency became stronger through natural selection, from being advantageous to a species in process of formation, so the sterility consequent on prepotency ·would at the same time be augmented ; and the final result would be various degrees of sterility, such as occurs with existing species. This view might be extended to animals, if the female before each birth received several males, so that the sexual element of the prepotent male of her own variety obliterated the effects of the access of previous males belonging to other varieties; but we haYe no reason to believe, at least |
