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Show 266 HYBRIDISM. CHAP. VIII. b e I.n appreCi·a ble by us., in the other ca.s e, or that of hybrids, the external conditions have remained th~ same, but the organisation has been dis~urbed by two d1ffe~ent structures and constitutions having been blend~d ~nto one. F Or I· t is scarcely possible that two organidS'a tiOnbs sh oul d b e compounded into one, without .s odm' e Istur - 1 . ance occurring in the development, or peno 1ca actiOn, or mutual relation of the different p~rts and organs o?e t o ano ther, or to the conditions of lif.e . Wh h.e n .he£'y. bri• ds are able to breed inter se, they transmit to t mr ouspnng from generation to generation the same compounded organisation, and hence we need not be s~rprised that their sterility, though in some degree variable, rarely diminishes. It must, however, be confessed that we cannot und~r-stand, excepting on vague hypoth.eses, seve:al facts. with respect to the sterility of hybnds; for 1nstan~e, the unequal fertility of hybrids. ~ro~uced from r.empro?al crosses· or the increased stenhty In those hybrids whiCh occasio~ally and exceptionally resemble closely eit~er pure parent. Nor do I pretend that the forego~ng remarks go to the root of the matter : no explanatiOn is offered why an organism, when placed under unnatural conditions, is rendered sterile. All that I have attempted to show, is that in two cases, i~ some respects allied, sterility is the common result,-1~ the on~ case from the conditions of life having been disturbed, In the other case from the organisation having been ~isturbed by two organisations having been compounded 1nt? ~ne. It may seem fanciful, but I suspect th~t a similar parallelism extends to an allied yet very different cl.ass of facts. It is an old and almost universal . belief, founded I think on a considerable body of evidence, that sli~ht chan~es in the conditions o~ life are bene· ficial to all living things. We see this acted on by CHAP. VIII. FERTILITY OF MONGRELS. 267 farmers and gardeners in their frequent exchanges of seed, tubers, &c., from one soil or climate to another and ba?k again. During the convalescence of animals: we plainly see that great benefit is derived from almost any change in the habits of life. Again, both with • plants and animals, there is abundant evidence, that a c:oss betw~en very distinct individuals of the same speCies, that Is between members of different strains or sub-b.reeds.' gives vigour and fertility to the offspring. I believe, Indeed, from the facts alluded to in our fourth chapter, tha~ a certain amount of crossing is indispensable even With hermaphrodites ; and that close interbreeding continued during several generations between the nearest rel~t~ons, espe.cially if these be kept under . the same conditions of hfe, always induces weakness and sterility in the progeny. . Hence it ~e:ms that~ on the one hand, slight changes In the conditions of hfe benefit all organic beings, and on the other hand, that slight crosses, that is crosses bet~een the m~les and females of the same species ":hiCh .have van:~ and become slightly different, give VIgour and fert1hty to the offspring. But we have seen that greater changes, or changes of a particular nat~re, often render organic beings in some degree sterile; and that greater crosses, that is crosses between ~ales an~ fem.al~s which have become widely or speCifically different,. produce hybrids which are generally st~rile in some d~gree. I cannot persuade myself that thi~ p~rallelism i~ an accident or an illusion. Both senes of facts seem to be connected together by some common but unknown bond, which is essentially related to the principle of life. Fer~ility of Varieties when crossed, and of their Mongrel 0ff8pnng.-It may be urged, as a most forcible argu- N2 |
