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Show 166 DARWINISM CHAP. Parallelism between Crossing and Change of Conditions. In the whole series of these phenomena, from the beneficial effects of the crossing of different stocks and the evil effects of close interbreeding, up to the partial or complete sterility induced by crosses between species belonging to different genera, we have, as Mr. Darwin points out, a curi?us para.l~e.lism with the effects produced. by change of physiCal cond1t10ns. It is well known thc-tt slight changes in the conditions of life arc beneficial to all living things. Plants, if constantly grown in one soil and locality from their own seeds, arc greatly benefited by the importation of seed from some other locality. The same thing happens with animals ; and the bcncfi t we ourselves experience from " change of air " is an illustration of the same phenomenon. But the amount of the change which is beneficial has its limits, and then a greater amount is InJUrious. A change to a climate a few degrees warmer or colder may be good, while a change to the tropics or to the arctic regions might be injurious. Thus we see that, both slight changes of conditions and a slight amount of crossing, are beneficial; while extreme changes, and crosses between individuals too f~tr removed in structure or constitution, arc injurious. And there is not only a. parallelism but an actua.l connection between the two classes of facts, 'for, as we have already shown, many species of animals a.nd plants arc rendered infertile, or altogether sterile, by the change from their natural conditions which occurs in confinement or in cultivation; while, on the other hand, the increased vigour or fertility which is invarinbly produced by a judicious cross may be also effected by a judicious change of climate and surroundings. W c shall sec in a subsequent chapter, that this interchangeability of the bcneficialcfl'ccts of crossing and of new conditions, serves to explain some very puzzling phenomena in the forms and economy of flowers. Rema1'!.·s on the Facts of Jlyb1·idity. The facts that have now been adduced, though not very numerous, arc sufficiently conclusive to prove that the old bel1ef, of the universal sterility of hybrids and fertility of mongrels, is incorrect. The doctrine that such a universal VII ON THE INFERTILITY OF CROSSES 167 law existed wa.s never more than a plausible generalisation, founded on a few inconclusive facts d ri vcd from dome. ticatcd animaJs and cultivated plants. The fact.· were, and still arc, inconclusive for sevcml reasons. They are founded, primarily, on what occurs among animals in domestication ; and it has been shown that domestication both tends to increase fertility, and was itself rendered possible by the fertility of those particular species being little affected by changed conditions. The exceptional fertility of all the varieties of domesticated animals docs not prove that a similar fertility exists among na,tural varieties. In the next place, the generalisation is founded on too remote crosses, as in the case of the horse and the ass, the two most distinct and widely separated species of the genus Eqnus, so eli. tinct indeed that they have been held by some naturali. ts to form distinct genera. Crosses between the two species of zebra, or even between the zcbm and the quagga, or the quagga and the ass, might have led to a very different result. Again, in preDarwinian times it was so universally the practice to argue in a circle, and declare that the fertility of the offspring of a cross proved the identity of species of the parents, that experiments in hybridity were usually made between very remote species and even between species of different genera, to avoid the possibility of the reply : "They are both really the same specie. ; " and the sterility of the hybrid offspring of such rcmoLe crosses of course served to strengthen the popular belief. Now that we have arrived at a different standpoint, and look upon a species, not as a distinct entity due to spedal creation, but as an assemblage of individuals which have become somewhat modified in structure, form, and constituLion so as to adapt them to slightly different conditions of life; which can be differentiated from other allied assemblages; which reproduce their like, and which usually breed together-we require a fresh set of experiments calcula.tcd to determine the matter of fact,-whethcr such species crossed with their ncar allies do always produce offspring which are more or less sterile inter se. Ample materials for such experiments exist, in the numerous "·represcnta.tive species" inhabiting distinct areas on a continent or different islands of a group ; or even |