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Show 154 DARWINISM CHAP. and numbers of species of rodents, very rarely breed in confinement; while other species do so more. or less freely. Hawks, vultures, and owls hardly eve!' breed m confinement; neither did the faJcons kept for hawking e~er br~e~. Of the numerous small seed-eating birds kept m av1anes, hardly any breed, neither do parrots. Gallinaceous birds usually breed freely in confinement, but some do not ; and e_ven the guans and curassows, kept tame by the South ~mencan Indians, never breed. This shows that cha~gc of climate has nothing to do with the phe~omenon ; and, m. fact, the same species that refuse to breed m Eur~pe do_ so, m ah~ost every case when tamed or confined in the1r nat1Ve countnes. This inability to reproduce is not due to ill- health, since many of these creatures are perfectly vigorous and live very long. With our true domestic animals, on the other hand, fertility is perfect, and is very little affected by changed conditions. Thus, we see the common fowl, a native of tropical India, living and multiplying in almost every part of the world ; and the same is the case with our cattle, sheep, and goats, our dogs and horses, and especially ':ith ~~mestic pigeons. It therefore seems probable, that th1s facihty for breeding under changed conditions was an original property of the species which man has domesticated - a property which, more than any other, enabled him to domesticate them. Yet, even with these, there is evidence that great changes of conditions affect the fertility. In the hot valleys of the Andes sheep are less fertile ; while geese taken to the high plateau of Bogota were at first almost sterile, but after some generations recovered their fertility. These and many other facts seem to show that, with the majority of animals, even a slight change of conditions may produce inferWity or sterility; and also that after a time, when the animal has become thoroughly acclimatised, as · it were, to the new conditions, the infertility is in some cases diminished or altogether ceases. It is stated by Bechstein that the canary was long infertile, and it is only of late years that good breeding birds have become common; but in this case no doubt selection has aided the change. As showing that these phenomena depend on deep-seat r<l causes and are of a very general nature, it is interesting VII ON THE INFERTILITY OF CROSSES 155 to note that they occur also in the veaetable kinadom All . 0 b . owmg for all the circumstances which are known to prever~t the prod~1ction of seed, such as too great luxuriance ?f foliage, too little or too much heat, or the absence of msects to cross-fertilise the flowers, Mr. Darwin shows that many species which grow and flower with us, appareptly in perfect health, ye~ never produce seed. Other plants are affected_ by very ~hght changes of conditions, producing seed frccl~ m one sOil and _not in another, though apparently g~'owmg equally. ~vell m ~oth ; while, in some cases, a difference of pos1t10n even m the same o·arden produces a similar result.l b Reciprocal C1·osses. . Another indication of the extreme delicacy of the adJ~s.tmCI_lt between the sexes, which is necessary to produce fert_Ih~y, IS afforded. by the behaviour of nuwy species and vanet10s when reciprocally crossed. This will be best illust1:1ted by a few_ o! the examples furnished u. by Mr. Darwm. Th? two distmct spe~ies of plants, Mirabilis jalapa and 1 M. longiflO~'a, can .be easily crossed, and will produce hea1thy and fertile hybnds when the pollen of the latter is applie_d to the stigma of the former plant. But the same expenmenter, IGHreuter, tried in vain, more than two hundred times ~uring eight y~ars, to cross them by applying the pollen of M. Jalapa to the stigma of M. longiflora. In other cases two plants are so closely allied that some botanists class them as varieti_es (as with Matthiola annua and M. glabra), and yet there 1s the same great difference in the result when they are reciprocally crossed. Individual Differences in respect to Cross-Fertilisation. A still more remarlmble illustration of the delicate balance_ of. o_rganisa~ion needful for reproduction, is afforded by the m?Ividual diff~rences of animals and plants, as regards both thmr power of mtercrossing with oth er individuals or other species, ~nd t~e fertility ?f the offspring thus produced. Among domesttc :tmmals, Darwm states tha,t it is by no means rare to find certain males and females which will not breed 1 Darwin's Animals ctnd Plants under Domestication, vol. ii. pp. 163-170. |