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Show , I 110 DOMESTICATION ELIMfNATES STERILITY. CnAP. XVI. experiment would in all cases be surrounded by ~any d~fficulties. Dureau de la :M:alle, who has so closely stud1ed classical literature, states 2i that in the time of the Romans the common mule was produced with more difficulty than at the present day; but whether this statement may be trusted I know .not .. A much more important, though somewhat different, case 1.s ?1ven by :M:. Groenland,28 namely, that plants, known from thmr I~termediate character and sterility to be hybrids between ~plops and wheat, have perpetuated themselves under culture since 1857, with a rapid but varying increase of f ertility in e.a~h gene?·~tion. In the fourth generation the plants, still retammg their intermediate character, had become as fertile as common cultivated wheat. The indirect evidence in favour of the Pallasian doctrine appears to me to be extremely strong. In the earlier chapters I have attempted to show that our various breeds of dogs are descended from several wild species; and this probably is the case with sheep. There can no longer be any doubt that the Zebu or humped Indian ox belongs to a distinct species from European cattle : the latter, moreover, are descended from two or three forms, which may be called either species or wild races, but which co-existed in a state of nature and kept distinct. We have good evidence that our domesticated pigs belong to at least two specific types, S. scrofa and Indica, which probably lived together in a wild state in South-eastern Europe. Now, a widely- extended analogy leads to the belief that if these several allied species, in the wild state or when first reclaimed, had been crossed, they would have exhibited, both in their first unions and in their hybrid offspring, some degree of sterility. Nevertheless the several domesticated races descended from them are now all, as far as can be ascertained, perfectly fertile together. If this reasoning be trustworthy, and it is apparently sound, we must admit the Pallasian doctrine that long-continued domestication tends to eliminate that sterility which is natural to species when crossed in their aboriginal state. 27 • Aunales des Sc. Nat.,' tom. xxi. (1st series), p. 61. 2s 'Bull. Bot. Soc. de France,' Dec. 27th, 1861, tom. viii. p. 612. CnAr. XVI. INCREASED FERTILITY FROM DOMESTICATION. 111 On -increased Fertility from Domestication and Cultivation. Increased fertility from domestication, without any reference to crossing, may be here briefly considered. This subject bears indirectly on two or three points connected with the modification of organic beings. As Buffon long ago remarked/9 domestic animals breed oftener in the year and produce more young at a "birth than wild animals of the same species; they, also, sometimes breed at an earlier age. The case would hardly have deserved further notice, had not some authors lately attempted to show that fertility increases and decreases in an inverse ratio with the amount of food. This strange doctrine has apparently arisen from individual animals when supplied with an inordinate quantity of food, and from plants of many kinds when grown on excessively rich soil, as on a dunghill, becoming sterile; but to this latter point I shall have occasion presently to return. With hardly an exception, our domesticated animals, which have long been habituated to a regular and copious supply of food, without the labour of searching for it, are more fertile than the corresponding wild anim~1.ls. It is notorious bow frequently cats and dogs breed, and bow many young they produce at a birth. The wild rabbit is said generally to breed four times yearly, and to produce from four to eight young; the tame rabbit breeds six or seven times yearly, and produces from four to eleven young. The ferret, though generally so closely confined, is more prolific than its supposed wild prototype. The wild sow is remarkably prolific, for she often breeds twice in the year, and produces from four to eight and sometimes even twelve young at a birth ; but the domestic sow regularly breeds twice a year, and would breed oftener if permitted; and a sow that produces less than eight at a birth "is worth little, and the sooner she is fattened for the butcher the better." The amount of food affects the fertility even of the same individual : thus sheep, which on mountains never produce more than one lamb at a b:llth, when brought ~9 Quoted by I sid. Geofrroy St. Hilaire, 'Hist. N aturelle Generale,' tom. iii. p. 476. Since this MS. has been sent to press a full discussion on the present subject has appeared in 1\Ir. Herbert Spencer's 'Principles of Biology,' vol. ii. 1867, p. 457 et seq. |