Variation of animals and plants under domestication, v.2

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86 ON CROSSING AS A CAUSE CHAP. XV. same thm. g as t h e1. r separat I' on. In sava ge and semi-civilised countries, where the inhabitants have not the means of sepa-rating their animals, more than a single br~ed of the s.ame species rarely or never exists. In former times, even. 1~ a country so civilised as North America, there were no distmct races of sheep, for all had been mm· g1 e d t oge ther· 1 The cele-brated agriculturist Marshall 2 remarks that "sheep t~at are " kept within fences, as well as shephe:ded flocks .m o~en " countries have generally a similarity, If not a umformity, " of cha1;a~ter in the individuals of each flock ; " for they breed freely together, and are prevented from crossing with other kinds· whereas in the unenclosed parts of England the un­sheph~ rded sheep, even of the same flock, are far from true or uniform, owing to various breeds having mingled and cro~s~d. vV e have seen that the half-wild cattle in the several Bntish parks are uniform in character in each; but i~ the different parks, from not having mingled and crossed durmg many gene-rations, they differ in a slight degree. . . We cannot doubt that the extraordinary number of vanet1es and sub-varieties of the pigeon, amounting to at least one hun­dred and :fifty, is partly due to their remaining, differently from other domesticated birds, paired for life when once matched. On the other hand, breeds of cats imported into this country soon disappear, for their nocturnal and rambling habits render it hardly possible to prevent free crossing. Rengger 3 gives an interesting case with respect to the cat in Paraguay: in all the distant parts of the kingdom it has assumed, apparently from the effects of the climate, a peculiar character, but near the capital this change has been prevented, owing, as he asserts, to the native animal frequently crossing with cats imported from Europe. In all cases like the foregoing, the effects of an occasional cross will be augmented by the increased vigour and fertility of the crossed offspring, of which fact evidence will hereafter be given; for this will lead to the mongrels increasing more rapidly than the pure parent-breeds. 1 Communications to the Board of Agriculture, vol. i. p. 367. ~ 'Review of Reports, North of Eng-land,' 1808, p. 200. 3 'Saugcthiere von Paraguay,· 1830, s. 212. CHAP. XV. OF UNIFORMITY OF CHARACTER. 87 . When distinct breeds are allowed to cross freely, the result Will be a heterogeneous body; for instance, the dogs in Para­gu~ y are far from uniform, and can no longer be affiliated to th~Ir parent-races.4 'l1he character which a crossed body of ammals will ultimately assume must depend on several con­tingencies,- namely, on the relative numbers of the individuals belonging to the two or more races which are allowed to mingle ; on the prepotency of one race over the other in the transmission of character; and on the conditions of life to which they are exposed. When two commingled breeds exist at :first in nearly equal numbers, the whole will sooner or later become intimately blended, but not so soon, both breeds being equally favoured in all respects, as might have been expected. The following cal­culation 5 shows that this is the case : if a colony with ari. equal number of black and white men were founded, and we assume that they marry indiscriminately, are equally prolific, and that one in thirty annually dies and is born; then "in 65 " years the number of blacks, whites, and mulattoes would be "equal. In 91 years the whites would be 1-lOtb, the blacks "1-10th, and the mulattoes, or people of intermediate degrees of "colour, 8-10ths of the whole number. In three centuries not "1-100th part of the whites would exist." When one of two mingled races exceeds the other greatly in number, the latter will soon be wholly, or almost wholly, absorbed ~nd lost.6 .Thus E~ropean pigs and dogs have been largely mtroduced mto the Islands of the Pacific Ocean, and the native races have been absorbed and lost in the course of about :fifty or sixty years ; 7 but the imported races no doubt were favoured. Rats may be considered as semi-domesticated animal~. Some snake-rats (Mus alexandrinus) escaped in the Zoological Gardens of London, " and for a long time afterwards "the keepers frequently caught cross-bred rats, at :first balf-breds "a_fterwards with less and less of the character of the snake-rat: " till at length all traces of it disappeared." 8 On the other hand, 4 Rcngger, 'Saugethiere,' &c., s. 154. 5 White, 'Regular Gradation in Man,' p. 14:6. 6 Dr. W. F. Edwards, in his' Charac­tcres Pbysiolog. des Races Humaines,' p. 23, first called attention to this sub-ject, and ably discussed it: 7 Rev. D. Tyerman, and Bennett 'Journal of Voyages,' 1821-1829, vol: i. p. 300. 8 1\llr. S. J. Salter, 'Journal Linn. Soc.,' vol. vi., 1862, p. 71.