The period of gestation and of maturity, as shown in the earlier chapters,--the season and the frequency of the act of breeding,--have all been greatly modified under domestication. With the Egyptian goose the rate of change with respect to the season has been recorded. (24/38. Quatrefages 'Unite de l'Espece Humaine' 1861 page 79.) The wild drake pairs with one female, the domestic drake is polygamous. Certain breeds of fowls have lost the habit of incubation. The paces of the horse, and the manner of flight of certain breeds of the pigeon, have been modified and are inherited. Cattle, horses, and pigs have learnt to browse under water in the St. John's River, East Florida, where the Vallisneria has been largely naturalised. The cows were observed by Prof. Wyman to keep their heads immersed for "a period varying from fifteen to thirty-five seconds." (24/39. 'The American Naturalist' April 1874 page 237.) The voice differs much in certain kinds of fowls and pigeons. Some varieties are clamorous and others silent, as the Call and common duck, or the Spitz and pointer dog. Every one knows how the breeds of the dog differ from one another in their manner of hunting, and in their ardour after different kinds of game or vermin.
With plants the period of vegetation is easily changed and is inherited, as in the case of summer and winter wheat, barley, and vetches; but to this subject we shall immediately return under acclimatisation. Annual plants sometimes become perennial under a new climate, as I hear from Dr. Hooker is the case with the stock and mignonette in Tasmania. On the other hand, perennials sometimes become annuals, as with the Ricinus in England, and as, according to Captain Mangles, with many varieties of the heartsease. Von Berg (24/40. 'Flora' 1835 b. 2 page 504.) raised from seed of Verbascum phoeniceum, which is usually a biennial, both annual and perennial varieties. Some deciduous bushes become evergreen in hot countries. (24/41. Alph. de Candolle 'Geograph. Bot.' tome 2 page 1078.) Rice requires much water, but there is one variety in India which can be grown without irrigation. (24/42. Royle 'Illustrations of the Botany of the Himalaya' page 19.) Certain varieties of the oat and of our other cereals are best fitted for certain soils. (24/43. 'Gardener's Chronicle' 1850 pages 204, 219.) Endless similar facts could be given in the animal and vegetable kingdoms. They are noticed here because they illustrate analogous differences in closely allied natural species, and because such changed habits of life, whether due to habit, or to the direct action of external conditions, or to so-called spontaneous variability, would be apt to lead to modifications of structure.
ACCLIMATISATION.
From the previous remarks we are naturally led to the much disputed subject of acclimatisation. There are two distinct questions: Do varieties descended from the same species differ in their power of living under different climates? And secondly, if they so differ, how have they become thus adapted? We have seen that European dogs do not succeed well in India, and it is asserted (24/44. Rev. R. Everest 'Journal As. Soc. of Bengal' volume 3 page 19.), that no one has there succeeded in keeping the Newfoundland long alive; but then it may be argued, and probably with truth, that these northern breeds are specifically distinct from the native dogs which flourish in India. The same remark may be made with respect to different breeds of sheep, of which, according to Youatt (24/45. Youatt on 'Sheep' 1838 page 491.), not one brought "from a torrid climate lasts out the second year," in the Zoological Gardens. But sheep are capable of some degree of acclimatisation, for Merino sheep bred at the Cape of Good Hope have been found far better adapted for India than those imported from England. (24/46. Royle 'Prod. Resources of India' page 153.) It is almost certain that all the breeds of the fowl are descended from one species; but the Spanish breed, which there is good reason to believe originated near the Mediterranean (24/47.