294 LAWS OF VARIATION. Chap. XXIV. 



be fed ; but they would not touch swill, which was devoured 

 by the other pigs. An animal when once accustomed to an 

 unnatural diet, which can generally be effected only during 

 youth, dislikes its proper food, as Spallanzani found to be 

 the case with a pigeon which had been long fed on meat. 

 Individuals of the same species take to new food with different 

 degrees of readiness ; one horse, it is stated, soon learned to 

 eat meat, whilst another would have perished from hunger 

 rather than have partaken of it. 36 The caterpillars of the 

 Bombyx hesperus feed in a state of nature on the leaves of the 

 Cafe diable, but, after having been reared on the Ailanthus, 

 they would not touch the Cafe diable, and actually died of 

 hunger. 37 



It has been found possible to accustom marine fish to live 

 in fresh water : but as such changes in fish and other marine 

 animals have been chiefly observed in a state of nature, they 

 do not properly belong to our present subject. 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. 33 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 natural- 

 ised. The cows were observed by Prof. Wyman to keep their 

 heads immersed for " a period varying from fifteen to thirty- 

 five seconds." 39 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 



36 This and several other cases are 563. 



given by Colin, ' Physiologie Comp. 38 Quatrefages, ' Unite de l'Espece 



des Animaux Dom.,' 1854, torn. i. p. Humaine,' 1861, p. 79. 



426. 39 ' The American Naturalist,' Ap> 



3 ' M. Michely de Cayenne, in ' Bull. 1874, p. 237. 

 Soc. d'Acclimat.,' torn, viii., 1861, p. 



