[7] ‘On the Varieties of Wheat,’ p. 52.
[8] Mr. Spencer has fully and ably discussed this whole subject in his ‘Principles of Biology,’ 1864, vol. ii. ch. x. In the first edition of my ‘Origin of Species,’ 1859, p. 267, I spoke of the good effects from slight changes in the conditions of life and from cross-breeding, and of the evil effects from great changes in the conditions and from crossing widely distinct forms, as a series of facts “connected together by some common but unknown bond, which is essentially related to the principle of life.”
[9] ‘Essais de Zoologie Générale,’ 1841, p. 256.
[10] Since the appearance of the first edition of this work, Mr. Sclater has published (‘Proc. Zoolog. Soc.,’ 1868, p. 623) a list of the species of mammals which have bred in the gardens from 1848 to 1867 inclusive. Of the Artiodactyla 85 species have been kept, and of these 1 species in 1·9 have bred at least once during the 20 years; of 28 Marsupialia, 1 in 2·5 have bred; of 74 Carnivora, 1 in 3·0 have bred; of 52 Rodentia, 1 in 4·7 have bred; and of Quadrumana 75 species have been kept, and 1 in 6·2 have bred.
[11] Du Rut, ‘Annales du Muséum,’ 1807, tom. ix. p. 120.
[12] ‘Saugethiere von Paraguay,’ 1830, s. 49, 106, 118, 124, 201, 208, 249, 265, 327.
[13] ‘The Naturalist on the Amazons,’ 1863, vol. i. pp. 99, 193; vol. ii. p. 113.
[14] ‘Embassy to the Court of Ava,’ vol. i. p. 534.
[15] ‘Journal,’ vol. i. p. 213.
[16] ‘Säugethiere,’ s. 327.