Whether sexual selection is also a factor is still a matter of opinion. Without it the specific character and constancy of secondary sexual features are at present unexplained. If inherited use and disuse are admitted as factors in origin, they must also be admitted as important factors in guidance. Questions of origin and guidance should, so far as is possible, be kept distinct. These terms, however, apply to the origin and guidance of variations. In the origin of species guidance is a factor, no doubt a most important factor. The title of Darwin's great work was, therefore, perfectly legitimate. And those who say that natural selection plays no part in the origin of species are, therefore, undoubtedly in error. NOTES [CI] It is beyond the scope of this book to give the _evidences_ of evolution. Such evidence from embryology, from distribution, and from palæontology, is now abundant. For palæontological evidence, see Nicholson's "Manual of Palæontology," 3rd edit., especially the second volume on "Vertebrates," by R. Lydekker. [CJ] Weismann, "Essays on Heredity," p. 24. [CK] Ibid. p. 140. [CL] Weismann, "Essays on Heredity," p. 90. [CM] Ibid. p. 292. See also a discussion in _Nature_, in which Mr. Romanes and Professor Ray Lankester took part, beginning vol. xli. p. 437. [CN] Weismann, "Essay on Heredity," p. 140. [CO] "Origin of Species," p. 110. [CP] With regard to blind cave-fish, Professor Ray Lankester has suggested that some selection has been effected. Those animals whose sight-sensitiveness enabled them to detect a glimmer of light would escape to the exterior, leaving those with congenitally weak sight to remain and procreate in the darkness of the cave. [CQ] Darwin, "Descent of Man," pt. ii. chap. viii. [CR] "Darwinism," chap. x. [CS] "Darwinism," p. 295. Messrs. Geddes and Thomson, "The Evolution of Sex," p. 28, also contend that "combative energy and sexual beauty rise _pari passu_ with male katabolism." [CT] "Darwinism," p. 293. [CU] Mr. Poulton, who takes a similar line of argument in his "Colours of Animals," lays special stress upon the production of _white_ (see p. 326). [CV] See Chapter VIII. [CW] "Darwinism," p. 172. [CX] See "Animals and Plants under Domestication," vol. ii. p. 80. [CY] "Darwinism," p. 332. [CZ] "The Colour-Sense," by Grant Allen, p. 95. [DA] That on "The Emotions of Animals" (X.). [DB] "Darwinism," p. 318. [DC] Natural History Society of Wisconsin, vol. i. (1889). [DD] "Darwinism," p. 286. [DE] On the negative character of disuse, see p. 196. [DF] Cope, "Origin of the Fittest," p. 374.