as the female exercises a choice, it is not because the males are beautiful, but because they are "the most vigorous, defiant, and mettlesome." The view of Wallace is supported in the main by that of Westermarck, who especially emphasizes the fact that colors and the other secondary characters are "upon the whole advantageous, inasmuch as they make it easier for the sexes to find each other." They exist to be _seen_. By association of ideas it is natural that the females should find them pleasing, for to them they are the "symbols of the most exciting period of their lives."[658] Furthermore, "the greatest advantage is won with the least possible peril;" for "usually they occur in males only, because of the females' greater need of protection. They are not developed till the age of reproduction, and they appear, in a great many species, only during the pairing season."[659] It follows, therefore, that sexual selection is but another aspect of natural selection, and the secondary sexual characters are perpetuated in harmony with the law of survival of the fittest. Whichever view is accepted, the fact with which we are especially concerned remains: the female exercises the function of choice. [651] DARWIN, _op. cit._, chap. xiii, 367; chap. viii, 214 (prolonged courtship of animals). _Cf._ WESTERMARCK, _op. cit._, 159. [652] DARWIN, _op. cit._, chap. xxi, 614. [653] _Ibid._, chap. viii, 211; _cf. ibid._, 496, 554. [654] WESTERMARCK, _op. cit._, 241. [655] DARWIN, _op. cit._, chap. xvi, 496. [656] WALLACE, _Darwinism_, 268-300; also his _Tropical Nature_, 221-48. [657] Accepting TYLOR'S results in _Coloration of Animals and Plants_ (London, 1886). [658] WESTERMARCK, _op. cit._, 252, 249. WALLACE has also noted the use of colors as a means of recognition: _Darwinism_, 217 ff.; and admits that the sexual colors may become _pleasing_ to the females, though they may be devoid of an æsthetic sense. This alleged inconsistency is criticised by POULTON, _Colours of Animals_, 286. [659] WESTERMARCK, _op. cit._, 240-52, especially 241, 244, 251, 252. For a comparison of the different theories of sexual selection see GEDDES AND THOMPSON, _Evolution of Sex_, 3-30, who think the truth lies between the views of Darwin and Wallace; POULTON, _op. cit._, 284-335, who sustains Darwin's view; and FINCK, _Primitive Love_, 229 ff., who attempts "to demolish the theory of sexual selection in reference to the lower races of man as Wallace demolished it in reference to animals." _Cf._ ESPINAS, _Des sociétés animales_, 290 ff.; BROOKS, _Law of Heredity_ (1883), 166-241; GROOS, _Die Spiele der Thiere_, 230 ff., 267 ff., who takes a medial position between Darwin and Wallace; WEISMANN, _Studies in the Theory of Descent_ (London, 1882), I, 161 ff.; EIMER, _Die Entstehung der Arten_ (1888); and GEDDES, articles "Reproduction," "Sex," "Variation and Selection," in _Encycl. Brit._