Chapter XXII: already gave the reason (see p. 335, above). This world (3)

[569] 'Adaptive' changes are those produced by the direct effect of outward conditions on an organ or organism. Sunburned complexion, horny hands, muscular toughness, are illustrations.

[570] For these and other facts cf. Th. Ribot: De l'Hérédité; W. B. Carpenter: Contemporary Review, vol. 21, p. 295, 779, 867; H. Spencer: Princ. of Biol. pt. ii. ch. v, viii, ix, x; pt. iii. ch. xi, xii; C. Darwin: Animals and Plants under Domestication, ch. xii, xiii. xiv; Sam'l Butler: Life and Habit; T. A. Knight: Philos. Trans. 1837; E. Dupuy: Popular Science Monthly, vol. xi. p. 332; F. Papillon; Nature and Life, p. 330; Crothers, in Pop. Sci. M., Jan. (or Feb.) 1889.

[571] [Because, being exhibited by neuter insects, the effects of mere practice cannot accumulate from one generation to another.--W. J.]

[572] Origin of Species, chap. vii.

[573] Princ. of Psychol., ii. 561.

[574] _Ibid._ p. 263.

[575] Ueber die Vererbung (Jena, 1883). Prof. Weismann's Essays on Heredity have recently (1889) been published in English in a collected form.

[576] Best expressed in the Essay on the _Continuitat des Keimplasmas_ (1885).