[Footnote 16: _Foraminifera_, preface, p. x.] [Footnote 17: _United States Geological Survey of the Territories_, 1874.] [Footnote 18: _Proceedings of the Entomological Society of London_, 1875, p. vii.] [Footnote 19: _Ann. des Sci. Nat._, tom. xvi. p. 50.] [Footnote 20: See _Winter Birds of Florida_, p. 206, Table F.] [Footnote 21: See Table I, p. 211, of Allen's _Winter Birds of Florida_.] [Footnote 22: _Proc. Zool. Soc._, 1864, p. 64.] [Footnote 23: J.A. Allen, on Geographical Variation among North American Mammals, _Bull. U.S. Geol. and Geog. Survey_, vol. ii. p. 314 (1876).] [Footnote 24: _Proc. Zool. Soc. Lond._, 1864, p. 700, and 1868, p. 28.] [Footnote 25: See _Trans. Entomological Society of London_, 1887, p. 24.] [Footnote 26: _Nature_, vol. xix. p. 554.] [Footnote 27: _Nature_, vol. xvi. p. 163; and vol. xi. p. 227.] [Footnote 28: _Ibid._, vol. xxxi. (1885), p. 533.] [Footnote 29: _Nature_, vol. xxvi. p. 81.] [Footnote 30: _Animals and Plants under Domestication_, vol. ii. p. 258.] CHAPTER IV VARIATION OF DOMESTICATED ANIMALS AND CULTIVATED PLANTS The facts of variation and artificial selection--Proofs of the generality of variation--Variations of apples and melons--Variations of flowers--Variations of domestic animals--Domestic pigeons--Acclimatisation--Circumstances favourable to selection by man--Conditions favourable to variation--Concluding remarks. Having so fully discussed variation under nature it will be unnecessary to devote so much space to domesticated animals and cultivated plants, especially as Mr. Darwin has published two remarkable volumes on the subject where those who desire it may obtain ample information. A general sketch of the more important facts will, however, be given, for the purpose of showing how closely they correspond with those described in the preceding chapter, and also to point out the general principles which they illustrate. It will also be necessary to explain how these variations have been increased and accumulated by artificial selection, since we are thereby better enabled to understand the action of natural selection, to be discussed in the succeeding chapter. _The facts of Variation and Artificial Selection._