Some mode of organic combination (analogous to chemical combination) might afford an explanation of the occurrence of new variations and the increase of existing characters. In the protozoa there may be a summation of the effects of the environment in succeeding generations. There is no convincing evidence that in the metazoa special modifications of the body so influence the germ as to become hereditary. But there is no reason why such influence should not be assumed as a provisional hypothesis. NOTES [BD] "Animals and Plants under Domestication," vol ii. p. 239. [BE] Or in certain "physiological units" (Herbert Spencer), or "plastidules" (Haeckel), which may be regarded as organic molecules exhibiting their special properties under vital conditions. [BF] _Nature_, vol. xxxix. p. 486. [BG] Darwin, "Animals and Plants under Domestication," 2nd edit., vol. ii. chap. xxvii., from which the following description and quotations are taken. [BH] For an excellent account of the genesis and growth of the modern views of heredity, see Mr. J. Arthur Thomson's paper on "The History and Theory of Heredity:" Proceedings of the Royal Society of Edinburgh, 1889. [BI] Geddes and Thomson, "The Evolution of Sex," p. 92. [BJ] Weismann, "Essays on Heredity," English translation, p. 173. [BK] Weismann, "Essays on Heredity," p. 205. [BL] A few pages earlier (p. 200) in the same essay, Professor Weismann says, "A sudden transformation of the nucleo-plasm of a somatic cell into that of a germ-cell would be almost as incredible as the transformation of a mammal into an am[oe]ba." This at first sight does not seem quite consistent with the subsequent sentence which I have quoted in the text; for here, at any rate, the daughters of "mammals" are said to be converted into "am[oe]bæ." But this is no doubt because the am[oe]bæ (germ-plasms) are _contained in_ the mammals (body-cells). (See the quotations that follow in the text.) [BM] Weismann, "Essays on Heredity," p. 207. [BN] Weismann, "Essays on Heredity," p. 179. [BO] It will, of course, be understood that a minute fragment of germ-plasm is capable of almost unlimited growth by assimilation of nutritive material, its properties remaining unchanged during such growth.