The most notable attempt to supplement and strengthen the theory of natural selection has been made by Weismann in his theory of Germinal Selection. In Weismann's hypothesis, which has furnished in a sense the philosophical basis for the popular Eugenics movement, the struggle for existence is transferred to a struggle among the constituents of the germ-plasm. The minute invisible "determinants" of the germ-plasm, which give rise to the variations in the organ, or cell, which they determine, are unequally nourished by the nutritive stream. A determinant at first favoured by chance may at length gain strength actively to nourish itself to the detriment of its fellow-determinants, and thus attain a permanent upward movement. With Weismann the fluctuations within the germ-plasm "are the real root of all hereditary variations, and the preliminary condition for the occurrence of the Darwin-Wallace factor of selection." The struggle for existence, or the struggle for possession of the mate in sexual selection, practically goes back to "the struggle between the determinants within the germ-plasm"[57] for food and space. 57: "Darwin and Modern Science": "The Selection Theory," p.49. Let us see how this theory of determinants will apply to the famous case of the antlers of the elk or stag. The antlers would increase in size, in this case, only because the determinants, corresponding in the germ-plasm to the antlers in the adult organism, attracted nourishment to themselves, and withdrew it to a certain extent from their fellows. Instead, therefore, of a corresponding strengthening in the whole anterior half of the animal, which Weismann admits would be necessary,[58] we should have, with the increased weight of the antlers, a decrease in weight and strength of other parts of the body. The problem, instead of being solved, seems to be involved in deeper mystery, and there will be hesitation in accepting the statement that "thus in our time the great riddle has been solved--the riddle of the origin of what is suited to its purpose without the coöperation of purposive forces."[59] T. H. Morgan thinks it unfortunate that Weismann should seek to supply the deficiencies of Darwin's theory by new speculative matter skilfully removed from the field of verification.[60] 58: "Darwin and Modern Science": "The Selection Theory," p. 32. 59: "The Evolution Theory," II, p. 391. 60: "Evolution and Adaptation," pp. 165 f.