Speaking in 1883[36] of Darwin’s theory of pangenesis, Weismann remarks: “We become lost in unfounded hypotheses.” I think this is a true and allowable criticism, but I also think that it applies in far greater measure to the theory of the germ plasm developed by Weismann himself. It may be pointed out that Darwin placed little store upon his theory, and apologised for its speculative nature, not only in the letters already referred to, but in his work upon “The Variations of Plants and Animals under Domestication,” where it appears in a single chapter at the end of a large work of 800 pages. Darwin here says (vol. ii., p. 349): “I am aware that my view is merely a provisional hypothesis or speculation; but until a better one be advanced, it will serve to bring together a multitude of facts which are at present left disconnected by any efficient cause.” Weismann’s speculations, equally unfounded on fact, are nevertheless viewed by him as being of sufficient importance to demand the most detailed elaboration. Do not suppose for an instant that I am decrying the use of fancy or provisional hypotheses; the world advances upon the steps, generally the ruined steps, of hypotheses. The value of an hypothesis depends, however, entirely upon whether we can put it to the test of experiment. If it is intangible, then it remains an hypothesis, while the great body of fact workers go on building their sciences, and in the completion of these it has no part. Darwin’s pangenesis has no value as an hypothesis, it seems to me, apart from its being a pictorial way of illustrating how use and disuse might be inherited. This latter is a question which can be solved experimentally; pangenesis was a mental picture present to Darwin’s mind, and he threw it out for what it was worth. In this picture the gemmules were supposed to pass from the cells of the body through the blood into the reproductive cells, and the experiments which Galton, and subsequently Romanes, undertook to disprove pangenesis by showing that certain samples of blood did not contain gemmules, and that therefore pangenesis did not occur, appear to me to show a want of appreciation of the fact that Darwin intended this theory merely as a temporary mental picture, and nothing more.