Summing up now the origin of variations, apart from those which are merely individual, on the hypothesis that particular modifications of the body-cells cannot be transmitted to the germ-cells, we have-- 1. In protozoa, the direct influence of the environment and the induced development of faculty. 2. In metazoa-- (_a_) Some direct and merely general influence of the environment on the germ, including under the term "environment" the nutrition, etc., furnished by the body. (_b_) The combination and recombination of elementary protoplasmic faculties (specific molecular groupings) acquired by the protozoa. (_c_) Influences on the germ, the nature of which is at present unknown. * * * * * We may now pass on to consider the position of those who give an affirmative answer to the question--Can the body affect the germ? Two things are here required. First, definite evidence of the fact that the body does so affect the germ; i.e. that acquired characters are inherited. Secondly, some answer to the question--How are the body-cells able to transmit their modifications to the germ-cells? We will take the latter first, assuming the former point to be admitted. Let us clearly understand the question. An individual, in the course of its life, has some part of the epidermis, or skin, thickened by mechanical stresses, or some group of muscles strengthened by use, or the activity of certain brain-cells quickened by exercise: how are the special modifications of these cells, here, there, or elsewhere in the body, communicated to the germ, so that its products are similarly modified in the offspring? The following are some of the hypotheses which have been suggested:-- (_a_) Darwin's pangenesis. (_b_) Haeckel's perigenesis; Spencer's physiological units. (_c_) The conversion of germ-plasm into body-plasm, and its return to the condition of germ-plasm (Nägeli). (_d_) The unity of the organism. (_a_) Concerning pangenesis, nothing need be added to what has already been said. Although, as we have seen, it has been adopted with modifications by Professor Brooks; although Mr. Francis Galton, a thinker of rare ability and a pioneer in these matters, while contending for continuity, admitted a little dose of pangenesis; although De Vries has recently renewed the attempt to combine continuity and a modified pangenesis;--this hypothesis does not now meet with any wide acceptance.