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).