Heredity and the Origin of Variations. 159 



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. 



(&) 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 (Nageli). 



(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. 



(&) With the pamphlet in which Professor Haeckel 

 brought forward his hypothesis termed the perigenesis of 

 the plastidule, I cannot claim first-hand acquaintance. 

 According to Professor Eay Lankester, who gave some 

 account of it in Nature* protoplasm is regarded by Haeckel 

 as consisting of certain organic molecules called plasti- 

 dules. These plastidules are possessed of special undulatory 

 movements, or vibrations. They are liable to have their 

 undulations affected by every external force, and, once 



* July 15, 1876. Since reprinted in " The Advancement of Science," p. 273. 



