WEISMANN1SM AND OTHER THEORIES 399 



develop into cells like those from which they originally migrated, in 

 this manner building up a new individual like the parent. Some of the 

 gemmules do not function until a comparatively late stage in the onto- 

 geny, and others may remain latent through several generations: on 

 these two assumptions it is possible to account for the late appearance of 

 certain characters and for the fact that others may "skip" one or more 

 generations. It is further supposed that some gemmules remain latent 

 in the individuals of one sex: thus, for instance, the characters normally 

 present only in the male may be transmitted through the female. 



In the many criticisms of this hypothesis the tendency has been to 

 judge and condemn it solely on the ground of the supposed transportation 

 of the gemmules from the body cells to the germ cells, for which no direct 

 evidence has ever been discovered. It should not be forgotten, however, 

 that Darwin suggested an explanation for the phenomena of heredity 

 on the basis of representative material units in the cells, a conception 

 which was of the greatest importance in that it constituted the starting 

 point for later fruitful investigations and theories. The migration of 

 the gemmules was postulated largely to account for the phenomena of 

 regeneration and the inheritance of acquired somatic modifications. 

 Since regeneration may be explained as well on other grounds, and since 

 the evidence for the inheritance of acquired somatic modifications is 

 for the most part of such extremely doubtful value, such a migration of 

 representative units, first denied by Galton (1875), has come to be re- 

 garded as unnecessary. A theory postulating representative particles 

 but no such migration was that of de Vries. 



De Vries's Theory of Intracellular Pangenesis. According to de 

 Vries (1889) the particles of hereditary substance, or pangens, do not 

 represent different kinds of cells as Darwin thought, but stand rather for 

 different elementary characters or qualities out of which the many visible 

 characters of the organism are built up. Furthermore, these living 

 elements or pangens do not pass from cell to cell, but merely circulate 

 between the nucleus, where a complete outfit of them is conserved, and 

 the other parts of the cell hence the term " intracellular pangenesis." 

 In this way the characters brought into the new individual through the 

 nucleus are delivered to the cell as a whole. Contrary to the idea of 

 Darwin and especially to that of Weismann (see below), all the cells (or 

 nuclei) of the body contain pangens for all the hereditary characters: 

 they are not sorted out as development proceeds. 



Nageli's Idioplasm Theory. rA highly speculative theory of a some- 

 what different type was that formulated by Nageli (1884) five years 

 before that of de Vries. Protoplasm was thought by Nageli to be made 

 up of a vast number of fundamental living units; these he called micellce. 

 As a result of the ways in which these molecular complexes or micellae 

 may be arranged, there are in the cell two kinds of protoplasm : in nutri- 



