6 THE GERM-PLASM 



is not diminished by the fact of his having been less startled by 

 the consequences of his ' gemmule ^-hypothesis when seeking for 

 a purely formal explanation, than he would have been had he 

 tried to adapt his hypothesis to the facts. The hypothesis, as 

 stated by him, could not be regarded as a real solution of the 

 problem of heredity, if only because it leaves unexplained the 

 giving off of the gemmules into the blood, their circulation 

 through the body, and intrusion into the germ- and other cells. 

 All these are assumptions without a basis in fact. This is evi- 

 dently the reason why modifications of the theory of pangenesis 

 were repeatedly made very soon afterwards. 



Before considering these modifications, I should like once 

 more to state clearly the relation of Spencer's ' physiological 

 units' to Darwin's 'gemmules.' Darwin himself considered 

 the former to be closely related to his gemmules ; and, in 

 fact, he would have regarded Spencer's ideas as essentially 

 coinciding with his own, had he not noticed certain passages 

 in Spencer's book which seemed to point to something quite 

 different.* 



It will be apparent, I think, from what has already been said, 

 that these two views are entirely different. What is common 

 to both is that they assume the existence of minute living units, 

 multiplying by fission : but the part taken by them in the con- 

 stitution of the body is quite differently conceived. Spencer's 

 units are the elements which exclusively compose the living 

 body ; while Darwin's gemmules only give rise to cells, i.e., 

 they are elements which are present for the special purpose of 

 bringing about heredity, without anything being specified as 

 to their share in the composition of the living body. As will 

 be shown more clearly later on, Spencer's hypothesis is superior 

 in this respect to Darwin's. On the other hand, Spencer's simi- 

 lar units are the bearers of all the characters of the species, 

 owing to their complex molecular structure ; while the Dar- 

 winian gemmules are primary constituents of individual cells, 

 which are to be considered as differing in a manner correspond- 

 ing to the difference of the individual cells. Spencer's theory 

 is epigenetic, Darwin's evolutionary ; in this respect the latter is, 

 in my opinion, superior to the former. 



* Charles Darwin, ' The Variation of Animals and Plants under Domes- 

 tication," 2nd ed., vol. ii., London, 1888; note on p. 371. 



