THE LAMARCKIAN THEORY 



423 



plants and animals gives the idea of germinal continuity a plausi- 

 bility which it does not deserve. In the hght of the facts cited 

 briefly above, it is evident that continuity is merely a corollary 

 of reproductive function, and anything which is capable of bridging 

 the gap between generations will be continuous. 



The idea has, nevertheless, influenced the thought of modern 

 supporters of Lamarckian evolution. It has appeared necessary 

 to explain the reciprocal association of the soma and germ plasm 

 whereby the in- 

 dividual might 

 make some con- 

 tribution to the 

 heritage of the 

 species, and va- 

 rious hypotheses 

 have been ad- 

 vanced with this 

 end in view. 



Pangenesis. 

 Darwin, while 

 he was at first 

 vigorously op- 

 posed to the 

 views of Lamarck, 

 cited the effects 

 of use and disuse 

 frequently in his 

 writings on evolution. Like his followers he looked upon the 

 body as a thing more or less definitely separated from the germ 

 cells that it produces, and to meet the diflficulty of associ- 

 ating somatic characters with the heritage he proposed the theory 

 of pangenesis. This theory assumed that every part of the body 

 constantly gave rise to minute units characteristic of itself. These 

 units were supposed to be taken up l:)y the blood stream and 

 transmitted throughout the body. The units, called gemmules, 

 were supposed to collect in the germ cells, thus influencing them 

 according to the development attained by the somatic structures 

 so that they in turn might carry this influence to the next genera- 

 tion. The acquired characters of Lamarckian theory, according 

 to this hypothesis, would be provided with a vehicle whereby 



B 



Fig. 210. — Diagrams to illustrate the behaviour of the 

 germ plasm in a succession of generations. Black 

 indicates the germ plasm, the outlined triangle the 

 soma. The succession is from top to bottom. A, 

 continuity of the germ plasm in ordinary sexual re- 

 production, involving the confluence of two lines at 

 each generation; B, continuity of both germ plasm and 

 soma in fission; C, continuity of soma and discon- 

 tinuity of germ plasm in plants and flatworms. 



