EVOLUTION AND HEREDITY. I35 



Observe that all these hypotheses sprang from the 

 evolution standpoint, and were constructed to account 

 not only for the repetition phenomena but for what 

 was by each author considered a prime factor in evo- 

 lution, namely, the transmission of acquired charac- 

 ters, or the inheritance by the offspring of some of 

 the influences which environment and habit have 

 exerted upon the body cells of the parent. All reason- 

 able hypotheses, as these certainly were, have their 

 value in stimulating research ; and the main service ren- 

 dered to science by the pangenesis doctrines has been 

 a negative one, namely, they have shown that it is 

 extremely difficult to construct even an a priori zvorking- 

 hypotJiesis of heredity which will explain the trans- 

 mission of acquired character's. 



Weismann is the most brilliant of the post-Darwinian 

 writers, and no one at the present time has so great a 

 following or is exerting such a wide influence. He ap- 

 proached the heredity problem purely from the embryo- 

 logical side : '* How is it that a single cell of the body 

 can contain within itself all the hereditary tendencies of 

 the whole organism .? " There can be but two alterna- 

 tives, either the substance of the germ cells is derived 

 from the body of the new individual, or directly from the 

 parent germ cells. His theory of the " continuity of the 

 germ plasm " supports the latter alternative according 

 to which the germ cells of the parent must give rise to 

 " somatic " cells forming the body of the offspring, and 

 to ''germ" cells. Each generation has an identical 

 starting-point in ceUs of the latter kind which are in a 

 sense immortal, the individuals being mere offshoots from 

 the continuous germ plasm stem. Some of the details 



