18 THEORY OF EVOLUTION 



offspring of a pair of animals are not affected 

 by the structure or the activities of their par- 

 ents, but the germ plasm is the unmodified 

 stream from which l)oth tlie parent and the 

 young have arisen. Hence their resemblance. 

 Xow, it has been found that a variation arising 

 in the germ plasm, no matter what its cause, 

 may affect any stage in tlie development of the 

 next individuals that arise from it. There is 

 no reason to supj^ose that such a change pro- 

 duces a new character that always sticks it- 

 self, as it were, on to the end of the old series. 

 This idea of germinal variation therefore car- 

 ried with it the death of the older conception 

 of evolution by superposition. 



In more recent times another idea has be- 

 come current, mainly due to the work of 

 Bateson and of de Vries — the idea that varia- 

 tions are discontinuous. Such a conception 

 does not fall easily into line with the statement 

 of the biogenetic "law": for actual experience 

 with discontinuous variation has taught us that 

 new cliaracters that arise do not add themselves 

 to the end of tlie line of alreadv existing char- 

 acters ])ut if tliev affect the adult characters 



