220 DEGENERATION sec. v 



no means for all cases. But I must insist that since the action 

 of sexual mixing assumed by Weismann presupposes differ- 

 ences among the mingling individuals, the periods of time 

 necessary for that action presuppose the time required for the 

 evolution of these differences. 



It is thus clear, that modification by sexual mixture 

 requires not only the time which is necessary for the accom- 

 plishment of the mixture, but in addition, the time the external 

 conditions have needed in order to produce the differences 

 which such action starts from. 



I by no means deny that sexual mixture has any influence 

 at all in the production of new forms ; but I believe that it 

 cannot possibly be the only determining cause, and therefore 

 I contend against it in detail. If it were all-powerful, great 

 differences in the production of species would exist among the 

 various gi^oups of the animal kingdom, for those animals 

 which possess but slight powers of locomotion would neces- 

 sarily produce more species than the others, because among 

 them sexual mixture is more limited. Moreover, some 

 animals are more stationary than others, even when they 

 possess similar powers of locomotion. What I previously 

 said of the individuals of a nation must apply still more to 

 the innumerable animals which are confined to the narrowest 

 limits of habitat, and which keep to such limits more than is 

 usually supposed. Since in these cases, e.g. in reptiles,^ slight 

 local barriers set limits to migration for a long time, if the 

 view of the all-powerful effect of sexual mixture were correct, 

 thousands of species and varieties would have arisen where 

 only one exists. 



Sexual mixture is, in fine, only one of the agencies which 

 may promote the formation of species. 



^ Cf. my paper on the " Variation of the AVall-Lizard," p. 261 et seq. 



