LAW OF MIGRATION. 



159 



but that the formation of species can take place only 

 with the assistance of isolation has been effectively 

 refuted by Weismann.*^ He has shown that an " inter- 

 crossing of the incipient variety with the aboriginal form 

 is not avoided by isolation;'' through the instance de- 

 rived from the Steinheim lake, of the formation of a 

 new species amid the older formations. On Haeckel's 

 remark that in the asexual propagation of the lower 

 beings, the influence of intercrossing was not to be 

 feared, Wagner had already restricted the necessity of 

 isolation to the higher organisms with separate sexes. 

 But Weismann most justly insists that Wagner's " law 

 of migration " is deprived of all foundation by one of 

 the most remarkable examples of the formation of 

 varieties on the same territory, namely, the fact of the 

 separation of the sexes, as to the derivation of which 

 from species once hermaphrodite, all (the believers 

 in Creation naturally excepted) are assuredly of one ac- 

 cord. 



As we have already mentioned, it seems as if the 

 impulse to form varieties once exists, the tendency 

 spreads rapidly. Proof of such periods of variation is 

 to be found in the Palseontological investigations on 

 page 96. If isolation coincides with such a period, it 

 effects the establishment of new varieties into species 

 without the aid of natural selection. As Darwin ad- 

 mits in his work on the origin of Man, he formerly be- 

 stowed too little attention on the formation of so- 

 called morphological species. By this we mean, species 

 not distinguished from their aboriginal stocks by any 

 physiological advantages, and hence not superior to them, 

 in which , therefore the principle of selection in the strict 



