LAW OF MIGRATION. 1 59 



but that the formation of species can take place only 

 with the assistance of isolation has been effectively 

 refuted by Weismanii/* He has shown that an " inter- 

 crossing of the incipient variety with the aboriginal form 

 is not avoided by isolation;" and by the very favourable 

 instance of the lake of Steinheim, among others, he has 

 exhibited the formation of new species in the midst of 

 the old ones. On Haeckel's remark that in the asexual 

 propagation of the lower beings, the influence of inter- 

 crossing was not to be feared, Wagner had already re- 

 stricted 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 deriva- 

 tion of which from species once hermaphrodite, all (the 

 believers in Creation naturally excepted) are assuredly 

 of one accord. 



As we have already mentioned, it seems that if the im- 

 pulse to form varieties once exists, the tendency spreads 

 rapidly. Steinheim, with its Planorbis multiformis, is 

 specially propitious to the demonstration of these 

 periods of variation. If isolation coincides with such 

 a period, it effects the establishment of new varieties 

 into species without the aid of natural selection. As 

 Darwin admits in his work on the origin of Man, he 

 formerly bestowed too little attention on the forma- 

 tion 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 

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