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 ;” 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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