2$O EVOLUTION OF TO-DAY. 



isolation implies an amount of geographical change 

 which is absurd. Moreover, many facts tell us that 

 such isolation is not necessary. It is not difficult to 

 find examples of the formation of two or more 

 varieties in the same locality, and if varieties are 

 incipient species, evidently isolation is not necessary 

 for their formation. In the Steinheim lake already 

 referred to, we have an instance of a number of new 

 species being formed in the same locality without 

 the remotest possibility of isolation. Or if it be 

 claimed that the Steinheim species are simply varie- 

 ties, the argument is still the same, for cross-breed- 

 ing will prevent the formation of varieties just as 

 truly as it will prevent the formation of species. 

 Wagner's theory of isolation, which finally led its 

 author to reject natural selection entirely, must be 

 admitted as a factor in the production of species. 

 Isolation will in many cases be a true cause of the 

 production of species by preventing the interbreed- 

 ing of modified and unmodified varieties. Still no 

 one but Wagner would claim for it any thing more 

 than a subordinate position. 



Meehan has suggested that a practical isolation of 

 individuals occurs in the extreme limits of the distri- 

 bution of any species. Consider, for instance, any 

 plant extending in great numbers over a large terri- 

 tory. In the centre of this region, so numerous will 

 be the individuals that any variation will be im- 

 mediately checked by cross-breeding. But upon the 

 boundary of the territory the individuals will be 

 so few in numbers that the offspring of the same in- 

 dividual will breed with each other, and thus would 



