Evidences of Physiological Selection. 95 



gamy, i. e. not only geographical isolation, but also 

 by sexual preference in pairing, and the several 

 other forms of homogamy, which Mr. Gulick has 

 shown to arise in different places as the result of 

 intelligence. 



Evidence from Special Cases. 



Hitherto I have been considering, from the most 

 general point of view, the most widespread facts 

 and broadest principles which serve to substantiate 

 the theory of physiological selection. I now pass 

 to the consideration of one of those special cases in 

 which the theory appears to have been successfully 

 applied. 



Professor Le Conte has adduced the fossil snails 

 of Steinheim as serving to corroborate the theory of 

 physiological selection \ 



The facts are these. The snail population of this 

 lake remain for a long time uniform and unchanged. 

 Then a small percentage of individuals suddenly began 

 to vary as regards the form of their shells, and this in 

 two or three directions at the same time, each affected 

 individual, however, only presenting one of the varia- 

 tions. But after all these variations had begun to 

 affect a proportionally large number of individuals, 

 some individuals occur in which two or more of the 

 variations are blended together, evidently, as Weis- 

 mann says, by intercrossing of the varieties so blended. 

 Later still, both the separate varieties and their 

 blended progeny became more and more numerous, 

 and eventually a single blended type, comprising 

 in itself all the initial varieties, supplanted the 



' Evolution and its Relations to Prlis^ous Thought, &c. pp. 2;?6-7. 



