THE ORIGIN OF SPECIES AND GENERA 273 



Lake Tanganyika offer us a striking example of 

 the extent of these divergences derived from 

 geographical isolation. Several of the genera of 

 freshwater shells peculiar to this lake remind one 

 by their external form of certain genera of marine 

 Molluscs, such as Trochus, Turbo, Littorina, etc., 

 to such a degree that it has sometimes been main- 

 tained that we are really dealing with a residual 

 marine fauna enclosed by a continental depression, 

 and adapted by degrees to waters becoming gradu- 

 ally less salt. It seems more probable that the 

 genera of the Tanganyika should be considered 

 as very divergent Melanidse, that is to say, as types 

 of fresh or slightly brackish waters, separated for 

 several geological periods from their congeners, and 

 having acquired through isolation very specialized 

 characteristics giving them the value of genera 

 and perhaps even of distinct families. 



In the case of animals of marine habitat the 

 conditions of isolation are more difficult to realize 

 than in the case of terrestrial and freshwater 

 animals, so that the divergences which separate 

 regional forms are, as a rule, much less marked. 

 This constantly observed fact is a very solid 

 argument in favour of the influence of geographical 

 isolation on the formation of species and genera 

 by means of lateral divergence. 



But this influence of isolation, important as it 

 may be in the creation of new phyletic branches, 

 is certainly not exclusive of other causes of varia- 

 tions still more speedy in their effects. I refer to 

 the phenomenon of abrupt variation, or saltation, 

 to which the attention of naturalists has so forcibly 



