166 OKIGIN OP LIFE IN AMERICA 



instructive case of convergence. The two very distinct forms 

 have converged because of the similarity of their environ- 

 ment, and especially owing to the absence of those elements 

 in their environment that lead to external protective 

 adaptations. 



The family Amblyopsidae is confined to North America, 

 and its nearest relations are no doubt the Poeciliidae, a group 

 of fresh-water fishes with a wide distribution in America, 

 southern Europe, Asia and Africa. There are therefore no 

 grounds for the supposition that the blind fishes of the sub- 

 terranean waters of North America are descended from 

 marine ancestors. 



