166 
ORIGIN OF 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. 
