45 
of a race wMcli once occupied every river and sea. We 
may attribute their diminution to the introduction and ulti- 
mate predominance of more favoured forms. In the open 
seas the Ganoids can no longer hold their own, but a few 
species, representing, as if by accident, diverse types of 
structure, linger on in isolated river-basins, protected from 
extermination by the mountains and seas which now enclose 
them, and at the same time limit the area within which 
destructive competitors may arise. 
