294 ANIMAL LIFE 



covers a wider range of climate. Kentucky has a richer 

 fauna than Iowa because it includes a greater variety of 

 conditions. New England was called by Professor Agassiz 

 a " zoological island," because of the relatively small num- 

 ber of its native animals, especially of species inhabiting its 

 rivers. The cause of this is found in its isolation, being 

 shut off from the Middle States by mountain ranges, while 

 it is bounded on two sides by the sea. 



156. Barriers affecting fresh-water animals. The animals 

 inhabiting fresh-water streams are affected by differences in 

 temperature and elevation much as land animals are. They 

 tend to spread from stream to stream whenever they can 

 find their way. An isolated stream is likely to have its 

 peculiar fauna just as island life is likely to differ from 

 that of the mainland. The same species wanders widely 

 within the limits of a single river basin. If a kind of fish 

 establishes itself anywhere in the Mississippi Valley, it may 

 find its way to every stream in the whole basin. If it likes 

 cold spring water, as the rainbow-darter does, we may look 

 for it in any cold spring. If, like the long-eared sun-fish, 

 it frequents deep pools in the brooks, we may look for it 

 under roots of stumps and in every " swimming hole." If, 

 like the channel-cat, it chooses the ripples of a river, we 

 may fish for it wherever ripples are. The larger the whole 

 river basin the more species find their way into it, and 

 therefore the greater the number of species in any one of 

 its streams. 



Each species finds its habitat fitted to its life, and then 

 in turn is forced to adapt itself to this habitat. Any other 

 kind of habitat then appears as a barrier to its distribu- 

 tion. Thus to a fish of the ripples a stretch of still water 

 becomes a barrier. A species adapted to sandy bottoms 

 will seldom force its way through swift waters or among 

 weeds or rocks. The effect of waterfalls as barriers is else- 

 where noticed. In some streams the dam made by a colony 

 of beavers has the same effect. Mill-dams and artificial 



