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 muchas land animalsare. 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. Ifa 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 
