GEOGRAPHICAL DISTRIBUTION. 131 
and breaking through every barrier, it spreads out into a 
vast inland sea. The body of water is then so great, 
that weeks elapse before it can be drained off through 
the usual channels, and the country remains during this 
time covered with it. When these inundations are thus 
excessive and long continued, which usually only hap- 
pens after intervals of years, they must in a great de- 
gree destroy animal life, and reduce, if they do not ex- 
terminate, the species most exposed to them. Thus the 
undue extension of these animals is checked in a region 
otherwise particularly adapted to their increase, and 
they are kept within more restricted numerical limits. 
Another and very different influence of rivers in 
then- ordinary action is to bring down into the plains 
and lowlands, and thus aid in distributing, the species 
occurring in the more elevated regions. We may 
suppose that a species, having by its own powers at- 
tained the summit of a range of mountains, may, when 
aided accidentally by the current of rivers, be very 
rapidly diffused through the country on the other side ; 
and thus a much shorter period of time be required 
for its extension than would have been necessary under 
other circumstances. 
Fires. It was the custom of the aboriginal inhabit- 
ants of the country to burn over, annually, large 
tracts of land, by setting fire to the rank, dry grass 
and herbage on the prairies and in the more open parts 
of the woods. This practice, repeated every autumn, 
insured a fresh and luxuriant growth of plants in the 
