GEOGRAPHICAL DISTRIBUTION. \^1 



and breaking tlirougli 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. Wlien these mimdations 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 Hmits. 



Another and very different influence of rivers in 

 their 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 growtli of plants in the 



