ANIMAL LIFE AS A GEOLOGICAL AGENCY. §3 
a brutal triumph to professedly civilized hunters—if the herds of 
the American bison, which are numbered by hundreds of thou- 
sands, do not produce visible changes in the forms of terrestrial 
surface, they have at least an immense influence on the growth 
and distribution of vegetable life, and, of course, indirectly 
upon all the physical conditions of soil and climate between 
which and vegetation a mutual interdependence exists. 
In the preceding chapter I referred to the agency of the 
beaver in the formation of bogs as producing sensible geograph- 
ical effects. 
I am disposed to think that more bogs in the Northern States 
owe their origin to beavers than to accidental obstructions of 
rivulets by wind-fallen or naturally decayed trees; for there 
are few swamps in those States, at the outlets of which we may 
not, by careful search, find the remains of a beaver dam. The 
beaver sometimes inhabits natural lakelets and even large rivers 
like the Upper Mississippi, when the current is not too rapid, but 
he prefers to owe his pond to his own ingenuity and toil. The 
reservoir once constructed, its inhabitants rapidly multiply so 
long as the trees, and the harvests of pond lilies and other 
aquatic plants on which this quadruped feeds in winter, suffice 
for the supply of the growing population. But the extension 
of the water causes the death of the neighboring trees, and the 
annual growth of those which could be reached by canals and 
floated to the pond soon becomes insufficient for the wants of 
the community, and the beaver metropolis now sends out expe- 
ditions of discovery and colonization. The pond gradually fills 
up, by the operation of the same causes as when it owes its exist- 
ence to an accidental obstruction, and when, at last, the original 
settlement is converted into a bog by the usual processes of 
vegetable life, the remaining inhabitants abandon it and build 
on some virgin brooklet a new city of the waters.* 
* T find confirmation of my own obseryations on this point (published in 
1863) in the Worth- West Passage by Land of Milton and Cheadle, London, 1865, 
These travellers observed ‘‘ a long chain of marshes formed by the damming 
up of a stream which had now ceased to exist,” Chap. X. In Chap. XII, 
