ANIMAL LIFE AS A GEOLOGICAL AGENCY. 81 



-n Lis ecorKest known stages of exi>n'iice, was probably a destruc- 

 tive power upon the earth, though perhaps not so emphatically as 

 are his present representatives. 



The larger wild animals are not now numerous enough in any 

 one region to form extensive deposits by their remains ; but they 

 have, nevertheless, a certain geographical importance. If the > 

 myriads of large browsing and grazing quadrupeds which wander i 

 over the plains of South Africa — and the slaughter of which by I 

 thousands is the source of a ferocious pleasure and a brutal tri- 

 umph to professedly civilized hunters — if the herds of the Ameri- ' 

 can bison, which are numbered by hundreds of thousands, 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 hfe^ and, of course, indirectly upon all the physical 

 conditions of soil and climate between which and vegetation a I 

 mutual interdependence exists. 



In the preceding chapter I referred to the agency of the beaver 

 in the formation of bogs as producing sensible geographical 

 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 inliabits natural lakelets and even large rivers like the 

 Upper Mississippi, when the current is not too rapid, but he pre- 

 fers to owe his pond to his own ingenuity and toil. The reser- 

 voir once constructed, its inhabitants rapidly multiply so long as 

 the trees, and the harvests of pond lihes 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 expeditions of discovery and 

 colonization. The pond gradually fills up, by the operation of 

 the same causes as when it owes its existence to an accidental ob- 

 struction, and when, at last, the original settlement is converted 



into a bog by the usual processes of vegetable life, the remaining 

 4* 



