DWELLINGS. 229 



dence upon it, and afterwards contributed by their 

 labour to its maintenance. There is no satisfactory 

 evidence that the American beavers either live or 

 work in colonies ; and if some such cases have been 

 observed, it will either be found to be an exception 

 to the general rule, or in consequence of the sudden 

 destruction of a work upon the maintenance of which 

 a number of families were at the time depending. 



" The great age of the larger dams is shown by their 

 size, by the large amount of solid materials they 

 contain, and by the destruction of the primitive forest 

 within the area of the ponds ; and also by the extent 

 of the beaver-meadows along the margins of the 

 streams where dams are maintained, and by -the 

 hummocks formed upon them by and through the 

 annual growth and decay of vegetation in separate 

 hills. These meadows were undoubtedly covered 

 with trees adapted to a wet soil when the dams were 

 constructed. It must have required long periods of 

 time to destroy every vestige of the ancient forest by 

 the increased saturation of the earth, accompanied 

 with occasional overflows from the streams. The 

 evidence from these and other sources tends to show 

 that these dams have existed in the same places for 

 hundreds and thousands of years, and that they have 

 been maintained by a system of continuous repairs. 



" At the place selected for the construction of a dam, 

 the ground is usually firm and often stony, and when 

 across the channel of a flowing stream, a hard rather 

 than a soft bottom is preferred. Such places are 

 necessarily unfavourable for the insertion of stakes 

 in the ground, if such were, in fact, their practice in 

 building dams. The theory upon which beaver-dams 

 are constructed is perfectly simple, and involves no 



