138 ASSOCIATION OF ORGANISMS THE WEB OF LIFE 



burrows in their banks, it is, in itself considered, a remarkable 

 fact that he should have voluntarily transferred himself, by means 

 of dams and ponds of his own construction, from a natural to an 

 artificial mode of life. Some of these dams are so extensive as 

 to forbid the supposition that they were the exclusive work of a 

 single pair, or of a single family of beavers; but it does not 

 follow, as has very generally been supposed, that several families, 

 or a colony, unite for the joint construction of a dam. After care- 

 ful examination of some hundreds of these structures, and of 

 the lodges and burrows attached to many of them, I am alto- 

 gether satisfied that the larger dams were not the joint product 

 of the labour of large numbers of beavers working together, 

 and brought thus to immediate completion; but, on the contrary, 

 that they arose from small beginnings, and were built upon 

 year after year, until they finally reached that size which ex- 

 hausted the capabilities of the location; after which they were 

 maintained for centuries, at the ascertained standard, by constant 

 repairs. So far as my observations have enabled me to form 

 an opinion, I think they were usually, if not invariably, com- 

 menced by a single pair, or a single family, of beavers; and 

 that when, in the course of time, by the gradual increase of the 

 dam, the pond had become sufficiently enlarged to accommo- 

 date more families than one, other families took up their resi- 

 dence upon it, and afterwards contributed by their labour to its 

 maintenance. There is no satisfactory evidence that the Ameri- 

 can beavers either live or work in colonies; and if some such 

 cases have been observed, it will either be found to be an ex- 

 ception to the general rule, or in consequence of the sudden 

 destruction of a work upon the maintenance of which a number 

 of families were 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 all 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 



