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 
