er careful examination of some hundreds of
these structures, and of the lodges and burrows attached to many of
them, I am altogether 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 exhausted
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, commenced 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 accommodate more families than one, other families took up their
residence 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 ar