THE BEAVERS OF NORTH AMERICA 39 
that the pond would be lowered and the entrances 
to the lodges exposed, perhaps even the beaver 
would be left without any place of retreat. The 
subsidiary dams would greatly lessen the dangers 
as they would retain the water to a greater or lesser 
depth according to the conditions. There would 
be still another advantage in holding back the 
water, as it would make the repair work or 
rebuilding of the main dam a matter of much less 
difficulty owing to the decreased force of the 
current. Taking all things into consideration, we 
can see how important are these secondary or 
supporting dams and how greatly they reflect 
credit on the animal’s power of reasoning and 
application of this power. For by doing what 
apparently is a vast amount of extra and, at first 
glance, almost unnecessary work, the beaver is 
taking steps to prevent a possible catastrophe. 
Surely he must reason this out, for otherwise how 
would he know that great rains do come, and even 
the greatest dams do burst. Does he learn it by 
seeing some of the very small structures give way 
under pressure slightly more than normal? But 
who shall answer these questions ? 
In building the smaller dams, the method of 
construction differs in no way from that already 
described ; only in point of size are they different. 
As a rule they are not large, seldom more than 
about thirty feet in length. Exceptions there are 
to this, but speaking generally, they are below this 
