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 



