THE BEAVER. 135 



existence depends ; indeed, the bark on which lie lives requires 

 to be moistened before it becomes fit for food. When instinct 

 teaches a colony of beavers that the water is not of sufficient 

 depth to escape freezing throughout, they provide against the 

 evil by making such a dam as has been mentioned, across the 

 stream, or the outlet of the lake, at a convenient distance from 

 their habitations. The plan of these dams varies according to 

 the character of the lake or stream. If the current is but slight, 

 they build the dam almost straight ; but where the water runs 

 at a rapid rate, it is almost always constructed with a consider- 

 able curve, the convex side towards the stream. Frequently, in 

 such cases, if there is any small island in the centre, it is taken 

 advantage of, and the dam is built out to it from either bank. 

 They make use of a variety of materials ; employing drift- 

 wood when it can be obtained, to save themselves the trouble 

 of cutting down trees. This they tow to the spot, and sink 

 it horizontally with mud and stones. They also employ 

 pieces of green willows, birch, and poplars, intermixing the 

 whole with mud and gravel, in a manner which contributes 

 greatly to the strength of the dam. They observe, however, 

 no order or method in the work, placing their materials as 

 they can obtain them, except that they make the dam main- 

 tain its regular sweep, and form all parts of equal strength. 

 They carry the mud and stones in their fore-paws ; and in 

 one night will collect as much as amounts to many thousands 

 of their little loads. When drift-wood is not to be found, 

 they obtain the timber they require from the groves skirting 

 the lake or pond. To do this, they squat on their hams, 

 and rapidly gnaw through the stems of trees from six to 

 twelve or fourteen inches in diameter, with their powerful 

 incisors. Sometimes a tree will not fall prostrate, the boughs 



