37-1 ANIMAL INTELLIGENCE. 



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. 



In external appearance there are two distinct kinds of 

 dams, although all are constructed on the same principle. 

 One, the more common, is the ' stick dam,' which is com- 

 posed of interlaced stick and pole work upon the lower 

 face, with an embankment of earth mixed with the same 

 materials on the upper face. The other is the ' solid-bank 

 dam,' which differs from the former in having much more 

 brush and mud worked into its construction, especially 

 upon its surfaces ; the result being that the whole formation 

 looks like a solid bank of earth. In the first kind of dam 

 the surplus water percolates through the structure along 

 its entire length ; but in the second kind the discharge 

 takes place through a single furrow in the crest, which, 

 remarkable though the fact unquestionably is, the beavers 

 intentionally form for this purpose. 



In the construction of the dam, stones are used here 

 and there to give down-weight and solidity. These stones 

 weigh from one to six pounds, and are carried by the 

 beavers in the same way as they carry their mud namely, 

 by walking on their hind legs while holding their burden 

 against the chest with their fore-paws. The solid dams 

 are much firmer in their consistence than the stick dams ; 

 tor while a horse might walk across the former, the weight 

 of a man would be too great to be sustained by the latter. 

 Each kind of dam is adapted to the locality in which it is 

 built, the difference between the two kinds being due to 

 the following cause. As a stream gains water and force in 

 its descent, it develops banks, and also a broader and 

 deeper channel. These banks assume a vertical form in 

 the level areas where the soil is alluvial. Tnus, an open 

 stick-work dam could not in such, places be led off 

 from either bank ; and even if it could, the force and depth 

 of the stream would carry it away. Therefore in such 

 places the beavers build their solid-bank dams, while in 

 shallow and comparatively sluggish waters they content 



