228 THE INDUSTRIES OF ANIMALS. 



the animal from pursuit and capture, the surface- 

 level of the pond must, to a greater or less extent, be 

 subject to their immediate control. As the dam is 

 not an absolute necessity to the beaver for the main- 

 tenance of his life, his normal habitation being rather 

 natural ponds and rivers, and burrows in their banks, 

 it is, 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. 



" Some of these dams are so extensive as to forbid 

 the supposition that they were the exclusive work of 

 a single pair, or of a single family of beavers ; but it 

 does not follow, as has very generally been supposed, 

 that several families, or a colony, unite for the joint 

 construction of a dam. After careful examination of 

 some hundreds of these structures, and of the lodges 

 and burrows attached to many of them, I am alto- 

 gether satisfied that the larger dams were not the 

 joint-product of the labour of large numbers of 

 beavers working together, and brought thus to im- 

 mediate 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 resi- 



