BUILDING INSTINCT OF BEAVER. 537 



Sylvia sutoria, or Tailor-bird, a little eastern bird allied to 

 our linnet ; which, by the aid of filaments of cotton drawn 

 from the cotton-plant, sews leaves together with its beak and 

 feet, in such a manner as to conceal the nest which they 

 enclose from the observation of its enemies (fig. 279). 



706. The association of a number of individuals of a certain 

 species, for the performance of labours in which they all unite 

 to one common end, is another most remarkable example of 

 the operation of instinct. Several Mammals exhibit this 

 tendency in a greater or less degree ; but the most interesting 

 of all, in this point of view, is the Beaver (fig. 280), which is 



Fig. 280. BEAVER. 



now chiefly found in Canada, though it formerly abounded 

 on the Continent of Europe. During the summer it lives 

 solitarily in burrows, which it excavates for itself on the 

 borders of lakes and streams ; but as the cold season ap- 

 proaches, it quits its retreat, and unites itself with its fellows, 

 to construct, in common with them, a winter residence. It is 

 only in the most solitary places that their architectural in- 

 stinct fully developes itself. Having associated in troops of 

 from two to three hundred each, they choose a lake or river 

 which is deep enough to prevent its being frozen to the 

 bottom ; and they generally prefer running streams, for the 

 sake of the convenience which these afford in the transporta- 

 tion of the materials of their erection. They begin by 

 constructing a sloping dam, whereby the water is kept-up to 



