SOCIAL BACKBONED ANIMALS 139 



earth, accompanied with occasional overflows from the stream. 

 The evidence from these and other sources tends to show that 

 these dams have existed in the same places for hundreds and 

 thousands of years, and that they have been maintained by a 

 system of continued repairs. 



" At the place selected for the construction of a dam the 

 ground is usually firm and often stony, and when across the 

 channel of a flowing stream, a hard rather than a soft bottom is 

 preferred. Such places are necessarily unfavourable for the in- 

 sertion of stakes in the ground, if such were, in fact, their practice 

 in building dams. The theory upon which beaver-dams are con- 

 structed is perfectly simple, and involves no such necessity. Soft 

 earth, intermixed with vegetable fibre, is used to form an em- 

 bankment, with sticks, brush, and poles imbedded within these 

 materials to bind them together, and to impart to them the re- 

 quisite solidity to resist the effects both of pressure and of satura- 

 tion. Small sticks and brush are used, in the first instance, with 

 mud and earth and stones for down-weight. Consequently these 

 dams are extremely rude at their first commencement, and they 

 do not attain their remarkably artistic appearance until they have 

 been raised to a considerable height, and have been maintained 

 by a system of annual repairs for a number of years." 



The beaver-house or "lodge" is a domed structure, con- 

 structed of clay and wood on the upper side of the dam. A 

 lodge is from 6 to 8 feet high, and its base is 9 to 12 feet broad, 

 or rather more. The living-room within, which also serves as 

 a " nursery ", is lined with grass, and its floor is at water-level. 

 There are several entrances which open well below the surface, 

 so that there is no danger of blockage by ice during the winter. 

 The beaver does not hibernate, and its pond is deep enough to 

 enable it to swim about under the surface of the ice in the most 

 rigorous seasons. The lodge includes a store-chamber, in which 

 large quantities of the succulent rhizomes of certain water-lilies 

 are heaped up for winter use. Tree-branches are also accumu- 

 lated in the deeper parts of the pond for the same purpose. 

 Martin (in Castorologia) makes the following interesting remarks 

 regarding the popular misconception as to the highly-finished 

 character of the lodge, and also as to the way in which it has 

 evolved from the simple burrow: " The beaver-lodge is gener- 

 ally included in the list of marvels reserved for the investigation 



