SOCIAL BACKBONED ANIMALS 137 



quite possible that the habit of bark-feeding was the original one, 

 and that the use of the wood for constructive purposes has been 

 evolved subsequently. The process described provides for the 

 framework of a dyke, and twigs, &c., are added, together with 

 much earth. The nature of the work is adapted to the local 

 conditions: in the case of brooks or large streams with ill-marked 

 banks there is a "stick-dam", in which the earth is of compara- 

 tively small amount; where the water is deeper and the banks 

 clearly defined there is a " solid-bank dam", with enough earth 

 to cover up all the woody framework. In the former case no 

 special channel is made through which the water may make its 

 way, while in the latter case a suitable exit is scooped out on 

 the top of the dyke. In order the better to resist water-pressure, 

 the upper side of the barrier is sloping; its lower surface is ap- 

 proximately vertical. The foundation is from about 9 to 12 

 feet, the upper part about 2 feet in thickness. The dam is built 

 straight across if the current is gentle, while the greater pressure 

 existing in a rapid stream is compensated by making the work 

 curved, with its convex side facing upwards. The following 

 careful account of this preliminary engineering work is given 

 by Lewis H. Morgan (in The American Beaver and his Works], 

 and it will be noted that he does not favour the common view 

 that several or a large number of families co-operate for the 

 benefit of the "village": "The dam is the principal structure 

 of the beaver. It is also the most important of his erections 

 as it is the most extensive, and because its production and pre- 

 servation could only be accomplished by patient and long-con- 

 tinued labour. In point of time, also, it precedes the lodge, 

 since the floor of the latter and the entrances to its chamber are 

 constructed with reference to the level of the water in the pond. 

 The object of the dam is the formation of an artificial pond, 

 the principal use of which is the refuge it affords to them when 

 assailed, and the water connection it gives to their lodges and 

 to their burrows in the banks. Hence, as the level of the pond 

 must, in all cases, rise from one to two feet above these entrances 

 for the protection of 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 ab- 

 solute necessity to the beaver for the maintenance of his life, 

 his normal habitation being rather natural ponds and rivers, and 



