494 The Outline of Science 



deep. They usually communicate between clumps of trees and 

 the pond above the dam, but they may form a short cut between 

 two loops of the river, or they may go right through an island. 

 In the last case the work would not be justified until there was an 

 open waterway from end to end. In some other cases a moist 

 roadway between the pond and a pool in the wood might be gradu- 

 ally converted into a canal. Instances of "locks" have been 

 recorded, but there is a tendency to forget that animals are more 

 likely to take advantage of what exists or is hinted at in Nature 

 than to discover new ideas or principles ! 



Beavers are notably gregarious, for there may be many 

 lodges near a suitable wood. When there is overcrowding a mi- 

 gration occurs, the old houses being left to related new couples. 

 Isolated males are often found, and some naturalists say that 

 these have been expelled from the community for laziness or mis- 

 behaviour. There are no beavers left in Britain, but they flourish 

 in Russia, in Siberia, and in Canada and other parts of North 

 America. It is interesting to notice that in many places from 

 which beavers have been gone for centuries, evidences of their 

 work remain as "beaver-meadows" and the like. 



Mutual Aid 



Prince Kropotkin did a notable service in his book, Mutual 

 Aid, a Factor in Evolution (1904), for he showed in a scholarly 

 way the frequency of gregariousness, combination, co-operation, 

 and sociality among animals. One answer-back that pays in the 

 struggle for existence is to sharpen teeth and claws, i.e., to inten- 

 sify competition ; but another successful answer-back is to practise 

 mutual aid. Even the individualistic carnivores may form packs 

 as in the case of wolves and jackals; but there is more elaboration 

 among the grazing herds. All kinds of beasts and birds of prey 

 have proved powerless against the colonies of Russian sousliks. 

 Combination gives strength to the sociable musk-rats of North 

 America and to the prairie-dogs. 



