ANIMAL LIFE AND EVOLUTION 347 



Another kind of association among animals is that based on a 

 greater or lesser degree of mutual aid derived from this associa- 

 tion. It appears in two types, one exemplified by com- 

 mensalism and symbiosis, in which two different kinds of 

 animals live together temporarily or permanently to their 

 mutual benefit or at least to the benefit of one kind and 

 little or no injury to the other, and the other exemplified by a 

 gregarious and social or communal life, in which a number of 

 individuals of the same kind live together in temporary or 

 permanent bands or in large family communities. 



The first type is illustrated by the hermit crabs that carry 

 colonies of hydroid polyps on their shells, the little crab being 

 protected from enemies by the stinging threads of the polyps, 

 while the sessile polyps gain an advantage by the crab's 

 movements and probably by getting small bits from the crab's 

 clumsy tearing up of its own food. Especially developed is 

 the commensal relation between certain insects called ant- 

 guests and the ants which serve as their hosts. Over 1500 

 species of insects, mostly small beetles and flies, live in the 

 nests of various ants, some as robbers, some as tolerated para- 

 sites, and some as desired commensals. These latter provide 

 the ants with certain kinds of special food produced by their 

 bodies, and aid in cleaning both the nests and the ants them- 

 selves, while they in turn receive food from the storerooms of 

 the ants, or even by regurgitation from their mouths. 



Of the second type the life of the communities of the social 

 wasps and bees and ants, already described in some detail 

 elsewhere in this book, is the best example. The highly 

 specialized communities of the honey-bee and of the ants are 

 a splendid illustration of the possibilities and great advantages 

 of adaptations based on the principle of mutual aid. It is 

 of course the extreme development of the mutual aid factor 

 in human life that makes man the highly successful and noble 

 animal he is. 



