122 ZOOLOGY 



158. The individuals of one species of animals may often 

 be practically indifferent to the presence of those of other species. 

 Their relation is simply that of competing for the general food 

 supply and thus assisting in the elimination of the unfit in all 

 species. They may graze in the same pasture, swim in the same 

 pool, or even be parasitic on the same host, and have no other 

 relation. From this as the simplest relationship we may pass 

 by gradual stages to the most intimate friendships and the most 

 bitter antagonism. Every species is indifferent to some and 

 hostile to other of the species which surround it; and man is no 

 exception to the rule. It is a perversion of manifest fact to 

 pretend that all animals are of some use to man. 



159. We have seen that the individuals of a given species 

 are engaged in a struggle among themselves for the means of 

 subsistence, and that in certain cases they form communities 

 or colonies a kind of organic corporation in order to meet 

 more successfully the demands made upon them by their en- 

 vironment. Similar partnerships may be formed by animals 

 of different species. The simplest of these associations are 

 known as commensalism or " mess-mateism," in which the degree 

 of dependence and mutual advantage is perhaps not very great. 

 As instances may be cited the occupancy of the same burrows 

 by the prairie dog and a species of owl; the attachment of 

 barnacles to whales and sharks; the hundreds of species of 

 other insects which live in the nests of ants; the lodging of 

 fishes and other animals in the body-cavity of some of the 

 large tropical sea-anemones or among the tentacles of some 

 of the Hydrozoa. Each member of the association can live 

 without the other, but for some reason they often occur to- 

 gether. The way in which species of rats and mice follow 

 man and occupy his habitations perhaps may be considered 

 as illustrating a similar condition. 



1 60. Symbiosis. Under this term are included even closer 

 relationships between members of different species, where there 

 seems to be a distinct advantage accruing to both members of 

 the partnership sufficient to account for it. The relation 

 of the ants to the aphides or plant-lice which they capture may 



