124 ZOOLOGY 



in connection with which organs of offense and defense are 

 found. Such are the horns, tusks, spurs, manes, and even the 

 excessive size of the males as compared with the females. In a 

 similar way temperament, courage, fierceness and the like play 

 a part. These qualities depend in part upon the glands of 

 internal secretion. Manifestly the same qualities which make 

 a male a formidable rival to another are likely to be of service 

 to himself, his mates, and his young, and thus to the species, in 

 protecting them from the attack of their enemies among other 

 species. The competition between males is not all of this stress- 

 ful kind, however. It is believed by many naturalists that, in 

 those instances where simple mating rules, those, males with the 

 most striking colors, pleasant voices, and winning ways displace 

 their less favored rivals and thus tend to accumulate by natui'al 

 (sexual) selection the adaptations of this class. 



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 — by which they 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 commensaUsm 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 



