INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 



pared by parental care. Many fishes are known to devour 

 their own young. We have all had occasion to wonder what 

 becomes of the small frogs in a box containing large ones. 

 The struggle between the males for the possession of the 

 females has resulted in the development of many interesting 

 adaptations. The struggle may take the form of actual com- 

 bat 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. 

 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 com- 

 petition between males is not all of this stressful 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 natural (sexual) selec- 

 tion the adaptations of this class. 



154. 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. 



155. 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 



