RELATIONS AND SURROUNDINGS. 137 



the female, in those species where monogamy prevails. 

 The thought is that they displace their rivals not by 

 strength, but by diplomacy. 



153. The individuals of one species of animals may 

 often be indifferent to the presence of another. Their 

 relation is merely that of a kind of competition for food 

 and air and water. 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. Starting with this as 

 the simplest relationship, we may pass by gradual stages 

 to the most intimate friendships on the one hand and 

 to the most bitter antagonism on the other. Probably 

 to every species there are some other species that are 

 friendly, some that are harmful, and some that do not di- 

 rectly influence it one way or the other. Man is no excep- 

 tion to this statement . There is no truth in the idea that all 

 other animals \vere made for him and are of some use 

 to him. 



154. Friendly Relations Among Different Species. 



Occasionally we find animals of different species forming 

 a kind of partnership. An instance of this may be seen 

 in the association of the prairie dog and a species of owl 

 in the same burrow. There are hundreds of species of 

 insects which live in the nests of ants. Each member 

 of these associations can live without the other, but for 

 some reason they usually occur together. This is known 

 as commensalism or " mess-mateism." 



A somewhat more intimate relationship is found between 

 the ants and the aphides or plant-lice. The ants capture 

 the aphides and either nourish them outright or place them 

 on plants of which the lice are fond, and in return the 

 ants get the sweet secretion produced by the aphides. In 



