106 GENERAL ZOOLOGY 



the struggle for existence by mutual aid and cooperation. 

 Prince Kropotkin has recently called attention to cases of 

 mutual aid in the animal kingdom. He says : " As soon as 

 we study animals not in laboratories and museums only, 

 but in the forest and the prairie, in the steppe and the moun- 

 tain we at once perceive that though there is an immense 

 amount of warfare and extermination going on amidst vari- 

 ous species, and especially amidst various classes of animals, 

 there is, at the same time, as much, or perhaps even more, of 

 mutual support, mutual aid, and mutual defense amidst ani- 

 mals belonging to the same society. Sociability is as much a 

 law of nature as mutual struggle." Illustrations may be seen 

 in the complicated communities of the ants,Jbees, and wasps 

 among the insects ; others occur among the birds and fur- 

 bearing animals (see Chapters XXIX and XXXI). 



Sexual Selection. The principle of sexual selection, also 

 formulated by Darwin, is an extension of the principle of 

 selection to account for the secondary sexual characters which 

 exist in many animals. In most insects, where there is sexual 

 dimorphism, the male, though usually smaller, is more brightly 

 colored ; it is armed or ornamented with spines, which the 

 female does not possess, or it has special sound-producing 

 'organs. In the common stag-beetle (Luca'nus, Fig. 56) the 

 mandibles of the male are of larger size than those of the 

 female. Among the birds, in those cases where the sexes are 

 differently colored, the males are usually more brilliant ; they 

 often have spurs, wattles, crests, or plumes, while the females 

 are without these structures, or have them in less degree. 

 It is only the male birds, too, which possess the gift of song. 

 Among the fur-bearers special characteristics, such as horns, 

 antlers, and tusks, often occur. These various secondary 

 sexual differences are ascribed by Darwin to sexual selection, 

 which "depends, not on a struggle for existence in relation 

 to other organic beings or to external conditions, but on a 



