186 



GENERAL ZOOLOGY 



believe that but a single method produces a given result. 

 It seems entirely probable that natural selection has played 

 an important part in organic evolution. It is at the same 

 time true that natural selection cannot explain all the changes 

 that have taken place in the various types of animals. 



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 



Fig. 100. Stag beetles (male left, female right). (Natural size) 



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 (Lu- 

 ca'nus, Fig. 100) 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 struc- 

 tures, 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 



