84: EVOLUTION OF LIVING OEGAMSMB. 



and the strong scents given off by female hawk-moths, 

 male musk-deer and stags, and other animals at the time 

 of maturity. Conspicuous sexual differences are often 

 produced by the development in one sex of special organs 

 to receive stimuli from the other, as for instance in the 

 Crustacea and Insecta, where the olfactory organs, the 

 antennae, may be greatly enlarged and modified in the 

 males. 



But there are many other secondary sexual characters 

 the use of which is by no means so obvious such as the 

 beautiful patterns and colours developed in male butter- 

 flies and other insects, the marvellous wealth of orna- 

 mental colours and plumage in birds, and offensive 

 weapons like the antlers of deer and it is one of Dar- 

 win's greatest triumphs to have g^en us in the theory 

 of sexual selection a rational explanation of the evolu- 

 tion of these apparently useless characters. No hard 

 and fast line can be drawn between natural and sexual 

 selection. Sexual selection may be considered as sub- 

 ordinate to natural selection, as a special kind of natural 

 selection taking place within the limits of an interbreed- 

 ing set of animals or "species." It is due to the com- 

 petition between individuals of one sex for the possession 

 of the other. Almost always it is the males which com- 

 pete for the females, either because they are more numer- 

 ous or because they are polygamous. This kind of selec- 

 tion takes place only among highly organised animals, 

 and is almost entirely restricted to the vertebrates and 

 arthropods (insects and spiders). It has been aptly 

 called the struggle for wife as opposed to the struggle for 

 life ; but failure means, in the end, extinction in the one 

 case as in the other. 



Darwin has convincingly shown how severe is the com- 

 petition between the rival males, how universal is the 

 law of battle, leaving the most agile the strongest and 

 the best-armed in possession of the field. The general 

 superiority of the male sex in strength, pugnacity, and 



