24 SEXUAL DIMORPHISM 



Again, if the legs on the abdomen of the caterpillar behind 

 the three pairs of thoracic legs are in any way related to the 

 abdominal appendages of the ancestor, it is all the more 

 certain that the maggots of the flies, or of the ants, bees, and 

 wasps, having no legs at all, cannot resemble the ancestor. 

 In such cases, the structure of the larva corresponds to its 

 mode of life, and is much more different from any possible 

 ancestor than is the adult. The individual does not here 

 climb its own genealogical tree, unless it may be said to 

 begin at the top and climb downwards. As for the origin of 

 the modifications in the young stages, we have no evidence 

 that their appearance was independent of the conditions: 

 the fact that the special structure only lasts as long as the 

 special larval habits last, suggests strongly enough the direct 

 dependence of the modifications on the conditions of life. 



I may possibly in the future be able to undertake a more 

 detailed study of the evidence concerning the action of the 

 external conditions of life in producing the differences of 

 structure in different kinships of animals, and in different 

 stages of the individual life. But the present work is de- 

 voted especially to the consideration of the commonest case 

 of Polymorphism, namely, Sexual Dimorphism, and the 

 general aspects of this subject are to be exhibited in the 

 remainder of this Introduction. 



Darwin's theory of Sexual Selection is a logical corollary 

 of his more general theory of Natural Selection. The facts on 

 which it is based cannot be denied. Fierce combats between 

 mature male animals for the possession of the females are 

 known to occur in large numbers of species. The average re- 

 sult of such struggles must be the victory of those individuals 

 best provided with weapons, and with the strength and skill 

 to use them. Rivalry of another kind in the endeavour to 

 please the female and win her admiration and consent, is 

 displayed abundantly in the higher vertebrates generally. 



