Phenomena of Variation and Geographical Distribution, 173 



sexual selection. For instance, in most polygamous animals the males 

 fight foi' the possession of the females, and the victors, always becom- 

 ing the progenitors of the succeeding generation, impress upon their 

 male offspring their own superior size, strength, or unusally developed 

 offensive weapons. It is thus that we can account for the spurs and the 

 superior strength and size of the males in gallinaceous birds, and also 

 for the large canine tusks in the males of the fruit-eating Apes. So 

 the superior beauty of plumage and special adornments of the males of 

 so many birds can be explained by supposing (what there are many 

 facts to prove) that the females prefer the most beauty of plumage and 

 perfect-plumaged males, and that thus slight accidental variations of 

 form and colour have been accumulated till they have produced the 

 wonderful train of the peacock, and the gorgeous plumage of the bird 

 of paradise. Both these causes have no doubt acted partially in insects, 

 so many species possessing horns and powerful jaws in the male sex 

 only, and still more frequently the males alone rejoicing in rich colors 

 or sparkling lusti-e. But there is here another cause which has led to 

 sexual differences, viz., a special adaptation of the sexes to diverse 

 habits or modern life. This is well seen in female butterflies (which 

 are generally weaker and of slower flight), often having colors better 

 adapted to concealment ; and in certain South American species {Fapilo 

 torquatus) the females, which inhabit the forest, resemble the jEneas 

 group, which abound in similar localities, while the males, which 

 frequent the sunny, open river-banks, have a totally different coloration. 

 In these cases, therefore, natural selection seems to have acted inde- 

 pendently of sexual selection ; and all such cases may be considered 

 as examples of the simplest dimorphism, since the offspring never offer 

 intermediate varieties between the parent forms. 



The distinctive character therefore of dimorphism is this, that the 

 union of these distinct forms does not produce intermediate varieties, 

 but reproduces them unchanged. In simple varieties, on the other 

 hand, as well as when distinct local forms or distinct species are crossed, 

 the offspring never resemljles either parent exactly, but is more or less 

 intermediate between them. Dimorphism is thus seen to be a 

 specialized result of variations, by which new physiological phenomena 

 have been developed ; the two should therefore, whenever possible, be 

 kept separate.'!' 



* The phenomena of diviorphism a.nd polymorphism may be well illustrated by supposing that 

 a blue-eyed, flaxen-haired Saxon man had two wives, one a black-haired, red-skinned Indian 

 squaw, the other a wooly-headed, sooty-skinned negress — and that instead of the children 

 being mulattoes of brown or dusky tints, mingling the separate characteristics of their parents 

 in varying degrees, all the boys should be pure Saxon boys like their father, while the girls 

 should altogether resemble their mothers. This would be thought a sufficiently wonderful 



