158 A BOOK OF INSECTS 



connected variations show that it is at least possible for 

 very elaborate colour specialisations to arise, in process of 

 time, from very small beginnings — as Darwin pointed out 

 many years ago in regard to the feathers of the peacoek. 

 Variableness is, as it were, the motive force behind the 

 evolution of all living things ; and it is only as this force 

 is restrained, harnessed, driven along a definite course by 

 the law of natural selection that evolution becomes 

 possible. It is easy, therefore, to imagine how those 

 females of H. misippus which originally approached most 

 nearly to the warningly coloured models were benefited 

 in the struggle for existence, and how in the end they 

 became firmly established to the exclusion of all other 

 forms ; while in the case of H. bolina we are free to 

 believe that an identical process of selection is even now 

 in progress. 



Equally interesting is the case of certain swallow-tail 

 butterflies which are confined to Africa and Madagascar. 

 They are usually spoken of collectively as Papilio dardanus, 

 though there are many local forms, or sub-species, each of 

 which has been accommodated with a name. The males 

 have always angular, sulphur-yellow wings, marked with 

 black, while each hind-wing ends in a long tail. The 

 females, however, usually have rounded wings, without 

 tails, while their colours resemble those of various dis- 

 tasteful butterflies which abound in the same localities. 

 This is the rule. But the Madagascan representatives of 

 these butterflies, and one or two East African forms, have 

 black-and-yellow tailed females which are almost identical 

 with their males in shape, colour, and pattern. Why have 

 these isolated races failed to participate in the scheme of 

 mimicry from which their congeners presumably derive 

 benefit ? The question is not easy to answer. Yet in the 

 absence of contradictory evidence we may assume that 



