110 THE EVOLUTION THEORY 



ditions, but must gradually disappear. We can understand, then, that 

 the primitive form of the Papilio merope female may persist even for 

 a long time side by side with the mimetic form in certain habitats. 

 It is, probably, not a mere chance, that this should have happened 

 just in Abyssinia, for, in that region, the mimetic female is still 

 tailed — that is, she has not yet reached the highest degree of resem- 

 blance to her immune model. In the whole of the rest of Africa the 

 process of the transformation of the female has already reached its 

 highest point, and on the east and west coasts, as well as in South 

 Africa, the primitive form of the species is now represented only by 

 the male. 



The gradual dying out of the less favourably conditioned forms 

 of a species is a law which follows as a logical necessity from the 

 essence of the process of selection, but its reality may be inferred 

 from the phenomena themselves. On it depends, as far at least as 

 adaptations are concerned, the transformation of species. 



A beautiful example of the crowding out of a less favoured form 

 of a species by a more favoured one is afforded by a butterfly of 

 North America, of which the two female forms have long been 

 known, although the reason for their dimorphism was not understood. 

 A yellow butterfly, Papilio turnus, not unlike our swallow-tail, has 

 yellow females in the north and east of the United States, but black 

 ones in the south and west. There was much guessing as to what 

 the cause of this striking phenomenon might be, and it was for 

 a time thought that this difference was directly due to the influence 

 of climate, and, later, the black form of female was regarded as 

 protectively coloured, because of the supposed greater persecution 

 b}^ birds in the south, since the female would be less easily recog- 

 nized if of a dark colour, and would thus be better protected. This 

 last explanation could hardly be looked upon as satisfactory, for 

 a black butterfly in flight would be very easily seen by sharp-sighted 

 birds; indeed, against a light background, it would be even more 

 readily seen than a light one. 



Since we have acquired a more exact knowledge of the immune 

 species of Papilio this case has become clear to us. For on those 

 stretches of country on which the black female of Papilio turnus 

 lives there occurs another Papilio which is black in both sexes, 

 Papilio pMlenor, and this is one of those species which are protected 

 by their unpleasant taste and odour. Here, therefore, we have a case 

 of mimicry, the female of Papilio turnus imitates the immune Papilio 

 philenor, and thereby secures protection for itself ; but as the immune 

 model only occurs in the southern half of the distribution of Papilio 



