648 Studies in the Theory of Descent. 



an instance which will even be conceded by Von 

 Hartmann as being only explicable by natural 

 selection, viz., a case of mimicry. 



Supposing that among the South American 

 Heliconiida there occurred a species of Pieris 

 which had no resemblance to these protected 

 butterflies, either in form, marking, or colouring ; 

 who can deny that it would be most useful to this 

 species to acquire the form and colouring of a 

 Heliconide, and thus, by taking to new conditions 

 of life, to avoid the persecutions of its foes ? But 

 if the physical nature of the Pieride concerned 

 precluded the occurrence of Heliconoid Variations, 

 would this incapability of insinuating itself into 

 these new conditions necessitate the decline of the 

 species ? Could not its existence be secured in 

 some other manner ? could not the destruction of 

 numerous individuals by foes be compensated for 

 by increased fertility? to say nothing of the 

 numerous other means through which the number 

 of surviving individuals might become increased, 

 and the existence of the species secured. This 

 case is not arbitrarily chosen ; in the districts 

 where the Heliconiidie occur there are actually a 

 large number of Whites which do not possess the 

 protective colours of the former nauseous family. 

 In the adoption of these new life conditions we 

 have not to deal therefore with survival or ex- 

 termination, but only with amelioration. It is not 

 every species of " White " that can become 



