284. THE THEORIES OF EVOLUTION 



against the theory of natural selection. And yet 

 there is no direct antagonism between Wagner's and 

 Darwin's theories, although the Neo-Darwiuians are 

 far from agreeing with Wagner. It is true that Dar- 

 win does not regard the distribution of organic be- 

 ings as an indispensable factor in species-forming 

 and only mentions geographical distinctions and the 

 affinity of the various fauna? as arguments subsidiary 

 to his main thesis — the common origin of all species. 

 He recognises, however, the importance of natural 

 barriers and he gives many illustrations of interf aunal 

 differences which are in proportion to the difficulty 

 with which intervening barriers can be crossed. 1 



On the other hand, Wallace, a Darwinian, empha- 

 sised in a paper read before the Congress of the 

 British Association in 1876, the importance of local 

 conditions. He mentioned the case of South Amer- 

 ican moths, several subfamilies of which, JDanaince, 

 Acraenice and Heliconince, present differences in pat- 

 tern and colouration according to the regions where 

 they are found. Thus the South- Andean species 

 (Peru and Bolivia) are orange and black; in the 

 North- Andean species, the orange colour is replaced 

 by yellow. There could be no question of mimicry 

 in those cases, for the three species are equally well 

 protected against insectivorous birds by an objection- 

 able secretion. In tropical Africa two groups of but- 



i The Origin of Species, Chap. XIII. 



