150 CONCLUSION [ch. 



closely to the common Danaid type represented by 

 Danais vulgaris and other species, but apart from these 

 none of the many Pierids in Asia resemble any of the 

 recognised models. Africa is apparently destitute of 

 Pierids which mimic species belonging to other groups. 

 Yet no group of butterflies is more persecuted by 

 birds. Of all the instances of bird attacks collected 

 together by Marshall 1 more than one-third are instances 

 of attacks upon this group alone. If birds are the 

 agents by which mimetic likenesses are built up through 

 the cumulative selection of small variations, how can 

 the rarity or absence of mimetic Pierids in the Old 

 World be accounted for? For the species of Pierids, 

 like the species of other families, shew considerable 

 variation, and if this process of selection were really 

 at work one would expect to find many more Pierid 

 mimics in these regions than actually occur. It is 

 true that the white, yellow, and red pigments found 

 in Pierids differ from those of other butterflies in being 

 composed either of uric acid or of some substance 

 closely allied to that body 2 . These substances are 

 generally found between the two layers of chitin, of 

 which the scale is composed, whereas the black pigment 

 is intimately associated with the chitin of the scale 

 itself. What is perhaps the principal factor in the 

 formation of a mimetic likeness is the distribution of 

 the black pigment with reference to the lighter pig- 

 ments ; and although the latter are chemically distinct 



1 Trans. Ent. Soc. Lond. 1909. 



2 Cf. F. G. Hopkins, Phil. Trans. Roy. Soc. 1895. 



