MIMETIC AND NON-MIMETIC SPECIES 275 



afforded by the theory of Natural Selection. It is a real 

 difficulty to the theories of External and Internal Causes ; 

 for, as regards the first, we should expect the closely 

 related species of a genus to react similarly to the local 

 conditions, rather than that certain of them should react 

 quite differently from the others but quite similarly to 

 the species of distantly related Sub- Families ; and, as 

 regards the second, we should expect such species to 

 reach nearly the same evolutionary stage together, rather 

 than that some should reach one stage and others another 

 entirely different one, but the same as that reached by 

 certain species of remote affinity. With Natural Selection 

 for our explanation such differences are at once intelligible. 

 The variation which formed the foundation for selection 

 to build upon may well have been present in certain 

 species of a genus but not in others ; or slight differences 

 in life-history or the methods of adaptation, or the attacks 

 of enemies, may have rendered Mimicry advantageous for 

 this species but not for that. 



When we pass from Mimicry among butterflies to 

 Mimicry between butterflies and moths, the difficulties 

 encountered by all theories except Natural Selection 

 become greater because of the wider structural difference 

 between model and mimic. To take an example, certain 

 species of day-flying Chalcosid moths of Borneo mimic 

 Danaine butterflies while others mimic Pierinae : another 

 mimics an Agaristid moth.^ Why should part of the 

 Heteroceran group be acted on by external conditions so 

 as to cause a superficial resemblance to Danainae, the 

 others so as to cause a resemblance to Pierinae'^ Why 

 out of the same closely related set of species should some 

 reach the evolutional stage of Danainae, the others of 

 Pierinae ? Why should the models happen to differ from 

 butterflies in general in their slow flight and conspicuous 

 appearance, in the similarity of the patterns on the under 

 side of the wings to those on the upper side, in the fact 

 that they are distasteful to the generality of insect-eating 



^ See the beautiful plate (xxi) illustrating these resemblances in 

 R. Shelford's paper in Proc. Zool. Soc. Lond., 1902 : also pp. 256, 257, 

 and 259, 260 of the text. 



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