274 MIMICRY AND NATURAL SELECTION 



Mimicry among Rhopalocera (butterflies) is much less 

 common as we pass into northern regions, but there is 

 one excellent example in temperate North America which 

 serves to show how superficial an interpretation is that 

 offered by the theory of External Causes, and how com- 

 pletely it breaks down when examined with a little care. 

 With comparatively few exceptions the insect fauna of 

 North America is that of the great northern circumpolar 

 land-belt. These exceptions are intruders from the 

 tropical South, and among them is the large Danaine 

 butterfly Anosia plexippus which now ranges over the 

 United States and a large part of Canada. In tropical 

 America closelysimilar representative species, sub-species, 

 or forms still persist. This abundant Danaine butterfly 

 affords the model which is closely resembled by an 

 indigenous Nymphaline butterfly which we should place 

 in the genus Limenitis, although some American natura- 

 lists prefer to put the Nearctic species in a separate genus, 

 Basilarchia. There are also other mimics among the 

 species of the North American Limenitis (Basilarchia), 

 but two of them are non-mimetic and enable us to 

 reconstruct the appearance of their close ally before the 

 intrusion of the great Danaine model. In the New 

 World the genus Limenitis is confined to the Nearctic 

 Region with the exception of a single species, a form of 

 the mimetic L. aslyanax (Fabr.), which just enters the 

 borders of Mexico. If butterfly colours and patterns are 

 the expression of the direct influences of the environment, 

 then it is clear that the indigenous non-mimetic species of 

 Limenitis (Basilarchia) are an expression of Nearctic 

 (temperate North American) conditions, and according to 

 the theory of External Causes, the invader from the 

 South should have come to resemble them, instead of 

 drawing an ancient Nearctic species far away from its 

 ancestral colours and patterns into a close superficial 

 likeness to itself. The fact that certain species of a 

 single genus should thus be entirely mimetic, while others 

 are entirely non-mimetic and preserve the ancestral 

 appearance, has been sometimes urged, for example by the 

 late Professor J. O. Westwood, against the interpretation 



