210 ANIMAL COLORATION. 



pattern as some poisonous or uneatable creature. A re- 

 markable instance of this is the genus Basilarchia, among 

 North American butterflies : with one exception the species 

 are dark-coloured with blue spots round the margins of the 

 wings ; the exception is B. hipparchus, which has a tawny 

 brown colour diversified by black bands and marks. This 

 butterfly almost exactly copies Anosia plexippus* ; the re- 

 semblance is more striking in this and some other similar 

 instances from the very fact that it is only one out of a number 

 of species which is coloured after the pattern of an insect 

 belonging to another family, and in acquiring this coloration 

 has departed widely from the plan of colour found in its- 

 immediate relatives. On the theory of mimicry we interpret 

 this divergence as due to natural selection ; and if an insect 

 is found whose coloration is strikingly unlike that of its allies 

 we immediately assume that some other species will be found! 

 which it imitates. This assumption, however, is not always 

 borne out ; and the existence of such forms constitutes to a 

 certain extent a difiiculty in the way of accepting the currently 

 received theory of mimicry : if a divergence from the normal 

 style of colour and marking does not always go with the 

 existence of a prototype belonging to a different genus or 

 group, a certain amount of doubt is thrown upon the validity 

 of the explanation in other cases where there is a prototype. 

 This difficulty has been suggested, and has been grappled with, 

 by Mr. Scudder in his magnificent work upon American 

 Butterflies to which I have so often had, and shall have, 

 occasion to refer. He quotes two suggestions with reference 

 to the matter : the first is the obvious one that the prototype 

 which is imitated does exist, but has not been found ; this 

 explanation is easy to make and difficult to disprove. Another 

 • I follow Soudder's nomenclature of the species. 



