264 THEORIES OF MIMICRY 



that this loss is recent, and to trace the steps by which it 

 has been reached from a condition in which normal scales 

 were present. 



In the very large convergent group of tropical American 

 Lepidoptera which has sprung up round the best known 

 species of the Ithomiine genera Methona and Thyridia, 

 transparency of a large part of the wings is a charac- 

 teristic feature. Throughout the group the ground-colour 

 of the wings is transparent, with a black border which 

 generally passes inwards as two transverse bands in the 

 fore-wing, separating the transparent part into three 

 areas, and as one band in the hind-wing, separating it 

 into two areas. The group consists of Ithomiinae (Neo- 

 tropinae) of many genera, of Danainae, of Pierinae, and 

 of moths of the genera Anthomyza and Hyelosia, belong- 

 ing to the Pericopidae (Hypsidae), and the widely separated 

 genus Castnia (Castniidae). 



Under the theory of External Causes we should expect 

 that the transparency would be attained in a similar 

 manner throughout, by the reduction of scales to hairs, 

 by the complete loss of scales, or by some other uniform 

 method. Under the theory of Natural Selection we 

 should expect the methods would be different in the 

 different groups. There are many ways in which trans- 

 parency can be attained, and we should expect that one 

 group would submit one set of variations making towards 

 the resemblance, another a different set, to the operation 

 of N atural Selection. 1 1 will be shown that this prediction 

 is abundantly justified. 



Microscopic examination of the Ithomiinae (Neotro- 

 pinae) showed that the scales on the wings are of two 

 kinds, broad and narrow, which alternate more or less 

 regularly. In the transparent parts both kinds of scale 

 can still be detected, the narrow being frequently 

 reduced to fine simple hairs, and, in the most extreme 

 cases, the broad scales being reduced to Y" s ^ a P ec ^ 

 hairs. 



The two commonest species of the whole group, 

 probably forming the centre towards which the others 

 have converged, are the Ithomiine butterflies Methona 



