576 PEOF. E. B. POULTON: KATIIRAL SELECTION 



of external and internal causes are unable to ofEer an adequate 

 explanation of certain facts which are clearly explicable by 

 natural selection. In the males of the Pieriue group Bismor- 

 fliina, the long-and-narrow-winged appearance of an Ithomiine 

 butterfly is largely produced by the excessive overlap of the 

 upper upon the under wings. This results in the concealment of 

 a large part of the upperside of the under and of the underside 

 of the upper wing; and it will be found that the mimetic 

 patterns are withheld from these hidden surfaces, which often 

 retain some distinct trace of the old Pierine character, viz. an 

 opaque white appearance. 



The male o^ DismorpMa praxinoe is shown on Plate 40. fig. 4, 

 the underside in fig. 5 ; the upper and under sides of the female, 

 for comparison, in figs. 6 and 7. This fact holds true even for 

 such a specialized and perfect mimic as Dismorphia orise (see 

 Plate 40. figs. 8 & 9, for the appearance of the male). 



P. D. Godman and 0. Salviu, in the ' Biologia Centrali- Ameri- 

 cana ' (Ehopalocera, p. 17,3), speak of these hidden chalky 

 patches, surrounded by a silky area covered by peculiar scales, 

 as a character of the Dismorphina. The interpretation of the 

 patch as a sexual brand perhaps with the nature of a scent- 

 producing organ, does not in any way disprove the suggestion 

 here adopted — that the white pigment in the scales is a survival 

 from an ancestral condition still found over the greater part of 

 the wing-surface in so many non-mimetic Pierince, as well as in 

 the males of many mimetic species {e.g. in the genus Mylothris). 

 It should be noted, however, that the patch is not white in 

 certain species of the Bismorpliina. Mr. Belt (' Naturalist in 

 Nicaragua,' pp. 384, 385) states that the white patch is usually 

 concealed by the males, as indeed may be inferred from the 

 change in character of the surface, which indicates the normal 

 amount of overlap of the fore upon the hind wing. Mr. Belt 

 also suggested that the white appearance is ancestral and has 

 been retained by the operation of sexual selection. 



The restriction of the effects to those parts of the surface 

 which can be seen tells very strongly against any theory which 

 is not based on the principle of selection. 



(7) 'Essential Nature of these Jtesemlla^ices : their Analysis 



into the several kinds of Effect produced. 

 The resemblances under discussion are made up of elements 



