16 THE COMMONElt BUTTERFLIES. 



morphism or ])olymorpliism of various kinds, that is, the 

 existence of a given species under recognizably distinct 

 forms (two or more, even sometimes to five or six) is by no 

 means nncommon. 



This distinction is often sexual; indeed there are relative- 

 ly fcAV species in which the ontward aspect of the two sexes 

 does not differ, in some cases to a remarkable degree. It is 

 universal in the numerous species of Eurymus, for example, 

 where in general the inner margin of the dark outer bor- 

 dering of the wings is sharp and ])reci8e in the male, con- 

 fused and irregular in the female. In very many cases, 

 however, it is accompanied by a simple dimorphism, some- 

 times affecting one sex only (and then usually the female), 

 as in many species of Eurymus, where one form of female 

 has the bright ground color of the male, the other a pallid 

 ground color ; at other times affecting both sexes, as in 

 some species of Polygonia : in P. interrogatlonis, for ex- 

 ample, there are four sets of individuals differing in the 

 general coloring of both surfaces of the wings and even in 

 the form of the wings — differences all of which may occur 

 in the progeny of a single individual and fed on the same 

 plant. 



But these differences are very often correlated with, 

 generally confined to, differences of brood. One of the 

 most striking and at the same time one of the simplest 

 examples is in the double-brooded European species 

 Arasclinia pro?'^a, where the first brood is composed of 

 individuals of one type with highly variegated markings 

 (levana), the second of a very distinct type with more 

 sharjdy-contrasted coloring (prorsa), which, until they 

 were bred from each other, were universally, and reason- 

 ably, regarded as distinct species. This is called seasonal 

 dimorphism. 



Numerous striking examples occur in this country, not 

 a few of which are excellently shown in Edwards's Butter- 



