164 THE COLORING OF BUTTERFLIES, 



either continent, thus far without real success. 

 Within a few years the cabbage butterfly [see 

 Figs. 110, 11 1] was accidentally introduced from 

 Europe, and has now spread widely ; nor does any 

 difference exist between European and ordinary 

 American examples of this insect. But there has 

 suddenly appeared in this country a variety un- 

 known to Europe, or, if known, excessively rare, 

 in which the normal chalky white, which forms 

 almost the only color of the wings, is replaced by 

 a pale sulphur tint ; probably few of our ento- 

 mologists have not seen this variety, although 

 from its resemblance on the wing to our common- 

 est of butterflies, the Clouded Sulphur, it would 

 ordinarily escape observation. 



At the close of the last chapter, I promised to 

 discuss a new element of complexity which arises 

 in considering the different broods of butterflies ; 

 this complexity, it would seem, is already suffi- 

 ciently embarrassing, when we have lethargy of the 

 caterpillar, or premature hibernation of the chrys- 

 alis, with the consequent commingling of broods ; 

 but in addition to this the successive broods of 

 the same butterfly which appear in a single season 

 almost always differ from each other ; often so 

 incredibly as to be mistaken for distinct species. 

 This phenomenon is termed alternate, or, better, 

 seasonal dimorphism dimorphism signifying the 

 existence of an animal under two distinct forms. 



