SEASONAL DIMORPHISM. 1629 



after tliis ajux. The first two, liowever, do not represent distinct broods ; 

 for tclanionides is not tlie direct conseasonal produce of marcellus, but both 

 are made up of butterflies which have wintered as ciirysalids, those which 

 disclose their inmates earliest prodiieiujr niarcelhis, tiie others tchmionidcs ; 

 wliile all buttertlies produced from eggs of tlie same season, and there are 

 several successive broods, belong to ajax. These forms differ in the length 

 of the long tails upon the hind wings, in the clothing of the front of the 

 head, in the extent of the blood-red spots upon the hind wings and in other 

 markings, and before their relation was known were regarded by all 

 naturalists as distinct species. 



So, too, we have an admiralile example in Cvaniris pseudargiolus. In 

 New England, there first appears a blue butterfly, in which the spots of the 

 under surface are very large and often blended into great patches. Later 

 on, comes what is apparently another species, in which the spots of the 

 under surface, still tolerably large and distinct, are never blended ; and 

 finally a third in which all the markings are very feeble and faint; so too, 

 as the season advances the females grow paler and paler. 



These forms of seasonal dimorphism and polymorphism have attracted the 

 greatest attention of naturalists, and an explanation has long been sought of 

 the causes which have induced them. Numerous experiments have been 

 made which have brought to light the great probability that in some way 

 the retardation of development through cold has had much to do with 

 these changes. The argument in favor of this view is very much strength- 

 ened when we recall the fact stated at the outset, a fact which does not 

 seem to have been j^articularly noted, that, although seasonal dimorphism 

 appears between the earliest and later broods of the season without always 

 affecting all the members of each, and although it appears to be wholly in- 

 dependent of whether the winter is passed in the imago, the chrysalis or the 

 larval stage, it is nevertheless true that no case of any kind of dimorphism 

 which affects alike both sexes has yet been noted, in which the species is single 

 brooded. In support of this statement no instance in our own country can 

 be more striking than that of Polygonia, all the species which are double 

 brooded showing this form of dimorphism between the two bi'oods, while 

 no sign of the same appears in the sjiecies of which there is a single brood 

 in the course of the year. 



There would seem therefore, to be ground for believing that temperature 

 has much or aU to do with seasonal dimorphism ; and the manufactm'e, so to 

 speak, of the spring type from chrysalids which should normally produce 

 the summer type (by the artificial application of cold) lends strength to this 

 supposition, at least as regards seasonal dimorphism in the temperate 

 zones, where the contrasts of temperature are great. And on this account 

 Pryer and some others have chosen to denominate the resulting varieties 

 as "temperature forms." 



