88 THE CANADIAN ENTOMOLOGIST. 



that the wing-pigments are not formed until a short time before emergence. 

 In all the cases of which I have recollection this has been so. 



But I imagine that the ratio between the growth of the wing and the 

 metabolism of its pigment is not always the same, nor is that between the 

 wing-formation and the growth of the vital and reproductive organs con- 

 stant, and herein I believe lies the key to the solution of the problem. 



Rapid metabolism produces darkness of colour, while slow change 

 accompanied by growth gives rise to a larger expanse of wing, on which 

 the pigment is paler, lighter, and often more brilliant. 



The vital and reproductive organs of a butterfly will develop sooner 

 and at a lower temperature than the pigment of the wings ; and hence 

 in a country where the winters are cold and the summers hot, the hiber- 

 nating pupae will have reached a nearly full development by the time the 

 warm weather comes on, except as regards the pigment of the wings. 

 This will undergo very rapid metabolism to be ready by the time of emer- 

 gence, and the result will be a dusky and small winged form. On the 

 other hand, if the spring comes gradually, and the winter is warm, the 

 wing-pigment will develop more slowly, the wings will have longer to grow, 

 and consequently the vernal brood will be paler even than that of the 

 summer. 



And this is precisely what we find ; Pier is virginiensis is a pale spring 

 form taking the place of the dark oleracea-hyemalis of the more northern 

 portion of the continent, while P. rapes and P. brassicoj, which do not 

 exhibit dusky vernal broods, are natives of Europe, where the winters are 

 milder and the advent of spring more gradual than in North America. 



It may here be objected, why are not tropical species, whose develop- 

 ment is often excessivly rapid, uniformly dusky or black ? That they are 

 in many cases darker than their representatives in more temperate regions 

 will I think be admitted, but I would point out that they are not by any 

 means in the same position as North American vernal forms. The vital 

 organs must in any case take a certain time for development, which is 

 always longer than that necessary for the metabolism of the pigment. So 

 that in the case of any summer brood, however rapidly developing, the 

 temperature being high enough to allow the development of the pigment 

 at the same time as the organs of the body, it has ample time for sufficient 

 metabolism— less indeed than in the case of a warm and gradual spring, 

 but more than in that of a frigid winter and quick coming summer, where 



