MOEPHOLOGY OF THE LEPIDOPTEROUS PUPA. 255 



shrinkage in the pupal wings until the imaginal organs had become smaller than the 

 former. The various stages of shrinkage are well exemplified in the genus Ilybernia 

 alone. In S. frogemmaria the wings of the female are certainly far larger than those 

 of the pupa ; in fact the expansion does take place, but it is much limited ; and the 

 resulting organs are entirely useless. In K. rupicapraria and JS. aurantiaria the wings 

 seem to be as nearly as possible of the same size as those of the pupa : the wings of the 

 female on emergence are of the same size as those of other females, but there is no 

 expansion. Finally, in H. leucophcaria and R. defoUaria the wings of the female are 

 almost invisible and much smaller than those of the pupa. In this last case alone is 

 there any probability of the pupal wings having shrunk, and I should imagine that such 

 shrinkage, if any, would be small, because the transition described above in nearly 

 allied species belonging to one genus seems to show that the whole character is very 

 recent. Since writing this sentence, I have been enabled to obtain evidence of the 

 validity of the conclusion ; for I have now shown that the pupal wings of the female 

 H. defoUaria remain of considerable size (Plate XXVII. fig. 7)- 



I wish to point out clearly how much and how little is contained in this explanation 

 and history of degeneration. I have shown the causes which always tend in this 

 direction and the chief conditions which prevent their action. I have pointed out that 

 certain species become partially, others completely, relieved from these conditions, and 

 thus seem to be free to follow the tendencies, and we see that all the instances of 

 degeneration occur among such species. But while it has thus been, I think, sufficiently 

 explained how it is that degeneration of the female wings becomes jjossible, it is quite 

 a different thing to say why certain species and not others should have availed them- 

 selves of the possibility. There are certain other small species of Bombi/cince and 

 TlneiniB to which degeneration seems to be as possible as to those in which this change 

 has actually occurred. The probable answer is a very general one — that the struggle 

 for existence would prevent a large number of species from taking one uniform line of 

 development, even though it may apx^ear beforehand to be very directly pointed out as 

 the path of least resistance for all of them. Eurthermore we may fairly concede that in 

 many instances it is now impossible for us to ascertain the exact conditions which led 

 certain species to take the line we are considering to tlie exclusion of others, to which 

 it was also open, as far as the general conditions would admit. In the Geometrince, how- 

 ever, I think I may claim that the demonstration of the cause and coui'se of degeneration 

 has been complete. The number of species to which the line was open being limited, 

 they appear almost without exception to have availed themselves of the opportunity. 



3. The Form of the Imaginal Fore Wing detected on that of the Pupa. — Altliough the 

 wing of the imago expands into a size far beyond that reached by the pupal organ within 

 which it is developed, the former does not entirely fill the latter l)efore emergence. The 

 margin of the imaginal wing lies well within the corresponding margin of the pupal 

 wing which encloses it. Not only are the two margins separated by an interval, l)ut 

 their contours are frequently very different. Furthermore, in many species the position 

 of the future margin of the imaginal wing can be distinctly made out on tlie pupal 



3b* 



