59 



been cast by the pupa (" Proceedings," igo8-g, PI. Ill, figs. 

 3 — 8), and this seems to be generally the case, if not always, 

 in other species with similar projections. Then, again, the 

 various hooks, teeth and beaks, all of which aid, in one way 

 or another, the escape of the imago from the cocoon in those 

 species which spin such domiciles, all appear to be brought 

 about by pupal development. 



The imagines I think afford us many examples of what I 

 might call secondary evolution, affecting only one stage of 

 the life of the insect as a whole. Take, for instance, the 

 genus Bryophila. Our two species, /'^r/rt and uinralis, vary 

 immensely in the ground colour, and also in the strength 

 of the markings. B. perla, ab. fiavescens, Tutt, occurs on 

 walls covered with orange-coloured lichens, and on some 

 walls is almost as common as the type (" Brit. Noct.," vol. i, 

 p. 8). It seems probable, therefore, that if we found ova, 

 larvae or pupae on one of these walls, we could not tell 

 whether we should breed from them the type, orBh.flavcscens, 

 or both. Again, in Gnophos obsctiraria we have light and 

 dark forms, generally from light or dark soils : but I believe 

 these forms would freely interbreed, and that if we received 

 larvae without knowing whence they came, we could not tell 

 what form the resulting imagines would take. In both these 

 instances we see causes, which need Jiot apparently affect 

 the earlier stages, resulting in the variation of the imago. 

 There is another form of variation which apparently affects 

 only the imagines — that of dimorphism. 



In Dryas paphia we have the var. valesina, which is, I 

 believe, generally considered the more ancestral form of the 

 female, which inhabited the ancient forests when they were 

 larger, more moist, and perhaps less sun-lit, than are our 

 smaller woods of to-day, in which that form of the female, 

 coloured like the male, is now more usually found. Yet we 

 know that from eggs laid by the var. valcsina we can 

 breed both forms of the female. Only last year Mr. E. C. 

 Joy bred from eggs laid by one female var. valesina, forty- 

 one males, twenty-three ordinary females, and thirteen var. 

 valesina. These were exhibited here on October 14th. The 

 late J. A. Clark had in his collection a specimen inter- 

 mediate between the two forms. 



Hcpiahis humiili has two forms of the male, the possibly 

 ancestral form, var. thnlens (lictlilandica), which occurs abun- 

 dantly in the Shetland Islands, where it flies in daylight and 

 therefore does not need to be so conspicuous an attraction 

 to the female as does the more ordinary white form which 



