238 [March, 
rot express myself veiy clearly on the subject, and must therefore trespass on the 
space of the Magazine to add a few explanatory remarks. 
Everybody knows that there are two forms of Selenia illustraria in appearance 
80 different from one another that, did we not know positively to the contrary, we 
should still be regarding them in the light of distinct species. In Mr. Doubleday's 
Catalogue these are respectively designated — the one illustraria — the other delu- 
naria (yar. wst.), by which names in the present note, for the sake of convenience 
and greater perspicuity, the so-called spring and summer broods will be respectively 
indicated. 
It is pretty well known, too, that in the natural sequence illustraria reproduces 
itself in the form o? delimaria and vice vers'i. But what I assert is, that whenever 
(whether, at large, owing to exceptionally hot or long summer seasons, or, in cap- 
tivity, from warmth, assisted, perhaps, by what Mr. Crewe has happily termed 
" feeding up quickly") the completion of the pupal stage is accelerated, then dehi- 
naria produces delunaria, not illustraria. Further, it is my belief that the converse 
will be found to hold good, viz., that should the completion of the pupal stage be 
retarded either by cold seasons or climates in a state of nature, or artificially by the 
aid of an ice-well, illustraria — not delunaria — would be found to result from illus- 
traria ; and I feel the more confident of this from the fact that its congener Junoria 
is single-brooded in northern localities, and the spring form is the only one known 
in those parts of the country. I am unaware if illustraria occur in Scotland ; but 
if it do, I venture to say that not only will it be found to be single-brooded, but the 
form delunaria wUl be unknown there. 
I therefore feel justified in coming to the conclusion (not my own idea, by the 
way) that acceleration or retardation of the completion of the pupal stage from 
thermic influences are causes of greater or less degree of maturation or (to use 
another word) variation in the imago. 
Certainly, against this view, I have myself adduced the case of P. rapw, and 
Mr. Greene has added that of E. lanestris as well as some of the " prominents "; but 
these by no means do away with the fact that when the " purple thorn" emerges 
after pupal hybernation the result is always illustraria, — when before pupal hyberna- 
tion, delunaria. 
Touching the subject of boreal variation in Lepidoptera, it is well known to our 
continental friends that the intensity of the colour and markings of the long-lived 
C. delphinii is proportionate to, and it is reasonable to suppose dependent upon, 
the duration of the pupal state ; nearer home, among other familiar instances, we 
find Colias edusa, after a long absence, during which we may rationally presume it 
has lain dormant in the pupal state, reappearing more beautiful than ever, with 
iridescent tints. 
Any discussion repecting the influence of light to produce variation I leave to 
those good men who have had opportunities of testing it, and who know its value 
better than I. For myself, I have simply stated it to be worth trial. 
On other points Mr. Greene and I are much of the same opinion ; I understand him 
to admit the power of hereditary influence, i.e., the " selection " (whether natural 
or artificial) theory to cause variation ; and as for phytophagic varieties, he goes, 
further than I did when he states that intentional administration of food to the larva 
^th a view of producing variation in the imago is in almost all cases a total failure 
