82 
about, and why, on the rocks, naturally dark or artificially darkened 
by rain, black forms are generally found.” And a little further on : 
“ Although humidity or wetness, by darkening the surfaces on which 
the insects rest, would intensify the colour, yet the particular 
geological form is here the prevailing factor, or why should we get 
white forms on the purer cretaceous strata, grey on the impure lime¬ 
stones, red on the red sandstones, black on the slate and schists, and 
brownish-black on peat ? ” 
Thus Mr. Tutt admitted, even at a time when he may almost be 
regarded as having been a special pleader for a moisture-melanism 
theory which he had brought .forward, that the influences producing 
the variation in Sciadion obscurata were mainly other than those which 
his theory assumed. I am not preparing my present paper with a 
view to opposing the view which Mr. Tutt held at that time—or, at 
least, a little earlier—for we find that his position gradually, and 
almost imperceptibly, shifted as his articles proceeded. Inasmuch as 
he has now probably abandoned the theory himself*, it were quite 
unnecessary to waste words in an endeavour to refute it. 
As compared with the majority of our most variable Geometers, 
and also with so many Agrotids, etc., the speciality of D. obscurata 
strikes one as consisting in its comparative lack of striking variability 
in any single locality. I say “ comparative” advisedly, for I am not 
unmindful of the interesting and well varied series which I have seen 
from Folkestone, nor of the existence side by side of two very different 
forms on the coast near Babbacombe, and probably in a few other 
places. But I think I shall carry you with me in the general state¬ 
ment if you will reflect on your own experiences with the species ; at 
any rate, I myself should no more expect to find a black or reddish 
specimen at Sandown, or a white or reddish one on White Moor (New 
Forest), than I should expect to find Smerinthus ocellata var. atlanticus 
in a London garden. Contrast the case of some other variable species 
which will readily occur to us. It was only at our last meeting that 
our President was remarking (a propos of an exhibit of Agrochola 
limosa) that it appeared that the whole range of known forms could 
readily be obtained in any single locality where it occurred. I made 
a similar statement in our London fauna list (Trans. City Lond. Ent. 
Soc., ix., p. 72) in connection with our ubiquitous friend Triphaeha 
pronuba. 1 am at present breeding a very interesting variable series 
of T. comes-', the offspring of a black 5 taken at Findhorn; and 
although they include black and deep-red specimens which one could 
not hope to match from any south British locality, yet there are also 
a number of specimens as pale as any which we get in the south. 
Among geometrid species, I need only instance Xanthorhoe Jiuctuata, 
on whose variation I read a paper before this Society on April 21st, 
1896 (vide Ent. Fwc., viii., 54, et, seq.). 
It is true there are many local forms in various Noctuids and 
* I do not reckon the admission of the fact that melanism is often due to a 
darkening of tree trunks by moisture as equivalent to an acceptance of the theory 
with which Mr. Tutt set out, that melanism is (physiologically) due to moisture. 
To regard the two ideas as identical would be to land oneself in a labyrinth of 
logical difficulties. 
* The last word has not yet been said on the nomenclature of this unhappy 
species, and I fear I was rash in accepting Snellen’s synonymy in our London list. 
I provisionally revert to the well-known and safe name. 
