34 
place, and although we may reasonably assume that the prevalent form 
for the locality is the one which we are in the habit of finding there, 
yet this is not to say that the divergent ones are absent in a state of 
nature—maybe they are snapped up by other enemies before we have 
seen them. I am inclined to suspect, however, that we should find the 
forms remaining tolerably constant. Their evolution cannot have been 
at all recent, even if that of some of our melanic lepidoptera in the 
manufacturing centres can be regarded as recent, but must have had 
an enormous length of time to acquire stability in an unaltering, 
or almost unaltering, environment; and one would not expect such 
stability to be easily upset. 
The same circumstances upon which I have been dwelling, go far 
to excuse our old English entomologists for their mistake in splitting 
up Sciadion obscurata into two or more species. If an entomologist 
collected in only two localities, say Lewes and Lyndhurst, he might 
reasonably say, after the longest experience, that he had found his 
“ two species ” perfectly constant in their own localities, and had 
never taken a specimen of one mixed amongst the other. It was the 
discovery of some of those (comparatively few) localities where the 
conditions were more inconstant, and light and dark forms blended, 
which finally led to the abandonment of the theory that there was more 
than one species. It would be interesting to study such localities very 
closely. Unfortunately, I have never collected the species at Folke¬ 
stone, where the interesting banded forms occur, and also, I believe, 
occasional pretty dark specimens among the usual chalk forms ; but I 
fancy I have been told that the place is not a pure unmixed chalk 
locality, and that the darker admixture can be logically accounted for 
by those who know the spot. It is undoubtedly so at Petitor, near 
Babbacombe, where the sandy (or almost reddish) var. argillacearia 
occurs together with a grey form ; when I say that the spot is exactly 
at the junction of the limestone with the red sandstone, I need not 
add any further explanation. A few miles away (at Hope’s Nose), 
where one is fairly off the red sandstone, I have never met with a 
single example of var. argillacearia. 
On account, too, of the “comparative fixity of the forms of 
S. obscurata in most districts,” we often hear collectors speak of “ the 
New Forest form,” “ the Lewes form,” and so on ; and this makes a 
satisfactory nomenclature up to a certain point; but it is obviously 
more concise, and in many ways better, to use a proper varietal 
nomenclature in Latin. This has been partly done in Staudinger’s 
Catalog, etc., and probably would have been more used in Britain if 
some confusion had not crept in as to the application of the names. 
I now propose to give a very brief historical account of the nomencla¬ 
ture, and to draw up a classification of the principal forms with their 
right names; and I trust that in so doing I shall be helping some 
who have hitherto been—as I was myself until I set myself to work 
for this paper—in uncertainty regarding the application of the names. 
Geometra obscurata, as it was then called, was first made known as 
a species by Schiffermuller and Denis in their work on the Lepidoptera 
of Vienna, in 1776*, but their description (if such it may be called) 
* Vide Ann. Mag. Nat. Hist. (7) vi., p. 158, for this date and bibliographical 
details. 
