460 Mr. R. M‘Lachlan’s Observations on 
frequent there than in other parts. It appears to me then not dif- 
ficult to imagine that should this district suddenly become isolated, 
these forms would of necessity develope into what we should very 
fairly call species. Many North American insects are very 
similar to ours; some of these are considered identical, others 
possessing rather wider differences are called distinct species ; 
surely here is a very clear case of “developmental” handywork. 
Again, to slightly diverge from the subject of British Lepidoptera, 1 
would add that a very slight acquaintance with exotic Rhopalocera 
has convinced me of the probability of the developmental theory, 
for we find there the greatest difficulty in distinguishing between 
what are to be considered as species and what as varieties, because 
differences of locality produce forms which, though closely allied, 
present certain minor differential characters; no doubt these are 
rightly termed “species,” but still we cannot doubt their com- 
mon origin. In Europe the genus Erebia is an instance of this, 
and precisely the one in which we should expect to find such a 
result, because as these insects appear to require a certain low 
average temperature, they naturally are chiefly found in mountain 
districts, which from upheaval, or from sinking of the surrounding 
country, have become isolated, and present the most favourable 
conditions for gradual development. To return to my subject, I 
may ‘say that there are some few British species which present 
dimorphic forms of the female only, as in the familiar instances 
of Colas Edusa and Argynnis paphia. Others again present almost 
endless variation, without regard to locality, such as most of the 
species of the genus Miana, several Geometrid@, the genus Peronea, 
&c. In another British insect there exist two forms, which 
though I think without doubt of common origin, are yet entitled 
to rank as distinct,—I allude to Lasiocampa quercus, which on the 
northern moors, and in some similar isolated positions in the south, 
presents the form known as L. callune, which differs remarkably 
in habit from Z. quereus, and in which the larva differs slightly, 
correlated with still smaller differences in the imago. Metrocampa 
margaritaria, which is double-brooded in the south of England, has 
only one brood in Scotland, and I believe there are other analogous 
instances; it is also well known to all Scotch Lepidopterists that 
many species habitually remain there in the pupa state for two, 
three or four years, although in the south this would form quite 
the exception in the same species; this retardation of development 
may probably have some effect in causing variation, according to 
the observations of M. Bellier de la Chavignerie. 
I now come to consider variation in the larval condition. I 
have been kindly favoured by Messrs. Hellins and Buckler (than 
