274 Distribution of Moths of the Sub-family Bistoninae. 
genus ; they, too, form a natural little group of two, differing 
slightly in certain points of structure from the first three. 
Four of the species possess almost apterous females; of the 
fifth, P. liquidaria, the female is unknown. 
The most primitive, or phylogenetically oldest form, is P. 
pomonaria which, in consequence, is a fairly widespread insect. 
The insect being attached to oak, and not refusing such foods 
as hawthorn, it seems curious that its range is not greater. 
Careful study of the map will reveal the facts that the lhmits 
are those set by the winter isotherm of 35°F or, more plainly, 
it only inhabits regions in which there is a minimum of two 
and a half months of frost. With most species such factors 
are not of serious importance, but in the case of forms such 
as those comprised in the Non-Boarmioid Bistoninae, which 
pass the winter as a fully formed imago inside the pupa case, 
a very open winter spells disaster* as direct experiment has 
shown. I took about eighty pupe of P. pomonaria as soon as 
they had hardened and exposed them to the weather through- 
out the late Summer and Autumn. The imagines commenced 
to emerge in December and were over by the end of January. 
Ova laid under these conditions would hatch far in advance of 
the leafing of oak and the larvae would therefore perish. Any 
hold then that the species has on regions other than those 
with a fairly rigorous continental climate must be very pre- 
carious. If this state of affairs holds now, it is reasonable to 
assume that the same physiological peculiarities have been 
potent in the past. Therefore, when Poecilopsis was developed 
in the Boreal home of the group, we must suppose that it took 
its origin at some point to the North-east far from the insular 
climates produced by the Atlantic Ocean and possible Gulf 
Stream of early or middle Pliocene times. Now it was pos- 
tulated in tracing the wanderings of Lycia that it arose nearer 
Europe than America; Poecilopsis, therefore, came into being 
similarly nearer to Europe but to the east of the centre of dis- 
persal of Lycia. In consequence, with the deterioration of the 
climate in late Pliocene and in Glacial times, it would retreat via 
Scandinavia and North Russia until the sunnier and warmer 
Pannonian Hills of Hungary and Southwest Russia were 
reached, whence it issued finally at the close of the Glacial Period. 
Many attempts to advance would be made as so-called Inter- 
glacial conditions intervened ; such attempts, owing to the 
more temperate character of its dominating food plant, oak, 
would be foiled for all suitable points, even in Central Europe, 
were overwhelmed later by the coalescence of the local glaciers 

* Unless they have developed the habit, as in the case of all the genus 
Nyssia, Lycia hivtavia, and Poecilopsis lapponaria of not responding 
readily to the stimulus of a rise in temperature. 
Naturalist, 
