Distribution of Moths of the Sub-family Bistoninae. 277 
The two remaining species, P. rachelae and P. lapponana, 
now claim our attention. They are a closely related, but 
perfectly distinct pair of species, and seem, in spite of their 
inhabiting such different geographical areas, not to be 
“representative” species. P. rachelae is the most primitve 
form and shows most relation to the other group of three. 
Both are northern forms and extend far beyond the Arctic 
‘Circle and both, in nature, feed on such Arctic or Boreal 
forms as Betula nana, Myrica gale and the low growing 
Salices peculiar to Northern and Alpine regions. From 
biological and other considerations, I judge that P. rachelae 
originated as a break from the pomonaria stem and, arguing 
from the food plants, possibly as a Northern form of that 
species and this is confirmed by the fact that convergence has 
caused the much younger Alpine form (P. isabellae) of pomonaria 
to assume many of the outward characters of P. rachelae. 
Being the oldest break, and also more Arctic in its nature than 
P. pomonaria, P. rachelae, had time to press far to the north 
west in the Arctic Archipelago, giving off as a new break, 
soon after it commenced to spread, the species which has 
yielded what is now known as P. lapponania. 
With the approach of the Ice Age, P. rachelae retreated but, 
as the insect was of Arctic origin, its retreat was postponed far 
longer than that of Lycia and P. pomonaria ; these passed south- 
ward and reached refuges, one both in Europe and America 
and the other, on account of its limited range and attachment 
to oak, only in Europe. P. rachelae, on the contrary, gave 
ground before the oncoming glacial conditions so slowly that, 
when it had to flee far to the south, its way was barred in 
the Eastern areas of Arctic America by the newly formed 
Baffin Bay, the formation of which was no doubt almost con- 
temporaneous with the separation of Spitzbergen from Green- 
land. Only one course of escape was now open and that was 
south westward over the northern portions of the Arctic 
Archipelago to the North Canadian coast near the Mackenzie 
river mouth, up the valley of which it slowly withdrew as 
climatic conditions deteriorated. Along the foothills of the 
Rockies, it worked its way into what is now the States of 
Montana and Wyoming, attaining a haven of rest there. From 
this haven, it once more issued as the ice-sheets waned and 
favourable conditions returned, retracing its steps as far as 
possible, but at the same time giving off offshoots which 
passed along the Valleys of the Saskatchewan, Qu’appelle and 
Assiniboine Rivers eastward. Finally, it reached its northern 
limits and rounded the foothills of the Mountains of Alaska to 
reach its most westerly stations. 
With lapponaria, the case was different. This, also, is of 
great age for, from it, the whole genus Nyssia has been evolved 
1916 Sept. 1. 
