278 Distribution of Moths of the Sub-family Bistoninae. 
and that at a time when the larval characters were not greatly 
removed from those of P. vachelae but when, nevertheless, 
the biology of the imago was as specialised as it is now. From 
which facts, as P. lapponaria is even more Arctic in character 
than P. vachelae as the abnormal period its pupe can lie over 
and its attachment to Betula nana in Lapland tell us, it follows 
that lapponaria must have arisen long before P. rachelae had 
reached its western areas. It must have spread at once to the 
north of the vachelae stations of those days. In all probability, 
then, the ‘metropolis’ of P. lapponaria was in some long 
submerged lands in the neighbourhood of, but to the north of, 
Spitzbergen and Franz Josef Land. It, too, would feel the 
pinch of the approach of the glacial period, but in a far less 
degree than any of the others. Hence, its retirement was still 
slower than that of P. vachelae; so slow was it that, ere the in- 
sect fell back, the ice smothered the whole of the Scandinavian 
mountains, and when they were reached it found the conditions 
so inclement that it bifurcated* as it struck them, one horde 
passing down to the British Islands, utilising ice-free plains of 
the continuous Scandinavian and British coasts as a causeway 
and the other wending its way down the eastern margin of the 
then much more extensive Baltic Inland Sea through Lapland, 
Finland and Livonia to Central Russia. Somewhere on islands 
fringing the coast, and not impossibly on ice clear spots inland, 
the western branch of the species survived the Glacial Period, 
reaching its present position (which the Arctic nature of the 
insect demands) when the ice disappeared. The other division, 
similarly, passed through the Ice Age on non-glaciated areas 
of Russia, extending its range slightly as the climate periodically 
improved, but finally emerging as the last phase of the Glacial 
Epoch, the Great Baltic Glacier, melted away. Steadily pur- 
suing a northward course, but limited by the Baltic Sea, which 
was slightly more extensive then, it reached its present posts. 
This completes the history of the wanderings of the species 
we call P. lapponaria, but it is well to note that the Scottish 
insect which I have provisionally described as a subspecies 
under the name P. scotica, owing to its long geographical 
separation from the other colony is clearly diverging from it 
specifically and, if allowed to exist, will finally attain specific 
rank. The Continental insect is a dull, heavy, shaggy insect, 
whereas our own form is more brightly coloured and neater 
' reminding one vividly of P. rachelae. 

* Evidences of such bifurcation we see in the present stations of the 
plants Potentilla fruticosa, Saxifraga nival.s, etc., and this is emphasised 
by their absence from the Alps. Many other such examples are masked 
by the fact that many Alpine forms have gained access to their present 
stations in Scandinavia, N. Russia and the Alps by direct migration from 
the east in late geological times. 

Naturalist, 
