Distribution of Moths of the Sub-family Bistoninae. 197 
Such an advance of Northern forms, driven westward and flow- 
ing southward in Britain, in the absence of great climatic 
changes in these islands, I have shown elsewhere to be a possible 
cause of the curious commingling of northern and southern 
forms in certain Pleistocene deposits of our own and contiguous 
areas. 
Whilst these events, mainly of local importance, were occur- 
ring in the west, almost simultaneously, events of far reaching 
importance developed in the east. The Arctic-Aralo-Caspian 
arm of the sea had slowly dried up, opening the way for the pent- 
up horde of Siberian and other Asiatic forms which streamed 
westward as an all conquering flood, giving us some of what 
we regard as our most typical animals and plants. But the 
removal of the barrier had other effects, for the reduced European 
Flora and Fauna also used the narrow gateway to gain new 
ground and a wedge of migrating L. hirtaria passed forth to its 
present Asiatic home in the Issi Kul and Ili districts. With 
this, we have traced L. hirtaria to its present stations and, with 
the final waning of the Baltic Ice, the various sets of conflicting 
settlers assumed a state of equilibrium and, with but slight 
changes, resulting from temporary climatic modifications, have 
remained so until the present day—as far as the hand of man 
has allowed them. 
Next we must take up the problem of the Nearctic L. wrsaria 
left penned up by the ice in the Southern Appalachian region. 
With the retreat of the wide-spread icesheet, the species pressed 
slowly to the north, guided along the coast by the ever present 
and impassable Appalachian Mountain system until it reached 
what is now the state of New York, where the outflowing 
species could outflank the mountains over the low-lying land 
between them and Lake Erie; this a division attempted to 
do, only to be brought up by the barrier interposed by Lake 
Erie which caused the stream once more to bifurcate, one 
branch passing into Canada over the Niagara River and the 
other moving westward.t Thus access was gained to the 
broad area of the Great Central Plains to the south of the 
Great Lakes, and L. ursaria was only bound by the climatic 
conditions which limit its northern and southern trend and 


Scandinavian Flora and Fauna were quite extirpated. If so, what was 
the position of such plants as Artemisia norvegica at the period in question ? 
+ Here it is important to note that insects such as Pieris rapae, Phytono- 
mus punctatus and Cryptorhynchus lapatht which have been introduced 
at various points on the Atlantic seaboard have kept exactly to the lines 
of advance mapped out here as careful observation has shown. Such 
observations have been used to predict where and when a given alien 
pest will reach the state of Ohio. I have not considered the valley of the 
Big Kanawaka in West Virginia or the Cumberland Gap as giving access 
to the plains beyond the Alleghanies as no introduced species have ever 
been observed to use them, 
1916 June 1. 
