378 ENTOMOLOGY 
all directions and does so spread until its further progress is 
prevented, in one way or another. The paths along which a 
species spreads are determined, then, by the absence of barri- 
ers. ‘The diffusion of insects in our own country has received 
much attention from entomologists, especially in the case of 
such insects as are important from an economic standpoint. 
The accessions to our insect fauna have arrived chiefly from 
Asia, Central and South America, and Europe. 
Webster, our foremost student of this subject, to whom the 
author is indebted for most of his facts, names four paths along, 
which insects have made their way into the United States: 
(1) Northwest—Northern Asia into Alaska and thence south 
and east; (2) Southwest—Central America through Mexico; 
(3) Southeast—West Indies into Florida; (4) Eastern—tfrom 
Europe, commercially. 
Northwest. — The northern parts of Europe, Asia and 
North America have in common very many identical or closely 
allied species, whose distribution is accounted for if, as geol- 
ogists assure us, Asia and North America were once con-: 
nected, at a time when a subtropical climate prevailed within 
the Arctic Circle; in fact, the distribution is scarcely explic- 
able upon any other theory. Curiously enough, the trend of 
diffusion seems to have been from Asia into North America 
and rarely the reverse, so far as can be inferred. 
Coccinella quinquenotata, occurring in Siberia and Alaska, 
has spread to Hudson Bay, Greenland, Kansas, Utah, Califor- 
nia and Mexico; while C. sanguinea, well known in Europe 
and Asia, ranges from Alaska to Patagonia; and Megilla mac- 
ulata from Vancouver and Canada to Chile. About six hun- 
dred species of beetles are holarctic in distribution, as was 
mentioned. Some of them inhabit different climatal regions 
in different parts of their range; thus Melasoma (Lina) lap- 
pomica in the Old World “ occurs only in the high north and 
on high mountain ranges, whereas in North America it ex- 
tends to the extreme southern portion of the country,” being 
widely diffused over the lowlands (Schwarz). Similarly, 
