DISPERSAL OF BUTTERFLIES 
117 
lie quotes, I do not think there is sufficient evidence for the 
belief that insects as a whole are influenced in their dispersal 
by that agency. His statement that the influence of high 1 
winds on insects is illustrated by the great number of butter¬ 
flies that are sometimes encountered by ships at sea at long 
distances from land, is somewhat misleading. As a matter 
of fact, remarkably few species of butterflies have been 
observed far out at sea, and these always belong to species 
that are in the habit of migrating. It is a well-known 
phenomenon that certain species of butterflies and moths, 
such as the painted lady (Vanessa cardui), the milk-weed 
butterfly (Anosia archippus) and the moth Urania leilus, 
congregate into flocks or swarms and migrate in a body at 
certain times of the year. And it is such swarms that are 
occasionally scattered by storms and carried out to sea. 
These are, however, altogether exceptional instances, and we 
are not justified in drawing conclusions from them and apply¬ 
ing them to insects as a whole, very few of which possess any 
migrating instincts. On the contrary, the facts of the 
geographical distribution of insects are, as a rule, quite 
in conformity of those of mammals. Even the distribu¬ 
tion of the strongly-winged Sphingidae in North America 
shows a distinct division into an Atlantic and Pacific sub- 
region. 
I have already alluded, on p. 90, to the range of the genus 
of butterflies Parnassius in North America, pointing out that 
it had apparently entered the continent in Alaska and had 
then spread along the Rocky Mountain chain. Altogether, the 
butterflies and moths of the Rocky Mountains show a close 
resemblance to those of the Old World ; among them we meet 
with the familiar genera Colias, Argynnis, Erebia and 
Oeneis.* The main advance has apparently taken place in a 
southward direction from the north along the crest of the 
mountains. 
Much remains to be done before we can obtain even a 
general idea of the beetle fauna of the Rocky Mountains, but 
it is certain that many Old World genera and even species 
have travelled southward along this chain for a considerable 
Pagenstecher, A., “ Lepidopteren d. Ilochgebirges,” p. 145. 
