DISPERSAL OF. BUTTERFLIES 117 



he 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 

 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 thpse 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 Eocky 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. Hochgebirges," p. 145. 



