Charlie <i 
DISTRIBUTION 
I. GEOGRAPHICAL 
Importance of Dispersion.—Dispersion enables species to 
mitigate the intense competition and the rigid selection that 
result from crowded numbers; hence the tendency to disperse, 
being self-preservative, has become universal. Some species 
habitually emigrate in prodigious numbers: the African migra- 
tory locust, the Rocky Mountain locust, and the milkweed but- 
terfly, which annually leaves the Northern states for the South 
in immense swarms, in autumn, and in the following spring 
straggles back to the North. Vanessa cardui occasionally mi- 
grates in immense numbers, as do also Pieris, some dragon 
flies and some beetles, notably Coccinellidz. 
Wide Distribution of Insects.—Insects have been found in 
almost every latitude and altitude explored by man.  Butter- 
flies and mosquitoes occur beyond the polar circle, the former 
ia Late83° NN. the Jatter ain Lat, 72> N.;-and a species var 
Emesa closely allied to our common £. longipes is recorded by 
Whymper from an altitude of 16,500 ft. in Ecuador, where, 
according to the same traveler, Orthoptera occur at 16,000 
ft., Pieris xanthodice ranges above 15,000 ft., and dragon flies, 
Hymenoptera and scorpions reach a height of 12,000 ft., while 
twenty-nine species of Lepidoptera range upward of 7,300 
ft. A very few species of insects inhabit salt water, Halobates 
being found far at sea; some kinds live in arid regions and a 
few even in hot springs, while caves furnish many peculiar 
species. In short, insects are the most widely distributed of 
all animals, excepting Protozoa and possibly Mollusca. 
While all the large orders of insects are world-wide in dis- 
tribution, the most richly distributed are Coleoptera, Thys- 
366 
