90 
NATUEAL HISTOEY OF 
greater, wltliin certain limits, is the heat, ttie more 
active will be their vital principle. On the Ameri- 
can continent, the extremes of heat and cold in the 
course of the year are, as is well known, incompa- 
rably greater than in places of the same latitude in 
Europe. We may therefore readily conceive how 
particular families of insects will inhabit a wider 
range of latitude in the former country than in the 
latter. We also see how insects may swarm in the 
very coldest climates, such as Lapland and Spitz- 
bergen, where the short summer can boast of ex- 
traordinary rises in the thermometer ; because the 
energy of the vital princijde in such animals is, 
within certain limits, proportionate to the degree of 
warmth to which they may be subjected, and escapes 
in a manner the severe action of cold.”* 
As heat is the principal agent in giving impulse 
and vigour to organic life, it will be found that 
these insects undergo as great a change under the in- 
creasing temperature of the earth and atmosphere, 
on approaching the equator, as is well known to take 
place in vegetables and the larger animals. Their 
numbers are prodigiously augumented, and they 
acquire considerable momentum from the great size 
of many of the species. The latter, too, are contin- 
ually varying even under the same parallel of lati- 
tude, so that countries similar to each other in soil, 
temperature, and all other circumstances which 
Horae Entomologicee, part i. p. 45. 
