90 NATURAL HISTORY OF 



greater, within certain limits, is the heat, tne 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 principle 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 



* Horse Entoraologicae, part i. p. 45. 



