DISTRIBUTION OF THE PETALOCERA. 45 



than extreme heat*. Such animals therefore as can avoid 

 this cold — either by passing it in a state of torpidity, or by 

 the habit of buiTOwing in the earth, or by living in the sea, 

 or by artificial clothing, — will in general be found the most 

 widely dispersed, and the least to affect local situations. 

 And insects which can escape the extremes of cold, not 

 only by passing them in the torpid state of pupa, but by 

 being generally, when in this torpid state, buried in the 

 earth, must in a special manner be little sensible to the cold 

 winters of northern climates. What they chiefly require is 

 the presence of heat during some period of their existence ; 

 and the greater, Avithin certain limits, is the heat, the more 

 active will be their vital principle. On the American con- 

 tinent the extremes of heat and cold in the course of the 

 year are, as is well known, incomparably greater than in 

 places of the same latitude in Europe. We may therefore 

 readily conceive how particular famihes of insects will in- 

 habit a wider range of latitude in the former country than 

 in the latter. AVe see also how insects may swarm in the 

 very coldest climates, such as Lapland and Spitzbergen, 

 where the short summer can boast of extraordinary 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. 



It is on the above principles also that I would account 

 for what may seem at first sight an extraordinary circum- 



^The white bear of Greenland, Ursus maritimus, L,, appears to dislike 

 warmer climates, from the great obstacles heencounters towards the enjoy- 

 ment of his favourite element, and the procurement of his food in cpuntries 

 where he cannot float about on the ice. This animal hates drought ratlier 

 than heat. 



