GEOGRAPHICAL DISTRIBUTION OF INSECTS. 47 



In comparing the productions of places situated under the 

 same parallel of latitude, but differing greatly in longitude, 

 reference must always be had to the inflexion of the isothermal 

 lines, and also to the difference between the mean summer and 

 mean winter temperature. If, in our idea of the mean tem- 

 perature of a place, we are guided solely by latitude, we shall 

 err most surprisingly. Pekin and Philadelphia are nearly 2° 

 more south than Rome, yet at Rome we find the mean tem- 

 perature 15*8° centigrade, whilst, at the two former, it is only 

 12"7° cent., a difference of 3*1° cent, degrees, about 5'6° of 

 Fahrenheit's scale. The mean summer and mean winter 

 temperature offer still greater differences : at Rome, the latter 

 is + 7°-7; at Pekin, — 3°-l ; at Philadelphia, + 1°-1 ; the 

 former, at Rome and Philadelphia, is 24°, at Pekin, 28 0, 1. 

 If we proceed westward, from the shores of the Atlantic, until 

 we arrive at the basin of the Mississippi, we shall find the mean 

 temperature about 2° Fah. less than on the coast at the same 

 latitude, a difference which would increase as we proceeded 

 towards the Rocky Mountains, were not the summers so ex- 

 tremely hot as in some degree to counterbalance the intense 

 cold of the winters. 3 The temperature of the western coast of 

 North America appears to differ but little from western 

 Europe. In the eastern parts of Europe the temperature 

 more nearly resembles that of America on its eastern shores : 

 Nicolaieff, on the Black Sea, about 5° of latitude south of us, 

 having a mean temperature of about 2° Fah. less than ours. 



In tracing the changes of form, which are observable in 

 insects, as we proceed towards the equator, we must remember 

 that it is only from those inhabiting the parts but little elevated 

 above the level of the sea that our inferences should be drawn. 

 If we disregard the effect of elevation we shall be sure to fall 

 into error. Styracijlua liquidambar , which, at Xalapa, clothes 

 the sides of the mountains at an elevation of three or four 

 thousand feet, in New England is met with only in the plains. 

 Its true climate, therefore, is not that of Mexico, but of the 

 northern parts of the United States. The same will hold 

 good with regard to insects ; and therefore we have no right to 

 call an insect tropical unless we know the elevation of the 



a At Council Bluffs, on the Missouri, the thermometer has a range of 129° 

 Fah., or from - 21° to + 108°. 



