48 THOUGHTS ON THE 



parts which it inhabits, and how far the effect of that elevation 

 is increased or diminished by pecuhar local circumstances. 



But we must not suppose that the insects of an elevated 

 region will altogether resemble those of a neighbouring more 

 northerly region where the mean temperature is the same. 

 The productions of a country are influenced, as I before 

 remarked, by its mean annual temperature, its mean summer 

 and mean winter temperature, and by the greater or less 

 difference between these two last. Hence, if we compare the 

 birds, insects, or plants of Europe, with those from the eastern 

 parts of North America, which have a corresponding mean 

 temperature, we shall find those of America to bear a much 

 greater resemblance to those from the tropical regions of that 

 continent than ours do to those of any part of Africa south of 

 the Great Desert. This may be accounted for by the great 

 heat of the summers in the Atlantic States, which fully equals, 

 if it does not exceed, the common temperature of the low 

 regions of the tropics. Perhaps, also, that great ocean of 

 sand which extends from the western shores of Africa to the 

 Persian Gulf, with scarce any interruption, may, conjointly 

 with the Mediterranean, have obstructed the spread both of 

 animals and plants towards the north. No species of that 

 lovely group, which may be called the humming-birds of the 

 Old World, has ever been found to visit Europe ; and our 

 summer visitants, finding in the northern parts of Africa, 

 amongst — 



— groups of lovely date-trees bending 

 Languidly their leaf-crowned heads 



Like youthful maids, when sleep descending, 

 Warns them to their silken beds, — 



a climate entirely conformable to their habits, never make the 

 fruitless attempt to cross the desert. 



But in the New World nothing occurs to prevent the spread 

 of species as far north as their organization will allow ; and 

 therefore we find some of the birds of its equinoctial regions, 

 summer visitants, even of the inhospitable regions of Canada. 

 Trochilus colubris, I believe, has been found as far north as 

 lat. 54«. 



On the eastern shores of America and Asia tropical forms 

 are intermixed with those of the temperate zone in an 

 extraordinary manner. Bamboos, Cycadem, Epidendra, 



