GEOGRAPHICAL DISTRIBUTION OF INSECTS. 51 



mountains. On the eastern side, the climate is dry to excess, 

 the vegetation scanty, scarce offering food sufficient for any of 

 the larger quadrupeds. The bison, which, more to the east, 

 roam in herds of fifteen or twenty thousand, no longer find 

 food for their countless numbers ; a few argalis {Americe, 

 big-horns) and a straggling antelope are the sole occupiers 

 of these sterile plains ; scarce a tree enlivens the desolate 

 landscape ; rain is rare, but at times descends in torrents. 

 Though the summers are extremely hot, the winter tempera- 

 ture often reaches the point at which mercury congeals, but is 

 rendered less difficult to bear by the great dryness of the 

 winter months. But how different is every thing to the west 

 of the mountains. Close to their base the climate is mild and 

 dry ; but, as we advance to the shores, it becomes more and 

 more rainy, resembling much that of Ireland. The winters 

 are mild, scarcely ever frosty, but the rains are continual ; the 

 summer moderately warm, with frequent rains. The vege- 

 tation is here totally different ; and, instead of being barren of 

 trees, the firs often exceed three hundred feet in height. Can 

 any suppose this to be rightly considered as a subclimate, the 

 insects of which are to resemble one another as much as those 

 of the department of the Seine resemble those of Prussia ? 



Again ; let us turn to South America, and survey the space 

 bounded by the 79th and 55th meridians, and by the 12th 

 northern parallel and the equator. Is this an insect climate 

 or subclimate ? assuredly not. But first how are we to under- 

 stand these terms? I should define them thus. Nature has 

 given to each species certain assigned limits ; these it cannot 

 pass. In some the greater degree of flexibility of organiza- 

 tion enables them to extend over a wide space, but of others 

 the range is much more confined. Certain forms also are 

 peculiar to certain regions. Supposing that we start from 

 any fixed point, we will say the extreme northern limit of 

 insects, and proceed southward until the greater portion of 

 species differ from those we have left behind us, and the 

 general form is materially altered ; we have now arrived at a 

 new subclimate ; proceed farther, until we find the whole, or 

 nearly the whole, of the species different, and these new 

 species presenting a different general form, we have now 

 reached a new climate. The same will apply, if we suppose 



