GEOGRAPHICAL DISTRIBUTION OF INSECTS. 285 



Suppose, that when unprovided with instruments, or without 

 sufficient time to make use of them, we wish to ascertain the 

 temperature of some one of the plains on the sides of these 

 colossal mountains, — 



Whose heads in wintry grandeur tower, 



And whiten with eternal sleet; 

 While Summer, in a veil of flowers, 



Is sleeping rosy at their feet. 



Let us look around us. The breeze plays wantonly through 

 the dense dark-green foliage of the cacao, and the silky leaves 

 of the banana, whilst the tree-ferns wave gracefully above thick 

 groves of Cinchonas and arborescent Melastomce. The coffee 

 flourishes, exposed to full radiance of the sun. There are 

 no rows of plantains or of Erythrince, whose scarlet blossoms 

 are frequented by humming-birds, to shelter it from its too 

 great power. The presence of the cacao indicates a mean heat 

 between 84° and 74° Fah., whilst that of the Cinchoncs indicates 

 a much cooler temperature than the former of these. This 

 shows us that we are near the limit, — as one ceases, the other 

 begins to thrive. The coffee-shrub will only flourish without 

 shelter from the sun where the mean temperature does not 

 exceed 75° Fahv Comparing this with the limits of the cacao, 

 we arrive at the conclusion, that the temperature is either 74° 

 or 75° Fah. ; and knowing this, and the latitude, we can easily 

 deduce the elevation. 



In plants, we find equinoctial forms extending much farther 

 south of the equator than north of it ; and this also appears to 

 be the case in insects. Latreille states, that Trinidad is the 

 extreme northern limit of Morpho Menelaus and other equi- 

 noctial insects, whilst these are well known to be far from rare 

 at Rio, in lat. 20° 59' south ; and, if I mistake not, Spix and 

 Martius state, that they occur at an elevation of 2,000 or 

 0,000 feet on the campos in that latitude. 



Although we may lay it down as a rule, that the insects of 

 two countries enjoying the same temperature, but widely dif- 

 fering in latitude and longitude, will be totally different, yet we 

 shall find a certain resemblance between them, — a kind of 

 representation of one another, — which will be more strong in 

 proportion as the soil and general outline of the two countries 

 resemble one another. Thus we find the extraordinary South- 



