52 THOUGHTS ON THE 



that our course is directed parallel to the equator, or rather if 

 it follows the direction of the isothermal lines. 



In the want of a sufficient knowledge of the country, we 

 may be guided by its natural boundaries, the course of rivers, 

 the direction of mountains, the interposition of deserts, &c. 

 Spix and Martins remark, that most of the great tributaries of 

 the Maranon have a peculiar flora; Burchell remarks, that 

 the Great Karro is the limit of Cape plants, nearly all the 

 plants from the banks of Gariep and the country adjoining 

 being entirely different; and lastly, Latreille observes, that 

 lofty chains of mountains are mostly real limits in the geogra- 

 phy of insects, and therefore it is not surprising that the 

 insects of New Granada are totalis/ different from those of 

 Cayenne and Denierara. This last remark is true, very 

 true ; but we shall see how entirely in his distribution of 

 subclimates he has overlooked this circumstance. I have 

 before observed, that in all things relating to the division of 

 the earth into insect-climates we have nothing to do with 

 mountains, except in so far as they form natural boundaries 

 which prevent the spread of species beyond them. 



Let us return from this long digression to that subclimate 

 which includes the Republic of New Granada and Venezuela, 

 part of that of the equator (del Ecuador), Demerara, Berbice, 

 parts of Surinam, of French and of what was Portuguese 

 Guiana. First, we have the shores of the Pacific ; and what 

 relation have the insects from those shores to those of the 

 shores of Cape Paria ? Are not these regions separated by 

 snow-clad mountains, whose summits are never looked down 

 upon by any earthly being save the condor ? Do not these 

 mountains mark the limit which the western species cannot 

 pass ? If we follow the shores of the Atlantic as far as the 

 55th meridian, we arrive at that very country the insects of 

 which Latreille has pronounced to be totally different from 

 those of New Granada, even to the east of the Magdalena ; 

 and moreover, this region, which stretches from the mouths 

 of the Oronoco to that of the Amazons, is divided by a line 

 which separates Cayenne from Berbice and Demerara. Guiana, 

 or the country included between the Rio Negro, the Oronoco, 

 and the Maranon, is a vast Hylasa, a level and almost unin- 

 terrupted tract of forest, which cannot be better described than 

 in the words of the illustrious Prussian traveller :— " Sylvan 



