On the Geographical Distribution of Insects. 347 



regions, and the rule of division, of which we now speak, ne- 

 cessarily applies to them likewise. From this it follows, that 

 their number increases in an inverse ratio to that of the spe- 

 cies ; or, in other words, the number of genera in a given 

 country will be relatively greater the fewer species that coun- 

 try possesses, and reciprocally. 



Famihes being all represented in the same regions, except 

 the Xylophagi and Pselaphidae, they cannot be made the sub- 

 jects of similar calculations. We might descend, it is true, to 

 the groups immediately inferior, that is to say, to tribes ; but 

 it is well known how few entomologists are agreed on this 

 point. 



If, laying aside calculations, the precision of which is liable 

 to suspicion, we desire to class the different regions of the 

 globe according to their entomological riches, without attempt- 

 ing an imattainable exactitude, it will require to be done in 

 the following manner. 



In the first rank will be placed intertropical America ; and 

 at the head of the countries which it contains, Brazil ; after 

 which wiU come Mexico, then Guiana, and Colombia. 



The isles of Sunda, the portion of the Indian Continent lying 

 in their vicinity, Madagascar, Cafraria, and the eastern coasts 

 of intertropical Africa, will occupy the second rank ; but it is 

 impossible in the present state of our knowledge to determine 

 to which of these the preference ought to be assigned. 



In the third grade we may place Europe, including the sides 

 of the bason of the Mediterranean. Germany, taking that 

 word in its widest acceptation, appears the richest country of 

 this division. 



North America seems to be much less prolific than Europe 

 in the same latitude, and considering it as a whole, it appears 

 to be, in the view we are now taking, on a level with Asia, 

 which, although situate in part beneath the tropics, includes 

 too many extensive sterile tracts to be so productive of insects 

 as its geographical position would lead us to expect. 



The same reason renders it necessary to place in the fifth 

 rank Northern Africa, Chili, Tucuman, Peru, and in general 

 the countries of America lying to the west of Brazil and south 

 of Colombia. 



