342 On the Geographical Distribution of Insects. 



periods, characterized by the simultaneous appearance of cer- 

 tain species of flowers and insects.* 



Habitations. — This department of the subject, more im- 

 portant than any hitherto touched upon, divides itself into the 

 four following branches : — \st, The determination of the num- 

 ber of species of insects existing on the globe. 2d, The pro- 

 portion according to which the various famiUes are spread 

 over different countries. 3f/, The extent of the habitation of 

 species and groups. 4?/*, The division of the surface of the 

 globe into entomological regions, characterized by the insects 

 which predominate in each of them. 



In regard to the first subject of inquiry, it wiU appear ob- 

 vious, that, in the present state of entomology, it is impossible 

 to estimate the number of actually existing species, except by 

 way of induction, that is to say, hy setting out from a point 

 better known. Plants, which have always been collected with 

 greater ease than insects, and which have a most intimate rela- 

 tion to these animals, have properly been assumed as the point 

 of departure by authors engaged in this calculation. By com- 

 paring the number of insects with that of the plants of a given 

 country, we obtain the relation which subsists between these 

 two classes of organized beings ; and, by applying this rela- 

 tion to the total number of plants supposed to exist, we arrive 

 at the approximate result sought. 



Now there exist in France, according to the Botanicon 

 Gallicum of MM. Decandolle and Duby, 7194 species of plants : 

 it may now be considered to be 7400, with the addition of 

 those discovered since the publication of that work. The 

 number of insects in the same country, as far as it can be de- 

 termined from the study of authors and the inspection of the 

 richest collections, is not less than 15,000, which gives about 

 two insects for each plant. This number will appear too low, 

 for there are vegetables, such as the oak, which support twenty 

 times that number ; but if we reflect that the Cryptogamia, 

 which supply food to a very small number of these animals, 



* We have omitted M. Lacordaire's account of Kirby and Spence's views 

 on this subject, as being comparatively familiar to an English reader, or at 

 all events readily accessible to him in their Introduction, vol. iv. p. 512. 



