46 THOUGHTS ON THE 



in Dejean's Catalogue, only about one-fourth belong to the 

 tropical parts, whilst in the terrestrial Adephaga the proportion 

 is about one-third. In the Libellulce, those from equatorial 

 regions yield in bulk to our own, although in some species the 

 abdomen is of extraordinary length. Those Lepidoptera also, 

 which in the larva state may be almost termed aquatic, offer 

 no striking difference in size between those from within the 

 tropics, and from the northern parts of the temperate zone. 

 We have therefore good reason for believing that aquatic 

 insects are but little influenced by climate, a conjecture ren- 

 dered the more probable by the wide range of certain species 

 which are found to extend from lat. 45° north to lat. 10° south ; 

 and to be common to the Old and New World. 



The proportion of aquatic Hemiptera is so small, and so 

 nearly that of Coleoptera, that we cannot suppose this order 

 to be less influenced by climate ; and the Orthoptera, being 

 altogether terrestrial, are, of all classes, the most exposed 

 to this influence. 



Had we sufficient data, it would be well worth inquiring 

 what proportion the number of species in each of the great 

 divisions of the globe bears to the whole, and also in what 

 ratio the species in a given space increase in number as we 

 proceed towards the equator. Perhaps, if we reason upon 

 what has been observed with regard to plants, we may arrive 

 at more correct conclusions than if we trust to our more 

 imperfect knowledge of foreign species. 



Humboldt states, that, of 38,000 species of plants described 

 and preserved in Herbaria, 7,000 belong to Europe, 6,000 to 

 Asia, 3,000 to Africa, 5,000 to New Holland and the Isles of 

 the Pacific, and 17,000 to America. The ratio of increase in 

 proceeding southwards, for latitudes 68°, 45°, 0°, is as 1 : 4 : 

 12. This is, in all probability,, nearly the case in insects. 



Another point to which our attention should be directed is 

 the proportion which the number of genera bears to that of 

 species. In plants, we find, whether we proceed towards the 

 poles, or the summits of lofty mountains, that the number of 

 species diminishes much faster than that of genera: " Nam in 

 regiones cum frigidas, turn aridas genera zonarum propin- 

 quarum semper unam alteramve speciem quasi colonos immit- 

 tunt: unde fit numerum generum magis ibi crescere quam 

 specierum." 



