DISTRIBUTE INSECT VARIETY. 273 



deposited sand, limestone, and chalk over the land, as we have 

 shown to be the case in respect of our own island, where around 

 the feet of the mountains of Scotia and Cambria from the 

 earliest times the invading waves have been stayed. 



We will now turn and inquire what evidence of the geo- 

 logical past is furnished by a study of Insecta, and how far the 

 modern cosmological theories of descent are vindicated. The 

 present distribution of insects on the terrestrial superficies is con- 

 veniently treated in respect to six great zoological regions, namely, 

 the Palaearctic Region, comprising the whole of Asia-Europe except- 

 ing the south-east of Asia, Northern Arabia, and North Africa 

 as far as the Sahara ; the Ethiopian Region, including Africa 

 south of the Sahara, and the southern portion of Arabia; the 

 Indian Region, including South-Eastern Asia ; the Australian 

 Region, comprising Celebes, New Guinea, Australia, and New 

 Zealand, with the Polynesian Archipelago ; the Neotropical 

 Region, comprising South and Central America, the West Indies, 

 and a great part of ^Mexico ; lastly, the Nearetic Region, or 

 North America. Since certain species may be selected as 

 characterising either division^ and others are more or less widely 

 spread or cosmopolitan, superficial harmony is presented to the 

 classifier, who allots his various kinds different localities thus indi- 

 cating their distribution, and proceeds to associate them with the 

 soils and similarly dispersed natural productions. But deeper 

 insight tends to dispel this crude idea of constancy ; for we notice 

 a species invariable in one district in another assume so changed 

 an aspect that until its transformation is understood, it is made 

 to rank distinct. Others, again, are so prone to vary, that on 

 one spot two specimens are seldom alike — a circumstance which 

 has signally foiled the most expert, who experience an acknow- 

 ledged difficulty in defining types from their varieties. 



Insects, indeed, vary in all their stages, and in the final or 

 perfect condition they do so uniformly or by differentiation of 

 the sexes, producing some varieties, seasonal or of geographical 

 limitation, that are selected or become permanent, and others 

 resulting from change in larval diet ; nor is hybridism unknown, 

 and species vary generally on the limits of their horizontal and 

 vertical distribution. This variation, which preceding geologists 

 consider marks extinction or, as has been believed, mutation of 

 s 



