On the Geographical Distribution of Insects. 173 



Continuous areas are most common, but they do not afford 

 so many interesting considerations as the other sort. Dis- 

 joined areas become more rare, in proportion as Ave descend to 

 inferior groups, and acquire great importance when we ar- 

 rive at species, for if the individuals of any one of them exist 

 in two countries remote from each other, we are often greatly 

 perplexed to explain the fact ; in such a case we are almost ob- 

 liged to admit that this separation has taken place with the 

 origin of that species, in other words, that it must have had a 

 multiplied origin. 



A group is said to be sporadical when it inhabits many re- 

 gions, and endemical when found in one only. It will be ob- 

 served that these words have only a relative meaning depend- 

 ing on the extent assigned to regions. 



Notwithstanding their powers of locomotion, entomological 

 species appear less sporadical than vegetable. About 1th of 

 the phanerogamous plants of the United States reappear in 

 Europe, and the proportion is much higher in regard to the 

 cryptogamia, while it falls greatly short of this in the case of 

 insects. The reason of this no doubt is, that the seeds of 

 plants are liable to be transported to a distance by a multi- 

 tude of causes, which is not the case with the eggs of the lat- 

 ter. But locomotion plays an important part in their spora- 

 dicity, when they are compared with each other. We iind 

 that it is the Lepidoptera which furnish the greatest number 

 of sporadical species, and after them the IRmenoptera. 



The kind of nourishment also exercises no small influence 

 on this phenomenon. Phytophagous species are more remark- 

 able in this respect than creophagous, although there are a 

 considerable number of the latter whose geographical exten- 

 sion is very great. 



It happens, as might naturally be expected, that species ex- 

 tend more in a parallel direction than in the opposite one. The 

 temperature is much more uniform in the former than in the 

 latter case. Thus we find at Japan a great number of the 

 kinds which occur in the neighbourhood of Paris ; Thunberg 

 has mentioned 50 in his catalogue of Japanese insects of which 

 this holds true. The same thing is observed in all latitudes, 



