98 Geographical Distribution of Insects. 



these agents. Insects being for the most part very active, are, 

 of course, in the latter condition, and we must therefore take 

 into account their faculty of locomotion. 



The external conditions whose influence requires to be 

 examined are, nourishment, temperature, light, soil, and or- 

 ganized beings. 



Influence of Nourishment. — When vegetable life ceases, ani- 

 mal life likcAvise becomes extinct ; but this is the case only 

 with land species. Such as are aquatic are independent of 

 vegetable life, and perhaps no part of the sea furnishes a 

 greater number of living beings than the polar regions.* In- 

 sects are everywhere subject to the law in question, and they 

 cease to occur in the same latitude as phanerogamous plants, 

 with which their existence is very closely connected. Mel- 

 ville Island (75" N. Lat.), which possesses only a few vegetables 

 of this sort, furni.shed only six insects to Sir E. Parry's expe- 

 dition, during the eleven months of their sojourn upon it. 



When phytophagous species cease, it follows as a necessary 

 consequence that creophagous species, which live at their ex- 

 pense, must likewise cease. 



The inverse of this is observed in proportion as we recede 

 from the poles. Phytophagous species augment along with 

 vegetables, and their number attains its maximum under the 

 Tropics at the same tune as the latter. But this progressive 

 increase does not take place with all the creophagous species, 

 particularly those of the coleopterous order, as will afterwards 

 be shewn more particularly. Of these the equatorial regions 

 possess an infinitely smaller number, viewed both absolutely 

 and relatively, than the temperate regions of the nox'thern 

 hemisphere. 



Other considerations may be deduced from the relations 

 ■which exist between plants and insects. 



It is doubtful whether any insect is to be foxmd whose 



* The animalcules of the order Acalephxis are to other marine animals 

 what vegetables are upon land to terrestrial animals. It is their innume- 

 rable multitudes which renders life possible in the seas of these dismal re- 

 gions. Mr Scoresby has calculated that a surface of two square miles con- 

 tains 23,888,000,000,000,000 of these animalcula, and they are met with in 

 greater part of the Polar Seas. 



