ECONOMY OF NUTRITIVE MATTER. 51 



sible medusa,* through endless tribes of mollusca, and of 

 fishes, up to the huge Leviathan of the deep. 



Not even are these portions of organic matter, which, in 

 the course of decomposition, escape in the form of gases, 

 and are widely diffused through the atmosphere, wholly 

 lost for the uses of living nature: for, in course of time, 

 they, also, as we have seen, re-enter into the vegetable sys- 

 tem, resuming the solid form, and reappearing as organic 

 products, destined again to run through the same never end- 

 ing cycle of vicissitudes and transmutations. 



The diffusion of animals over wide regions of the globe 

 is a consequence of the necessity which prompts them to 

 search for subsistence wherever food is to be met with. 

 Thus while the vegetation of each different climate is regu- 

 lated b}' the seasons, herbivorous animals are in the winter 

 forced to migrate from the colder to the milder regions, 

 where they may find the pasturage they require; and these 

 migrations occasion corresponding movements among the 

 predaceous tribes which subsist upon them. Thus are con- 

 tinual interchanges produced, contributing to colonize the 

 earth, and extend its animal population over every habitable 

 district. But in all these changes we may discern the ulti- 

 mate relation they ever bear to the condition of the vegeta- 

 ble world, which is placed as an intermediate and necessary 

 link between the mineral and the animal kingdoms. All 

 those regions which are incapable of supporting an exten- 

 sive vegetation, are, on that account, unfitted for the habita- 

 tion of animals. Such are the vast continents of ice, which 

 spread around the poles; such are the immense tracts of 

 snow and of glaciers, which occupy the summits of the 

 highest mountain chains; and such is the wide expanse of 

 sand, which covers the largest portions both of Africa and 



* The immensity of the numbers of these microscopic medusae, which peo- 

 ple every region of the ocean, may be judged of from the phenomenon of 

 the phosphorescent light which is so frequently exhibited by the sea, when 

 agitated, and which, as 1 have already observed, is found to arise from the 

 presence of an incalculable multitude of these minute animals. 



