452 



EXTINCTION OF SPECIES. 



[Ch. XLII. 



generally supposed to amount to eiglit linndred millions^ so 

 that we may easily understand liow great a nnmber of beasts 

 of prey^ birds^ and animals of every class^ this prodigious 

 population must have displaced^ independently of the still 

 more important consequences which have followed from the 

 derangement brought about by man in the relative numerical 

 strength of particular species. 



It may perhaps be said^ that man has, in no small de 

 compensated for the appropriation to himself of the food of 

 many animals by artificially improving the natural produc- 

 tiveness of soils, by irrigation, manure, and a judicious inter- 



mixture of mineral 



ingredients 



conveyed from dijfferent 



of organic life. 



cultural importance, though 



localities. But it admits of reasonable doubt whether, upon 

 the Avhole, we fertilise or impoverish the lands which Ave 

 occupy. This assertion may seem startling to many; because 

 they are so much in the habit of regarding tlje sterility or 

 productiveness of land in relation to the wants of man, and 

 not as regards the organic world generally. It is difficult, at 

 first, to conceive, if a morass is converted into arable land, and 

 made to yield a crop of grain, even of moderate abundance, that 

 we have not improved the capabilities of the habitable surface 



that we have not empowered it to support a larger quantity 



In such cases, however, a tract, before of no 

 utility to man, may be reclaimed, and become of high ag-ri- 



it may, nevertheless, yield a 

 scantier vegetation. If a lake be drained, and turned into a 

 meadow, the space will provide sustenance to man, and many 

 terrestrial animals serviceable to him, but not, perhaps, so 

 much food as it previously yielded to the aquatic races. 



The felling of dense and lofty forests, which covered, even 

 within the records of history, a considerable space on the 

 globe, now tenanted by civilised man, must generally have 

 lessened the amount of vegetable food throughout the space 

 where these woods grew. We must also take into our 

 account the area covered by towns, and a still larger smface 



occupied by roads. 



If we force the soil to bear extraordinary crops one year; 



we are, perhaps, compelled to let it lie fallow the next. But 

 nothing so much counterbalances the fertilising effects of 





