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effect has been likewise favoured by the diminution in the 

 numbers and luxuriancy of the vegetable kingdom, as plants 

 gradually exhausted the air of that stimulus to their growth, 

 — the carbonic acid. ''At length," says Liebig, "man 

 appeared upon earth, and the air since then has been 

 rendered constant in its composition." 



From these considerations, it is evident that each of the 

 great kingdoms of organized nature is mutually dependant 

 upon the other for its continued existence. Had animals 

 never been created, vegetation must in time have exhausted 

 the air of carbonic acid, and plants must then have ceased 

 to live. On the other hand, an animal kingdom alone would 

 in time so diminish the amount of uncombined oxygen, and 

 increase the amount of carbonic acid, as to render the air 

 irrespirable, and all its species must die suffocated. Liebig 

 says, " The presence of a rich and luxuriant vegetation 

 may be conceived without the concurrence of animal life ; 

 but the existence of animals is undoubtedly dependant upon 

 the life and development of plants." But it is evident that 

 neither vegetation nor animal life could be permanent without 

 the co-existence of the other. 



It is proper, however, to state here, that Dumas and 

 Boussingault have made a calculation, according to which 

 it would require a period of 800,000 years for animal 

 respiration entirely to convert all the free oxygen of our 

 atmosphere into carbonic acid, supposing the counteracting 

 effect of vegetation to be entirely arrested ; and that it would 

 only be after the lapse of 10,000 years, that the change 

 produced in the atmosphere would become at all appre- 

 ciable. From this it would appear that even had we not the 

 benefit of the reducing action of vegetation upon carbonic 

 acid, still, during the period over which the history of our 

 own race extends, any change in the composition of the 

 atmosphere would be quite inappreciable. 



