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and activity; but the composition of the atmosphere has 

 remained unaltered since man has appeared upon the earth — 

 probably since hot-blooded animals have peopled the surface 

 of our globe. Animals, like plants, exert a marked influence 

 upon the aerial medium in which they live, only this is of an 

 exactly opposite nature to that exerted by vegetation. A 

 portion of the oxygen drawn into the lungs of an animal at 

 each inspiration is retained, and expiration discharges into 

 the air, in its stead, an equal volume of carbonic acid gas. 

 The removal of pure oxygen, and the substitution of carbonic 

 acid, is the constant and invariable effect of all animal respi- 

 ration. An animal confined in a limited portion of air soon 

 deteriorates this so much by the process of respiration, that 

 the air is incapable of any longer subserving this function, 

 and the animal dies suffocated. It might be feared that the 

 constant action of the whole animal kingdom upon the entire 

 of our atmosphere, would in like manner so deteriorate the 

 whole mass as to render it unfit for respiration, when all 

 animal life must cease ; but we have seen that vegetable life 

 produces a precisely opposite effect, and by this mutual 

 counteraction, an unvaried state of the air is constantly 

 preserved during the present era of the world's history. 



The infrequency of animal remains in the strata which re- 

 present that early period in the geological history of our earth 

 to which we have before adverted, when our coal beds were 

 living forests, leads to the conclusion that animals were as yet 

 few in number ; and they were chiefly aquatic or cold-blooded 

 animals, whose slow respiration would be quite incapable of 

 countervailing the effects upon the atmosphere of an active and 

 crowded vegetation. Since that period, animal life is shown, 

 in like manner, by the examination of fossil remains in the 

 strata of more recent formation, to have been gradually on 

 the increase in respect to numbers, and to have presented 

 beings more highly organised, whose active respiration would 

 more decidedly antagonise the effects of vegetable life ; which 



