enabled gigantic species of plants to develope at a time 

 when there was little soil to support them, but also, in some 

 measure, prevented their dead remains becoming decomposed 

 by the action of the oxygen of the atmosphere."* " It is 

 further supposed," adds Dr. Lindley, " that the excess of 

 carbonic acid thus assumed to have existed, which would 

 have been fatal to air-breathing animals, was gradually 

 abstracted from the atmosphere by plants, until the air 

 became fit, in the first place, for the respiration of reptiles, 

 and then for that of mammalia." 



This latter particular is a most important one for our 

 present consideration. It is a well-established fact in the 

 physiology of plants, that the entire effect of vegetation upon 

 the atmosphere is the removal of carbonic acid gas, which is 

 absorbed by the green parts of plants into their tissues. Here 

 the decomposition of the gas is eff'ected, its carbon becoming 

 fixed in combination with the elements of water, to form the 

 constituents of freshly developing parts, whilst the pure 

 oxygen is again discharged into the atmosphere. 



Thus we can understand how the carbonic acid which 

 loaded the atmosphere in a more early period in the history 

 of our globe, was removed by the action of a luxuriant 

 vegetation, which, having fixed the carbon in its own tissues, 

 became itself converted into fossil coal; fitting the air for 

 the respiration of man and other hot-blooded animals, and 

 providing for him abundant stores of the most valuable fuel. 

 The coal which we burn upon our hearths and in our 

 furnaces, was formerly carbonic acid difi'used through the 

 air ; and the eff'ect of its combustion at the present day is, 

 to restore it to the atmosphere again in the same state of 

 gaseous combination. 



Vegetation is still exerting the same influence upon the 

 atmosphere as in former times, in proportion to its amount 



* Lindley and Hutton's Fossil Flora, vol i., preface, page 21. 



