6 LIFE ON THE EARTH. 



conceive of as possible. For though it may be urged, 

 and cannot be doubted, that other elements, other 

 forces, and other laws might be employed for other 

 systems of life ; that is a conjecture which may be 

 useful when thinking of the possible inhabitants of 

 other planets, as Mercury, or Jupiter, or Neptune, but 

 is not suited to the Land, Sea, and Air, with which our 

 History of Life is connected. Deprive the atmosphere 

 of its carbonic acid, plants disappear ; let phosphate 

 of lime be absent not only vertebrate animals vanish, 

 but a large part of both the animal and vegetable 

 races would languish and become unproductive 1 . 

 Without supposing oxygen or the other elements 

 to be entirely absent, any material change in their 

 relative quantity must greatly affect the relative 

 abundance of different races of living beings whose 

 dependence on these elements is different in degree. 

 Life is dependent on a continual loss and restora- 

 tion of parts in its organic fabric. One great part in 

 this process is maintained by the atmosphere, from 

 which all plants and all animals draw supplies of 

 gaseous elements suited to their constitution. Plants 

 absorb the carbonic acid, which exists in the at- 

 mosphere to the extent of nnroth part by weight, 



1 Phosphate of Lime occurs in so many animals, and in so 

 many plants, in some part or other, as to be regarded by eminent 

 writers as an invariable accompaniment of life. 



