LIFE ON THE EARTH. 7 



and yield oxygen in return ; while on the other 

 hand, animals inspire this oxygen, and evolve in 

 exchange carbonic acid. Thus appears a real and 

 necessary relation between the atmosphere as it is, 

 and the double system of life which is in operation. 

 If we change the constitution of the atmosphere, 

 all the relations in which it is so important must 

 be changed also, and amongst the most obvious of 

 these are the reciprocally dependent races of plants 

 and animals. It has been conjectured by Brong- 

 niart, that the very rich series of vegetable forms, 

 including many ferns, in the old carboniferous de- 

 posits, may have been favoured in their amazing 

 growth, not only by high temperature and humid 

 atmosphere, but by a greater proportion of carbonic 

 acid in the air. Dr Daubeny has submitted this 

 to a trial, in vessels properly supplied with a regu- 

 lated artificial atmosphere, and the result is not 

 unfavourable to the speculation. 



Again, animal life depends upon the previous 

 exercise of vegetable life; for ultimately all animals 

 subsist upon plants, as these feed upon the atmo- 

 sphere. Perhaps nothing is more surprising than the 

 immense diversity of the forms and qualities of 

 the plants, coupled with the almost equal depend- 

 ence of all vegetative life upon the same atmosphere 

 chemically everywhere almost identical. Upon this 



