THE ENVIRONMENT 61 



the solar system, and especially the eartli 

 among planets, are very favorable for life, 

 partly through apparently accidental circum- 

 stances. Putting aside, therefore, the biological 

 fitness of the special climates of the earth both as 

 a familiar fact, and as possibly in no small de- 

 gree accidental, we may more advantageously 

 give our attention at once to other phenomena 

 which appear to be of a far more general 

 character, — the occurrence of large quanti- 

 ties of water and carbon dioxide in the atmos- 

 phere, and the fundamental meteorological 

 processes which their presence involves. Ni- 

 trogen and various other substances auto- 

 matically find a place beside water and car- 

 bonic acid, but it will be convenient to pass 

 them by or to postpone the consideration 

 until other aspects of the subject have been 

 more fully developed. 



VI 



THE PRIMARY CONSTITUENTS OF THE 

 ENVIRONMENT 



Of course a consideration of the prop- 

 erties of water and carbonic acid might 

 be approached from a study of terrestrial 

 processes exclusively. But since the assump- 

 tion that such phenomena are common occur- 



