THE MEANING OF EVOLUTION 265 



activity tends always toward cessation, the oxygen of 

 the atmosphere would gradually disappear, first by 

 its combination with oxidisable substances, and second 

 by its combination with the nitrogen of the atmosphere 

 under the influence of electric discharges. Carbon 

 dioxide would either combine with materials in the 

 rocks, or would remain in the atmosphere along with 

 nitrogen and other inert gases in a stable condition. 

 Water, moved by the tides and winds, would gradually 

 plane down the surface of the land, unless along with 

 other gases it would gradually become dissipated into 

 outer space. We see, then, that the materials of the 

 earth tend to fall into stable combinations, and that 

 they approximate toward conditions in which potential 

 chemical energy becomes reduced to a minimum, the 

 whole energy possessed by matter being that of the 

 motions of the molecules, that is, kinetic energy un- 

 available for transformations of any kind. It would 

 be an earth devoid of phenomena. 



Vegetable life alone would be possible only for a 

 time on an earth such as we know it at present. The 

 green plant depends for its existence on the presence in 

 the soil of mineral substances such as salts of nitric 

 acid and of ammonia, and on the presence of water and 

 carbon dioxide in the atmosphere. The chlorophyllian 

 apparatus is essentially a mechanism whereby these 

 substances become built up into carbohydrates, like 

 starch and sugar ; hydrocarbons, like resins and oils ; 

 and proteids. The energy necessary for these syntheses 

 is obtained from solar radiation through the agency of 

 the chlorophyll plastids. The green plant would 

 depend for its supply of nitrate or ammonia on the 

 combination of the nitrogen of the atmosphere with 

 oxygen, or on the exhalations from volcanoes, and 

 these are irreversible processes which tend continually 



