THE ENVIRONMENT 59 



cold surface, and so no less certainly be de- 

 prived of a gaseous envelope and of oceans. 



Accordingly it appears safe to say that 

 really durable atmospheric conditions depend 

 upon sufficient size of the planet, the presence 

 of a sun, and rotation. 1 No doubt a host of 

 other factors which exist in the case of the 

 earth are only less important. In any event, 

 all such phenomena, though varied by chance, 

 are of automatic origin, and whatever may be 

 the peculiarities of our solar system there is 

 no reason to suppose that like conditions 

 are not of frequent occurrence. Throughout 

 space there must be thousands of planets 

 which, like the earth and Mars, are enveloped 

 in an atmosphere that endures through count- 

 less centuries, and that contains great quan- 

 tities of water and carbon dioxide. 



All such atmospheres must in greater or 

 less degree manifest general meteorological 

 phenomena. There must be winds and clouds, 

 rain and snow and ice, the formation of oceans 

 and ocean currents, streams and lakes, all in- 

 terrelated by complex cyclic processes which 

 endure. Tides, too, and magnetic and elec- 

 trical phenomena cannot be absent, while 



1 A full discussion of all such problems will be found in the 

 "Lehrbuch" of Arrhenius and in S. Giinther's "Handbuch 

 der Geophysik." Two volumes, Stuttgart, 1897-1898. 



