LIFE IN SPACE AND TIME 21 



though similar, by no means follow exactly the same routine, 

 is in full accord with this view, as is also the nature of the 

 observed variations in color. 



Though the hypothesis of vegetation offers a sufficient 

 explanation of the observations, it is by no means a necessary 

 one. Arrhenius, for example, has suggested that similar 

 variations in appearance would be presented by sahne 

 deserts, playas as they are called in the Southwest, which, 

 in the rainy season, are seas of dark mud, while in the 

 dry season the efflorescent salts come to the surface and 

 the color-tone is much hghter. Even in the absence of 

 actual rain, the absorption of moisture by hygroscopic 

 salts might produce a similar effect. That such areas should 

 exist in a desert planet, whose surface has been extensively 

 weathered, is highly probable, and the seasonal changes, 

 alone, can hardly be regarded as decisive evidence in favor 

 of vegetation. 



The presence of a considerable amount of oxygen in the 

 atmosphere, however, cannot be accounted for on the 

 salt-desert theory, while it is an immediate consequence 

 of the presence of vegetation. Indeed, as has already been 

 said, it is hard to see how the oxygen could have got into 

 the atmosphere by inorganic processes, or remained there 

 permanently if it had. In the writer's judgment, the com- 

 bined evidence makes the existence of vegetation upon the sur- 

 face of Mars highly probable, though it cannot be said that it 

 estabhshes it beyond all doubt. 



Granting this, the canals are simply interpretable in 

 accordance with W. H. Pickering's suggestion that they 

 represent narrow strips of vegetation along valleys or 

 water-courses of some sort, where there is more moisture 

 available than in the surrounding deserts. The canals 

 in the dark areas, whose existence appears to be well authen- 

 ticated, would then be bands of richer vegetation in a country 

 of sparse growth, and the seasonal changes in the canals 

 are immediately exphcable. 



The importance of such a conclusion to a general philos- 

 ophy of nature is obvious. Life, amazing as it is in the 

 complexity and dehcate adjustment of its processes, is 

 not confined to our world alone, where we might suppose 



