i 7 4 MAN'S PLACE IN THE UNIVERSE 



year at nearly the same date in the Martian calendar. 

 It may be that there is an inorganic explanation; the 

 spring rains moisten the surface and change its colour. 

 But it is perhaps unlikely that there is enough rain 

 to bring about this change as a direct effect. It is 

 easier to believe that we are witnessing the annual 

 awakening of vegetation so familiar on our own 

 planet. 



The existence of oxygen in the Martian atmosphere 

 supplies another argument in support of the existence 

 of vegetable life. Oxygen combines freely with many 

 elements, and the rocks in the earth's crust are thirsty 

 for oxygen. They would in course of time bring about 

 its complete disappearance from the air, were it not that 

 the vegetation extracts it from the soil and sets it free 

 again. If oxygen in the terrestrial atmosphere is main- 

 tained in this way, it would seem reasonable to assume 

 that vegetable life is required to play the same part on 

 Mars. Taking this in conjunction with the evidence of 

 the seasonal changes of appearance, a rather strong case 

 for the existence of vegetation seems to have been made 

 out. 



If vegetable life must be admitted, can we exclude 

 animal life? I have come to the end of the astronomical 

 data and can take no responsibility for anything further 

 that you may infer. It is true that the late Prof. Lowell 

 argued that certain more or less straight markings on 

 the planet represent an artificial irrigation system and 

 are the signs of an advanced civilisation; but this theory 

 has not, I think, won much support. In justice to the 

 author of this speculation it should be said that his own 

 work and that of his observatory have made a magni- 

 ficent contribution to our knowledge of Mars; but few 

 would follow him all the way on the more picturesque 



