The astronomer might be able to give a partial answer if he could find evidence of life on some other planet, for we should then at least know that life had occurred more than once in the history of the universe, but so far no convincing evidence has been forthcoming. Some astronomers interpret certain markings on Mars as canals, which they believe to be the handiwork of intelligent beings, but this interpretation is not generally accepted. Again, seasonal changes necessarily occur on Mars as on the earth, and certain phenomena accompany these which many astronomers are inclined to ascribe to the growth and decline of vegetation, although they may represent nothing more than rains watering the desert. There is no definite evidence of life, and certainly no evidence of conscious life, on Mars—or indeed anywhere else in the universe. It seems at first somewhat surprising that oxygen figures so largely in the earth’s atmosphere, in view of its readiness to enter into chemical combination with other substances. We know, however, that vegetation is continually discharging oxygen into the atmosphere, and it has often been suggested that the oxygen of the earth’s atmosphere may be mainly or entirely of vegetable origin. If so, the presence or absence of oxygen in the atmospheres of other planets should shew whether vegetation similar to that we have on earth exists on these planets or not. Oxygen certainly exists in the Martian atmosphere, but its amount is small. Adams and St John estimate that there cannot be more than 15 per cent. as much, per square mile, as on earth. On the other hand it is either completely absent, or of negligible amount, in the atmosphere of Venus. If any is present at all, St John estimates that the amount above the clouds which cover the surface of Venus is less than 0·1 per cent. of the terrestrial amount. The evidence, for what it is worth, goes to suggest that Venus, the only planet in the solar system outside Mars and the earth on which life could possibly exist, possesses no vegetation and no oxygen for higher forms of life to breathe.