As will be gathered from his theories regarding vegetation, Professor Pickering does not deny the existence of a form of life upon Mars. But he will not hear of civilisation, or of anything even approaching it. He thinks, however, that as Mars is intermediate physically between the moon and earth, the form of life which it supports may be higher than that on the moon and lower than that on the earth. In a small book published in the latter part of 1907, and entitled _Is Mars Habitable?_ Dr. Alfred Russel Wallace sets himself, among other things, to combat the idea of a comparatively high temperature, such as Professor Lowell has allotted to Mars. He shows the immense service which the water-vapour in our atmosphere exercises, through keeping the solar heat from escaping from the earth's surface. He then draws attention to the fact that there is no spectroscopic evidence of water-vapour on Mars[21]; and points out that its absence is only to be expected, as Dr. George Johnstone Stoney has shown that it will escape from a body whose mass is less than one-quarter the mass of the earth. The mass of Mars is, in fact, much less than this, _i.e._ only one-ninth. Dr. Wallace considers, therefore, that the temperature of Mars ought to be extremely low, unless the constitution of its atmosphere is very different from ours. With regard to the latter statement, it should be mentioned that the Swedish physicist, Arrhenius, has recently shown that the carbonic acid gas in our atmosphere has an important influence upon climate. The amount of it in our air is, as we have seen, extremely small; but Arrhenius shows that, if it were doubled, the temperature would be more uniform and much higher. We thus see how futile it is, with our present knowledge, to dogmatise on the existence or non-existence of life in other celestial orbs. As to the canals Dr. Wallace puts forward a theory of his own. He contends that after Mars had cooled to a state of solidity, a great swarm of meteorites and small asteroids fell in upon it, with the result that a thin molten layer was formed all over the planet. As this layer cooled, the imprisoned gases escaped, producing vents or craterlets; and as it attempted to contract further upon the solid interior, it split in fissures radiating from points of weakness, such, for instance, as the craterlets. And he goes on to suggest that the two tiny Martian satellites, with which we shall deal next, are the last survivors of his hypothetical swarm. Finally, with regard to the habitability of Mars, Dr. Wallace not only denies it, but asserts that the planet is "absolutely uninhabitable."