tion on either
side of them. In support of this view he draws attention to the
existence of long "steam-cracks," bordered by vegetation, in the deserts
of the highly volcanic island of Hawaii. We have already seen, in an
earlier chapter, how he has applied this idea to the explanation of
certain changes which are suspected to be taking place upon the moon.

In dealing with the Lowell canal system, Professor Pickering points out
that under such a slight atmospheric pressure as exists on Mars, the
evaporation of the polar caps--supposing them to be formed of
snow--would take place with such extraordinary rapidity that the
resulting water could never be made to travel along open channels, but
that a system of gigantic tubes or water-mains would have to be
employed!

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 unif