justified in affirming that life is ubiquitous, although we are thus
justified in saying that it must be, in a general sense, universal. We
might liken the universe to a garden known to contain every variety of
plant. If on entering it we see no flowers, we examine the species
before us and find that they are not of those which bloom at this
particular season, or perhaps they are such as never bear flowers. Yet
we feel no doubt that we shall find flowers somewhere in the garden,
because there are species which bloom at this season, and the garden
contains all varieties.

While it is tacitly assumed that there are planets revolving around
other stars than the sun, it would be impossible for us to see them
with any telescope yet invented, and no instrument now in the
possession of astronomers could assure us of their existence; so the
only planetary system of which we have visual knowledge is our own.
Excluding the asteroids, which could not from any point of view be
considered as habitable, we have in the solar system eight planets of
various sizes and situated at various distances from the sun. Of these
eight we know that one, the earth, is inhabited. The question, then,
arises: Are there any of the others which are inhabited or habitable?
Since it is our intention to discuss the habitability of only one of
the seven to which the question applies, the rest may be dismissed in
a few words. The smallest of them, and the nearest to the sun, is
Mercury, which is regarded as uninhabitable because it has no
perceptible supply of water and air, and because, owing to the
extraordinary eccentricity of its orbit, it is subjected to excessive
and very rapid alterations in the amount of solar heat and light
poured upon its surface, such alterations being inconsistent with the
supposition that it can support living beings. Even its average
temperature is more than six and a half times that prevailing on the