The spectroscope shows that, so far as the chemical elements go,
other worlds are composed of the same elements as ours. Hydrogen
especially exists everywhere, and we have reason to believe that
the same is true of oxygen and nitrogen. Calcium, the base of
lime, is almost universal. So far as chemical elements go, we may
therefore take it for granted that the conditions under which life
begins are very widely diffused in the universe. It is, therefore,
contrary to all the analogies of nature to suppose that life began
only on a single world.

It is a scientific inference, based on facts so numerous as not to
admit of serious question, that during the history of our globe
there has been a continually improving development of life. As
ages upon ages pass, new forms are generated, higher in the scale
than those which preceded them, until at length reason appears and
asserts its sway. In a recent well-known work Alfred Russel
Wallace has argued that this development of life required the
presence of such a rare combination of conditions that there is no
reason to suppose that it prevailed anywhere except on our earth.
It is quite impossible in the present discussion to follow his
reasoning in detail; but it seems to me altogether inconclusive.
Not only does life, but intelligence, flourish on this globe under
a great variety of conditions as regards temperature and
surroundings, and no sound reason can be shown why under certain
conditions, which are frequent in the universe, intelligent beings
should not acquire the highest development.

Now let us look at the subject from the view of the mathematical
theory of probabilities. A fundamental tenet of this theory is
that no matter how improbable a result may be on a single trial,