space. If it began at a certain stage of evolution by
a natural process, the question will arise, what conditions are
favorable to the commencement of this process? Here we are quite
justified in reasoning from what, granting this process, has taken
place upon our globe during its past history. One of the most
elementary principles accepted by the human mind is that like causes
produce like effects. The special conditions under which we find life
to develop around us may be comprehensively summed up as the existence
of water in the liquid form, and the presence of nitrogen, free perhaps
in the first place, but accompanied by substances with which it may
form combinations. Oxygen, hydrogen, and nitrogen are, then, the
fundamental requirements. The addition of calcium or other forms of
matter necessary to the existence of a solid world goes without saying.
The question now is whether these necessary conditions exist in other
parts of the universe.

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,
inte