ur judgment as
to their conditions of habitability.

No doubt the natural tendency of the mind is to regard all the planets
as habitable worlds, for there seems to be deeply implanted in human
nature a consciousness of the universality of life, giving rise to a
conviction that one world, even in the material sense, is not enough for
it, but that every planet must belong to its kingdom. We are apt to say
to ourselves: "The earth is one of a number of planets, all similarly
circumstanced; the earth is inhabited, why should not the others also be
inhabited?"

What has been learned of the unity in chemical constitution and
mechanical operation prevailing throughout the solar system, together
with the continually accumulating evidence of the common origin of its
various members, and the identity of the evolutionary processes that
have brought them into being, all tends to strengthen the _a priori_
hypothesis that life is a phenomenon general to the entire system, and
only absent where its essential and fundamental conditions, for special
and local, and perhaps temporary, reasons, do not exist.

If we look for life in the sun, for instance, while accepting the
prevalent conception of the sun as a center of intense thermal action,
we must abandon all our ideas of the physical organization of life
formed upon what we know of it from experimental evidence. We can not
imagine any form of life that has ever been presented to our senses as
existing in the sun.

But this is not generally true of the planets. Life, in our sense of it,
is a planetary, not a solar, phenomenon, and while we may find reasons
for believing that on some of the planets the conditions are such that
creatures organized like ourselves could not survive, yet we can not
positively say that every form of living organism must necessarily be
excluded from a world whose environment would be unsuited for us and our
contemporaries in terrestrial life.

Although our sole knowledge of animated nature is confined to what we
learn by experience on the earth, yet it is a most entertaining, and by
no means unedifying, occupation, to seek to apply to the exceedingly
diversified conditions prevailing in the other planets, as astronomical
observations reveal them to us, the principles, types, and limitations
that govern the living creatures of our world, and to judge, as best we
can, how far those types and limits may be modified or extended so that
those other planets m