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 may reasonably be included among the probable abodes