are really not compelled to imagine special adaptations by means of
which it may be brought into accord with that prevailing upon the earth.
As long as the temperature does not rise to the _destructive_ point,
beyond which our experience teaches that no organic life can exist, it
may very well attain an elevation that would mean extreme discomfort
from our point of view, without precluding the existence of life even in
its terrestrial sense.

And would it not be unreasonable to assume that vital phenomena on other
planets must be subject to exactly the same limitations that we find
circumscribing them in our world? That kind of assumption has more than
once led us far astray even in dealing with terrestrial conditions.

It is not so long ago, for instance, since life in the depths of the sea
was deemed to be demonstrably impossible. The bottom of the ocean, we
were assured, was a region of eternal darkness and of frightful
pressure, wherein no living creatures could exist. Yet the first dip of
the deep-sea trawl brought up animals of marvelous delicacy of
organization, which, although curiously and wonderfully adapted to live
in a compressed liquid, collapsed when lifted into a lighter medium, and
which, despite the assumed perpetual darkness of their profound abode,
were adorned with variegated colors and furnished with organs of
phosphorescence whereby they could create for themselves all the light
they needed.

Even the fixed animals of the sea, growing, like plants, fast to the
rocks, are frequently vivid with living light, and there is a splendid
suggestion of nature's powers of adaptation, which may not be entirely
inapplicable to the problems of life on strange planets, in Alexander
Agassiz's statement that species of sea animals, living below the depths
to which sunlight penetrates, "may dwell in total darkness and be
illuminated at times merely by the movements of abyssal fishes through
the forests of phosphorescent alcyonarians."

In attempting to judge the habitability of a planet such as Venus we
must first, as far as possible, generalize the conditions that govern
life and restrict its boundaries.

On the earth we find animated existence confined to the surface of the
crust of the globe, to the lower and denser strata of the atmosphere,
and to the film of water that constitutes the oceans. It does not exist
in the heart of the rocks forming the body of the planet nor in the void
of space surrounding it