The generally accepted hypothesis of the origin of the solar system
leads us to believe that Venus has experienced the same process of
evolution as that which brought the earth into its present condition,
and we may fairly argue that upon the rocky shell of Venus exists a
region where chemical combinations and recombinations like those on the
surface of the earth are taking place. It is surely not essential that
the life-forming elements should exist in exactly the same states and
proportions as upon the earth; it is enough if some of them are
manifestly present. Even on the earth these things have undergone much
variation in the course of geological history, coincidently with the
development of various species of life. Just at present the earth
appears to have reached a stage where everything contributes to the
maintenance of a very high organization in both the animal and vegetable
kingdoms.
So each planet that has attained the habitable stage may have a typical
adjustment of temperature and atmospheric constitution, rendering life
possible within certain limits peculiar to that planet, and to the
special conditions prevailing there. Admitting, as there is reason for
doing, that different planets may be at different stages of development
in the geological and biological sense, we should, of course, not expect
to find them inhabited by the same living species. And, since there is
also reason to believe that no two planets upon arriving at the same
stage of evolution as globes would possess identical gaseous
surroundings, there would naturally be differences between their organic
life forms notwithstanding the similarity of their common phase of
development in other respects.
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