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. Thus a departure from the terrestrial
type in the envelope of gases covering a planet, instead of precluding
life, would only tend to vary its manifestations.

After all, why should the intensity of the solar radiation upon Venus be
regarded as inimical to life? The sunbeams awaken life.

It is not impossible that relative nearness to the sun may be an
advantage to Venus from the biologic point of view. She gets less than
one third as much heat as Mercury receives on the average, and she gets
it with almost absolute uniformity. At aphelion Mercury is about two and
four tenths times hotter than Venus; then it rushes sunward, and within
forty-four days becomes six times hotter than Venus. In the meantime the
temperature of the latter, while high as compared with the earth's,
remains practically unchanged. Not only may Mercury's temperature reach
the destructive point, and thus be too high for organic life, but
Mercury gets nothing with either moderation or constancy. It is a world
both of excessive heat and of violent contrasts of temperature. Venus,