The result is that, neglecting the effects of atmospheric refraction, which would tend to expand the borders of the domain of sunlight, about one quarter of the entire surface of Mercury is, with regard to day and night, in a condition resembling that of our polar regions, where there is but one day and one night in the course of a year--and on Mercury a year is eighty-eight days.

One half of the remaining three quarters of the planet's surface is bathed in perpetual sunshine and the other half is a region of eternal night.
And now again, what of life in such a world as that?
On the night side, where no sunshine ever penetrates, the temperature must be extremely low, hardly greater than the fearful cold of open space, unless modifying influences beyond our ken exist.

It is certain that if life flourishes there, it must be in such forms as can endure continual darkness and excessive cold.

Some heat would be carried around by atmospheric circulation from the sunward side, but not enough, it would seem, to keep water from being perpetually frozen, or the ground from being baked with unrelaxing frost.

It is for the imagination to picture underground dwellings, artificial sources of heat, and living forms suited to unearthlike environment.
What would be the mental effects of perpetual night upon a race of intelligent creatures doomed to that condition?
Perhaps not quite so grievous as we are apt to think.