us forests, or the age of the gigantic reptiles who ruled
both land and sea.

But, without making any assumptions as to the phase of evolution which
life may have attained on Venus, it is also possible to think that the
planet's thick shell of air, with its abundant vapors, may serve as a
shield against the excessive solar radiation. Venus is extraordinarily
brilliant, its reflective power being greatly in excess of Mercury's,
and it has often been suggested that this may be due to the fact that a
large share of the sunlight falling upon it is turned back before
reaching the planet's surface, being reflected both from the atmosphere
itself and from vast layers of clouds.

Even when viewed with the most powerful telescopes and in the most
favoring circumstances, the features of Venus's surface are difficult
to see, and generally extremely difficult. They consist of faint shadowy
markings, indefinite in outline, and so close to the limit of visibility
that great uncertainty exists not only as to their shape and their
precise location upon the planet, but even as to their actual existence.
No two observers have represented them exactly alike in drawings of the
planet, and, unfortunately, photography is as yet utterly unable to deal
with them. Mr. Percival Lowell, in his special studies of Venus in 1896,
using a 24-inch telescope of great excellence, in the clear and steady
air of Arizona, found delicate spokelike streaks radiating from a
rounded spot like a hub, and all of which, in his opinion, were genuine
and definite markings on the planet's surface. But others, using larger
telescopes, have failed to perceive the shapes and details depicted by
Mr. Lowell, and some are disposed to ascribe their appearances to
Venus's atmosphere. Mr. Lowell himself noticed that the markings seemed
to have a kind of obscuring veil over them.

In short, all observers of Venus agree in thinking that her atmosphere,
to a greater or less extent, serves as a mask to conceal her real
features, and the possibilities of so extensive an atmosphere with
reference to an adjustment of the peculiar conditions of the planet to
the requirements of life upon it, are almost unlimited. If we could
accurately analyze that atmosphere we would have a basis for more exact
conclusions concerning Venus's habitability.

But the mere existence of the atmosphere is, in itself, a strong
argument for the habitability of the planet, and as to the temperature,
we