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 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.