(25) Each hemisphere undergoes this metamorphosis in turn.
(26) That it is recurrent is again proof positive of an atmosphere.
(27) The changes are metabolic, since those in one direction are later reversed to a restoration of the original status. Anabolic as well as katabolic processes thus go on there; that is, growth as well as decay takes place. This proves them of vegetal origin.
(28) The existence of vegetation shows that carbonic acid, oxygen, and undoubtedly nitrogen, are present in the Martian atmosphere, since plants give out oxygen and take in carbonic acid.
(29) The changes in the dark areas follow upon the melting of the polar caps, not occurring until after that melting is under way;
(30) And not immediately then, but only after the lapse of a certain time.
(31) Though not seas now, from their look the dark areas suggest old sea bottoms, and when on the terminator appear as depressions (whether because really at a lower level or because of less illumination is not certain).
(32) That they are now the parts of the planet to support vegetation hints the same past office, as water would naturally drain into them. That such a metamorphosis should occur with planetary aging is in keeping with the kinetic theory of gases.
(33) Terminator observations prove conclusively that there are no mountains on Mars, but that the surface is surprisingly flat.
(34) But they do reveal clouds which are usually rare and are often, if not always, dust-storms.