The melting of the caps shows that water vapor must be a constituent of the Martian atmosphere. Moreover, as the molecular weight of water vapor is less than that of oxygen or nitrogen or carbon dioxide, if the former can exist in the atmosphere of the planet, _a fortiori_ must these other gases. So that from this we have knowledge of the possibility of the presence of oxygen, nitrogen, and carbon dioxide there. From (28) we saw that their actual existence is demonstrated.
The next step is the ascertainment that the water is in very small amount. The extensive melting of the caps, (7), shows their quantity to be inconsiderable, which is the first fact pointing to a dearth of water. The second comes from the aspect and behavior of the reddish-ochre regions which proclaim them deserts, (18) and (19); the third from the detection of the character of the blue-green areas as not seas, (20), (21), and (22). In several different ways, study of these regions asserts their non-aquatic constitution, the easiest to appreciate being that they are traversed by permanent dark lines and other equally sedentary markings, (22). No bodies of water, therefore, are to be seen outside of the ephemeral polar seas, immediately surrounding the caps as they melt.
This leads us to the third presence on Mars indicative of a living world: vegetation. The other two spoke of substances necessary to life, the premises in the case, this one of organic existence itself, its conclusion. The evidence consists of static testimony from the look of the blue-green areas, (23), and of kinematic derived from their behavior, (24), (25), (26), and (27). Vegetation would present exactly the appearance shown by them, and nothing that we know of but vegetation could. But suggestive as their appearance is, it is as nothing compared with the cogent telltale character of their behavior. The seasonal change that sweeps over them is metabolic, constructive as well as destructive, that is, and proclaims an organic constitution for them such as only vegetation could produce. In tint their metamorphoses are those of the same substance. For the blue-green lapses into ochre and revives again to blue-green just as vegetation does on our own Earth at the proper season of the year, taking both the Sun and the advent of water into the reckoning. Furthermore, certain of the largest dark areas turn to a chocolate-brown at times, which is the color of fallow ground and suggestive, at least, as occurring where the blue-green at other seasons is the most pronounced. Lastly, the change occurs at the epoch at which, from a knowledge of the melting of the polar caps, theory demonstrates that it ought to take place if it be due to the action of vegetation.