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all forms of life of which we have any conception, two things in nature are vital, air and water. A planet must possess these two requisites to be able to support any life at all upon its surface. For there is no creature, no plant, no anything endowed with the possibility of that kind of change we call life, which is not in some measure dependent upon both of them. How, then, is Mars off for air?

Fortunately for an answer to this question, air, in the post-chaotic part of a planet's career, plays as vital a rôle in the inorganic processes of nature as in the organic ones. By the post-chaotic period of a planet's history we may designate that time in its evolutionary existence which follows the parting with its own inherent heat. After its heat has gone from it, atmosphere becomes essential, not only to any form of life upon its surface, but to the production of any change whatever there. Without atmosphere all development, even the development