tween Mars and our globe is intimately connected with certain points of contrast which it offers to the earth.
Of these the most important is that which concerns the atmosphere. When we consider what the qualifications for a globe as a possible abode for organic beings should be, it is natural to inquire first into the presence or the absence of an atmosphere. Seeing that our earth is enveloped by a copious covering of air, it follows that the beings which dwell upon its surface must be specially adapted to the conditions which the atmosphere imposes.
_-_Figure_17.jpg)
Fig. 17.—Mars and one of its Satellites.
Most, if not all, animals utilise this circumstance by obtaining a proximate source of energy in the union of oxygen from the atmosphere with oxidizable materials within their bodies. In this respect the atmosphere is of such fundamental importance that it is difficult for us to imagine what that type of life best adapted for existence on an airless globe would be. In other respects which are hardly less important, the conditions of life are also dependent on the fact that we live at the bottom of an
