Page:Is Mars habitable - Wallace 1907.djvu/105

 which was kept rigid by the weight of the overlying layers."

Now, adopting this theory as the last word of science upon the subject of the origin of planets, we see that it affords immense scope for diversity in results depending on the total amount of matter available within the range of attraction of an incipient planetary mass, and the rates at which this matter becomes available. By a special combination of these two quantities (which have almost certainly been different for each planet) I think we may be able to throw some light upon the structure and physical features of Mars.

The Probable Mode of Origin of Mars.

This planet, lying between two of much greater mass, has evidently had less material from which to be formed by aggregation; and if we assume—as in the absence of evidence to the contrary we have a right to do—that its beginnings were not much later (or earlier) than those of the earth, then its smaller size shows that it has in all probability aggregated very much more slowly. But the internal heat acquired by a planet while forming in this manner will depend upon the rate at which it aggregates and the velocity with which the planetismals' fall into it, and this velocity will increase with its mass and consequent force of gravity. In the early stages of a planet's growth it will probably remain cold, the