Page:The evolution of worlds - Lowell.djvu/220

 CHAPTER VII A PLANET'S HISTORY Sun-sustained Stage WO stages have characterized the surface history of the Earth,—stages which may be likened to the career of the chick within and without the egg. In the first of them the Earth lay screened from outside influence under a thick shell of cloud, indifferently exclusive of the cold of space or of the heating beams of the Sun. Motherless, the warmth of its own body brooded over it, keeping its heat from dissipating too speedily into space, and so fostering the life that was quickening upon its surface.

The second stage began when the egg-shell broke and the chick lay exposed to the universe about it, to get its living no longer from its little world within, but from the greater one without. One and the same event ended the old life to make possible the new. So soon as the cloud envelope was pierced, both the Earth's own heat escaped and the Sun's rays were permitted to come in.

It is not surprising that under such changed conditions development itself should have changed, too. In 182