Thus the youth of a world, like the babyhood of an individual, is passed screened from immediate contact from without. That this is the only way that life can originate on a planet we cannot say, but that it is away in which it does occur, our own Earth attests, and that, moreover, it is the way with all planets of sufficient size, the present aspect of the major planets shows. It may well be that with celestial bodies as with earthly species, some swaddle their young, others cast them forth to take their chance, and that those that most protect them rear the higher progeny in the end. What glories in evolution thus await the giant planets when they shall have sufficiently cooled down, we can only dimly imagine. But we can foresee enough to realize that we are not the sum of our solar system’s possibilities, and by studying the skies read there a future more wonderful than anything we know. CHAPTER VII A PLANET’S HISTORY _Sun-sustained Stage_ Two 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 fact, the transformation was marked. That its epochal character has failed to impress itself generally on geologists, is perhaps because they look too closely, missing the march of events in the events themselves, and because, too, of the gradual nature of its processional change. We can recall only De Lapparent as having particularly signalled it; although not only in its cause, but for its effects, it should have delimited two great geologic divisions of time. [Illustration: EARTH AS SEEN FROM ABOVE—PHOTOGRAPHED BY DR. LOWELL AT AN ALTITUDE OF 5500 FEET.] Astronomy and geology are each but part of one universal history. The tale each has to tell must prove in keeping with that of the other. If they seem at variance, it behooves us very carefully to scan their respective stories to find the flaw where the apparent incongruity slipped in. Each, too, fittingly supplements the other, and especially must geology look to astronomy for its initial data, since astronomy deals with the beginning of our own Earth.