II THE CELESTIAL BODIES, IN PARTICULAR THE EARTH, AS ABODES OF ORGANISMS There is no more elevating spectacle than to contemplate the sky with its thousands of stars on a clear night. When we send our thoughts to those lights glittering in infinite distance, the question forces itself upon us, whether there are not out there planets like our own that will sustain organic life. How little interest do we take in a barren island of the Arctic Circle, on which not a single plant will grow, compared to an island in the tropics which is teeming with life in its most wonderful variety! The unknown worlds occupy our minds much more when we may fancy them inhabited than when we have to regard them as dead masses floating about in space. We have to ask ourselves similar questions with regard to our own little planet, the earth. Was it always covered with verdure, or was it once sterile and barren? And if that be so, what are the conditions under which the earth can fulfil its actual part of harboring organic life? That “the earth was without form” in the beginning is unquestionable. It does not matter whether we assume that it was once all through an incandescent liquid, which may be the most probable assumption, or that it was, as Lockyer and Moulton think, formed by the accumulation of meteoric stones which became incandescent when arrested in their motion. We have seen that the earth probably consists of a mass of gas encased within a shell which is solid on the outside and remains a viscid liquid on the inner side. We presume with good reason that the earth was originally a mass of gas separated from the sun, which is still in the same state. By radiation into cold space the sphere of gas which, on the whole, would behave as our sun does now, would gradually lose its high temperature, and finally a solid crust could form on its surface. Lord Kelvin has calculated that it would not require more than one hundred years before the temperature of this crust would sink to 100°. Supposing, even, that Kelvin’s calculations should not quite be confirmed, we may yet maintain that not many thousands of years would have elapsed from the time when the earth assumed its first crust at about 1000° till the age when this temperature had fallen below 100° (212° F.). Living beings certainly could not exist so long, since the albumen of the cells would at once coagulate at the temperature of boiling water, like the white of an egg. Yet it has been reported that some of the hot springs of New Zealand contain algæ, although at a temperature of over 80°. When I went to Yellowstone Park to inquire into the correctness of this statement, I found that the algæ existed only at the edge of the hot springs, where the temperature did not exceed 60° (140° F.). The famous American physiologist Loeb states that we do not meet with algæ in hot springs at temperatures above 55°.