The Evolution of Worlds

by Percival Lowell

Its denizens responded. The organisms that already inhabited it proceeded to change their character and crawl out upon the land. For in Devonian times the Earth was the home of fishes. The land was not considered a fit abode by anything but insects, and not over-good by them. But it looked different when the Sun shone. Some maritime dwellers felt tempted to explore, and proceeded in the shape of amphibians to spy out the land. They have left very readable accounts of their travels in footnotes by the way. As one should always inspect the original documents, I will reproduce the footnotes of one early explorer. It is one of the few copies we have, as the type is worn out. But it tells a pretty full story as it stands. The ripple-marks show that a sea beach it was which the discoverer trod in his bold journey of a few feet from home and friends, and the pits in the sandstone that it was raining at the time of his excursion. No Columbus or Hakluyt could have left a record more precise or more eminently trustworthy. The pilgrims found it so good that their eventual collaterals, the great reptiles, actually took possession of the land and held it for many centuries by right of eminent domain. Yet throughout the time of these bold adventurers, their skies were only clearing, as the pitting of the sandstone eloquently states.

It was not till the chalk cliffs of Dover were being laid down that we have evidence that seasons had fully developed, in the shape of the first deciduous trees.[23] Cryptogams, cycads, and, finally, conifers had in turn represented the highest attainments of vegetation, and the last of these had already recognized the seasons by a sort of half-hearted hibernation or annual moulting; deeming it wise not to be off with the old leaves before they were on with the new. But finally the most advanced among them decided unreservedly to accept the winter and go to sleep till spring. The larches and ginkgo trees are descendants of the leaders of this coniferous progressive party.

At the same time color came in. We are not accustomed to realize that nature drew the Earth in grays and greens, and touched it up with color afterward. Only the tempered tints of the rocks and the leaden blue of the sea, subdued by the disheartening welkin overhead to a dull drab, enlivened their abode for the oldest inhabitants. But with Tertiary times entered the brilliantly petalled flowers. Beginning with yellow, these rose through a chromatic scale of beauty from white through red to blue.[24] They decked themselves thus gaudily because the Sun was there to see by, as well as eyes to see. For without the Sun those unconscious horticulturists, the insects, could not have exercised their pictorial profession.

To the entering of the Sun upon the scene this wondrous revolution was due; and once entered, it became the dominant factor in the Earth’s organic life. We are in the habit of apostrophizing the Sun as the source of all terrestrial existence. It is true enough to-day, and has been so since man entered on the scene. But it was not always thus. There was a time when the Sun played no part in the world’s affairs.

As its heat is now all-important, it becomes an interesting matter to determine the laws governing its amount. That summer is hotter than winter we all know from experience, pleasurable or painful as the case may be. This is due to the fact that the Sun is above the horizon for a greater number of hours in summer and passes more directly overhead. But not so many people are aware that on midsummer day, so far as the Sun is concerned, the north pole should be the hottest place on earth. That Arctic explorers, who have got within speaking acquaintance of it, assure us it is not so, shows that something besides the direct rays of the Sun is involved. Indeed, we learn as much from the extensively advertised thermometers of winter resorts which, judiciously placed, beguile the stranger to sojourn where it is just too cold for comfort. The factor in question is the blanketing character of our air. Now a blanket may keep heat out as well as keep it in. Our air acts in both capacities. It is by no means simply a storer of heat, as many people seem to suppose; it is a heat-stopper as well. What it really is is a temporizer, a buffer to ease the shocks of sudden change like those comfortable, phlegmatic souls who reduce all emotion to a level. For the heating power of the Sun, even at the Earth’s distance away, is much greater than appears. Knowledge of this we owe most to Langley, and then to Very, who continued his results to yet a finer determination, the best we have to-day. In consequence we have learnt that the amount of heat we should receive from the Sun, could we get above our air,—the solar constant, as it is called,—would be over three times what it is on the average in our latitude at the surface, and is rising still, so to speak. For as man has gone higher he has found his inferences rising too, and the limit would seem to be not yet. We see then that the air to which we thought ourselves so much indebted, actually begins its kindly offices by shutting off two-thirds of what was coming to us. As it plays, however, something of the same trick to what tries to escape, we are really somewhat beholden to it after all.

But not so much as has been thought. We used to be told that the Moon’s temperature even at midday hardly rose above freezing, but Very has found it about 350° F., which even the most chilly of souls might find warm. By the late afternoon, however, he would need his overcoat, and no end of blankets subsequently, for during the long lunar night of fourteen days the temperature must fall appallingly low, to -300° F. or less.

As the determination of temperature is a vital one, not only to any organic existence, but even to inorganic conditions upon a planet, it behooves us to look carefully into the question of the effective heat received from the Sun. Until recently the only criterion in the case was assumed to be distance from the illuminating source, about as efficient a mode of computation as estimating a Russian army by its official roll. For as we saw in our own case, not all that ought to ever gets to the front, to say nothing of what is lost there. Yet on this worse than guesswork astronomic text-books still assert as a fact that the temperature of other bodies—the Moon and Mars, for example—must be excessively low.

Let us now examine into this most interesting problem. It is intricate, of course, but I think you will find it more comprehensible than you imagine. Indeed, I shall be to blame if you do not. For if one knows his subject, he can always explain it, in untechnical language, technical terms being merely a sort of shorthand for the profession. The physical processes involved can be made clear without difficulty, although their quantitative evaluation is less forthrightly demonstrable. Let me, then, give you an epitome of my investigation of the subject.