V. The Terrestrial Stage, when the oceans have disappeared, and water is scarce, the one in which Mars is now.

VI. The Dead Stage, where are already the Moon and the satellites of other planets.

On the question of the origin of life Percival took the mechanistic view: “Upon the fall of the temperature to the condensing point of water, occurred another event in the evolution of our planet, the Earth, and one of great import to us: life arose. For with the formation of water, protoplasm (the physical basis of all plants and animals) first became possible, what may be called the life molecule then coming into existence. By it, starting in a simple, lowly way, and growing in complexity with time, all vegetable and animal forms have since been gradually built up. In itself the organic molecule is only a more intricate chemical combination of the same elements of which the inorganic substances which preceded it are composed.... There is now no more reason to doubt that plants grew out of chemical affinity than to doubt that stones did. Spontaneous generation is as certain as spontaneous variation, of which it is, in fact, only an expression.”

Life, he believed, began in the oceans soon after they had cooled below the boiling point, and spread all over them; seaweeds and trilobites existed in France, Siberia and the Argentine, their nearest relatives being now confined to the tropics; coral reefs, now found only in warm equatorial seas, have left their traces within eight degrees of the pole. This looks as if in paleozoic times the oceans were uniformly warm. The same record he finds in the plants of the carboniferous age. Gigantic ferns and other cryptogams grew to an immense size, with vast rapidity and without stopping, for there are no annual rings of growth, no signs of the effect of seasons, no flowers, and little or no color. “Two attributes of the climate this state of things attests. First, it was warm everywhere with a warmth probably surpassing that of the tropics of to-day; and, second, the light was tempered to a half-light known now only under heavy clouds. And both these conditions were virtually general in locality and continuous in time.” In the later volume he adds, to corroborate the general darkness, that many of the earlier trilobites, who lived in shallow water, were blind, while others had colossal eyes.

Various theories have been advanced to explain the carboniferous age, which he reviews, showing why they do not account for the facts. His own is that while the oceans were still hot a vast steaming must have gone up from them, forming clouds of great density that would keep the sun’s heat and light out, and the warmth of the Earth in. “In paleozoic times, then, it was the Earth itself, not the Sun, to which plant and animal primarily stood beholden for existence. This gives us a most instructive glimpse into one planetologic process. To the planet’s own internal heat is due the chief fostering of the beginnings of life upon its surface.”[30]

But he points out that a time must have come when the Earth, and especially its seas, had cooled, the envelope of dense cloud had gradually been pierced, and the sun’s rays let in. Then began the sharp alternation of day and night, the changes in the seasons and the diversity of climates, when the palms descended to the tropics, and the flora and fauna as we know them started to develop. This is the period when the Sun was dominant, or the Sun-Sustained Stage, the one in which we live.

Later the Earth went through another experience of which the facts are well known, but the date and cause have puzzled astronomers and geologists alike, for it lies in the twilight zone between the regions they illuminate. It is the Glacial Periods. He discusses the theory of Croll, once largely accepted but now abandoned, that these periods were due to a change in the eccentricity of the Earth’s orbit, combined with a progression of the equinoxes, which so altered the seasons that the northern hemisphere would have summers hot but too short to melt the snow and ice accumulated in the long cold winters. In fact Percival had already reviewed this theory some years before in a paper presented to the American Philosophic Society (Proc. Vol. XXXIX, No. 164) in which he showed that the eccentricity and inclination of axis in Mars are very close to those Croll had attributed to the Earth, and yet a glacial period does not exist there. In the case of Mars it is the southern hemisphere that should be glaciated, but in fact, although that pole has the larger extent of snow in winter this sometimes disappears wholly in the summer, which is never true at the northern pole. If, indeed, the amount of ice formed were much larger it would not be melted, so that the amount of water falling and frozen, and not the eccentricity or inclination of the axis, would be the cause of an ice age.

But he had another reason for rejecting Croll’s theory, and, indeed, for disbelieving in a general ice age altogether. It was that the glaciation does not appear to proceed from the pole, but from various distinct centres, moving from them in all directions, north as well as south; while some places, like northern Siberia, that one would expect to be covered with ice, were not so covered. Nor was the greater cold confined to the northern hemisphere, for on some mountains at the equator, and even at the south pole, there was more ice and snow than there is to-day. His explanation is that certain parts of the Earth’s surface were for some reason raised higher than they are now; and from the snow mountains or plateaus so formed the sheets of ice flowed down.

The remainder of the book on “Mars as the Abode of Life”—and it is the larger part of it—contains the reasons for believing that Mars is inhabited, the canals artificial, and that the Earth will in like manner gradually lose its supply of water. But this argument need not be retraced here, because with it the reader has already been made familiar. “The Evolution of Worlds” ends with a chapter entitled “Death of a World”; for to him the whole theory of planetary evolution is a vast drama, albeit with a tragic close. He describes four ways in which a planet, and all life thereon, may be destroyed. Three of these are: the effect of tidal action that would bring the same face always toward the Sun; the loss of water and atmosphere; and the cooling and final extinction of the Sun. All these things he cheerfully reminds us are sure to happen, but at a time enormously distant. The other is a collision with a star—“That any of the lucent stars, the stars commonly so called, could collide with the Sun, or come near enough to amount to the same thing, is demonstrably impossible for aeons of years. But this is far from the case for a dark star. Such a body might well be within a hundredth of the distance of the nearest of our known neighbors.... Our senses could only be cognizant of its proximity by the borrowed light it reflected from our own Sun.” A collision of this kind might happen at any time, but he consoles us by saying that “judged by any scale of time we know, the chance of such occurrence is immeasurably remote.” In an earlier part of the book he describes what its advent would be:

“We can calculate how much warning we should have of the coming catastrophe. The Sun with its retinue is speeding through space at the rate of eleven miles a second toward a point near the bright star Vega. Since the tramp would probably also be in motion with a speed comparable with our own, it might hit us coming from any point in space, the likelihood depending upon the direction and amount of its own speed. So that at the present moment such a body may be in any part of the sky. But the chances are greatest if it be coming from the direction toward which the Sun is travelling, since it would then be approaching us head on. If it were travelling itself as fast as the Sun, its relative speed of approach would be twenty-two miles a second.

“The previousness of the warning would depend upon the stranger’s size. The warning would be long according as the stranger was large. Let us assume it the mass of the Sun, a most probable supposition. Being dark, it must have cooled to a solid, and its density therefore be much greater than the Sun’s, probably something like eight times as great, giving it a diameter about half his or four hundred and thirty thousand miles. Its apparent brightness would depend both upon its distance and upon its intrinsic brightness or albedo, and this last would itself vary according to its distance from the Sun.... We shall assume, therefore, that its brilliancy would be only that of the Moon, remembering that the last stages of its fateful journey would be much more resplendently set off.

“With these data we can find how long it would be visible before the collision occurred. As a very small telescopic star it would undoubtedly escape detection. It is not likely that the stranger would be noticed simply from its appearance until it had attained the eleventh magnitude. It would then be one hundred and forty-nine astronomical units from the Sun or at five times the distance of Neptune. But its detection would come about not through the eye of the body, but through the eye of the mind. Long before it could have attracted man’s attention to itself directly its effects would have betrayed it. Previous, indeed, to its possible showing in any telescope the behavior of the outer planets of the system would have revealed its presence. The far plummet of man’s analysis would have sounded the cause of their disturbance and pointed out the point from which that disturbance came. Celestial mechanics would have foretold, as once the discovery of another planet, so now the end of the world. Unexplained perturbations in the motions of the planets, the far tremors of its coming, would have spoken to astronomers as the first heralding of the stranger and of the destruction it was about to bring. Neptune and Uranus would begin to deviate from their prescribed paths in a manner not to be accounted for except by the action of some new force. Their perturbations would resemble those caused by an unknown exterior planet, but with this difference that the period of the disturbance would be exactly that of the disturbed planet’s own period of revolution round the Sun.