CHAPTER XV
LATER EVOLUTION OF THE PLANETS
After the planets had been formed through the aggregation of revolving fragments driven off by the catastrophic collision from the Sun, and after they had attained their maximum heat in the process, they began, he says, to go through six stages:
I. The Sun-Stage, when they were white-hot and gave out light. This could have been true only of the largest ones if any.
II. The Molten Stage, when they were still red-hot, but not enough to give light, in which are now the four great outer planets.
III. The Solidifying Stage, when a crust formed, and the surface features of the planet began to assume their character. Here the science of geology takes its start with the metamorphic rocks, and it is the dividing line between the inner, smaller, and the outer, larger, planets.
IV. The Terraqueous Stage, when the surface has become substantially stable, there are great oceans gradually diminishing in size, and land gradually increasing. This is the stage of the sedimentary rocks, the time when the planet passes from its own supply of heat to dependence upon that of the sun; the stage when life begins, and the one in which the Earth is now.
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.