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.
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.
“Our exterior sentinels might fail thus to give us warning of the
foreign body because of being at the time in the opposite parts of their
orbits. We should then be first apprised of its coming by Saturn, which
would give us less prefatory notice.
“It would be some twenty-seven years from the time it entered the range
of vision of our present telescopes before it rose to that of the
unarmed eye. It would then have reached forty-nine astronomical units’
distance, or two-thirds as far again as Neptune. From here, however, its
approach would be more rapid. Humanity by this time would have been made
acquainted with its sinister intent from astronomic calculation, and
would watch its slow gaining in conspicuousness with ever growing alarm.
During the next three years it would have ominously increased to a first
magnitude star, and two years and three months more have reached the
distance of Jupiter and surpassed by far in lustre Venus at her
brightest.
“Meanwhile the disturbance occasioned not simply in the outer planets
but in our own Earth would have become very alarming indeed. The seasons
would have been already greatly changed, and the year itself lengthened,
and all these changes fraught with danger to everything upon the Earth’s
face would momentarily grow worse. In one hundred and forty-five days
from the time it passed the distance of Jupiter it would reach the
distance of the Earth. Coming from Vega, it would not hit the Earth or
any of the outer planets, as the Sun’s way is inclined to the planetary
planes by some sixty degrees, but the effects would be none the less
marked for that. Day and night alone of our astronomic relations would
remain. It would be like going mad and yet remaining conscious of the
fact. Instead of following the Sun we should now in whole or part,
according to the direction of its approach, obey the stranger. For
nineteen more days this frightful chaos would continue; as like some
comet glorified a thousand fold the tramp dropped silently upon the Sun.
Toward the close of the nineteenth day the catastrophe would occur, and
almost in merciful deliverance from the already chaotic cataclysm and
the yet greater horror of its contemplation, we should know no
more.”[31]