Are the Planets Inhabited? Part 6

CHAPTER XI

WHEN THE MAJOR PLANETS COOL

The question has been asked: "It is evident that life cannot exist at the present time on the outer planets, since they are in a highly heated and quasi-solar condition; but when they cool down, as cool they must, and a solid crust is formed, may not a time come when they will be habitable? It seems impossible to think that worlds so beautiful to our eyes and so vast in scale are destined never to be peopled by intelligent beings."

It is clearly difficult to answer satisfactorily a question that requires so deep a plunge into the recesses of the unknown future; yet, so far as our knowledge goes, there is no reason to think that Jupiter will be more habitable then than it is now. The difficulty of the small supply of light and heat received from the Sun would apparently still remain, if indeed, the cooling of the Sun itself would not increase it. We do not know of any means by which our Sun could so increase its radiation as to supply to Jupiter from 25 to 30 times as much heat as it now receives, and this would be necessary to place it in the same favoured condition as the Earth. If so great a change were to take place in the Sun, life would be scorched out of existence on all planets nearer than Jupiter, and, similarly, if the solar emission were increased to meet the necessities of Ura.n.u.s or Neptune, even Jupiter would fall a victim.

But we may consider it as a conceivable case that a planet of the exact dimensions of Jupiter may be revolving in an annual period of the same length as his, round some star that is capable of affording it adequate nourishment; and so with the three other giant planets. The actual Jupiter and Saturn of the solar system have, so far as we can tell, neither present nor future as habitable worlds, but we can consider what would be the case of imaginary bodies of similar dimensions in systems where the supply of heat would be sufficient. Or we can neglect the question of temperature altogether, as we did at first in the case of Mars.

All the four planets must shrink much in volume before their solidification will take place. Their average density at present but little exceeds that of water; indeed, Saturn is not so dense as water; yet we must suppose that the same elements are in general common to the Earth and to them all. If we a.s.sume, then, that the four planets all cool to the point of solidification, their densities must be much increased, and their volumes correspondingly diminished. Since all four greatly exceed the Earth in ma.s.s, it is but natural to expect that, when they have a.s.sumed the terrestrial condition, they will be more closely compacted than the Earth, and their densities in consequence will be greater. It will, however, be simpler if we a.s.sume exactly the same density for them as for the Earth. Jupiter will then have shrunk to about one-fourth of its present volume, and the statistics for the four planets will run as in the following Table:

STATISTICS OF THE FOUR OUTER PLANETS IF WITH THE SAME DENSITY AS THE EARTH

PROPORTIONS OF THE PLANETS:-- Ura.n.u.s Neptune Saturn Jupiter Diameter in miles 19300 20400 36000 54000 do [Symbol] = 1 244 257 456 682 Surface, [Symbol] = 1 60 66 208 466 Ma.s.s and Volume, [Symbol] = 1 146 170 948 3177 Gravity at surface, [Symbol] = 1 244 257 456 682 Rate of Fall, Feet in the First Second 392 413 733 1097

ATMOSPHERE, a.s.suming the total ma.s.s of the atmosphere to be proportional to the ma.s.s of the planet:--

Pressure at the surface in lb.

per square inch 882 970 3058 6850 Pressure at the surface in "atmospheres" 60 66 208 466 Level of half-pressure in miles 137 130 073 049 Boiling point of water at surface 127C 129C 148C 164C

Jupiter offers two peculiarities. In its shrunken condition, its diameter, instead of being eleven times that of the Earth, will be not quite seven, and the force of gravity at the surface will be greater than that of the Earth in the same proportion. A man who here weighs 150 lb. will there weigh over 1000 lb.; and the muscular effort of movement will be increased in the same ratio. The athlete who here can clear a height 5 ft. 8 in.

will there, with like pains, surmount 10 inches; and other efforts will be in the same proportion. The atmosphere, supposing it to be in proportion to the ma.s.s of Jupiter, will exercise a pressure of 46-1/2 "atmospheres,"

or more than 680 lb., to the square inch. Following on this enormous pressure at the surface would be the rapidity with which the atmosphere would thin out in the upward direction. The level of half-pressure would be attained by ascending less than half a mile in height; that is to say, there would be a difference of pressure of 340 lb. on the square inch from that experienced at the sea-level. We know from the fact that fishes live at enormous depths in the ocean, that living organisms can be constructed to endure great pressures, but they are not constructed to endure great alterations of pressure. The deep-sea fishes are as instantly killed by being brought up to the surface, as the surface fishes or the land animals would be if they were plunged into the depths. And it is clear that on Jupiter a low range of hills that on the Earth would be considered only an easy climb, would be an impa.s.sable barrier, not only from the immense exertion of mounting it, but chiefly from the unendurable change of pressure which the ascent would involve.

The sevenfold gravity of Jupiter, taken in connection with this enormous atmospheric pressure, would tend to make the meteorological disturbances of the planet violent far beyond anything of which the Earth can furnish an example. The atmosphere would possess a high viscosity, and differences in condition, pressure and saturation would tend to acc.u.mulate, until at length the balance would be restored with explosive suddenness and force.

Here our most violent tornadoes may reach a speed of 100 miles an hour; on Jupiter, gales of five or six times that velocity would be common. We cannot conceive that living organisms would be able to grow, flourish and multiply where the conditions were so cataclysmic.

This difficulty must always exist where the planet is great in ma.s.s, and the force of gravity high at the surface. The case of Saturn is not so extreme as that of Jupiter, though it is probably sufficiently severe to exclude it from the ranks of worlds that could ever be dwelt in. The atmospheric pressure would be about 21 "atmospheres," or more than 300 lb.

on the square inch. The level of half-pressure would be reached at about three-quarters of a mile, and the force of gravity be nearly 4-1/2 times that of the Earth.

But the serious condition for Saturn would come from that feature which renders it by far the most attractive of all the planets seen in the telescope, the presence of the wonderful Ring system.

To us, viewing Saturn from afar, and from practically the same direction as the Sun, the Rings are seen lit up; but to a dweller on Saturn, the Rings during the day are between his world and the Sun, and hence turn their dark side toward him. More than that, the telescope shows us that the Rings cast a shadow on the planet; in other words, they eclipse part of it; and this shadow changes its position with the progress of the Saturnian year. Proctor computed that if the Rings were a hundred miles in thickness, the equator would suffer, in consequence, total eclipse for nearly ten days at each equinox, and partial eclipse for about forty days more. Moving away from the equator, each higher lat.i.tude would have a longer and longer period of eclipse in the winter half of its year; the higher the lat.i.tude, the later after the autumnal equinox the eclipse would begin, and the longer it would last, until about lat.i.tude 40 was reached. Here the eclipses would begin nearly three terrestrial years after the time of the autumnal equinox. At first the Sun would be eclipsed only in the morning and evening of each day, but the length of the daily eclipse would increase, until the Sun was hidden the whole day long. This period of total eclipse would last for about 6 years 8 months, terrestrial reckoning, or with the periods of partial eclipse, 8 years and nearly 10 months. Whatever the efficiency of the Sun that afforded light and heat to such a planet, it is clear that such eclipses must be fatal to life in two ways: light and heat would be cut off from wide regions of the planet for long periods of time, and terrible meteorological convulsions must follow in the train. Here on the Earth, though a total eclipse generally lasts only two or three minutes, the atmospheric disturbance is perceptible, and the fall of temperature very marked, and it does not require much reflection to see that the a.n.a.logous disturbance in an atmosphere twenty times as dense must be terrific indeed during an eclipse that lasts not a few minutes only, but for more than six of our years.

The case of Ura.n.u.s introduces us to another cla.s.s of conditions fatal to habitability. The equator of Jupiter is inclined only 3 to the plane of its...o...b..t; the difference in its seasons is, therefore, almost imperceptible; there is hardly any alteration in the incidence of the solar rays; it is, as if on the Earth, the height of the Sun at noon in mid-winter were what it actually is on the 14th of March, and its height at midsummer the same as we observe on March 28. The inclination of the equator of Saturn is considerably greater than that of Mars or the Earth, so that its seasons are more p.r.o.nounced, but not to an extent that would introduce any radical difference. But for Ura.n.u.s, the inclination of the equator to the plane of the orbit is 82. If this were the case for the Earth, the noonday sun for London would be, at the spring equinox, 38-1/2 high as at present, but its alt.i.tude day by day would increase with great rapidity, and before the end of April, the Sun at noon would be right in the zenith, and 13 above the horizon at midnight. At midsummer, indeed, it would be only 59 high at noonday, but it would be north of the zenith instead of south, and at technical midnight, it would still be 44 in alt.i.tude, thus moving round in a very small circle, only 15 in diameter.

From about April 18 to August 25--that is to say, for 129 days--the Sun would never set, and unlike the summer day of our own polar regions now, wherein the Sun, though always present, is always low down in the sky, for much of that period it would pa.s.s the meridian quite close to the zenith.

As the year of Ura.n.u.s is 84 times the length of our year, the London of Ura.n.u.s would have to endure not far short of 30 years continuous scorching.

And the winter would be as long; the perpetual day of summer would be replaced by a night as enduring. More than 29 years of unbroken darkness, of unmitigated cold, cannot possibly ever consist with the conditions necessary for life upon a planet. Whatever the brightness of the imagined sun of Ura.n.u.s, if for 29 years at a time that sun were below the horizon, the water on the planet must be congealed, and during the 29 years of unbroken day all the water would be as certainly evaporated.

Thus, though Ura.n.u.s is not burdened by the enormous ma.s.s of Jupiter, nor overshadowed, like Saturn, by a system of rings, the extraordinary inclination of its axis introduces a condition which is as fatal to it, as a world to dwell in, as any of the disabilities of the other planets.

It is curious that these four outer planets, that resemble each other so strikingly in many of their conditions--in their vast size, high albedo, low density, and vaporous envelopes, that show, in their spectra, not merely the lines of reflected sunlight, but also special lines due to their own atmospheres (the chief of these being common to all the four planets)--should yet, in the inclination of their axes to the plane of their orbits, display every possible variety. The axis of Jupiter is almost normal to its...o...b..t, that of Ura.n.u.s lies almost in the plane of its...o...b..t. The axes of Saturn and Neptune have a mean inclination, but it would appear that the rotation of Neptune is in the reverse direction to that of planets in general, so that the true inclination is usually taken as being the complement of the observed angle, as if the axis were turned right over. It is uncertain whether this would have any important effect upon the habitability of the planet, but it supplies the fourth possible case for the position of the axis.

CHAPTER XII

THE FINAL QUESTION

In pa.s.sing in review the various members of the solar system, it has been seen that there are many conditions that have to be fulfilled before a planet can be regarded as the possible abode of life, because there are many conditions necessary in order that water may exist on its surface in the liquid state. The size and ma.s.s of the planet are restricted within quite narrow limits; and a world much larger or much smaller than our own is necessarily excluded. The supply of light and heat received from the Sun must not fall much below that received by the Earth, nor greatly exceed it; in other words, the distance of the planet from its Sun is somewhat precisely fixed, since the light and heat vary inversely not as the distance, but as its square. Of course, in different systems, with suns of different power, the most favourable distance will not be the same in each; but in any system there will be one most advantageous distance, and no great departure from it will be possible. This condition further implies that the planetary orbits must be nearly circular; p.r.o.nounced eccentricity, such as the orbits of even our short-period comets display, would be fatal to the persistence of water in the liquid state, and hence to the continuance of life. A wide discordance between the planes of the planet's equator and of its...o...b..t, by rendering the seasons extravagantly diverse, would act as prejudicially as an eccentric orbit, and a rotation period equal to that of revolution would mean that one hemisphere was eternally frozen while the other was exposed to perpetual heat.

It follows that in any given system there can be at most only one or two planets upon which life can find a home, and this only where the right conditions of size and ma.s.s, of rotation period, inclination of axis, and shape of orbit, all co-exist in a globe at the proper distance. But the type of system offered by our Sun and his planets is not the only one that exists. A very large proportion of stars are binaries--two suns revolve round their common centre of gravity. In many cases the two suns are separable in the telescope, and their relative movements can be measured; in other cases, termed "spectroscopic binaries," we only learn that a star which appears absolutely single has two components from the evidence of its spectrum; the spectroscope revealing two sets of lines that vibrate to and fro with respect to each other. Yet, again, a third cla.s.s of double stars has made itself known in the "Algol variables." The optical double stars are cases where the two components are far distant from each other, and hence can be distinguished in our telescopes as separate points of light. The "spectroscopic binaries" are cases where the two components are too close to be separately perceived, but where the two are not greatly unequal in brightness, so that the spectrum of the one does not overpower that of the other. The "Algol variables" are cases where the two components are of very unequal brightness, and, being very close to each other, are so placed with respect to the Earth that the fainter partly eclipses the brighter in its revolution round it, and so causes a temporary diminution in its light at regular intervals. All these three cla.s.ses of binary systems are now known to be very numerous. Prof.

Campbell estimates that fully one star in six is a spectroscopic binary.

But there must be many binary systems that do not reveal themselves--double stars where the companion is too faint or too close to be detected, Algol systems where the companion does not pa.s.s before its primary--and it seems almost certain that simple systems, like that of which our Sun is the unchallenged autocrat, must be comparatively rare.

But the problem of the movements of a planet attendant upon two or more suns is one of amazing complexity, and our greatest mathematicians have as yet only been able to deal with the approximate solution of a few very special cases. These are, however, sufficient to show that the orbit of a planet so placed would be most irregular; the variations in the supplies of light and heat received would be as great as even comets experience within the solar system, and, what would be more disastrous still, these variations would not be periodic but irregular. One year would be unlike that which preceded it, and would be followed by changed conditions in the next. Plants and animals would never have the chance of acclimatizing themselves to these ever-changing vicissitudes. The stability of condition essential for the maintenance of water in a liquid state would be wanting; and, in consequence, Life could neither come into existence, nor persist if it once appeared.

So far, therefore, our line of thought has led us to recognize that Life can exist in comparatively few of the innumerable stellar systems strewn through infinite s.p.a.ce, and in any given system it can at best find only one or two homes. The conditions for a Life-bearing planet are thus both numerous and stringent--there is no elasticity about them. It is not sufficient that a planet might fulfil many or even most of these conditions; failure in one is failure altogether; "one black ball excludes;" the candidate who fails in a single subject is "ploughed"

without mercy. And in most cases the failure is final; no opportunity is given to the candidate to "sit" again.

But s.p.a.ce is not the only horizon along which our thought must be directed; there is also the horizon of Time. Every world must have its Past and its Future, as well as its Present. For some worlds the conditions are so fixed that, like Jupiter and Saturn, they are not now worlds that can be dwelt in, they never were in that condition, and they never can be; their enormous ma.s.s forbids it. Mercury and the Moon at the other end of the planetary scale are also permanently disabled; their insignificant size excludes them. There was also a time when the Earth was not a world of habitation; it was "without form and void"; hot and vaporous, even as the four outer planets are now. Now it is inhabited, but there may come a time when this phase of its history has run its course, and either from a falling off in the tribute of light and heat rendered to it by the Sun, or from the gradual desiccation of the surface, or, perchance, from the slow loss of its atmosphere, it may approach the condition of Mars, and in its turn be no longer an abode of life. Many planets are essentially debarred from ever entering on the vital stage; but of those to which such a stage is possible, it can only form an incident in the entire duration of the orb. And if our Earth is any type or example of the vital stage in general, vast aeons must run their course from the first appearance of the humblest germs of life up to the bringing forth of Life in conscious Intelligence. One hundred million years are freely spoken of in this connection by those who study the crust of the Earth and those who are occupied with the relations of the varied forms of life. Man is the latest arrival on this planet, and however far back we try to push the time of his earliest appearance, it is beyond question that that time, relatively to the entire duration of the Earth since a solid crust began to form, is but as yesterday. If, from some other globe in the depths of s.p.a.ce, this world of ours could have been watched during the long aeons that elapsed from its first separation from the solar nebula down to the time when it first possessed a surface of land and water, and from that time, again, throughout the hypothetical one hundred million years that preceded the advent of man, then, during all those aeons, those imagined observers would have had under their scrutiny a world as yet without inhabitant. The Earth now is in the inhabited condition, but science gives us no clue as to how long that condition will endure; rather such hints as are afforded us would seem to point to its lasting but for a brief season as compared with the indefinite duration which preceded it, and the indefinite duration which shall follow.

If this thought be sound, it places before us an entirely new and most serious consideration. The world predestined for habitation must not only have its size within certain narrow limits, its distance from its central sun in a certain narrow zone, its rotation period, the inclination of its axis, the eccentricity of its...o...b..t, all suitable alike, but even if in these and in all other necessaries it is perfectly adapted for habitation, yet it will be only during a relatively small fraction of its entire duration that Intelligent Life, clothed in material form, will find a place upon it.

Let us sum shortly what we know and what we conclude. We know that this, our Earth, is a habitable globe, for we ourselves are living upon it. We know what const.i.tutes the physical basis of our life, and under what conditions on this Earth it flourishes, and under what conditions it is destroyed. If we turn our eyes from this, our Earth, and look out upon the starry skies, we see the other planets of our system, and the suns which are the centres of other systems. From the consideration of the planets in our own system, we have seen how stringent and how many are the conditions imposed for Life to be possible. Round our Sun there is but a narrow zone in which a habitable world may circle; in this zone there is room for but few worlds, and we actually know of three alone, the Earth, the Moon, and Venus. We know that the Earth can be and is inhabited; that the Moon is not and cannot be inhabited; and that Venus, though of habitable size, may yet be subject to the fatal disqualification of always turning the same face to the Sun. Of other planetary systems than our own, we actually know of none, but we a.s.sume that there are such, and as numerous as there are suns in the starry depths. But of these planetary systems we can rule out, as containing no habitable member, all such as circle round double or multiple suns or, indeed, round any single star that, from whatever cause, is largely variable and, therefore, much less stable than our own. Mira Ceti, which in 5 months increases its brightness 1000 times, may stand as an example. Probably these disqualifications rule out of court the great proportion of the stellar systems. Of the few, comparatively speaking, single and stable suns that remain in the heavenly abyss, we must conclude, from what we know of our solar system, that they, too, have but a narrow zone, outside of which no world would be fit to dwell in; whilst in the zone the few worlds which might exist must violate no one of many strict conditions. If we a.s.sume that there are a hundred million stars within the ken of our telescopes, we may well believe that not more than one in a hundred of these would fulfil the condition of being a single and stable sun, such as ours. Of the planets revolving round these million suns--stable and efficient suns--can we expect that in more cases than one in a hundred there will be a planet in the habitable zone fulfilling all the other conditions of habitability, of size, ma.s.s, inclination of axis, circular orbit, and rotation? Of these ten thousand earths which may be made fit for the habitation of Man, can we a.s.sume that even one in a hundred is now at that epoch in its history when it is no longer "without form and void," when a division has been made between the waters under the firmament and those that are above the firmament; when the waters under the heaven have been gathered into one place, and the dry land has appeared, and when the earth and the waters have brought forth life abundantly? Out of a hundred million of planetary systems throughout the depths of s.p.a.ce, can we suppose that there are even one hundred worlds that are actually inhabited at the present moment? These numbers and proportions certainly are not, and cannot be, based on knowledge; they are given as ill.u.s.trations only; but, vague as they are, they suggest that our Earth may be neither one of many inhabited earths, nor yet unique, but one of a few--indeed of a very few.

And then the objection is raised: "If our own Earth is but one of, perhaps, two inhabited worlds in the solar system; and of perhaps one or two hundred inhabited worlds throughout the furthest s.p.a.ce that we can scan; why is all this waste?" Of all the countless millions of stellar systems without living organisms as inhabitants, we cannot tell the purpose for the simple reason that we do not know it; but of "waste" in the solar system, there is no question. Relatively speaking, this is quite insignificant, for we cannot consider that as "waste material" which is useful and, indeed, essential to existence. For, consider first the material in the Earth itself. Its total volume is 260,613,000,000 cubic miles, but man only lives _upon_ its surface of less than 200 million square miles in extent, and he can not probe down as far as ten miles below it, through the depths of ocean or by his deepest mine. Thus we are left with over 258 thousand million of cubic miles that man, or plant, or beast can never make direct use of. But without this 258 thousand million cubic miles that he can never sow nor reap, the overlying platform on which he dwells would be useless for retaining the air or the water by which he lives. No less essential is the Sun; its vast bulk of

2,000,000,000,000,000,000,000,000,000 tons

can, in no single unit, be counted "waste," for it is from this that the heat and light necessary for life on the Earth is derived. But the tonnage of all the planets combined is but 013 per cent of the Sun alone; and a wastage, if such it is, like this is insignificant from a material point of view.

There is a type of politician at the present day who is convinced that the highest purpose to which land can be put is to build upon it; that being, in general, the use giving the highest money return per square foot, though the return does not always fall to the builder. It has taken not a little agitation and popular pressure to enforce the truth that cultivated land is also of use. But there are few who realize that land that is neither built upon nor cultivated is also essential. Our barren moors and bleak hillsides, "wastelands" as we call them, are absolutely necessary as collectors of the water by which we live. From them our springs take their source; and they supply our cities with the first necessity of life.

We find, then, in this universe so far as we can know it, that s.p.a.ce is lavishly provided, Matter is lavishly scattered, Time is unsparingly drawn upon, but Life in any form, and especially in its highest form, is, relatively speaking, very spa.r.s.ely given. That very circ.u.mstance surely points to the overwhelming importance of conscious, intelligent Life, and the insignificance of lifeless matter in comparison with it. We have to exhaust arithmetic in computing the size, the ma.s.s, the output of heat and light of our Sun, yet it is but the hearth-fire and lamp of terrestrial life; and its amazing agglomeration of matter and energy is ungrudgingly devoted to this humble purpose. Whatever view we hold as to the scheme of the universe; whether with the unthinking we fail to recognize Thought and Purpose behind its marvellous manifestations, or, with the thoughtful, realize that only Infinite Thought could provide so wonderfully for the bringing forth of thought in living material organisms, the conclusion still remains: living intelligences are, by the direct testimony of the universe itself, its n.o.blest and most precious product.

The plea is often made that as we find life adapting itself to a great variety of conditions on this Earth, we must not set limits to its power of adaption to the conditions of other worlds. But this plea is an unthinking one. The range of conditions through which we find life on this Earth is as nothing to the range given by the varied sizes and positions of the different planets; and even on our Earth, life in the unfavoured regions--the tops of mountains, the polar snows, the waterless deserts, the ocean depths--is only possible because there are more favoured regions close at hand, and there are, as it were, "crumbs that fall from the rich man's table." A well-known litterateur in setting forth "a hundred ways of making money" gave great prominence to the method of living as caretaker in an empty house. But residing in an empty house does not, in itself, supply the means of sustenance; these have to be furnished by the wealthier man who employs the caretaker.

Another plea for vague sentiment in this matter is that we cannot expect that intelligent beings on other worlds would have the same form as man, and if not the same form, then, that the same conditions of existence would not hold good for them as for us. Both contentions are unsound.

Protoplasm is the physical basis of all the life that we know, whatever its form; though these forms are to be counted by the million, and are as diverse as they are numerous. And everywhere and always, water is found essential to protoplasmic life. Of life of any other kind we do not know any examples; we have no instance; if such exist, then they are beyond our ken.

And neither anthropologist nor biologist would admit that the form of intelligent life was an unrelated accident. Whether the form brought the intelligence, or the intelligence the form, or both were evolved together, the one reacting on the other, the human form and the human intelligence are a.s.sociated, and we feel this to be so of necessity. In 1891, Dr.

Eugene Dubois found in Java a molar tooth and a portion of a skull, and later the thigh bone of the left leg, and two more teeth. Such as they were, these relics appeared nearer in form to the corresponding fragments of an average Australian than to those of an ape, and on this ground intelligence was claimed for the creature of which they were the remains, and it was given the name of Pithecanthropus, or Ape-Man. The discovery aroused much discussion, but on all sides it was unhesitatingly a.s.sumed that the difference between the form of Pithecanthropus and that of the most similar ape was an index of its superior intelligence over the ape, just in so far as that difference was in the direction of the modern human form. The same remark applies to the recent discovery of very ancient human remains in Suss.e.x. Never at any time has it been supposed that the physical frame has followed any other path in the evolution of intelligence than that which brought forth man. The flesh-eating animals have attained efficiency in hunting and warfare by variation along many types of form; the herbivora have been not less varied in the forms by which as races they secured themselves from destruction; but Thought has been a.s.sociated with the development of one type or form only, and the entire future of Thought on this planet rested neither with mammoth nor cave-bear, but with the possessor of the erect stature, the upward look, the differentiation of hand and foot, even in their crudest and earliest stages.

Swift, in _Gulliver's Travels_, conceived of a land where the intelligence and conscience of Man dwelt in the form of the horse, and the human form tabernacled the instincts of the beast. H. G. Wells, in his _War of the Worlds_, attributed intelligence to monsters--half-cuttlefish and half-anemone,--and the human form to their helpless, unresisting prey.

Both conceptions are as scientifically absurd as they are gross and revolting; and if it were possible for the skeleton of creatures from other worlds to be brought to us here, then biologists would as confidently p.r.o.nounce on their intelligence as they do on the extinct forms of bygone ages--the nearer to the human form, the nearer to the human mind. We have found the figures of reindeer, horse, and mammoth scratched in outline on a mammoth tusk; but though the artist has left no other trace, we need no further evidence of his bodily form. Neither horse, nor reindeer, nor mammoth made those rough outlines; they were drawn by a man. More striking still, France yields us chipped flints by the million, flints so slightly shaped that it is in dispute whether they may not have been so broken by the action of torrents. But there are only two theories about them; either they were so chipped by natural action, or they were designedly so chipped by creatures resembling ourselves in head and hand.

The question that has been dealt with in this volume is a scientific one, and the attempt has been made to treat it as such, and to argue from known physical facts as to the conditions of worlds which we cannot visit. But by many the question is generally discussed wholly apart from physical facts at all, and it becomes one of sentiment and of religious sympathy.

Yet, curiously enough, the division between those who think that all worlds must be inhabited and those who think that our own world stands alone is not coincident with any line of theological divisions, but rather cuts across all such. Some believers in Christianity argue that since G.o.d has filled this world with Life, Life has been His purpose in the world, and must therefore have been His purpose in all other worlds--they too must be filled with Life in like manner. Other believers argue that this world was the scene of the Incarnation of Our Lord, and is therefore unique in that respect; and that this uniqueness sets its stamp upon this world in all respects. Opponents to Christianity are divided into the same two cla.s.ses, the one arguing that wherever there is matter the inevitable course of evolution will produce life, and eventually intelligent life.

The other cla.s.s are equally clear that all forms of life are special, the result of the particular environment, and that it is unreasonable to expect that any other world has had the same history as our own, or that the same special conditions have prevailed elsewhere. In other words the belief that there are other inhabited worlds has depended chiefly neither on science nor on religious belief, but upon sentiment. There are some who like to think themselves, and the race to which they belong, altogether exceptional; others delight in finding themselves reflected wherever they look. So far as Science has progressed and can return an answer to an enquiry that exceeds so far the bounds of our direct observation, it dissents from both orders of thought. The conditions of life are indeed narrow, special, restricted; intelligent, organic life must, relatively speaking, be a rarity in the universe, but we lack the information that would enable us to affirm with any confidence that such life is only to be found upon this world of ours. Heavy as the odds are against any particular world being an inhabited one, yet when the limitless extent of s.p.a.ce is considered, and the innumerable numbers of stars and systems of stars, it seems but reasonable to conclude that though inhabited worlds are relatively rare, the absolute number of them may be considerable; considerable, if not at one particular moment of time, yet when the whole duration of the universe is admitted.

But there is a religious question connected with this enquiry; one that goes down to the very roots of man's deepest thoughts and aspirations. As individuals our days on the Earth are as a shadow, and there is none abiding; as individuals we pa.s.s and disappear; and though the race remains, yet as far as science can guide us and enable us to penetrate the future, the same lot awaits the race as well. Slowly but surely the water of a planet will combine with its substance or disappear into its crust.

The cooling of the Sun, though it may be long delayed, would seem to be inevitable in the sequel.