Here, then, I think, we have found an adequate explanation of the very long-continued light and heat-emitting capacity of our sun, and probably of many others in about the same position in the solar cl.u.s.ter. These would at first gradually aggregate into considerable ma.s.ses from the slowly moving diffused matter of the central portions of the original universe; but at a later period they would be reinforced by a constant and steady inrush of matter from its very outer regions, and therefore possessing such high velocities as to materially aid in producing and maintaining the requisite temperature of a sun such as ours, during the long periods demanded for continuous life-development. The enormous extension and ma.s.s of the original universe of diffused matter (as postulated by Lord Kelvin) is thus seen to be of the greatest importance as regards this ultimate product of evolution, because, without it, the comparatively slow-moving and cool central regions might not have been able to produce and maintain the requisite energy in the form of heat; while the aggregation of by far the larger portion of its matter in the great revolving ring of the galaxy was equally important, in order to prevent the too great and too rapid inflow of matter to these favoured regions.

It appears, then, that if we admit as probable some such process of development as I have here indicated, we can dimly see the bearing of all the great features of the stellar universe upon the successful development of life. These are, its vast dimensions; the form it has acquired in the mighty ring of the Milky Way; and our position near to, but not exactly in, its centre. We know that the star-system _has_ acquired these forms, presumably from some simple and more diffused condition. We know that we _are_ situated near the centre of this vast system. We know that our sun _has_ emitted light and heat, almost uniformly, for periods incompatible with rapid aggregation and the equally rapid cooling which physicists consider inevitable. I have here suggested a mode of development which would lead to a very slow but continuous growth of the more central suns; to an excessively long period of nearly stationary heat-giving power; and lastly, an equally long period of very gradual cooling--a period the commencement of which our sun may have just entered upon.

Descending now to terrestrial physics, I have shown that, owing to the highly complex nature of the adjustments required to render a world habitable and to retain its habitability during the aeons of time requisite for life-development, it is in the highest degree improbable that the required conditions and adaptations should have occurred in any other planets of any other suns, which _might_ occupy an equally favourable position with our own, and which were of the requisite size and heat-giving power.

Lastly, I submit that the whole of the evidence I have here brought together leads to the conclusion that our earth is almost certainly the only inhabited planet in our solar system; and, further, that there is no inconceivability--no improbability even--in the conception that, in order to produce a world that should be precisely adapted in every detail for the orderly development of organic life culminating in man, such a vast and complex universe as that which we know exists around us, may have been absolutely required.

SUMMARY OF ARGUMENT

As the last ten chapters of this volume embody a connected argument leading to the conclusion above stated, it may be useful to my readers to summarise rather fully the successive steps of this argument, the facts on which it rests, and the various subsidiary conclusions arrived at.

(1) One of the most important results of modern astronomy is to have established the unity of the vast stellar universe which we see around us.

This rests upon a great ma.s.s of observations, which demonstrate the wonderful complexity in detail of the arrangement and distribution of stars and nebulae, combined with a no less remarkable general symmetry, indicating throughout a single inter-dependent system, not a number of totally distinct systems so far apart as to have no physical relations with each other, as was once supposed.

(2) This view is supported by numerous converging lines of evidence, all tending to show that the stars are not infinite in number, as was once generally believed, and which view is even now advocated by some astronomers. The very remarkable calculations of Lord Kelvin, referred to in the early part of this chapter, give a further support to this view, since they show that if the stars extended much beyond those we see or can obtain direct knowledge of, and with no very great change in their average distance apart, then the force of gravitation towards the centre would have produced on the average more rapid motions than the stars generally possess.

(3) An overwhelming consensus of opinion among the best astronomers establishes the fact of our nearly central position in the stellar universe. They all agree that the Milky Way is nearly circular in form.

They all agree that our sun is situated almost exactly in its medial plane.

They all agree that our sun, although not situated at the exact centre of the galactic circle, is yet not very far from it, because there are no unmistakable signs of our being nearer to it at any one point and farther away from the opposite point. Thus the nearly central position of our sun in the great star-system is almost universally admitted.

On the question of the solar-cl.u.s.ter there is more difference of opinion; though here, again, all are agreed that there is such a cl.u.s.ter. Its size, form, density, and exact position are somewhat uncertain, but I have, as far as possible, been guided by the best available evidence. If we adopt Lord Kelvin's general idea of the gradual condensation of an enormous diffused ma.s.s of matter towards its common centre of gravity, that centre would be approximately the centre of this cl.u.s.ter. Also, as gravitational force at and near this centre would be comparatively small, the motions produced there would be slow, and collisions, being due only to differential motions, when they did occur would be very gentle. We might therefore expect many dark aggregations of matter here, which may explain why we do not find any special crowding of visible stars in the direction of this centre; while, as no star has a sensible disc, the dark stars if at great distances would hardly ever be seen to occult the bright ones. Thus, it seems to me, the controlling force may be explained which has retained our sun in approximately the same orbit around the centre of gravity of this central cl.u.s.ter during the whole period of its existence as a sun and our existence as a planet; and has thus saved us from the possibility--perhaps even the certainty--of disastrous collisions or disruptive approaches to which suns, in or near the Milky Way, and to a less extent elsewhere, are or have been exposed. It seems quite probable that in that region of more rapid and less controlled motions and more crowded ma.s.ses of matter, no star can remain in a nearly stable condition as regards temperature for sufficiently long periods to allow of a complete system of life-development on any planet it may possess.

(4) The various proofs are next stated that a.s.sure us of the almost complete uniformity of matter, and of material physical and chemical laws, throughout our universe. This I believe no one seriously disputes; and it is a point of the greatest importance when we come to consider the conditions required for the development and maintenance of life, since it a.s.sures us that very similar, if not identical, conditions must prevail wherever organic life is or can be developed.

(5) This leads us on to the consideration of the essential characteristics of the living organism, consisting as it does of some of the most abundant and most widely distributed of these material elements, and being always subject to the general laws of matter. The best authorities in physiology are quoted, as to the extreme complexity of the chemical compounds which const.i.tute the physical basis for the manifestation of life; as to their great instability; their wonderful mobility combined with permanence of form and structure; and the altogether marvellous powers they possess of bringing about unique chemical transformations and of building up the most complicated structures from simple elements.

I have endeavoured to put the broad phenomena of vegetable and animal life in a way that will enable my readers to form some faint conception of the intricacy, the delicacy, and the mystery of the myriad living forms they see everywhere around them. Such a conception will enable them to realise how supremely grand is organic life, and to appreciate better, perhaps, the absolute necessity for the numerous, complex and delicate adaptations of inorganic nature, without which it is impossible for life either to exist now, or to have been developed during the immeasurable past.

(6) The general conditions which are absolutely essential for life thus manifested on our planet are then discussed, such as, solar light and heat; water universally distributed on the planet's surface and in the atmosphere; an atmosphere of sufficient density, and composed of the several gases from which alone protoplasm can be formed; some alternations of light and darkness, and a few others.

(7) Having treated these conditions broadly, and explained why they are important and even indispensable for life, we next proceed to show how they are fulfilled upon the earth, and how numerous, how complex, and often how exact are the adjustments needed to bring them about, and maintain them almost unchanged throughout the vast aeons of time occupied in the development of life. Two chapters are devoted to this subject; and it is believed that they contain facts that will be new to many of my readers.

The combinations of causes which lead to this result are so varied, and in several cases dependent on such exceptional peculiarities of physical const.i.tution, that it seems in the highest degree improbable that they can _all_ be found again combined either in the solar system or even in the stellar universe. It will be well here just to enumerate these conditions, which are all essential within more or less narrow limits:--

Distance of planet from the sun.

Ma.s.s of planet.

Obliquity of its ecliptic.

Amount of water as compared with land.

Surface distribution of land and water.

Permanence of this distribution, dependent probably on the unique origin of our moon.

An atmosphere of sufficient density, and of suitable component gases.

An adequate amount of dust in the atmosphere.

Atmospheric electricity.

Many of these act and react on each other, and lead to results of great complexity.

(8) Pa.s.sing on to other planets of the solar system, it is shown that none of them combine all the complex conditions which are found to work harmoniously together on the earth; while in most cases there is some one defect which alone removes them from the category of possible life-producing and life-supporting planets. Among these are the small size and ma.s.s of Mars, being such that it cannot retain aqueous vapour; and the fact that Venus rotates on its axis in the same time as it takes to revolve round the sun. Neither of these facts was known when Proctor wrote upon the question of the habitability of the planets. All the other planets are now given up--and were given up by Proctor himself--as possible life-bearers in their present stage; but he and others have held that, if not suitable now, some of them may have been the scene of life-development in the past, while others will be so in the future.

In order to show the futility of this supposition, the problem of the duration of the sun as a stable heat-giver is discussed; and it is shown that it is only by reducing the periods claimed by geologists and biologists for life-development upon the earth, and by extending the time allowed by physicists to its utmost limits, that the two claims can be harmonised. It follows that the whole period of the sun's duration as a light and heat-giver has been required for the development of life upon the earth; and that it is only upon planets whose phases of development synchronise with that of the earth that the evolution of life is possible.

For those whose material evolution has gone on quicker or slower, there has not been, or will not be, time enough for the development of life.

(9) The problem of the stars as possibly having life-supporting planets is next dealt with, and reasons are given why in only a minute portion of the whole is this possible. Even in that minute portion, reduced probably to a few of the component suns of the solar-cl.u.s.ter, a large proportion seems likely to be ruled out by being close binary systems, and another large portion by being in process of aggregation. In those remaining, whether they may be reckoned by tens or by hundreds we cannot say, the chances against the same complex combination of conditions as those which we find on the earth occurring on any planet of any other sun are enormously great.

(10) I then refer, briefly, to some recent measurements of star-radiation, and suggest that they may thus possibly have important effects on the development of vegetable and animal life; and, finally, I discuss the problem of the stability of the stellar universe and the special advantage we derive from our central position, suggested by some of the latest researches of our great mathematician and physicist--Lord Kelvin.

CONCLUSIONS

Having thus brought together the whole of the available evidence bearing upon the questions treated in this volume, I claim that certain definite conclusions have been reached and proved, and that certain other conclusions have enormous probabilities in their favour.

The conclusions reached by modern astronomers are: (1) That the stellar universe forms one connected whole; and, though of enormous extent, is yet finite, and its extent determinable.

(2) That the solar system is situated in the plane of the Milky Way, and not far removed from the centre of that plane. The earth is therefore nearly in the centre of the stellar universe.

(3) That this universe consists throughout of the same kinds of matter, and is subjected to the same physical and chemical laws.

The conclusions which I claim to have shown to have enormous probabilities in their favour are--

(4) That no other planet in the solar system than our earth is inhabited or habitable.

(5) That the probabilities are almost as great against any other sun possessing inhabited planets.

(6) That the nearly central position of our sun is probably a permanent one, and has been specially favourable, perhaps absolutely essential, to life-development on the earth.

These latter conclusions depend upon the combination of a large number of special conditions, each of which must be in definite relation to many of the others, and must all have persisted simultaneously during enormous periods of time. The weight to be given to this kind of reasoning depends upon a full and fair consideration of the _whole_ evidence as I have endeavoured to present it in the last seven chapters of this book. To this evidence I appeal.

This completes my work as a connected argument, founded wholly on the facts and principles acc.u.mulated by modern science; and it leads, if my facts are substantially correct and my reasoning sound, to one great and definite conclusion--that man, the culmination of conscious organic life, has been developed here only in the whole vast material universe we see around us. I claim that this is the logical outcome of the evidence, if we consider and weigh this evidence without any prepossessions whatever. I maintain that it is a question as to which we have no right to form _a priori_ opinions not founded upon evidence. And evidence opposed to this conclusion, or even as to its improbability, we have absolutely none whatever.

But, if we admit the conclusion, nothing that need alarm either the scientific or the religious mind necessarily follows, because it can be explained or accounted for in either of two distinct ways. One considerable body, including probably the majority of men of science, will admit that the evidence does apparently lead to this conclusion, but will explain it as due to a fortunate coincidence. There might have been a hundred or a thousand life-bearing planets, had the course of evolution of the universe been a little different, or there might have been none at all.

They would probably add, that, as life and man _have_ been produced, that shows that their production was possible; and therefore, if not now then at some other time, if not here then in some other planet of some other sun, we should be sure to have come into existence; or if not precisely the same as we are, then something a little better or a little worse.

The other body, and probably much the largest, would be represented by those who, holding that mind is essentially superior to matter and distinct from it, cannot believe that life, consciousness, mind, are products of matter. They hold that the marvellous complexity of forces which appear to control matter, if not actually to const.i.tute it, are and must be mind-products; and when they see life and mind apparently rising out of matter and giving to its myriad forms an added complexity and unfathomable mystery, they see in this development an additional proof of the supremacy of mind. Such persons would be inclined to the belief of the great eighteenth century scholar, Dr. Bentley, that the soul of one virtuous man is of greater worth and excellency than the sun and all his planets and all the stars in the heavens; and when they are shown that there are strong reasons for thinking that man _is_ the unique and supreme product of this vast universe, they will see no difficulty in going a little further, and believing that the universe was actually brought into existence for this very purpose.

With infinite s.p.a.ce around us and infinite time before and behind us, there is no incongruity in this conception. A universe as large as ours for the purpose of bringing into existence many myriads of living, intellectual, moral, and spiritual beings, with unlimited possibilities of life and happiness, is surely not _more_ out of proportion than is the complex machinery, the life-long labour, the ingenuity and invention which we have bestowed upon the production of the humble, the trivial, _pin_. Neither is the apparent waste of energy so great in such a universe, comparatively, as the millions of acorns, produced during its life by an oak, every one of which might grow to be a tree, but of which only _one_ does actually, after several hundred years, produce the _one_ tree which is to replace the parent. And if it is said that the acorns are food for bird and beast, yet the spores of ferns and the seeds of orchids are not so, and countless millions of these go to waste for every one which reproduces the parent form. And all through the animal world, especially among the lower types, the same thing is seen. For the great majority of these ent.i.ties _we_ can see no use whatever, either of the enormous variety of the species, or the vast hordes of individuals. Of beetles alone there are at least a hundred thousand distinct species now living, while in some parts of sub-arctic America mosquitoes are sometimes so excessively abundant that they obscure the sun. And when we think of the myriads that have existed through the vast ages of geological time, the mind reels under the immensity of, to us, apparently useless life.

All nature tells us the same strange, mysterious story, of the exuberance of life, of endless variety, of unimaginable quant.i.ty. All this life upon our earth has led up to and culminated in that of man. It has been, I believe, a common and not unpopular idea that during the whole process of the rise and growth and extinction of past forms, the earth has been preparing for the ultimate--Man. Much of the wealth and luxuriance of living things, the infinite variety of form and structure, the exquisite grace and beauty in bird and insect, in foliage and flower, may have been mere by-products of the grand mechanism we call nature--the one and only method of developing humanity.