[[p. 326]] Those among my critics who have expressed adverse opinions, usually agree that my proofs of the absence of human life in the other planets of our system are very cogent if not quite conclusive, but declare that they cannot accept my view that the unknown planets that may exist around other suns are also without intelligent inhabitants. They give no reasons for this view other than the enormous number of suns that appear to be as favourably situated as our own, and the probability that many of them have planets as suitable as our earth for the development of human life. Several of them consider it absurd, or almost ludicrous, to suppose that man, or some being equally well organised and intelligent, has not been developed many times over in many of the worlds which they assume must exist. But not one of those who thus argue give any indication of having carefully weighed the evidence as to the number of very complex and antecedently improbable conditions which are absolutely essential for the development of the higher forms of organic life from the elements that exist upon the earth or are known to exist in the universe. Neither does any one of them take account of the [[p. 327]] enormous rate at which improbability increases with each additional condition which is itself improbable. Now the numerous conditions and sub-conditions essential to the development of the higher organic life, which must all exist simultaneously and which must all have continued to exist for enormous periods of time, are each of them improbable in various degrees, as clearly shown by the fact that the great majority of them are not present in the degree required in any other planet of the solar system. The actual degree of improbability of each of these conditions cannot be determined except vaguely, but any one who will carefully consider and weigh my very imperfect exposition of them will, I think, admit that they are usually very considerable, the chances against each of them being in some cases perhaps ten to one, in others ten thousand to one or even more. But if we take the whole of the simultaneous conditions requisite as only fifty in number, and the chances against each occurring simultaneously with the rest to be only ten to one, an estimate which seems to me absurdly low because many of them are quantitative within narrow limits, then, the chances against the simultaneous occurrence of the whole fifty would be a million raised to the eighth power (1,000,000⁸), or a million multiplied by a million eight times successively to I. These figures are suggested merely to give some indication to the general reader of the way in which the chances against any event happening more than once mount up to unimaginable numbers when the event is a highly complex one. But to make this still clearer let us take a concrete example.

    [[p. 328]] The chances are certainly very large against two persons, strangers to each other, finding on getting into conversation in a railway carriage that they were born on the same day in the same year. Here are only two coincidences, and the chances may be roughly calculated. But if we add successively other coincidences, each not very improbable in itself, we shall soon arrive at a series which will be held to be quite impossible. Let it be supposed, for instance, that the two persons meet accidentally on a particular day named by a third person beforehand; that they both have the same not common Christian and surname; that their wives were also born on the same day; that they, too, had the same name though not related or known to each other; then let us add a final set of coincidences, that not two only, but ten persons with similar identities met together fortuitously on a day named beforehand in one compartment of a railway carriage; that their ten wives were found to be similarly identical in name and age; that the ten couples had each the same number of children born on the same days as those of the others, and each child having the same names as those of the corresponding children,--then the series of coincidences becomes so overwhelming, that though their occurrence is theoretically possible, their actual existence is so wildly improbable that hardly any sane person would believe that they ever had occurred or ever would occur.

    I will now point out the additional argument derived from the teachings of evolution, which when compounded with those set forth in the preceding pages produces a degree of improbability not unlike [[p. 329]] that above suggested, though, in my opinion, even less likely to have happened through the action of the known laws of nature.

    The extraordinary and endless modifications of form and structure in organisms which often live under apparently similar conditions, together with the great variety of species which inhabit very restricted areas, are themselves a proof of the rigidity of natural selection and the severity of the struggle for existence, leading to a close adaptation of every living form to certain portions of its ever changing environment. It is therefore held by almost all those biologists who are well acquainted with animals or plants in a state of nature, that no species has ever arisen independently in different places or at different times from the convergent modification of distinct ancestors. The fact that the environment as a whole, inorganic or organic, is never identical in any two distinct areas, or in the same area at any two distinct epochs--epochs comparable with those that we know have been required for the modification of species--is sufficient proof that an identical specific evolution cannot take place a second time. Some biologists have indeed held that the same species may, occasionally, have had two independent origins, but they have never grappled with the difficulties of such a supposition, and their views have not been adopted by those best acquainted with the ever-fluctuating conditions under which organic forms have been evolved.

    But if the independent development of any one species, more than once, is almost inconceivable, what are the probabilities that not one only, but a [[p. 330]] whole series, starting from the first dawn of life and extending in an unbroken succession till it culminated in the human organism, has been developed many times over? The course of development of organic life is almost universally admitted to be best represented by a gigantic forest tree, the shoots, leaves, and flowers now produced corresponding to the living species, while all the branches, twigs, leaves, and flowers which have been produced year by year since the first shoot appeared above the ground correspond generally with the extinct species since life first originated. Of course the diversity of actual species is not represented, but merely their enormous multitude and the ever branching and diverging succession in which they have appeared. Fully to appreciate this suggestive analogy the last fifteen pages of the fourth chapter of Darwin's Origin of Species should be carefully read.

    Now if we could have watched such a tree during its whole life from the first shoot that appeared above the ground, we should have seen in its early stages many twigs and branches eaten off by herbivorous animals, many buds destroyed by insects or pecked out by birds. And as it grew and became a lofty tree, storms and tempests would often tear off a branch, other branches would die for want of light and air, arboreal mammals would devour the young twigs as well as the fruits and flowers, and hundreds of species of birds and reptiles, insects and molluscs, would feed upon its tenderest growing buds and leaves. All this corresponds with the way in which whole groups of species have become extinct through many and varied causes, just as a tree has been modified in its [[p. 331]] growth and the number and position of its branches by the various agencies just enumerated. In both cases there has been a continuous struggle for existence.

    Any being who could have watched the whole course of development of life upon the earth, would have been able to follow out from beginning to end the exact succession of forms, from the primæval living germ through a long series of lowly marine organisms into the primitive vertebrate, supposed to be allied to the very low type of fish, the Amphioxus. Thence it would have progressed into a type which was the progenitor of both reptiles and mammals, and thence again into some lowly mammal, and through some type allied to the lemurs to the generalised ape which became modified, in one direction into the existing four types of the anthropoid apes, and in another direction, through a long series of unknown extinct forms, into man.

    The point to which I wish to call special attention is this: that if it is true that each species has arisen from one parental species, and one only, then the whole line of descent from any living species (and therefore from man) back to the earliest form of life, has been fixed and immutable; so that if any one of the thousands or millions of successive species in the line of descent had become extinct before it had been modified into the next species in the line of descent (or, which is the same thing, if it had been differently modified owing to some different change in the environment from what actually occurred), then that particular species which constitutes the last link in that particular line of descent--and [[p. 332]] this also applies to man--would never have come into existence.

    The ultimate development of man has, therefore, roughly speaking, depended on something like a million distinct modifications, each of a special type and dependent on some precedent changes in the organic and inorganic environments, or on both. The chances against such an enormously long series of definite modifications having occurred twice over, even in the same planet but in different isolated portions of it, as in the eastern and western hemispheres of the earth had they been completely separated from each other, are almost infinite, when we know how easily the balance of nature can be disturbed, as in those cases when man purposely or accidentally introduced pigs, rabbits, cats, or weeds into new countries. Even if it be said that those biologists may be right who deny the necessary unity of origin of every species, that would not make much difference, for it is never claimed that all species have had a multiple origin, but only that such an origin is not impossible, and that in rare cases it has actually occurred.

    But if such long-continued identity of the whole course of evolution is hardly conceivable on different parts of the same planet, where all the great and essential conditions are equally fulfilled, how infinitely improbable it becomes that such an identity should have arisen, and have been maintained during millions of ages, on other planets of other suns, where the whole series of fundamental conditions which I have shown to be essential for any high development of life, though they might in rare cases [[p. 333]] approximate those of the earth, could certainly never have been quite identical. And without absolute identity to the smallest details, any identity of development, resulting after millions of ages in the same forms of the higher animals, is manifestly impossible.

    Of course it may be said that a creature with a mind and spiritual nature equal to that of man might have been developed in a very different form. To discuss this question fully would need another volume, but I may briefly state why it seems quite inadmissible. In the first place, man differs from all other animals in the range and speciality of his mental nature even more than in his physical structure. It is generally admitted that his mental development has been rendered possible by a combination of three factors: the erect posture and free hand, the specialised vocal organisation rendering articulate speech possible, and the exceptional development of the brain. The combination of these has made man the superior of all other animals, has given him power to modify nature, to create the arts, and to develop to a marvellous and apparently inexhaustible extent his intellectual, moral, and idealistic faculties, enabling him to extend his vision to the remotest limits of the material universe, to discover its laws, and to penetrate ever deeper and deeper into the hidden secrets of nature.

    No other animal types make the slightest approach to any of these high faculties or show any indication of the possibility of their development. In very many directions they have reached a limit of organic perfection beyond which there is no apparent scope [[p. 334]] for further advancement. Such perfect types we see in the dog, the horse, the cat-tribe, the deer and the antelopes, the elephants, the beaver, and the greater apes; while many others have become extinct because they were so highly specialised as to be incapable of adaptation to new conditions. All these are probably about equal in their mental faculties, and there is no indication that any of them are or have been progressing towards man's elevation, or that such progression, either physically or mentally, is possible. The mere assertion, therefore, that a being possessing man's intellectual and moral nature combined with a very different animal form, might have been developed, is wholly valueless. We have no evidence for it, while the fact that no other animal than man has developed his special faculties even to a lower degree, is strong evidence against it.

    If all the aspects of this great problem are considered, it will be seen that the improbability of an organic development on any other planet resulting in some animal type that could possibly lead to such a very definite and peculiar mental and moral nature as that of man, is far greater than that of the simultaneous occurrence there of the numerous physical conditions which we have found to be essential for the existence and development of any of the higher forms of life. But the two improbabilities have to be compounded, that is, to be multiplied by each other. If the physical or cosmical improbabilities as set forth in the body of this volume are somewhere about a million to one, then the evolutionary improbabilities now urged cannot be considered to be less than perhaps a hundred millions to one; and [[p. 335]] the total chances against the evolution of man, or an equivalent moral and intellectual being, in any other planet, through the known laws of evolution, will be represented by a hundred millions of millions to one. This argument, I feel sure, will appeal to all biological students of evolution, who have not, so far as I know, taken any part in the discussion aroused by my work; while even the general reader will be able to see that the chances against the independent origin of man in other worlds, as explained in the first edition of this volume, are enormously increased by the additional and totally distinct series of improbabilities here set forth.

    I submit, therefore, that the improbabilities of the independent development of man, even in one other world--and far more in thousands or millions of worlds, as usually supposed--are now shown to be so great as to approach very closely, if not quite to attain, the actually impossible. Of course this whole argument applies only to those who believe that the entire material universe, inclusive of man himself, is the product of the immutable laws and forces of nature, and most of my critics appear to adopt this view--the view of pure science, or, as some prefer to call it, the monistic view.

    But to those who believe that the universe is the product of mind, that it shows proofs of design, and that man is the designed outcome of it, and who yet urge that other worlds in unknown numbers have also been designed to produce man, and have actually produced him--to these I reply, that such a view assumes a knowledge of the Creator's purpose and mode of action which we do not possess; that we have no guide to His purposes but the facts we [[p. 336]] actually know; that we do know that here, on our earth, man is the culmination of one line of evolution, not of many, and that the presumption, therefore, is, that no line of evolution in other worlds under other conditions could produce him.

    But, further, we have no reason to suppose that the purpose of man's existence in the universe requires him to exist in infinite numbers, or that the number of human beings that have been or will be produced here is insufficient. On a moderate estimate of the antiquity of man, there must already have been produced many millions of millions of human souls, and these millions may yet be enormously increased by such a course of development of the stellar universe as may keep up the needful supply of light and heat for a few more millions of centuries--a thing far more probable, far easier to conceive, and far more in accordance with the working of natural law than the independent development of a whole series of almost identical animal forms up to man, in many other worlds.

    Whether we look at this great problem from the agnostic or religious point of view; whether we study it as scientific monists or as philosophical spiritualists, our only safe guide is to be found in the facts and the laws of nature as we know them, and in the conclusions to be logically derived from an unbiassed application of those laws to the question at issue. Such an application I have attempted to make in the present volume.