Although the question as to whether there are, elsewhere in the universe, beings sufficiently like ourselves to be called human, must for ever remain an open one, it is well worth discussing, because it leads us to consider matters of great interest to every thoughtful person and well worth the attention of students of science.

      Mr. Alfred Russel Wallace, in his recent book, Man's Place in the Universe, gives a negative answer; while Professor Simon Newcomb is inclined to admit the possibility on the ground that, among the millions of stars now revealed to our telescopes, there may be some which afford their accompanying planets conditions sufficiently like those of our earth to enable human-like beings to flourish. In the present article I propose to debate the matter rather from the point of view of the biologist than from that of the physicist or the astronomer, and shall endeavor to show that, judged from what we find in him, man is literally of the earth, earthy. An examination into his past history proves that he is adapted, with the most minute precision, to his own proper sphere; and that in all his parts, mental and bodily, he is as much a product of the complex conditions of life on this planet as the features of a bronze image are a product of its mould. It will be seen that, looking at the question from this stand-point, even if we grant all Professor Newcomb's millions of planetary systems, the probabilities are overwhelming against the existence of men and women in any other world.

      Popular speculations as to the nature of the supposed inhabitants of Mars, which crop up whenever Martian discoveries are announced from Flagstaff Observatory and elsewhere, may here be alluded to in passing. Whatever the presumed Martians may be like, it would certainly be impossible for us, if we met one of them, to recognize him as a man and a brother. Beings who can perform gigantic labors, such as digging of "canals" compared with which the Mississippi is a mere gutter, with not more than one-eighth of our atmosphere to breathe meanwhile, must have a chest development which would distort them out of all semblance to humanity; while the low force of gravity in Mars would enable people of average weight to get about on legs not much stouter than those of a collie dog. According to some careful observers, such as Professor Campbell of the Lick Observatory, it is even an open question whether Mars has any more atmosphere than the moon. More than this, certain leading physicists quoted by Mr. Alfred Russel Wallace, have declared that no oxygen, hydrogen, or water could exist on so small a world without being dissipated into space and sucked up by ourselves and the sun. Hence it has been suggested that the "polar snow caps" of Mars may consist of solid carbonic acid gas. From this point of view our Martian neighbors must subsist upon an atmospheric regimen of carbonic acid instead of upon one of air, and hence would be more likely to resemble trees in their physical constitution than the higher animals. Such a notion opens up an inviting field for imaginative writers who wish to rival Mr. H. G. Wells. Here below we irrigate and cultivate passive and helpless vegetables. There, perhaps, an alert and enterprising vegetable population is watering and fertilizing the soil on its own initiative and for its own private ends.

      If we trace man's pedigree up from the lowest organisms to his present state, we find that there must have been, on innumerable occasions, a dividing of the ways, in which conditions absolutely peculiar to this planet determined the issue as to which path should lead upwards to humanity.

      The items of environment which have directed the plastic life-stream along this or that evolutionary channel were often as inconsiderable and as fortuitous as the utterly trivial events which, in every-day life, fatally determine our future. There is this difference, however. A man has his innate will-power (or what amounts to the same thing), which enables him in some measure to assent or to resist, while the life-stream under the changing gusts of environment is as smoke wafted by the wind. Often in threading life's numberless cross-roads, the main procession of living things goes one way, ending nowhere in particular, while a few individuals drift off through some casual influence along an obscure by-path, which, in the end, proves the only track leading upwards to the goal. Such influences, however, are ordained and limited all the time by certain physical conditions proper to our own planet. Gravity, air-pressure, temperature, moisture, and light are only a few of these. If we took account of them all, and of their interdependence one upon another, and took into account also the innumerable phases and tendencies of complex organic life, even Newcomb's millions of stars would be nowhere in balancing the chances against the evolution of man elsewhere in the universe. Let us look at a few of these controlling circumstances and conditions, remembering all the time that they are but samples of continuous happenings throughout millions of years. We may be guided in the sampling process by keeping in mind the main divisions among living things as they are classified to-day.

      Very early indeed some of the primitive forms divided into those that drew oxygen from the air, and those that drew carbonic acid. Such as took the latter course shut themselves off forever from all earthly chances of becoming active and versatile beings of the nature of man. Oddly enough, however, as I hope to show by and by, this dependence on atmospheric carbonic acid on the part of the chlorophyll-bearing and light-seeking vegetable world has contributed more than anything else to certain forces of environment which have given to man his most distinctive characteristics in the shape of clever hands and a calculating, reasoning brain.

      Then consider the division which early took place into vertebrate and invertebrate. The invertebrate portion of creation long took precedence on the earth, and even to-day some of its representatives are, in a sense, much more highly developed, both physically and socially, than is the branch to which we belong. Most wonderful and elaborate in their exact adjustment to environment are the present life-schemes of many of the spiders, bees, and ants; and, moreover, geologists assert that these creatures had reached their present perfection long before man took precedence upon the earth. Yet it can be demonstrated that, in dispensing with a backbone, their early progenitors took a fatal step as far as the higher possibilities of life were concerned. I think it was Professor Lloyd Morgan who, in a lecture for juveniles, distinguished vertebrates from invertebrates by saying that the former were made of "flesh and bone," and the latter of "skin and squash." By depending too much upon their skins as a protection and support for their organs, the invertebrates, with exception of the molluscs, handicapped themselves fatally as regards progress to higher grades of being. They are literally hide-bound, and when they desire to grow large, like certain crabs and lobsters, they are obliged periodically to burst off their outer covering — which, it must be remembered, is also the scaffolding upon which their muscles are hung — and remain long in a dormant state before the new skin is ready for service. It is as if we were, every few months, deprived of all our bones, and had to lie in bed a long time before we could resume our active habits. This would handicap us fatally as regards getting on in the world. By far the greater number of the invertebrates have avoided such troubles by remaining small, and contenting themselves with a span of life of merely a few months' duration. They overcome the growth difficulty by adopting several distinct stages or transformations, and finally are born from their pupæ as fully equipped adults. Such habits bar them from progress in several ways. In the first place you cannot possibly have much intelligence without a big store of brain cells — and brain cells take up room. The old idea that the tiny ganglia in the fore-end of an ant are almost equal in thinking capacity to a human brain has become a mere fairy tale to the modern biologist.

      Moreover, such creatures have, in their economy of life, one fatal condition which bars them for ever from intellectual progress. This is the lack of continuity of experience due to their different phases. Caterpillars must acquire vast experience in eating leaves, and doubtless some of them learn to excel in the art. They may also become well informed as to the best way to escape from ichneumon flies and other foes of the caterpillar. But all this youthful education, if not lost in the oblivion of his chrysalis sleep, can be of no possible use to a caterpillar when he emerges as a butterfly. Thus, when he suddenly becomes "full-grown," as the chrysalis bursts open, the fully fledged insect has to commence his life duties in a hostile world without any education or apprenticeship at all. Hence he flits round in his aimless, frivolous way, living a monotonous and narrow life (in spite of poetic notions to the contrary), without any hope or possibility of mental betterment.

      Another critical turning-point in ancestral history is marked by the first beginnings of the backbone. When certain small and gelatinous creatures swimming in the seas got a little gristly stiffening down their middle, it enormously increased their power of directing their movements. It afforded them also a vastly better prospect of high development than the invertebrate plan of obtaining rigid points of attachment for the muscles of locomotion by stiffening the outer covering with chitine or carbonate of lime. Still this mere gristly rod, which we now call the "notochord," and which we find in the lancelet and other primitive creatures, would not have afforded a very good chance of upward progress if it had not led on towards a genuine spinal column of jointed but rigid segments. The true vertebrates soon parted company from their forerunners, and have gone ahead marvellously; while those content with the first make-shift backbone have remained very low in the scale.

      It is true that some molluscs have shown considerable powers of upward development without a backbone, but their general pulpiness prevented them from doing much on dry land, where the pull of gravity is felt, and where they are now chiefly represented by the humble snail and slug. It is only their marine representatives, such as the cuttle-fishes, &c., which have increased in size and complexity so as to compete in some measure with the vertebrates.

      Now we come to the critical decision as to whether land or water should be the scene of future activity. A momentous question this, for we find that none of the creatures which remained purely aquatic in their habits have acquired much brain. Their environment, always cold and wet, was too uniform, and probably the forces which regulated their lives were too rigid and too mighty to give much chance of versatility or choice of action. In considering how the water-born vertebrate first came to dry land, we must take account of the influence of the lunar tides. In ancient times, as at the present day, it was the shallow seas which abounded with animal life. Such creatures as were near a sloping shore were liable to be left high and dry twice in the twenty-four hours at low tide. Very long ago, as Sir George Darwin has pointed out, the moon was much nearer the earth than it is now, and its attraction was much stronger. Hence vast areas were alternately flooded and dry, and myriads of creatures, which originally extracted a little air from the water by means of gills, found themselves obliged to take their air undiluted, or die for want of it. There were other circumstances, such as the periodical drying up of rivers and lakes, which led to a like alternative. Those who succeeded entered on the narrow path which led upwards to humanity.

      Thus, unless some earth-like planet possessed vast shallow oceans, and a moon closely resembling our own, upward progress during this critical stage would have to depend on wholly different circumstances — and the results also would be wholly different. As the great founder of the evolutionary doctrine pointed out, man still retains in his physical framework, and in the functions of his body, traces not only of gills for obtaining air from water, but also of the regular periodic recurrence of lunar influence.

      Now we come to another set of cross roads, or rather a maze of them, in which the devious, but ever upward way is very difficult to trace. During that enormous period comprised by the later primary and the secondary epochs, huge populations of cold-blooded amphibia and reptiles swarmed over the land.

      Somehow, from among the less conspicuous of these, two distinct sets of creatures were developed, with a greatly improved breathing apparatus and a more rapid circulation. These were (ultimately) the birds and mammals. Let us consider the latter first, for it was they who took the right turning. Their hearts had four chambers, instead of three or two, and their lungs had an increased capacity for getting oxygen from the air. Hence their tissues were supplied with blood in which the carbonic acid had been eliminated much better than under the old system of circulation. Owing to the more rapid and complete oxydization of waste products within their bodies their blood became warmer than that of their fellows; just as, in the laboratory, quick and strong chemical action produces a greater degree of heat than weak and slow action. This was an enormous upward step. All the vital processes were vivified by the increased supply of oxygen and the accompanying warmth. These new beings were no longer so dependent upon external heat as their old-fashioned rivals. Their hot, red blood, like the wine which stirred Geraint in Tennyson's legend, "made summer in their veins." Hence their bodies, and especially their vital internal organs, their hearts, brains, and stomachs, drew benefit from a summer temperature all the time, like plants in a stove house.

      Following upon this — and probably in consequence of it — it became the rule, instead of the exception, to bring forth young alive. This was another dividing of the ways which has had momentous results well worthy of our attention. But before we discuss these, let us consider the plight of those earlier mammals born into a world swarming with ferocious reptiles. They were little feeble beings, not much bigger than rats, and it seems wonderful that they should have escaped at all from their hungry swarms of enemies. In their long grapple for supremacy with the "monstrous efts," which were then lords and masters of earth, it was their warm blood full of oxygen which saved them. Although wanting in brute force, they were capable of more rapid and more sustained motion than any reptile could show. To be "a good goer, and a good stayer," plenty of lung room and a strong heart have always been necessary; and in this respect the reptiles were nowhere in comparison with them. Moreover, their brain cells were nourished and stimulated by fast-flowing warm blood instead of the sluggish and chilly fluid which fed the reptilian brain, and hence they became quicker of perception and more fertile in schemes of attack and defence. The contest was pretty much that of Tom Thumb against the giants, where the small hero, by means of his quickness and cunning, always bested his huge and stolid adversaries.

      Since the young warm-bloods were born alive in a very helpless state, provision had to be made for feeding them. Here another crux arose, and there is evidence that Nature tried various shifts before mammalian methods were invented. Relics of certain of the more successful schemes which fell short of perfection are in evidence to-day in the monotremes, such as the duck-bill platypus, and the marsupials, chiefly represented by the opossum and the kangaroo. Apparently their life economy involves conditions which bar the way to any high degree of development, for although they had it all their own way in Australia, they never advanced far; and wherever they are brought into competition with the true mammals, they (with the possible exception of the American opossum) go under in the battle of life.

      When the young had to be suckled and assiduously cared for by their parents for a long period, education began to come into play. Here was another great upward step. For the first time in the world's history, experience gathered during the lifetime of one generation was put at the disposal of the next. Hence a capacity for brain growth and a power of learning on the part of the young became more and more important if they were to profit by the cumulative experience of their kind.

      The same is also true to a great extent of the large section of the warm-blooded beings which never became viviparous, that is to say, the birds. Indeed, there is reason to think that the birds went far ahead of the mammals during the earlier periods of warm-blooded life. They are indeed ahead of them to-day in many respects as regards physical perfection; yet this very success in obtaining nearly all they wanted, as far as animal comfort was concerned, took away their chance of becoming the intellectual rivals of man. One may compare their case, in fact, with that of a man who has been so well started in life that he has not been obliged to use his wits and develop his faculties. He will not long remain the rival of another of equal capacity who has had a rougher time of it at the outset, and who has fought his way upwards by overcoming the difficulties which stood in his way.

      What might have happened, supposing this avian branch, which obtained such a good start among more highly evolved beings, had wrested the germ of intellectual supremacy from their mammalian competitors, offers a fascinating field for speculation. Even now, if we exclude ourselves from the list, it would be quite a debatable matter whether the birds or the mammals have the best of it as regards psychical development.

      Let us now follow the course of the mammalian life-stream from its early beginnings during the Triassic period. Allusion has been made to those feeble primitive mammals from which, apparently, all the higher forms have sprung. From such remains as have been found in the Oxford clay and in the Stonesfield slate, they are presumed to have been opossum-like creatures which were arboreal in their habits. Now most creatures which leave the solid earth, and make a habitat in the trees, do so to avoid their enemies. It seems extremely probable, considering the voracious reptiles which then swarmed everywhere, that these first weak mammals were no exception to the rule. Even if this were not so from the very first, the trees soon had a big refugee population.

      Now when one looks into the habits of arboreal animals one finds that two distinct methods of obtaining a hold have become fashionable. By far the greater number develop claws, and cling to the rough bark by means of them. A smaller number have developed long digits on their extremities which are capable of obtaining a good grip of the branches. There are also intermediate forms in all degrees. What determined possession of claws, or of hand-like extremities, we cannot say; but the divergence was of vital import as regards the future. A foot merely armed with sharp claws remains almost entirely a means of progression, and is capable of very limited usefulness in any other capacity; but when the flexible digits are lengthened, and are controlled by groups of strong and complex muscles, all the wonderful capabilities of the human hand at once become possible. It seems probable that, during a very long period indeed, the hand-possessing climbers, having lost the power of rapid progression on the ground, lived a purely arboreal life among the high branches, much as most forest-dwelling monkeys do to-day; while creatures with claws which enabled them to scamper up and down the trunks of trees like a squirrel were less purely arboreal. As far as development in size and bulk was concerned, the possessors of gripping hands had much the best of it, for claw-climbing is obviously best managed by animals which are small and light. Nowadays, with the one exception of the sloth, which has developed its claws into finger-like hooks, apes and monkeys are the only creatures of any bulk which live habitually in trees. Bears, leopards, and jaguars, it must be remembered, in spite of the wonderful climbing powers which they display on emergency, are not tree-dwellers at all, because none of them can for long exist in comfort off the solid ground.

      Now let us get back for awhile to the suggestion put forward in the earlier part of this article, to the effect that the avidity of the vegetable world for carbonic acid had a crucial influence upon our higher development. What is a tree? It is a long-lived plant which has acquired woody fibre and has grown upwards. Why does it grow upwards? The answer to this question is very well known to those practical folk who make plantations, where the young trees are put at first very closely together, "to draw them up." It is this competition of plant with plant for light and for carbonic acid which makes each of them, when so placed, seek to tower above its fellows. They develop the firm woody fibre of their trunks and branches in this struggle for what they all covet. The plant or tree which is able to overtop its competitors, and to expose the greatest surface of chlorophyll-charged leaves to the air and to the sunlight, extracts the most carbonic acid from the atmosphere to use in building up its own tissues. Given slightly different conditions as to atmosphere, moisture and soil, and this kind of vegetable competition, with its fruitful and far-reaching influence, would not occur. It is not every part, even of our fertile earth, where forest trees flourish without the aid of man. Vast regions such as the steppes of Russia and the prairies of America are thickly covered with grasses and small-herbage which obtain their light and carbonic acid without ever aspiring to be trees. If the creeping grasses had been evolved earlier, and if the whole planet had been covered with steppe-like plains during the Tertiary epoch, how different might have been the result as regards ourselves!

      When we come to think of what man owes to the trees, it is no longer a matter for surprise that tree-worship has become a cult among many different branches of the human family. The writer has long had a steadily growing conviction — which he puts forward here with diffidence because the subject confessedly needs a good deal more threshing out before it is fit for formal presentation — that some of the most distinctive faculties of the human mind, and especially the mathematical faculty, are directly based upon subconscious brain-processes evoked by the daily needs of a long-continued arboreal existence. If any one has watched active monkeys, and especially our nearer kinsfolk, the gibbons, progressing by long leaps and armswings among the branches, he must have been struck by the strange power that such creatures show of accurately calculating distance and adjusting their muscles to the practical solution of complex physical problems. Here accuracy in getting a "right answer" is a matter of life or death. In such locomotion the nervous processes involved can only be described as a succession of distinct mental efforts, and are not rhythmically mechanical, like those regulating the running of a quadruped. For, unlike progression on level ground, a scamper among the tree-tops affords no two successive movements of a like nature. The muscular combinations requisite for each leap, together with the force required, must be arranged for and estimated beforehand according to the ever-changing distances and angles of the branches. No doubt all this is an unconscious process; and it must be acknowledged that the same faculties seem to be shown, in a measure, by most of the lower animals. Still, I do not see how we can escape from the conclusion that it involves calculation, although the brain cells work their mathematical problems as unconsciously as a healthy man's stomach performs the surprising chemical feat of digesting his dinner. Should the conscious, introspective mind which is a characteristic of man — and more especially of civilized man — learn to dive down among the psychic machinery which controls such automatic mental processes, and find a way of partially expressing what goes on there by means of numbers (as artificial words partially express our unworded thoughts), may we not here have the basis of that wondrous mathematical aptitude which in civilized times has blazed forth in man's mental firmament as suddenly as a meteor in a dark sky, and which has proved such a dazzling mystery to Wallace and other students of mental evolution? Before leaving this curious and interesting hypothesis let me call attention to two kindred facts which seem to give it some justification. Nearly all the mathematical prodigies who crop up from time to time acknowledge that certain of their most difficult arithmetical feats, especially those performed during their earlier years, are achieved subconsciously, so that they have found themselves puzzled to explain how the results were obtained. At the other end of the scale are low-grade savages who cannot count half a dozen, but who undoubtedly have certain mysterious ways of their own for correctly estimating numbers, distances, &c., which must involve some kind of mental calculation.

      Certain other conditions of arboreal life can, without so much danger of controversy, be linked up with our human characteristics. While safe from his enemies high up in the trees there was no need for our remote ancestor to develop large olfactory nerves in order to detect the approach of dangerous neighbors. Hence, throughout the Primates, these nerves bear no comparison with such as are found in quadrupeds whose habitat has always been upon the ground. Sight and hearing, though much more important, did not need to be so acute as among such creatures as the deer or the fox. This deficiency in certain of the sense organs must have had an enormous influence on mental development when man took to living on the ground and became a hunter. It was a most fortunate deprivation; for, not being able to trail and kill like a hound, he was obliged to depend on his brain and versatile hands in following and capturing his quarry, as well as in protecting himself against enemies. Hence in every hunt in which he became engaged he was exercising and sharpening his nascent reasoning faculties; for although we usually speak with contempt of the mental powers of the desert Bushman and the Australian black, the arts in which these primitive savages proverbially excel, those of tracking and snaring game, involve, if rightly understood, a keen exercise of the powers of observation, coupled with elaborate reasoning processes in which data of ever-varying value have to be continually taken into account.

      The success of the early hunter probably depended much more upon such intellectual feats as these than upon mere bodily prowess. Throughout the immeasurable Stone Ages, man, like all modern savages who live by the chase, was continually in peril of death from hunger. When game was scarce and shy those successful in hunting, that is to say in exercising their reasoning faculties, survived and fed their hungry families; while those (and there must have been many) who had not brains enough to find quarry and track it down, were eliminated, and their race became extinct.

      There can be no doubt whatever that man owes a great deal of that physical helplessness which has compelled him to resort to art in the manufacture of dwellings, clothes, and weapons, to his association with the trees where his pristine forefathers sought safety. Had he been clothed by Nature as perfectly as are the birds, he would never have attained his present civilized state. Moreover, if flight, which is now one of his most ardent ambitions, had been granted to him at an early period, food and safety would have been easily within his reach without the continual exercise of reason; and, moreover, he would never have acquired his unrivalled hands.

      It has only been possible, in a short and popular sketch such as this, to point out a very few of the main factors which have contributed in bringing us to our present estate since the beginning of life upon the Earth. But every one who thinks about the matter, with the necessary information at his disposal — and in all probability many of the readers of this article have a much more extensive and accurate knowledge of physics, astronomy, and geology than the present writer — would be able to bring forward numerous other instances where conditions peculiar to this planet have directed the upward march of that thin procession of living creatures which has culminated in man.

      If every one of these physical conditions had been repeated on some other orb in the universe, and had the self-same chain of meteorological events, with all its myriad links, reacted on such living organisms as might there have sprung into being, even then the odds would remain incalculably great against the evolution of man. Vastly more important than the nature of the outward pressure in such a problem is the nature of the matter acted upon. Among sensitive and shifty living things the possibilities of variation are infinitely great, and the chances of the same results being obtained twice over after millions of evolutionary changes are infinitesimally small. Hence we may say with confidence that, whatever intelligent beings may exist elsewhere in the universe, they are totally different from human-kind.

The Nineteenth Century and After.

Louis Robinson.