What Our Earth Teaches Us As To Other Worlds
( Originally Published Early 1900's )
BEFORE proceeding to consider the various circumstances under which the worlds or systems which surround us appear to subsist, it may be well to inquire how far we have reason to conclude, from the consideration of our own earth, that other orbs in space support life.
It would not be just to argue directly from the fact that the earth is inhabited to the conclusion that the other planets are inhabited also, nor thence to the conclusion that other stars have, like our sun, their attendant worlds, peopled with various forms of life. An analogy founded on a single instance has no logical force. And it is doubtful whether we have not, in the moon, an instance which would as effectually serve to support a directly opposite conclusion. It seems all but certain, as we shall presently have occasion to show, that no part of the moon's globe is inhabited by living creatures. Certainly she is inhabited by none which bear the least resemblance to those existing on our earth. Thus it might fairly be urged that, since one of the two orbs respecting which we know most appears to be uninhabited, there remains no probable argument in favor of the view that other orbs besides our earth are the abode of living creatures.
Yet the earth in reality supplies an argument of great force, when we consider the evidence she presents in an-other light. The mere fact that this world is inhabited is, as we have seen, little; but we shall find that the way in which life is distributed over the earth's surface is full of significance.
If we range over the earth, from the Arctic regions to the torrid zone, we find that none of the peculiarities which mark the several regions of our globe suffice to banish life from its surface. In the bitter cold within the Arctic Circles, with their strange alternations of long summer days and long winter nights, their frozen seas, perennial ice, and scanty vegetation, life flourishes in a hundred various forms. On the other hand, the torrid zone, with its blazing heat, its long-continued droughts, its strange absence of true seasonal changes, and its trying alternations of oppressive calms and fiercely raging hurricanes, nourishes even more numerous and more various forms of life than either of the great temperate zones. Around mountain summits as in the depth of the most secluded valleys, in mid-ocean as in the arid desert, in the air as beneath the surface of the earth, we find a myriad forms of life.
But this is far from being all. Various as are the physical habitudes which we encounter as we travel over the surface of our globe, we are able to trace the existence of other varieties even more remarkable. The geologist has been able to turn back a few leaves of the earth's past history, and though the pages have been defaced and mutilated by Time's unsparing hand, he is yet able to read in them of many strange vicissitudes to which the continents and oceans of our globe have been exposed. But, far back as he can trace the earth's history—and already he counts her age by millions of years —he finds no evidence of an epoch when life was absent from her surface. Nay, if he reads aright the mysterious lesson which the blurred letters teach him, he is led to believe that, at the most distant epoch to which his re-searches have extended, there was the same wonderful variety in the forms of life as at the present day. He can, indeed, find the scattered remains of only a few of those old-world creatures ; but he recognizes in those which have been preserved the clearest evidence that thousands of others must have existed around them. He knows that of a million creatures now existing scarcely one will leave to future ages any record of its existence; he sees whole races vanishing from the earth, leaving no trace behind them; and he is thus able to form an estimate of the enormous extent by which the creatures and races of which he can learn nothing must have outnumbered those whose scattered remains attest their former existence upon the earth.
Here, then, we have analogies which there is no mistaking. We see that not only is Nature careful to fill all available space with living forms, but that no time over which our researches extend has found her less prodigal of life. We see that, within very wide limits, she has a singular power of adapting living creatures to the circumstances which surround them. Nor is this lesson affected—like the general lesson drawn from the mere fact of the earth's being inhabited—by anything we can learn from the aspect of our satellite. For the arguments against the presence of living creatures on the moon are founded on the evidence we have that the physical habitudes of that orb are outside the limits within which Nature effects the adaptation spoken of.
The moon teaches us, however, that all the celestial bodies are not at all times habitable. The sun also teaches the same lesson. And it is necessary that we should consider how far the evidence presented by our own earth may serve to elucidate this teaching. We shall see that terrestrial analogies afford a very sure guide in the midst of many perplexities presented by the study of the worlds around us.
Let us trace out the various degrees of fitness or unfitness for the support of particular forms of life, which we recognize in various regions of our earth.
Often, where there exists so slight a difference between two regions of the earth that, to ordinary observation, it would appear that the forms of life existing in one should be well adapted to the other also, we yet find that this is not the case. Some minute peculiarity of soil, or climate, or vegetation, will render one region absolutely uninhabitable by a race which lives and thrives in the other. Dar-win mentions several instances in which an apparently insignificant change in the circumstances under which a particular race has thriven, and sometimes a change which does not, at first sight, appear to be in the least connected with the well-being of the race, has led to its gradual disappearance. And it seems demonstrated that even the slow processes of change to which every part of the earth is subjected would suffice to destroy a number of the races now subsisting on its surface, were the characteristics of those races unalterable. But as the physical habitudes of their abode slowly change, the various races of living creatures slowly change also, so as to adapt themselves continually to the varying circumstances under which they live.
The lesson taught us by this peculiarity is very obvious. On the one hand, we see that it would be by no means sufficient to indicate a general resemblance between the physical habitudes of our earth and those of some far distant planet, in order to prove that that planet is the abode of living creatures resembling those on our own earth. But, on the other hand, we are taught that the existence of differences sufficient to render a distant planet an unsuitable abode for such creatures as we are familiar with cannot force upon us the conclusion that the planet is uninhabited. On the contrary, the circumstance we have been considering teaches us that such differences as would suffice to banish life of certain kinds are insufficient to banish life of all kinds, or even to render less abundant the forms of life which exist under those changed conditions.
And now we may proceed a step farther. Of our earth we find differences of climate and of physical habitudes generally, which are much more important than those hitherto dealt with. We see that not only would certain races perish in the long run, if removed from their own abode to other parts of the earth, but that, in some in-stances, the process of destruction would be very rapid indeed. If we were to remove the polar bears from their Arctic fastnesses to tropical, or even to the warmer parts of temperate regions, a very few years would see the end of the whole race. The races inhabiting steppes and prairies would quickly perish if removed to mountain regions. Those accustomed to a moisture-laden air and abundant vegetation would not survive long if removed to the desert.
In some races, indeed, we find a power of enduring such changes which very far exceeds that possessed by other races. Those creatures, for example, which man has domesticated seem capable of enduring a variety of climate or of circumstances, which would destroy the seemingly more vigorous races not yet subdued to the yoke of man.
Even man himself, however, though he possesses in an unrivalled degree the power of enduring in safety the most complete change of climate, scene, and circumstances, is yet limited, in a certain sense, in his power of migration. The Englishman, for example, can endure the fiercest heat of the tropics or the bitterest cold of Arctic and Antarctic regions. But he cannot safely attempt to found true colonies in every part of the earth's surface. The Englishmen in India must send their children to be reared in England, if they wish them to grow up strong and vigorous. There can be. little doubt that if a thousand men and women from England were to settle in certain parts of India (not at any time inter-marrying with the natives), the colony would disappear within a couple of centuries.
Here we have a second degree of unfitness, according to which certain countries would quickly become depopulated, if supplied with inhabitants from certain other countries. We are taught the same lesson as before, but in a more striking manner. We see that differences exist within the confines of our own earth which render particular countries absolutely uninhabitable by particular races, insomuch that, though the individual might survive, the race itself would quickly perish. And we see, on the other hand, that these countries are not uninhabited, or even less fully peopled with living creatures, than seemingly more fortunate abodes.
Now, if some impassable barrier prevented the inhabit-ants of one country from visiting others, while yet it was possible to learn something of the conditions prevailing in other regions, how readily the conclusion might be reached that some at least of those inaccessible regions must be wholly uninhabited, simply because their physical habitudes appeared unsuited to the wants of the only creatures with which the observer was familiar. Who would believe, for example, that men can live, and not only live but thrive and multiply, in the frost-bound regions within the Arctic Circle, if travellers had not visited the Esquimaux races, and witnessed the conditions under which they subsist? Again, if we knew nothing of India, and some one pictured to us the intense heat of the Indian sun, the strange alternations of weather which replace to the Indian the seasonal changes we are familiar with, and all the other circumstances which render tropical regions so different from the English home, who could believe that, amidst those seemingly unendurable vicissitudes, there are races of men that thrive and multiply, even as our people in their temperate zone?
Therefore, in examining the circumstances of other worlds than ours, it will not be sufficient to prove that certain orbs would obviously not be habitable by the races subsisting on the earth, in order to enforce the conclusion that no living creatures subsist at all upon their surface.
Yet another step farther, however. There are regions of the earth where the members of races belonging to other regions quickly perish. The air of England is death to many creatures. And, indeed, there is not a spot in the whole world which would not be fatal in a brief space to many animals and plants belonging to other regions. Yet each spot, though thus fatal to certain races, is inhabited by numbers of others which live and thrive upon its surface.
Here, then, is our third lesson. We are taught by the analogy of our earth that it is not even sufficient to show that a planet would be an abode quickly fatal to all the living creatures subsisting on our globe to prove that it is therefore uninhabited.
But we have yet a stronger argument to touch on. There are regions of our earth to which creatures from other regions cannot be removed without being immediately killed. The warm-blooded animal perishes if placed for a brief space under water. The fish perishes if placed for a brief space on the earth. What could be more wonderful to us, were we not familiar with the fact, than that there are living creatures within the depth of that ocean beneath whose surface we ourselves, and the land creatures we are familiar with, cannot remain alive many minutes'? If fishes could reason, how could they believe that creatures can live in comfort in that element which is death to them? Yet land and river and sea are alike peopled with living creatures, each race as well adapted as its fellows to the circumstances in which it is placed.
We are taught, then, yet another lesson. We see that even though we could prove that every living creature on this earth would at once perish if removed to another orb, yet we cannot thence conclude that that orb is uninhabited. On the contrary, the lesson conveyed by our earth's analogy leads to the conclusion that many worlds may exist, abundantly supplied with living creatures of many different species, where yet every form of life upon our earth—bird, beast, or fish, reptile, insect, or animalcule—would perish in a few moments.
There remains yet a last lesson to be drawn from terrestrial analogies. On the earth there are regions where no form of life exists or can exist. Within the flaming crater of the volcano, or in the frozen heart of the iceberg, no living creature has its being. Yet even here Nature proves to us that the great end and aim of all her working is to afford scope and room for new forms of life, or to supply the wants of those which already exist. The volcano will die out, and the scene of its activity will one day become the abode of myriads of living creatures who would have perished in a moment in its consuming fires. The iceberg will melt, and its substance will once again be peopled with busy life. But this is little. It is the work of which volcano and iceberg are the signs, which most significantly teaches us what is Nature's real aim. The volcano is the index of those busy subterranean forces which are remodelling the earth's frame, slowly changing the level of the land, making continents of oceans and oceans of continents, preserving and vivifying all things, while all things seem to suffer a gradual destruction. The iceberg, too, has its work in remodelling and fashioning the surface of new continents. It also acts an important part in the formation and maintenance of the system of oceanic circulation on which the welfare of land creatures and water creatures so largely depends. And so of a multitude of other phenomena, which appear at first sight significant rather of the destructive than of the life-preserving character of Nature. The tornado and the thunder-storm, the earthquake and the volcano—nay, even the dreaded returns of plague and pestilence,—have each a more powerful influence by far towards the preservation than they have towards the destruction of life.
We see, then, that even if we could prove that an orb in space is so circumstanced that no life could by any possibility exist upon its surface; if it were the scene of a fierce and destructive turmoil, one moment of which would suffice to destroy every living creature now existing upon the earth; if its whole mass were heated to a degree a thousandfold more intense than that of the fiercest heat we know of; if its surface were bound in a cold compared with which our Arctic frosts would seem like tropical heat ; or even if the most rapid alternation of these extremes took place upon and within it—even then we could not conclude that it has not been in long-past ages, or will not be in ages yet to come, the abode of life.
Lastly, even when we can safely assert of any celestial object that neither now, nor at any past or future time, could it serve as the abode of living creatures, we are led by terrestrial analogies to the conclusion that it yet sup-ports life in other ways. So that these very orbs, of which it seems safest to assert that they are, have ever been, and must ever remain uninhabited, speak to us, no less strongly than those which appear best suited for habitation, of the existence of other worlds than ours.