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The case, however, seems to be entirely different with regard to the atmosphered planets. There all the physical conditions of our organic life exist in kind at least, and the only difference is as to degree. A different station in the universe gives an almost certain presumption, without any reasoning upon it, that the physical constitution will be different, just as for almost every different habitat the botanist expects and finds a different plant, though he cannot discern the causes of the difference. The argument against the moon is so strong because the difference is not merely in variety of combination, but in the probably total absence of one radical condition that directly or indirectly affects the whole physical, most of the sensitive, and a great part of the intellectual life of man, as we have seen in the relation of the atmosphere to nature and the fine arts. There is, therefore, one radical element wanting, and the analogy is destroyed, just as it would be for the existence of vegetable life in a region which could be shown to have a surface of smooth, impenetrable metal. A plant without a root in the ground would to our botanists not be a plant: and so a man without lungs, or ears, or speech, with his senses reduced to touch and eyesight, without winds and clouds and seas, with half the materials or substratum of terrestrial thought removed, would scarcely be man, or at all events would be so different that he must be quite as physically unlike us in fundamental constitution, as lîke. It would not be a variation of the same type, but a totally different type. The same elements in different proportions would not exist, but fewer elements. The absence of any one universal condition of life, rightly undermines the argument from analogy. The varied proportion of those elements is quite in harmony with it. Columbus certainly never expected to find new races of men unless he found a new terra firma ; but an à priori demonstration that the terra firma, so found, would be very different in its properties (much heavier or lighter suppose), than the crust of the old world, would no more have surprised him than the discovery of quite new forms of vegetation surprised him, and would only have induced him to expect similar modifications in the races of inhabitants. Dr. Whewell's argument about the planets is very like to that of a man who had à priori means of proving that no known European tree was to be found indigenous in New Zealand (which is, we believe, true), and should then argue from it that New Zealand must be utterly devoid of timber, on the ground that it would be purely gratuitous to assume the existence of entirely new species.
What we object to is, not so much the over-statement of particular discrepancies between the state of Jupiter or the other planets and our earth, as the virtual assumption carried
all through Dr. Whewell's essay that such discrepancies are per se difficulties in the way of supposing that beings on the whole resembling us, because subject to the same fundamental laws, exist there. The variations proved only appear to us to be in kind and degree what the different station in the universe might lead one antecedently to expect in the constitution of such beings, if existing; and the same course of argument carried into terrestrial matters might enable us to disprove à priori almost any unknown form of human existence. It would have been much easier for a Dyak to have disproved the possibility of an Esquimaux, or an Esquimaux of a Dyak, given the law of communication of solar heat to the terrestrial poles and the equator, than for Dr. Whewell to show the impossibility that Mercury, or even Jupiter, should contain human beings. The Dyak might argue that no light or heat from the sun (instead of twenty-five times as little, which is the Jovian amount) could reach the Arctic regions by direct radiation during the long months of winter; while during the summer the solar rays, never falling less obliquely than at 45°, and generally far more obliquely, could not impart much of the heat which he himself receives. Adding to these facts his own dependence on the rapid growth and teeming life of the equatorial regions, which he might plausibly argue to be absent among the everlasting frosts, we think he could make out a better case against his possible Hyperborean neighbours than Dr. Whewell has achieved against a Martial or Jovian type of humanity. Besides, knowing, as Dr. Whewell does, how exceedingly cold unatmosphered space is, even vastly nearer to the sun than we are,-in short, that far more depends on the atmospheric wrapper than on the solar distance, Dr. Whewell might almost as well argue that no one could sleep in a fireless attic because it was so far from the kitchen-fire without having regard to the possible accumulation or thickness of the blankets in the remoter apartments, as argue that Jupiter must be uninhabitable from his remoteness to the sun. And if he says that we can only argue on terrestrial analogies, the reply is, that changes of this trivial nature, especially changes in the constitutional necessities of the inhabiting beings, are agreeable to terrestrial analogy without assuming any radical changes in the laws to which they are subject. It would be very much easier to make out an à priori case against the possibility of a Cornish miner living where he does, than against a human inhabitant of Neptune.
We think it is likely to be universally held, in spite of Dr. Whewell's argument, that rational life is the highest general end of physical organization, that where we discern clearly the existence of the only elements of a similar physical organization which we could, under the circumstances, possibly discern,and this in a large number of cases,—there we discern the traces of present or future similar rational life,-and that differences only affecting those minor physical conditions which it is utterly impossible to connect with more important differences in the character of the rational organization, do not, in the least, even tend to shake this presumption.
To illustrate the exceeding poverty of Dr. Whewell's argument, let us take a parallel case to his own, but one far more favourable to his side of the question in several circumstances. Suppose that a resident by the Lake of Tiberias were trying to investigate, on scientific principles, whether or not all seas contained fishes, like his own. We will suppose him and his countrymen unable actually to reach the Mediterranean, which they see in the distance, or to experiment on the mountain lakes from which Tiberias is fed. He knows that his own lake abounds in fish, and that these creatures seem to have the highest kind of organized life to which such accumulations of water are subservient. This has hitherto been sufficient with his fellow-countrymen. They have quietly assumed, that as it was in the Sea of Tiberias, so it probably was in all other seas, until reasons
were shown to the contrary. This does not satisfy the learned doubt of our investigator, and he writes an Essay on the Plurality of Fish-worlds to show that the argument lies the other way. He begins with stating that the whole force of the analogy is broken, if in any case it can be clearly shown that a sea exists, but not the fish.
The onus probandi will then lie with the other party who believe in the plurality of fish-reservoirs. Now, first, he finds that this must have been the case for an indefinitely long duration of time with all the known waters. It is a scientific certainty that the water must pre-exist before the fish. All the known (inland) waters spring up from a depth which implies a pressure under which no fish could live, as can be shown by actual experiment. The water then is a mineral, and geologically can be shown to exist long before the traces of their modern inhabitants can be found. Moreover, to gain a notion of the time during which Tiberias itself may have been unpeopled, he remarks that no new species of fish has appeared in it within the memory of man, though a few may have disappeared. Hence it is natural to suppose that animal creation is an age-long process, and it is impossible to say how long the inorganic water waited for the first organic inhabitant, probably for ages in comparison with which the life of the piscatory species is short. If, then, Tiberias may have been fishless during a period indefinitely longer than that during which it has sustained fishes, is not the presumption in the case of another sea against, not for, its possession of like tribes ? But this is not all. There is, he observes, fortunately for the special guidance of scientific thought, an actual sea within reach, of which it can be all but demonstrated that it is fishless, – that it, at least, was not created on purpose to sustain fish. In the Dead Sea, into which the Jordan empties itself, not the most minute observation has been able to detect living fish. Moreover, it can be shown that the nature of the water is such that it will not support the life of those tribes at least which inhabit Tiberias. Now, with these evidences against any à priori presumption in favour of fishes always existing wherever there is a sea, derived from the certainly indefinite duration of the water before the fish in the cases under their own observation, and the almost certain fact that there is at least one sea where fishes cannot live, is it reasonable, he asks, to assert that fishes live in all the waters which we cannot examine ? Look at the Mediterranean, he continues : from observation of the vessels riding on its surface, which seem to be of the same materials as the boats of Tiberias, it can be calculated that its density must be decidedly greater than that of the inland lake, and this points rather to a resemblance in composition to the Dead Sea, which is known to be fishless. Moreover, the colour of its water gives to some extent the same impression. Is there not, then, reason to suppose that at the first sources and springs of water in the earth, it is too mineral and too fresh, from great pressure, to support animal life,—that after a certain stage of its course it attains that power,—but that when waters have collected from many sources and press on each other in a basin from which there is no exit, the density again increases, and the chemical properties change, so that piscatory life again becomes impossible? This is clearly the case with the waters of the Jordan. At Tiberias they now support plentifully the tribes of fishes, but at their rising from the earth this would be impossible, so that Merom and the higher lakes are probably too near the source to allow of fishes. Again, it is known that in the Dead Sea that impossibility returns. Does it not seem a most probable theory that Tiberias is, so to say, the moderate or “temperate zone of the fish-creation? In the higher basins the water is too cold, dense, and pure,—in the lower it is again too dense from the accretion of foreign material or from other causes; and hence the existence of fishes, at least at all resembling the tribes here found, is most improbable in the gigantic sea on which observers can only gaze and guess ? Really we think the force of such an argument would be in many respects far stronger than that Dr. Whewell has presented with regard to a plurality of worlds. At all events the Dead Sea would afford a plausible ground of reasoning to such a theorist
a far stronger than the Moon to Dr. Whewell. For we know that the Moon is exceptional in not having an atmosphere, which all the planets from Venus to Saturn are almost certainly known to possess.
But the Dead Sea could not be shown to be thus exceptional in relation to fishes, and might be plausibly maintained to be rather an illustration of the rule. Yet we know that the scientific investigator would have been wrong, and the popular mind of Tiberias right.
The theory on which Dr. Wheweli has discussed the planets appears to have been to assume a constant degree of heat and light, and a fixed density in the material of the planet as the test of the possibility of human life upon its surface.* Yet there is nothing so likely to be variable as the amount of heat and light necessary to beings of a similar organization, or as the amount of internal animal heat they may have, and nothing so likely to vary as the specific gravity of animal bodies. Nothing varies more amongst animal tribes on the earth, even when they have organs similarly constituted in kind, than the amount of light and heat which they require for equal quicksightedness and comfort.
We have said how hazardous and random Dr. Whewell's theory appears to us to be ; its plausibility, however, appears to lie only in its giving a connected scale of thought of some
Yet even thus he often makes very unwarrantable assumptions. What case he makes against Mars it is difficult to see. The solar heat and light received there are only half that of the average terrestrial heat and light, probably considerably more than that on many parts of the earth. The inclination to the sun being much greater, the globe is more equably warmed than the earth; and the snow appears to extend only to six degrees from the pole even at the close of winter, and, if we may believe the astronomers, disappears during the summer season. If so, the arctic explorers in Mars are better off than our own. Again, Dr. Whewell always assumes that everything as little or less dense than water is fluid, even though it be a much more consistent assumption to suppose it a light solid. If it be lighter than Water, he supposes it vapour ; if as light, certainly water. Thus he makes Jupiter and Saturn big clouds with vapour wound round them. Yet good authorities say that Jupiter's surface must be far lighter than his centre, and if it follows the terrestrial analogy, it will be about twice as light as the average density. In this case it is greatly too light to be mostly water or ice, and greatly too heavy to be mostly vapour. And though even on the earth's surface there are several kinds of substance, and even of stone (mentioned by Sir David Brewster), which would suit this state of the case, Dr. Whewell is too anxious to make Jupiter out to be a gigantic bath or skating pool to lend countenance to anything but cloud and ice. Surely the thick permanent belts of cloud both indicate something warmer than ice, and tend to preserve that warmth by preventing radiation. Are not our coldest regions comparatively little clouded both as a cause and an efft of that cold?