From an essay originally published in the Fortnightly Review.

IN the absence of any definite knowledge as to the composition of the atmospheres by which the planets are surrounded, or as to the climates which they enjoy, it would certainly be idle for us to speculate as to how far they might possibly be tenanted by creatures resembling those found on this earth. It would also be impossible for us to form any conception as to the biological characteristics of creatures which would be adapted for residence on the several planets. There is, however, one merely mechanical matter which may be usefully mentioned, inasmuch as it depends on considerations which admit of demonstration.

  We are able to weigh the several planets. Indeed, the problem is a comparatively easy one, when applied to those bodies which are attended by satellites, inasmuch as the movements of the satellites contain indications of the weights of their primaries. But even when a planet has no satellites, it is still possible for an astronomer to find the weight of a body by the effect which its attraction produces on other planets. But the weight of a planet must stand in important relation to the framework of the organisms which are adapted to dwell upon it. Let me try to make this clear by a few illustrations.

  Suppose that a planet, while still retaining the same size, was to be greatly increased as to its mass. The consequences would be felt very seriously by all organized creatures. The most immediate effect would be to increase the apparent weight of everything. If, for instance, a globe the same size as the earth possessed double the mass of the earth, the effect would be that the weight of each animal on the heavier globe would be double that on the earth. A horse placed on the heavy globe would be subjected to a load which would oppress him as greatly as if while standing on our earth, as at present constituted, he bore a weight of lead on his back which amounted to as many pounds as the animal itself. Each leg of an elephant would be called upon to sustain just double the not inconsiderable thrust which at present such a pillar has to bear. A bird which soars here with ease and grace would find that the difficulty of such movements was greatly increased, even if they were not wholly impossible on a globe of equal size to the earth, but double weight. It would seem as if flying animals must be the denizens of light globes, rather than of heavy ones.

  It is also easy to show that in general, other things being equal, the size of an animal should tend to vary in an inverse direction to that of the mass of the globe on which it dwells. At first it might be supposed that big animals might be most appropriately located on big worlds, and small animals on small worlds. No doubt there are so many circumstances to be considered, of which we are in almost complete ignorance, that any statements of this kind must be received with considerable caution. We may, however, assert with some confidence that, so far as our knowledge goes, the truth lies the other way. It is the small animals which are adapted for the larger worlds; it is the big animals which are adapted for the smaller worlds. The proof of this involves an interesting point.

  The argument is as follows: Suppose that an animal on this earth, as it is at present, were to have every dimension doubled. To take a particular instance, conceive the existence of a giant horse which was twice as high and twice as long in every feature and detail as an ordinary horse. It is obvious that as all three dimensions of the animal are doubled, its volume and therefore its weight would be increased eightfold, and the weight that would have to be transmitted down each of the four legs would be increased eightfold. Each leg of the giant horse would, therefore, have to possess eight times the weight-sustaining power that would suffice for the leg of the ordinary horse. As the proportions are supposed to have been observed throughout, the leg of the giant horse would be of course considerably stronger than that of the ordinary horse, but it would not be so much stronger as to enable it to accomplish the task it would be called on to perform. The section of the leg of the giant horse would no doubt be double in diameter that of the normal individual. This would imply that the area of the section was increased fourfold. But we have seen that the weight transmitted was increased eightfold. Study the effect of this on the horse’s hoof in contact with the ground. In the giant horse the area of the surface of contact would be four times as great as in the normal horse. As, however, the weight transmitted is eight times as great, it follows that this wear and tear on each square inch of the foot, and this is the proper way to estimate it, would be just twice as destructive in the giant horse as it would be in the ordinary animal. If, then, as we may well suppose, the foot of the latter is just adapted for the work which it has to do, then the foot of the giant horse would be incapable of withstanding the wear and tear to which it would be subjected. It follows that an effective animal, on the scale we have suggested, would be an impossibility on our earth; at all events, when the materials from which it was made were the same as those out of which our animals are fashioned.

  Suppose this giant horse, instead of being left on this earth, were transferred to another globe, which only exerted half the gravitating effect experienced on the earth’s surface, then the effort the animal would have to make in supporting its own weight would only be half that which it has to put forth here. The consequence is that the framework of the giant horse would in such a case have to support a weight which was not more than four times that of an ordinary horse standing on the earth. As the area of the bases of support in the large animal was fourfold that in the normal horse, it would follow that, area for area, there would be a pressure transmitted through the foot of the giant horse on the less ponderous globe precisely equal to that of the normal horse on the earth. The materials of which the big horse is built ought, therefore, to be able to sustain him effectively when he was placed on the light globe. It, therefore, appears that, so far as gravitation is concerned, the big horse would be better adapted for the light globe, and the small horse for the heavy one. More generally we may assert that, regarding only the point of view at present before us, the limbs of smaller animals would be better adapted for vigorous movement on great planets than would those of large creatures.

  It is, however, proper to bear in mind the point to which attention was, so far as I know, first called by Mr. Herbert Spencer. He has shown that there are excellent biological reasons, quite independent of those mechanical considerations to which I have referred, why it would be impossible for an efficient animal to be constructed by simply doubling every dimension of an existing animal. The support of the creature’s life has to be effected by the absorption of nourishment through various surfaces of the body. But if all the dimensions are doubled, the bodily volume, as we have already mentioned, is increased eightfold, and therefore its sustenance would, generally speaking, require eight times the supply that sufficed for the original animal. On the other hand, supposing the same scale to be observed throughout the animal’s body, the available surface area for absorption of nourishment has only increased fourfold, and therefore each square inch would have to do double duty in the large animal. If, however, the surfaces are at present at full work, it would seem impossible that they should efficiently undertake double the work they now get through. On this account, therefore, a live animal would seem impossible on a simple specification of dimensions twice those of any existing animal. Great structural modifications of pattern would have to accompany the enlargement of bulk. This, be it observed, is wholly independent of all questions as to gravitation.

  No reasonable person will, I think, doubt that the tendency of modern research has been in favor of the supposition that there may be life on some of the other globes. But the character of each organism has to be fitted so exactly to its environment that it seems in the highest degree unlikely that any organism we know here could live on any other globe elsewhere. We cannot conjecture what the organism must be which would be adapted for a residence in Venus or Mars, nor does any line of research at present known to us hold out the hope of more definite knowledge.