Later the theory was started that the size of beings in various worlds depends on the amount of light received from the central sun. Thus Wolfius asserted that the inhabitants of Jupiter are nearly fourteen feet high, which he proved by comparing the quantity of sunlight which reaches the Jovians with that which we Terrenes receive. Recently, however, it has been noted that the larger the planet, the smaller in all probability must be the inhabitants, if any. For if there are two planets of the same density but unequal size, gravity must be greater at the surface of the larger planet, and where gravity is great large animals are cumbered by their weight. It is easy to see this by comparing the muscular strength of two men similarly proportioned, but unequal in height. Suppose one man five feet in height, the other six; then the cross section of any given muscle will be less for the former than for the latter in the proportion of twenty-five (five times five) to thirty-six (six times six). Roughly, the muscular strength of the bigger man will be half as great again as that of the smaller. But the weights of the men will be proportioned as 125 (five times five times five) to 216 (six times six times six), so that the weight of the bigger man exceeds that of the smaller nearly as seven exceeds four, or by three-fourths. The taller man exceeds the smaller, then, much more in weight than he does in strength; he is accordingly less active in proportion to his size. Within certain limits, of course, size increases a man's effective as well as his real strength. For instance, our tall man in the preceding illustration cannot lift his own weight as readily as the small man can lift his; but he can lift a weight of three hundred pounds as easily as the small man can lift a weight of two hundred pounds. When we get beyond certain limits of height, however, we get absolute weakness as the result of the increase of weight. Swift's Brobdingnags, for instance, would have been unable to stand upright; for they were six times as tall as men, and therefore each Brobdingnag would have weighed 216 times as much as a man, but would have possessed only thirty-six times the muscular power. Their weight would have been greater, then, in a sixfold greater degree than their strength, and, so far as their mere weight was concerned, their condition would have resembled that of an ordinary man under a load five times exceeding his own weight. As no man could walk or stand upright under such a load, so the Brobdingnags would have been powerless to move, despite, or rather because of, their enormous stature. Applying the general considerations here enunciated to the question of the probable size of creatures like ourselves in other planets, we see that men in Jupiter should be much smaller, men in Mercury much larger, than men on the earth. So also with other animals.