If, for example, we compare two such worlds as the Earth and Neptune, which differ very much as regards distance from the Sun, we cannot imagine for a single instant that the organic forms should have had the same development. The average temperature must be much lower on Neptune than on the Earth, and so must the intensity of light. The years and the seasons are 165 times longer than with us; the density of materials is three times less, and gravitation, on the other hand, is a little stronger. Under conditions so different from ours, the activities of Nature can only have been shown in other forms. The elements also are not found present in the same proportions, and spectrum analysis has even shown us that substances which prevail in the atmospheres of Jupiter, Saturn, and Uranus, as well as Neptune, are different from those which constitute our organisms. Lungs functioning in another atmosphere would have to be different from ours. The same applies to the stomach and the digestive organs. Chemical constitution is not even the same. Instead of carbon as a fundamental element associated with hydrogen, oxygen, and nitrogen, we can imagine with Scheiner silicon and other bodies. We must conclude that the organs and the senses cannot be the same as they are here. The optic nerve, for instance, which has been formed and developed here from the rudimentary organ of the trilobite to the marvels of the human eye, must on Neptune be incomparably more sensitive than it is in our blinding sunlight and must perceive radiations which we do not perceive here. It may even be replaced by another organ. The bodily forms, animal and human, can resemble nothing on earth. Certain _savants_ object that, if the conditions are too different from terrestrial conditions, life cannot exist at all. But we have no right to limit the powers of Nature by the narrowness of our sphere of observation, or to pretend that our planet and our human race are the model for all planets. That is an hypothesis as infantile as it is ridiculous.