If, for example, we compare between themselves two worlds such as the Earth and Neptune, utterly different from the point of view of distance from the Sun, we could not for an instant suppose that organic structures could have followed a parallel development on these planets. The average temperature must be much lower on Neptune than on the Earth, and the same holds for intensity of light. The years and seasons there are 165 times longer than with us, the density of matter is three times as weak, and weight is, on the contrary, a little greater. Under conditions so different from our own, the activities of Nature would have to translate themselves under other forms. And doubtless the elementary bodies would not be found there in the same proportions. Consequently we have to conclude that organs and senses would not be the same there as here. The optic nerve, for instance, which has formed and developed here from the rudimentary organ of the trilobite to the marvels of the human eye, must be incomparably more sensitive upon Neptune than in our dazzling solar luminosity, in order to perceive radiations that we do not perceive here. In all probability, it is replaced there by some other organ. The lungs, functioning there in another atmosphere, are different from our own. So, too, for the stomach and digestive organs. Corporeal forms, animal and human, can not resemble those which exist upon the Earth. Certain _savants_ contend that if the conditions differed too much from terrestrial conditions, life could not be produced there at all. Yet we have no right to limit the powers of Nature to the narrow bounds of our sphere of observation, and to pretend that our planet and our Humanity are the type of all the worlds. That is a hypothesis as ridiculous as it is childish. Do not let us be "personal," like children, and old people who never see beyond their room. Let us learn to live in the Infinite and the Eternal. From this larger point of view, the doctrine of the plurality of worlds is the complement and the natural crown of Astronomy. What interests us most in the study of the Universe is surely to know what goes on there. * * * * * These considerations show that, in all the ages, what really constitutes a planet is not its skeleton but the life that vibrates upon its surface. And again, if we analyze things, we see that for the Procession of Nature, life is all, and matter nothing. What has become of our ancestors, the millions of human beings who preceded us upon this globe? Where are their bodies? What is left of them? Search everywhere. Nothing is left but the molecules of air, water, dust, atoms of hydrogen, nitrogen, oxygen, carbon, etc., which are incorporated in turn in the organism of every living being.