Thus, for instance, we have seen that the variable star Algol owes its variations in brilliancy, which reduce it from second to fourth magnitude every sixty-nine hours, to the interposition of a body between itself and the Earth, and celestial mechanics has already been able to determine accurately the orbit of this body, its dimensions and its mass, and even the flattening of the sun Algol. Here, then, is a system in which we know the sun and an enormous planet, whose revolution is effected in sixty-nine hours with extreme rapidity, as measured by the spectroscope. The star [delta] of Cepheus is in the same case: it is an orb eclipsed in a period of 129 hours, and its eclipsing planet also revolves in the plane of our vision. The variable star in Ophiuchus has an analogous system, and observation has already revealed a great number of others. Since, then, a certain number of solar systems differing from our own have been revealed, as it were in section, to terrestrial observation, this affords us sufficient evidence of the existence of an innumerable quantity of solar systems scattered through the immensities of space, and we are no longer reduced to conjecture. On the other hand, analysis of the motions of several stars, such as Sirius, Procyon, Altaïr, proves that these distant orbs have companions,--planets not yet discovered by the telescope, and that perhaps never will be discovered, because they are obscure, and lost in the radiation of the star. * * * * * Some _savants_ have asserted that Life can not germinate if the conditions of the environment differ too much from terrestrial conditions. This hypothesis is purely gratuitous, and we will now discuss it. In order to examine what is happening on the Earth, let us mount the ladder of time for a moment, to follow the evolutions of Nature. There was an epoch when the Earth did not exist. Our planet, the future world of our habitation, slept in the bosom of the solar nebula. At last it came to birth, this cherished Earth, a gaseous, luminous ball, poor reflection of the King of Orbs, its parent. Millions of years rolled by before the condensation and cooling of this new globe were sufficiently transformed to permit life to manifest itself in its most rudimentary aspects. The first organic forms of the protoplasm, the first aggregations of cells, the protozoons, the zoophytes or plant-animals, the gelatinous mussels of the still warm seas, were succeeded by the fishes, then by the reptiles, the birds, the mammals, and lastly man, who at present occupies the top of the genealogical tree, and crowns the animal kingdom. Humanity is comparatively young upon the Earth. We may attribute some thousands of centuries of existence to it ... and some five years of reason!