It would be impossible even with the most careful examination to discover any point of comparison between the beings which live on the Neptunian world and those which we know on Earth. None of our classifications, whether of the animal kingdom, vast and diversified though it be, or of the vegetable kingdom, highly complex in itself, could be applied to them. It is another world, absolutely different from this one. Spectrum analysis indeed establishes the fact that its chemical composition is quite other than that of our terrestrial home. The organisms which live on the surface of the different planets are the resultant of the forces acting upon them. The origin of the human form lies in the ancestral forms of the long animal series whence it has gradually emerged, and of which it is the highest perfection, and these primitive animal forms go back in an unbroken chain to the rudimentary organisms unprovided with the senses which are the glory of man, organisms which inaugurated the manifestations of life, but which can hardly be described as living. They are neither animals nor plants. They appear to be organised substances, already distinct from the inorganic kingdom, but as yet only simple chemical combinations endowed with a sort of diffused vitality, an elementary protoplasm, the germ of all developments of terrestrial life, both animal and vegetable. The first organised beings were formed in the bosom of the warm waters of the oceans which covered the entire surface of the earth at the time when the geological periods began. Their intrinsic nature, their properties, their faculties, were already the resultant of the chemical composition of those waters, of the density and temperature of the surrounding medium; the variation of this medium and of the condition of existence have brought about corresponding changes in the development of this genealogical tree, and, according to the habitat of the organisms, whether in the deep, middle, or upper regions of the waters, on the sea-shore, in the low-lying plains, on sunny slopes or mountain-tops, the genealogical tree gave rise to more and more diversified organisms. Present-day terrestrial humanity is the last flower, the last fruit of this tree. But all this life is terrestrial from root to summit, and on every planet the tree is different. Life is Neptunian on Neptune, Uranian on Uranus, Saturnian on Saturn, Sirian on the system of Sirius, Arcturian on that of Arcturus, appropriate to every medium, or rather, more strictly speaking, produced and developed by each world according to its physical state and in harmony with that primeval law which all nature obeys: the law of progress.