Many of the stars, the light of which has taken thousands of years to reach us, are certainly suns like our own mother-sun, and are girt about with planets and moons, just as in our own solar system. We are justified in supposing that thousands of these planets are in a similar stage of development to that of our earth—that is, they have arrived at a period when the temperature at the surface lies between the freezing and boiling point of water, and so permits the existence of water in its liquid condition. That makes it possible that carbon has entered into the same complex combinations on those planets as it has done on our earth, and that from its nitrogenous compounds protoplasm has been evolved—that wonderful substance which alone, as far as our knowledge goes, is the possessor of organic life. The monera (for instance, chromacea and bacteria), which consist only of this primitive protoplasm, and which arise by spontaneous generation from these inorganic nitrocarbonates, may thus have entered upon the same course of evolution on many other planets as on our own; first of all, living cells of the simplest character would be formed from their homogeneous protoplasmic body by the separation of an inner nucleus from the outer cell body (cytostoma). Further, the analogy that we find in the life of all cells—whether plasmodomous plant-cells or plasmophagous animal-cells—justifies the inference that the further course of organic evolution on these other planets has been analogous to that of our own earth—always, of course, given the same limits of temperature which permit water in a liquid form. In the glowing liquid bodies of the stars, where water can only exist in the form of steam, and on the cold extinct suns, where it can only be in the shape of ice, such organic life as we know is impossible.
The similarity of phylogeny, or the analogy of organic evolution, which we may thus assume in many stars which are at the same stage of biogenetic development, naturally opens out a wide field of brilliant speculation to the constructive imagination. A favorite subject for such speculation has long been the question whether there are men, or living beings like ourselves, perhaps much more highly developed, in other planets? Among the many works which have sought to answer the question, those of Camille Flammarion, the Parisian astronomer, have recently been extremely popular; they are equally distinguished by exuberant imagination and brilliant style, and by a deplorable lack of critical judgment and biological knowledge. We may condense in the following thesis the present condition of our knowledge on the subject:
I. It is very probable that a similar biogenetic process to that of our own earth is taking place on some of the other planets of our solar system (Mars and Venus), and on many planets of other solar systems; first simple monera are formed by spontaneous generation, and from these arise unicellular protists (first plasmodomous primitive plants, and then plasmophagous primitive animals).
II. It is very probable that from these unicellular protists arise, in the further course of evolution, first social cell-communities (cœnobia), and subsequently tissue-forming plants and animals (metaphyta and metazoa).
III. It is also very probable that thallophyta (algæ and fungi) were the first to appear in the plant-kingdom, then diaphyta (mosses and ferns), finally anthophyta (gymnosperm and angiosperm flowering plants).
IV. It is equally probable that the biogenetic process took a similar course in the animal kingdom—that from the blastæads (catallacta) first gastræads were formed, and from these lower animal forms (cœlenteria) higher organisms (cœlomaria) were afterwards evolved.
V. On the other hand, it is very questionable whether the different stems of these higher animals (and those of the higher plants as well) run through the same course of development on other planets as on our earth.
VI. In particular, it is wholly uncertain whether there are vertebrates on other planets, and whether, in the course of their phyletic development, taking millions of years, mammals are formed as on earth, reaching their highest point in the formation of man; in such an event, millions of changes would have to be just the same in both cases.
VII. It is much more probable, on the contrary, that other planets have produced other types of the higher plants and animals, which are unknown on our earth; perhaps from some higher animal stem, which is superior to the vertebrate in formation, higher beings have arisen who far transcend us earthly men in intelligence.