On Laplace’s Nebular Hypothesis, the condensation of an original nebulous mass endowed with a motion of rotation would result not only in the formation of a sun, similar to ours, but also in a system of planets revolving round the central body. If, indeed, the primitive nebula had no rotation or motions of any kind, the result would be a sun without planets or satellites; but the motions with which all the stars seem to be animated lead us to suppose that this would be a case of very rare occurrence. We may therefore conclude, with a high degree of probability, that the stars—at least those with spectra of the solar type—form centres of planetary systems somewhat similar to our own. This being surmised, let us consider the conditions necessary for the existence of life on these planets. There are various conditions which must be complied with before we can imagine life, as we know it, to be possible on any planet. Perhaps the most important of these is the question of temperature. We know that in the universe a great range of temperature exists, from the cold of interstellar space—estimated at about 460° below the freezing-point of water—to the intense heat which rages in the solar photosphere. In this long thermal scale life is, at least on the earth, restricted within rather narrow limits. Below a certain low temperature life can not exist. The point is, however, far above the temperature of space. On the other hand, above a certain high temperature—a low one, however, compared with the intense heat of the solar surface—life is also impossible, at least for highly organized beings like man and the larger animals. For minute microscopic organisms the scale may, perhaps, be somewhat extended; but even in its widest limits, the range of temperature within which life is possible is, so far as we know, certainly a narrow one.