6. But, in fact, the view which we have given of the mode of existence of the human species upon the earth, as being a progressive existence, even in the development of the intellectual powers and their results, necessarily fastens down our thoughts and our speculations to the earth, and makes us feel how visionary and gratuitous it is to assume any similar kind of existence in any region occupied by other beings than man. As we have said, we have no insuperable difficulty in conceiving other parts of the Universe to be tenanted by animals. Animal life implies no progress in the species. Such as they are in one century, such are they in another. The conditions of their sustentation and generation being given, which no difference of physical circumstances can render incredible, the race may, so far as we can see, go on forever. But a race which makes a progress in the development of its faculties cannot thus, or at least cannot with the same ease, be conceived as existing through all time, and under all circumstances. Progress implies, or at least suggests, a beginning and an end. If the mere existence of a race imply a sustaining and preserving power in the Creator, the progress of a race implies a guiding and impelling power; a Governor and Director, as well as a Creator and Preserver. And progress, not merely in material conditions, not merely in the exercise of bodily faculties, but in the exercise of mental faculties, in the intellectual condition of a portion of the species, still more implies a special position and character of the race, which cannot, without great license of hypothesis, be extended to other races; and which, if so extended, becomes unmeaning, from the impossibility of our knowing what is progress in any other species;--from what and towards what it tends. The intellectual progress of the human species has been a progress in the use of thought, and in the knowledge which such use procures; it has been a progress from mere matter to mind; from the impressions of sense to ideas; from what in knowledge is casual, partial, temporary, to what is necessary, universal, and eternal. We can conceive no progress, of the nature of this, which is not identical with this; nothing like it, which is not the same. And, therefore, if we will people other planets with creatures, intelligent as man is intelligent, we must not only give to them the intelligence, but the intellectual history of the human species. They must have had their minds unfolded by steps similar to those by which the human mind has been unfolded; or at least, differing from them only as the intellectual history of one nation of the earth differs from that of another. They must have had their Pythagoras, their Plato, their Kepler, their Galileo, their Newton, if they know what we know. And thus, in order to conceive, on the Moon or on Jupiter, a race of beings intelligent like man, we must conceive, there, colonies of men, with histories resembling more or less the histories of human colonies; and indeed resembling the history of those nations whose knowledge we inherit, far more closely than the history of any other terrestrial nation resembles that part of terrestrial history. If we do this, we exercise an act of invention and imagination which may be as coherent as a fairy tale, but which, without further proof, must be as purely imaginary and arbitrary. But if we do not do this, we cannot conceive that those regions are occupied at all by intelligent beings. Intelligence, as we see in the human race, in order to have those characters which concern our argument, implies a history of intellectual development; and to assume arbitrarily a history of intellectual development for the inhabitants of a remote planet, as a ground of reasoning either for or against Religion, is a proceeding which we can hardly be expected either to assent to or to refute. If we are to form any opinions with regard to the condition of such bodies, and to trace any bearing of such opinions upon our religious views, we must proceed upon some ground which has more of reality than such a gratuitous assumption.