Although, in accordance with the requirements of the task before us, we have to restrict ourselves to giving the results of natural science only in their general outlines, still we believe that we have not overlooked any essential result which is of importance to the question of the origin of species and of man. We have now finished our scientific review; and the conclusion to which we see ourselves brought is that natural science, in its investigation of the origin of species, has arrived at nothing but problems which it is not able to solve. There is a very great probability of an origin of species, at least of the higher organized species, through descent; but whether through descent by means of gradual development or of metamorphosis of germs, or whether with one group of organisms it is in this way, with another in that, is not yet decided. The attempt to explain their entire origin exclusively by the selection theory, must be regarded as a failure; all indications rather show that, supposing the descent principle correct, the deciding agencies which formed new species did not approach the old species out of which the new ones originated from {109} without, but that they originated or were already in existence within them. But what these agencies were, natural science is at present unable to state; and not only those scientists who reject every idea of a descent, but also those who are favorable to the ideas of descent and of evolution, rejecting only the selection theory, are at one in silent or open acknowledgment of this limit of our knowledge, be it permanent or temporary.