For it lies in the nature of the question itself, that {28} these theories, in their historical progress, did not appear singly, but together. Those who inclined to the theory of a descent of species could claim for it the attention of scientific investigators only after having also made the attempt at conceiving this descent in a concrete way, and according to certain analogies of observation. The only analogy of the kind appeared in the sphere of individual development and individual differences on the one hand, and in that of closely related characters of allied species on the other; and thus led of itself to the evolution theory. As soon as the naturalists thought they had found the causes of such an evolution of the species, they naturally placed these causes in the foreground of their demonstrations, and erected upon them the structure of their entire theory; thus treating descent, evolution and selection as one single and indissolubly connected theory. But this manner of treating the question had also its dangers, which have already caused a great deal of confusion and misunderstanding, as well as much unprofitable controversy. Often friends and enemies of the theories placed that which was in favor of the theory of descent to the credit of the evolution or selection theory; and, on the other hand, that which seemed opposed to the selection theory was often held to be a weakening of the evolution and descent theory; and this was done, not only by amateurs, but often enough by the highest authorities also. In reality, however, it is quite conceivable that the idea of a descent may prove correct, and possibly the idea of an evolution of the species will have to be replaced by that of a heterogenetic generation, or by the theory that certain groups in the organic system are originated {29} by heterogenetic generation, and others by evolution; and so the evolution theory must share with the theory of heterogenetic generation, or of a metamorphosis of germs. On the other hand, it is conceivable that even where the evolution theory is confirmed, the evolution can be accounted for wholly or partly by other reasons than those derived from the selection theory. And even this result of present investigations is not inconceivable: that the reasons for and against the different theories will be found to balance one another to such a degree that they will sooner or later lead science, in answering the question of the origin of species, to the old confession of Socrates--"_Ignoramus_."