I am not of the number of those who believe that we already know all, or at least nearly all, that is essential, but am rather convinced that whole regions of phenomena are still sealed to us, and I consider it probable that the nervous system in particular is not yet exhaustively known to us, either in regard to its functioning or in regard to its finest structural architecture, although I gratefully recognize the advances in this domain that the last decades have brought about. In any case, such assumptions as I have just indicated, or similar ones, seem to me quite too improbable to furnish any foothold for progress. Yet we must always remain conscious that we cannot decide as to the possibility or impossibility of any biological process whatever from a purely theoretical standpoint, because we can only guess at, not discern, the fundamental nature of biological processes. At the close of this lecture I shall return to the question of the theoretical conceivability of an inheritance of functional adaptations; but first of all we must consider the facts and be guided by them alone. If they prove, or even make it seem probable, that such inheritance exists, then it must be possible, and our task is no longer to deny it, but to find out how it can come about. Let us therefore investigate the question whether an inheritance of acquired characters, that is, in the first place, of functional adaptations, is demonstrable from experience. We shall speak later on of the effect of climatic and similar influences in causing variation; the case in regard to them is quite different, because they undoubtedly affect not only the parts of the body but the germ-cells as well. When we inquire into the facts which have been brought forward by the modern adherents of the Lamarckian principle as proofs of the inheritance of acquired characters in this restricted sense, we shall find that none of them can withstand criticism. First, there are the numerous reputed cases of the inheritance of mutilations and losses of whole parts of the body. It is not without interest to note here how opinion in regard to this point has altered in the course of the debate. At the beginning of the discussion they were all brought forward as evidence of undoubted value for the Lamarckian principle.