We are all aware that Lamarck regarded the direct influence of use and disuse as the most essential factor in transformation, and that Darwin, though hesitatingly and cautiously, recognized and accepted this factor, which he believed to be indispensable. Indeed, it seems at first sight to be so. There is a whole range of facts which seem to be intelligible only in terms of the Lamarckian theory; in particular, the existence of numberless vestigial or rudimentary organs which have degenerated through disuse, the remains of eyes in animals which live in darkness, of wings in running birds, of hind legs in swimming mammals (whales), and of ear muscles in Man, who no longer points his ears, and so forth through a long list. According to Wiedersheim, there are in Man alone about two hundred of these vestigial or rudimentary organs, and there is no higher animal which does not possess some. In all, therefore, a piece of the past history of the species is embodied in the actually existing organism, and bears witness to the fact that much of what the ancestors possessed is now superfluous, and is either transformed, or is gradually set aside, or is still in process of being set aside. It seems obvious that this gradual dwindling and degeneration of an organ no longer needed cannot be explained through natural selection in the Darwin-Wallace sense, for the process goes on so exceedingly slowly that the minute differences in the size of an organ, which may occur among individuals of the species at any given time during the retrogressive process, cannot possibly have a selection value. Whether the degenerate and now functionless hind leg of the whale is a little larger or a little smaller can have no importance in the struggle for existence; the smaller organ cannot be considered either as a lesser hindrance in swimming or as a greater economy of material, and the case is the same in regard to most other instances of degeneration through disuse. We therefore require another interpretation, and at first sight this seems to be supplied by the Lamarckian principle. But the reverse process, the strengthening, the enlarging, and the more perfect development of a part, very often goes on proportionately to its more frequent use, and here again the Lamarckian principle seems to afford a simple explanation. For we know that exercise strengthens a part, as disuse weakens it, and if we could assume that these results of use and disuse were transmitted from the individual who brought them about or 'acquired' them in the course of his life to his offspring, then there would be nothing to object to in the Lamarckian principle. But it is precisely here that the difficulty lies. Can we assume such a transmission of 'acquired' characters? Does it exist? Can it be demonstrated?