or later resist the withdrawal of nourishment from them; but for the descending progression there are no limits except total disappearance, and this is actually reached in all cases in which the determinants are related to a part which has become useless. But both these movements, the upward and the downward alike, are quite independent of natural selection, i.e. of personal selection; they are processes of a unique kind which run their course purely in accordance with intra-germinal laws. Whether a determinant 'ascends' or 'descends' depends solely upon the play of forces within the germ-plasm, not upon whether the direction of the variation in question is useful or prejudicial, or on whether the organ in question, the determinate, is of value or otherwise. In this fact lies the great importance of this play of forces within the germ-plasm, that it gives rise to variations quite independently of the relations of the organism to the external world. In many cases, of course, personal selection intervenes, but even then it cannot directly effect the rising or falling of _the individual_ determinants--these are processes quite outside of its influence--but it can, by eliminating the bearers of unfavourably varying determinants, set a limit to further advance in such directions. This we shall consider in more detail later on. Personal selection operates by removing unfavourably varying individuals from the genealogical tree of the species, but at the same time the determinants which are varying unfavourably are also removed, and their variation is thus put a stop to for all time. I have called these processes which are ceaselessly going on within the germ-plasm, Germinal Selection, because they are analogous to those processes of selection which we already know in connexion with the larger vital units, cells, cell-groups and persons. If the germ-plasm be a system of determinants, then the same laws of struggle for existence in regard to food and multiplication must hold sway among its parts which hold sway between all systems of vital units--among the biophors which form the protoplasm of the cell-body, among the cells of a tissue, among the tissues of an organ, among the organs themselves, as well as among the individuals of a species and between species which compete with one another. If this be the case, we have here ready to hand the explanation of every heritable variation of a part, ascending and descending alike. Let us consider for a little the latter category--that is, the disappearance of functionless _or useless organs_. It is clear that, from the moment in the life of a species that an organ, _N_, becomes useless, natural selection withdraws her hand from it; individuals with better or worse organs _N_ are now equally capable of life and struggle, the state of panmixia is entered upon, and the organ _N_ of necessity falls somewhat below its previously attained degree of perfection.