For, in most retrogressive processes _active_ selection in Darwin's sense plays no part, and advocates of the Lamarckian principle, as above remarked, have rightly denied that active selection, that is, the selection of individuals possessing the useless organ in its most reduced state, is sufficient to explain the process of degeneration. I, for my part, have never assumed this, {39} and I enunciated precisely on this account the _principle of panmixia_. Now, although this, as I still have no reason for doubting, is a perfectly correct principle, which really does have an essential and indispensable share in the process of retrogression, still it is not _alone_ sufficient for a full explanation of the phenomena. My opponents, in advancing this objection, were right, to the extent indicated and as I expressly acknowledge, although they were unable to substitute anything positive in its stead or to render my explanation complete. The very fact of the cessation of control over the organ is sufficient to explain its _degeneration_, that is, its deterioration, the disharmony of its parts, but not the fact which actually and always occurs where an organ has become useless--viz., _its gradual and unceasing diminution continuing for thousands and thousands of years culminating in its final and absolute effacement._ If, now, neither the selection of persons nor the cessation of personal selection can explain this phenomenon, assuredly some other principle must be the efficient cause here, and this cause I believe I have indicated in an essay written at the close of last year and only recently published.[14] I call it _germinal selection_. The principle in question reposes on the application, made some fifteen years ago by Wilhelm Roux, of the principle of selection to the _parts_ of organisms--on the _struggle of the parts_, as he called it. If such a struggle obtains among organs, tissues, and cells, it must also obtain between the smallest and for us invisible vital particles, not only between those of the body-cells, strictly so called, but also between those of the {40} germinal cells. Roux himself spoke of the struggle of the molecules, by which he presumably understood the smallest ultimate units of vital phenomena--elements which De Vries designated pangenes, Wiesner plasomes, and I _biophores_, after Bruecke's ingenious conception[15] of these invisible entities had been almost totally forgotten, or at least had lain unnoticed for thirty years. No struggle, as that is understood in the theory of selection, could take place between real {41} molecules, for molecules are neither nourished, subject to growth, nor propagated. The gradual degeneration of organs grown useless may be explained, now, by the theory of determinants very simply and without any co-operation on the part of active personal selection, as follows.