I found myself in quite a different position. As my doubts regarding the Lamarckian principle grew greater and greater, I was obliged to seek for some other factor in modification, which should be sufficient to effect the degeneration of a disused part, and for a time I thought I had found this in panmixia, that is, in the mingling of all together, well and less well equipped alike. This factor does certainly operate, but the more I thought over it the clearer it became to me that there must be some other factor at work as well, for while panmixia might explain the deterioration of an organ, it could not explain its decrease in size, its gradual wearing away, and ultimate total disappearance. Yet this is the path followed, slowly indeed, but quite surely, by all organs which have become useless. If panmixia alone guided the deterioration of the organ, and it was thus only chance variations which were inherited through panmixia and gradually diffused over the whole species, how could it come about that all the variations were in the direction of smaller size? Yet this is obviously the case. Why should no variations in the direction of larger size occur? And if this were so, why should a useless organ not be maintained at its original size, if it be admitted that an increase in size would be prevented by natural selection? But this never occurs, and diminution in size is so absolutely the rule that the idea of a 'vestigial or rudimentary' organ suggests a 'small organ' almost more than an 'imperfect' one. There must then be something else at work which causes the minus-variations in a disused organ to preponderate persistently and permanently over the plus-variations, and this something can lie nowhere else than where the roots of all hereditary variations are to be found--in the germ-plasm. This train of thought leads us to the discovery of a process which we must call selection between the elements of the germ-plasm, or, as I have named it shortly, _Germinal Selection_. If the substance of the germ-plasm is--as we assumed--composed of heterogeneous living particles, which have dissimilar rôles in the building up of the organism, there must of necessity be among them a definite labile state of equilibrium, which cannot be disturbed without modifying in some way the structure of the organism itself which arises from the germ-plasm. But if our further view be correct, that these individual and different living units of the germ-plasm are 'determinants,' that is, are the primary constituents of particular parts of the organism, in the sense that these parts could not arise if their determinants were absent from the germ-plasm, and that they would be different if the determinants were differently composed, we can draw far-reaching deductions.