was still useful, be removed by personal selection, solely by this means is a further weakening effected in the following generations--in short, only by this means are the determinants of the useless organ brought upon the inclined plane, down which they are destined slowly but incessantly to slide towards their completed extinction. The foregoing explanation will be probably accepted as satisfactory _in a purely formal regard_, but it will be objected that, even granting this, it has not yet been proved to be the correct one. In answer I can of course adduce nothing except that it is at present the only one that can be given. It may be that the actual state of things in nature is different, but if it can be shown that a self-direction of variation merely from the need of it is at all conceivable by mechanical means, {44} that in itself, it seems to me, is a decided gain. It must also not be forgotten that some process or other _must_ take place in the germ-plasm when an organ becomes rudimentary, and that as the result of it this organ, and only this organ, must disappear. Now in what shall this process consist, if not in a modification of the constitution of the germ? And how could the effect of such a modification be limited only to _one_ organ which was becoming rudimentary if the modification itself were not a local one? These are questions which it is incumbent on those to answer who conceive the germinal substance to be composed of like units. Applying, now, the explanation derived from the disappearance of organs to the opposed transformation, namely, to the _enlargement_ of a part, the presumption lies close at hand that the production of the long tail-feathers of the Japanese cock does not repose solely on the displacement directly effected by personal selection, of the zero-point of variation upwards, but that _it is also fostered and strengthened by germinal selection_. Were that not so, the phenomena of the transmutation of species, in so far as fresh growth and the enlargement and complication of organs already present are concerned, _would not be a whit more intelligible than they were before_. We should know probably how it comes to pass that the constitutional predisposition (group of determinants) of a _single_ organ is intensified by selection, but the flood of objections against the theory of selection touching its inability to modify _many_ parts at once would not be repressed by such knowledge. The initial impulse conditioning the independent maintenance of the useful direction of variation in the germ-plasm must rather be sought {45} in the utility of the modification itself, and this also seems to me intelligible from the side of the theory. For as soon as personal selection favors the more powerful variations of a determinant, the moment that these come to predominate in the germ-plasm of the species, at once the tendency must arise for them to vary _still more strongly_ in the plus direction, not