perhaps we should even be able to draw more definite conclusions in regard to the composition and activity of the determinants in the germ. In the meantime we must be content with the knowledge that, on the determinant hypothesis, the disappearance of organs which have become useless may be regarded as a process of intra-selection going on between the 'primary constituents' (_Anlagen_) of the germ, and depending on the same principle of the 'struggle of parts' which William Roux introduced into science with such brilliant results. If a struggle for food and space actually takes place, then every passive weakening must lead to a permanent condition of weakness and a lasting and irretrievable diminution in the size and strength of the primary constituent concerned, unless personal selection intervenes, and choosing out the strongest among these weakened primary constituents, raises them again to their former level. But this never happens when the organ has become useless. This explains why not only parts with active function, like limbs, muscles, tendons, nerves, and glands, disappear when they cease to function, but also passive parts like the colouring of the external surfaces of animals, the lifeless skeletal parts of Arthropods and the exact adaptation of their thickness to the dwindling function, the disappearance of superfluous wing-veins, and of the hard chitinous covering of the abdomen when it is concealed in a protecting house, as in the case of hermit-crabs, Phryganidæ, and Psychidæ. Here too we find a sufficient explanation of the fact that parts which have become functionless, such as the wings of ants, can disappear even in the case of sterile workers. * * * * * The principle of germinal selection, however, can only be understood in its full significance if we take the positive aspect also into consideration. We had reached the conclusion that because of the fluctuations of the food-supply one set of the homologous determinants represented in the various ids may vary in a minus direction, and another set in a plus direction, and that this direction will be adhered to as long as no intra-germinal obstacles come in the way. As long as this does not happen the determinant concerned will pursue the path of variation it has once struck out, and indeed the tendency will be strengthened, because every passive variation, upwards or downwards, results in a strengthening or weakening of the determinant's power of assimilation.