simpler molecules and atoms constituting the protoplasm-molecule. Thus, in ultimate instance, all variations depend upon quantitative changes of the constituents of which the varying part is composed. It might be objected to this argument that chemistry has made us acquainted with isomeric combinations whose qualitative differences do not depend upon a different _number_ of the molecules composing them, but upon their different arrangement; it might be supposed that something similar would occur also in morphological relations. And, in point of fact, this seems to be the case. We may, for instance, imagine one hundred hairs as being at one time equally distributed on the back of a beetle, and at another standing close together and forming a kind of brush, but although this brush would be a new character of the beetle, yet its development would depend upon quantitative differences, namely, on the fact that the same skin-area, which in the first case bore perhaps only one hair, had in the second case a hundred. The quantity of hair cells has notably increased upon this small area. In the same way the characteristic striping of the zebra depends not on a qualitative change in the skin as a whole, but upon an increased deposit of black pigment in particular cells of the skin, therefore on a quantitative change. In relation to the whole animal it is a qualitative variation, as contrasted, for instance, with the horse, but in respect of the constituent parts which give rise to the qualitative variation it is purely quantitative. The character of the whole edifice is changed when the proportion of the stones of which it consists are altered. Thus the determinants of the germ may not only become larger or smaller as a whole, but some kinds of the biophors of which they are made up may increase more than others, under definite altered conditions, and in that case the determinants themselves will vary qualitatively, so that, from the changing numerical proportions of the different kinds of biophors, a variation of the characters of the determinants can arise, and consequently also qualitative variations of the organs controlled by the determinants--the determinates. But, since nothing living can be thought of as invariable, the biophors themselves may, on account of nutritive fluctuations, grow unequally, and thereby vary in their qualities. To follow this out in greater detail and attempt to guess at the play of forces within the minutest life-complexes would at present only be giving the rein to imagination, but in principle no objection can be made to the assumption that every element of life down to the very lowest and smallest can, by reason of inequalities in its nutrition, be not only started on an ascending or descending movement of uniform growth, but can also be caused to vary _qualitatively_, that is, in its characters, because its component parts change their proportions.