referred to their disposition side by side and common history in the germ-substance, although a far larger proportion of them are probably due to their adaptation to similar functions, and are therefore to be regarded as a phenomenon of convergence within the same organism. We have already seen that the first increase in the growth of one determinant means a withdrawal of nourishment, however slight, from its neighbours; this can, of course, be equalized again if the claims on the common nutritive stream from another quarter are at the same time diminished; but it is possible that the claims from another quarter may also be increased, and the withdrawal will then be more marked, and the determinants being thus injured from two directions at once will sink downwards with greater rapidity. But it is also conceivable that the majority of determinants of a part may vary upwards, and, by their combined increased power of assimilation, direct towards themselves such a greatly increased stream of nourishment that the whole organ--for instance, a particular feather in a bird--varies in an upward direction, and becomes larger and larger, as we see in the case of many decorative feathers; or that certain determinants vary only as far as some of their biophors are concerned, and similarly for their determinates, as when a group of scales on a butterfly's wing that had previously been black turn out a brilliant blue. It can probably also happen that such variations within the determinants are transmitted to neighbouring determinants because the nutritive conditions which caused the first to vary have extended to those about them. The increase of brightly coloured spots in birds and butterflies gives us ground for concluding that there are processes of this kind within the germ-plasm. I will refrain from following this idea into greater detail, and translating the observable relations and variations of the fully-formed parts of the body into the language of the germ-plasm; but so much may be taken as certain, that multitudinous inter-relations and influences exist between the elements of the germ-plasm, and that one variation brings another in its train, so that--usually at a very slow rate, that is, in the course of generations and of species-forming, definite variations occur from purely intra-germinal reasons--variations which as far as they remain outside the limits of good or bad may of themselves change the character of a species, but which when they are seized upon by personal selection may, by sifting and combination of the ids, be led on to still higher development.