If we consider further that the variation of a part must depend not only on the quality of the external stimulus but also upon the constitution, the reacting power of the part, we shall understand that similar nutritive variations may cause two different determinants to vary in different ways, and when we reflect that every nutritive change must extend from the point from which it started with diminishing strength in a particular direction, we have a further factor in the variation of determinants and one which influences even similar determinants differently. Finally, if we remember that determinants of different constitution will also extract different ingredients from the nutritive stream and thus set up in it different kinds of chemical change, thus causing an altered supply of nutritive substances to flow to the neighbour determinants, we get some insight into a very complex and delicate but perfectly definite set of processes, into a mechanism which we can certainly only guess at, but whose results lie plainly before us in the spontaneous variations of the organism. We understand in principle the possibility of saltatory variation, as a more or less widespread, more or less marked disturbance of the species-type in this or that group of characters, and we may acknowledge that those 'kaleidoscopic variations' which Eimer supposed to be the sole basis of the transformation of species, and which have been brought to the foreground again quite recently by De Vries[20], are probably factors in transmutation operative within a limited sphere. [20] See end of chap. xxxiii. But we must think of all these struggles and mutual influencings as taking place on the smallest possible scale, so that it is only by long summation that they can produce any visible effect, and we must never forget the essential significance of the plurality of ids, for these 'spontaneous' variations may take place in a different and quite independent manner in each individual id. If this were not so no intervention of personal selection would be possible, natural selection would not exist, and the adaptation of the organism from the single cell up to the whole would remain wholly unexplained. The whole crop of spontaneous germ-variations, whenever it ceases to be 'indifferent,' and becomes either 'good' or 'bad,' comes under the shears of personal selection and under its almost sovereign sway.