Let us take a case of positive variation of the determinants of an organ _N_, which would be more useful to the species if it were more highly developed than it had previously been. The variation in an upward direction is at first purely passive, having arisen from fluctuations in the food-supply, but it soon becomes active, since the determinants that have become stronger will have a stronger affinity for food and will attract more and more of the available supply. The increased food-stream is thus maintained, and its gradual result is such a strengthening of the determinants in the course of generations of germ-cells, that the parts controlled by these determinants--the determinates--must enter on a path of plus-variations. If to this there be added personal selection, either natural or artificial, any fluctuations of this primary constituent towards the minus side will be effectually prevented, the direction of variation will remain positive, and the continued intervention of personal selection may raise its development to its possible maximum, that is, so far that further development in the same direction would not make for greater fitness, and personal selection must call a halt. This will always happen as soon as further increase of the organ would be prejudicial to the living power of the whole, and when the harmony of the bodily parts would thereby be permanently disturbed.
That variation in an upward direction really can persist for a long time is shown by artificial selection as practised by Man in regard to his domesticated animals and cultivated plants. At first general variability, or at least variability in many directions, sets in as a result of the greatly altered conditions of life; the ordinary fluctuations of the determinants are intensified by the greater fluctuations in the nutritive stream, and it becomes possible for Man consciously or unconsciously to select for breeding whatever he prefers among the chance variations that arise in individual parts or in whole complexes of parts, and he may thus give rise to a long-continued, often apparently unlimited, augmentation of variations in the same direction, although he cannot exercise _any direct_ influence upon the germ-plasm or its determinants. When a determinant has assumed a certain variation-direction it will follow it up of itself, and selection can do nothing more than secure it a free course by setting aside variations in other directions by means of the elimination of those that exhibit them.