If this be the case, we have here ready to hand the explanation of every heritable variation of a part, ascending and descending alike. Let us consider for a little the latter category—that is, the disappearance of functionless or useless organs. It is clear that, from the moment in the life of a species that an organ, N, becomes useless, natural selection withdraws her hand from it; individuals with better or worse organs N are now equally capable of life and struggle, the state of panmixia is entered upon, and the organ N of necessity falls somewhat below its previously attained degree of perfection.
That this must be so will be admitted when it is remembered that each organ of a species is only maintained at its highest level because personal selection keeps ceaseless watch over it, and sets aside all the less favourable variations by eliminating the individuals which exhibit them. But this is no longer the case with a useless organ. When a weaker variant of a disused organ arises through the intra-germinal fluctuations of nutrition, this is transmitted to the descendants just as well as the normally developed organ, and in the course of generations will be inherited by a greater and greater number of individuals, and must ultimately be inherited by all in some degree or other. The objection has been urged from many sides that variations upwards would be quite as likely to arise as those downwards, but this is an error. Even if, at the beginning, the minus-variations were rarer than the plus-variations, in the course of generations the minus ones would preponderate because ascending variations of disused organs are not indifferent for the organism but injurious to it. Perhaps an increase in the size of the organ itself would do no harm, but in that of its determinant it certainly would, because an ascending determinant requires more nourishment than previously, and withdraws it from its surroundings, and thus from the determinants in its immediate neighbourhood; but these are those of functioning and indispensable parts. Individuals in whose germ-plasm the determinants of disused organs ascend, and thereby depress the determinants of organs which are still active, are subject to personal selection, and are eliminated. There thus remain only those with descending determinants; in other words, the chance of variants in the direction of weakness in useless determinants far outweighs that of variants in the direction of increased strength; the latter will soon cease to occur at all, for as soon as a determinant has fallen a little below its normal level, it finds itself upon an inclined plane, along which it glides very slowly but steadily downwards. This might be disputed if it could be maintained that, at every stage of the descent, a change of direction was possible. But this probably takes place rarely and only in the case of individual ids, and will therefore not be permanent because in general the stronger neighbour determinants will possess themselves of the superfluous nourishment, and a lasting ascent will thus be impossible to the weakened determinant. This is precisely what I have called Germinal Selection. The determinant whose assimilating power is weakened by ever so little is continually being robbed by its neighbours of a part of the nourishment which flows towards it, and must consequently become further weakened. As no more help will be given to it by natural selection, since the organ is no longer of any value to the species, the better among the weakened determinants of N are never selected out, and they must gradually give way in the struggle with the neighbouring determinants which are necessary to the species, becoming gradually weaker and ultimately disappearing.
This process can, of course, no more be proved mathematically than any other biological processes. No one who is unwilling to accept germinal selection can be compelled to do so, as he might be to accept the Pythagorean propositions. It is not built up from beneath upon axioms, but is an attempt at an explanation of a fact established by observation—the disappearance of disused parts. But when once the inheritance of functional modifications has been demonstrated to be a fallacy, and when it has been shown that, even with the assumption of such inheritance, the disappearance of parts which are only passively useful, and of any parts whatever in sterile animal forms, remains unexplained, he who rejects germinal selection must renounce all attempt at explanation. It is the same as in the case of personal selection. No one can demonstrate mathematically that any variation possesses selection value, but whoever rejects personal selection gives up hope of explaining adaptations, for these cannot be referred to purely internal forces of development.
The total disappearance of a part which has become useless takes place with exceeding slowness; the whales, which have existed as such since the beginning of the tertiary period, have even now not completely lost their hind-limbs, but carry them about with them as rudiments in the muscular mass of the trunk, and the birds, which are even older, still show in their embryonic primordia the five fingers of their reptilian forefathers, although even their bird-ancestors of the Jurassic period, if we may argue from Archæopteryx, had only three fingers like our modern birds. A long series of similar examples might be given, and modern embryology in particular has contributed much that, like this example of birds' fingers, points to a certain orderliness in the disappearance of the individual parts of an organ which has become superfluous. Parts which, in the complete animal, have disappeared without leaving a trace, appear again in each embryonic primordium, and disappear in the course of the ontogeny. Speaking metaphorically, we might express this on the basis of the determinant theory, by saying that the determinants, as they become weaker, can only control an increasingly short period of the whole ontogeny of the organ, so that ultimately nothing more than its first beginning comes into existence. But this is only a metaphor; we cannot tell what really happens as long as we are ignorant of the physiological rôle of the determinants, and even of the laws governing the degeneration of a useless organ. In respect of the latter, much might still be achieved if comparative anatomy and embryology were studied with this definite end in view, and 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.
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.