The fact that, as far as our experience goes, superfluous fingers are never inherited for more than five generations may be simply explained, for there has been no reason for the intervention of personal selection, either in the negative sense, for the six-fingered state does not threaten life, nor in the positive, since it is not of advantage. The deformity depends on spontaneous germinal variation, which must have taken place in a majority of ids or it would not have become manifest. But such a majority of 'polydactylous' ids is liable to become scattered again in every new descendant, and to be reduced again into a minority which can no longer make itself felt by the chances of reducing division and the admixture of normal ids in amphimixis. A polydactylous race of men could only arise through the assistance of personal selection; in that case there would doubtless be just as much chance of success in breeding a six-fingered race as there was in breeding the crooked-legged Ancon sheep from a single ram which was malformed in this manner. Without a gradual setting aside of the germs with normal ids, that is, without personal selection, such spontaneous deformities, and indeed all spontaneous variations, must fail of attaining to permanent mastery.
This must frequently be the case in free nature also, but we shall have to investigate later on, in the section devoted to the formation of species, whether external circumstances (inbreeding) may not also occur which make it possible for spontaneous variations to become constant breed-characters, even although they remain neither good nor bad, and are thus not subject to the action of personal selection.
In general, however, amphigony with its reduction of the ids and its constant mingling of strange ids will form the corrective to the deviations which may arise through the processes of selection within the id, and which lead to excessive or superfluous development of certain structures, to a complete disturbance of the harmony of the parts, and ultimately to the elimination of the species.
It must be admitted, however, that Emery was probably right when he directed attention to the possibility of a 'conflict between germinal and personal selection.' It is quite conceivable that in cases of useful variations, that is, of adaptations, the processes of selection within the germ-plasm may lead to excessive developments, which personal selection cannot control, because, on account of their earlier usefulness, they have in the course of a series of generations and species become fixed not only in a majority of ids, but in almost all the ids of the collective germ-plasm of the species. In this case a reversal must be difficult and slow, for the gathering together of ids with relatively weaker determinants can only take place slowly, and it is questionable whether the species would survive long enough for the slow process to take effect. But, apart from the question of time, such a reduction of an excessive development would sometimes be quite impossible, for the simple reason that there is nothing for personal selection to take hold of.
Döderlein has pointed out that many characters go on increasing through whole series of extinct species, and ultimately grow to such excess that they bring about the destruction of the species, as, for instance, the antlers of the giant stag or the sabre-like teeth of certain carnivores in the diluvial period. I shall have to discuss this in more detail in speaking of the extinction of species; it is enough to say here that such long-continued augmentations in the same direction can never be referred solely to germinal selection, since it is hardly conceivable that a species—much less a whole series of species—should arise with injurious characters; they would have become extinct while they were still in process of arising. Although we see that the Irish stag, with his enormous antlers over ten feet across from tip to tip, was heavily burdened, we are hardly justified in concluding that the size and weight of the burden on his head tended to his destruction from the first—for in that case the species would never have developed at all—but it may well be that at some time or other the life-conditions of the species altered in such a manner that the heavy antlers became fatal to it. In this case the variation-direction which had gained the mastery in all ids could no longer be sufficiently held in check by personal selection, because the variations in the contrary direction would be much too slight to attain to selective value. Sudden, or at least rapidly occurring changes in the conditions of life, such as the appearance of a powerful enemy, exclude all chance of adaptation by the slow operation of personal selection.
If we look into the matter more carefully, we see that it is not strictly true to say that germinal selection alone brings about the extinction of a species by cumulative augmentation of structures which are already excessive; it is the incapacity of personal selection to keep pace with the more rapid changes in the conditions of life and to reduce excessive developments to any considerable extent in a short time. This would always be possible in a long time, for the determinants of the excessive organ E can never be equally strong in all the ids; they always fluctuate about a mean, however high this mean may be. Here again it must still be possible that reducing-divisions and amphimixis may lead to the formation of majorities of ids with weaker E-determinants, and if sufficient time be allowed, artificial selection could, by consistently selecting the individuals with, let us say, weaker antlers, give rise to a descending variation-movement. There are no variation-movements which cannot be checked; every direction can be reversed, but time and something to take hold of must be granted. That was wanting in the case of the giant stag, for it would not have been saved even if its antlers had at once become a couple of feet shorter, and germinal selection can hardly make so much difference as that.
Analogous to hereditary deformities, and of special interest in connexion with the processes within the germ-plasm, are 'sports' variations of considerable magnitude which suddenly appear without our being able to see any definite external reason for them. I have already discussed these in detail in my Germ-plasm, and have shown how simply these apparently capricious phenomena of heredity can be understood in principle from the standpoint of the germ-plasm theory.
The chances of the transmission of the saltatory variation will be greater or less according to whether the variation of the relevant determinants involves a bare majority of ids or a large majority, for the more ids that have varied, the greater is the probability that the majority will be maintained throughout the course of ensuing reducing divisions and amphimixis, that is, that the seeds of the plant will reproduce the variation, and will not revert to the ancestral form. Although one of the most satisfactory results of the id-theory lies precisely in the interpretation of these conditions, I do not wish to enter into the matter here, but will refer to the details in my Germ-plasm, published in 1894, which I consider valid still. At that time I had not formulated the idea of germinal selection, but the explanation of the occurrence of such sport-variations which I gave was based upon the assumption of nutritive fluctuations in the germ-plasm, which gave rise to variations in certain determinants. There was still lacking the recognition that the direction of variation once taken must be adhered to until resistance was met with, and that the determinants stand in nutritive correlation with one another, so that changes in one determinant must re-act upon the neighbouring ones, as I shall explain more fully afterwards. I also showed from definite cases that such sports, though they are sudden—'saltatory'—in their mode of occurrence, are long being prepared for by intimate processes in the germ-plasm. This 'invisible prelude' of variation depends on germinal selection. When a wild plant is sown in garden-ground it does not require to vary at once; several, even many, generations may succeed each other which show no sports; suddenly, however, sports appear, at first singly, then, perhaps, in considerable numbers. It is not, however, by any means always the case that considerable numbers occur, for some varieties of our garden flowers have arisen only once, and then have been propagated by seed; and such saltatory sports in plants which are raised from seed are usually constant in their seed, and if they are fertilized with their own pollen they breed true—a proof that the same variations must have taken place in the relevant determinants in a large majority of ids.
In animals, it would appear, such saltatory variations occur much more rarely than in plants; the case examined in detail by Darwin of the 'black-shouldered peacock' which suddenly appeared in a poultry-yard is an example of this kind. Much more numerous, however, are the instances among plants, and especially among plants which are under cultivation. This indicates that we have here to do with the effect of external conditions, of nutritive influences which cause the slow variation of certain determinants, sometimes abetting and sometimes checking. As soon as a majority of ids varied in this way comes to lie in a seed, a sport springs up suddenly and apparently discontinuously—a plant with differently coloured or shaped petals or leaves, with double flowers, with degenerate stamens, or with some other distinguishing mark, and these new characters persist if the variety is propagated without inter-crossing.