LECTURE XXVI
GERMINAL SELECTION (continued)
Germinal selection, spontaneous and induced—Climatic forms of Polyommatus phlæas—Deformities—Excessive augmentation of variations—Can it lead to the elimination of a species?—Saltatory variations, copper-beech, weeping trees—Origin of sexual distinguishing characters—Formation of breeds among domesticated animals—Degenerate jaws—Human teeth—Short-sightedness—Milk-glands—Small hands and feet—Ascending variation—Talents, intellect—Combination of mental endowments—The ultimate roots of heritable variation—There are only plus- and minus-variations—Relations of the determinants to their determinates—The play of forces in the determinant system of the id—Germinal selection inhibited by personal selection—Objection on the score of the minuteness of the substance of the germ-plasm.
Hitherto we have derived the variations of the determinants of the germ-plasm, upon which we based the process of germinal selection, from chance local fluctuations in nutrition, such as must occur in an individual id, independently of the nutrition of the other ids of the same germ-plasm. But there are doubtless also influences which set up similar nutritive changes in all ids, and by which, therefore, all homologous determinants, in as far as they are sensitive to the nutritive change in question, are affected in the same manner. To this category belong changes in the external conditions of life, and particularly climatic changes. It is, then, germinal selection alone which brings about the presence of a majority of ids with determinants varying in the same direction, and personal selection has no part in the transformation of the species. Many years ago I instituted experiments with a small butterfly, Pararga egeria, and these showed that a heightened temperature so influenced the pupæ of this form that the butterflies emerged with a different and deeper yellow ground-colour, similar to that of the long-known southern variety Meione. More thoroughly decisive, however, were the experiments on Polyommatus phlæas, the small 'fire-butterfly,' which were carried on in the eighties by Merrifield in England and by myself almost at the same time. I shall discuss these later in more detail, and will only say here that this butterfly, whose range extends from Lapland to Sicily, occurs in two forms, the southern distinguished by a 'dusting' of deep black from the northern, in which the wing-surfaces are of a pure red-gold. The experiments showed that the southern form can be artificially produced by warmth, and the interpretation must be that the direct influence of higher temperature affects the quality of the nutritive fluids in the germ-plasm, and thereby at the same time the determinants of one or more kinds of wing-scales are caused to vary in all the ids in the same direction, in such a fashion that they give rise to black scales instead of the former red-gold ones. It is thus certain that there are external influences which cause particular determinants to vary in a particular manner. I call this form of germinal variation 'induced' germinal selection, and contrast it with 'spontaneous' selection, which is caused, not by extra-germinal influences, but by the chances of the intra-germinal nutritive conditions, and which will, therefore, not readily occur at the same time in all the ids of a germ-plasm, and so will not give rise to variation of the same kind in the homologous determinants of all the ids.
The two processes must also be distinguished from each other in their relation to personal selection, for induced germinal selection will go on increasing until the maximum of variation corresponding to the nature of the external influences and of the determinants concerned is reached. Since all the ids are equally affected and caused to vary in the same way, personal selection has nothing to take hold of, and the variation might go on intensifying even if it should become biologically prejudicial. But it is quite otherwise with spontaneous germinal selection, which has its roots not in all, but only in a majority of the ids. Here the variation may go on increasing by germinal selection alone, but only until it acquires a positive or negative biological value, that is, until it becomes advantageous or prejudicial to the life of the individual; then personal selection intervenes and decides whether it is to go on increasing or not. Spontaneous germinal selection can therefore only lead to the general variation of a whole species when it is supplemented by some external factor such as, especially, the utility of the variation.
This does not imply, however, that indifferent variations of large amount could not arise through spontaneous germinal selection, but they would remain confined to a small number of individuals, and would sooner or later disappear again. The congenital deformities of Man may in part fall under this category. If, for instance, certain determinants are, by reason of specially favourable local nutritive conditions, maintained for a long time in progressive variation, they will become so strong that the part which they determine will turn out excessive, perhaps double. Hereditary polydactylism in Man may perhaps be explained on this principle, and I had already referred it to the more rapid growth and duplication of certain determinants of the germ even before formulating the idea of germinal selection. In this I was at one with the pathologist Ernst Ziegler, who had designated polydactylism as a germ-variation, and in contrast to others had not interpreted it in an atavistic sense, as a reversion to unknown six-fingered ancestors. All excessive or defective hereditary malformations may be referred to germinal selection alone, that is, to the long-continued progressive or regressive variation of particular determinant-groups in a majority of ids.
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.
But it happens sometimes, though more rarely, that not the whole plant but individual shoots may exhibit the variation. To this class belong the 'bud-variations' of our forest trees, the copper-beeches, copper-oaks, and copper-hazels, the various fasciated varieties of oak, beech, maple, and birch, and the 'weeping trees'; also the numerous varieties of potato, plantain, and sugar-cane. It seems that only a few of these breed true when reproduced from seed, or in other words, they usually exhibit reversions to the ancestral form: on the other hand, in the weeping oak for instance, nearly all the seedlings exhibit the character of the new variety, though 'in varying degrees.' The records as to the transmissibility of bud-variations through seed are probably not all to be relied upon, and new investigations are much to be desired, but the fact that in many cases they may be propagated not only by means of layers and cuttings but by seed also, is most important in our present discussion, for it proves that here too the varied determinants must be contained in a majority of ids. As it is only a single shoot that exhibits the saltatory variation, only the germ-plasm which was contained in the cells of this one shoot can have varied, and it must have done so in so many ids that the variation prevailed and found expression. But that, in this case also, the variation does not appear in all, but only in a small majority of ids, is proved by the frequent reversion of bud-varieties to the ancestral form. I have already reported a case of this kind shown to me by Professor Strasburger in the Botanic Gardens in Bonn, where a hornbeam with deeply indented 'oak-leaves' had one branch which bore quite normal hornbeam leaves. In my own garden there is an oak shrub of the 'fern-leaved' variety, whose branches bear some leaves of the ordinary form; variegated maples with almost white leaves often exhibit in individual branches a reversion to the fresh green leaves of the ancestral form. We see from this that what is so energetically disputed by many must in reality occur—namely, differential or non-equivalent nuclear division—for otherwise it would be unintelligible how the ids of the new variety, if they once attain a majority in the tree, could give place in an individual branch to a majority of the ancestral ids. Only differential nuclear division, in the manner of a reducing division, can be the cause of this. Of course this implies only a dissimilar or differential distribution of the ids between the two daughter-nuclei, not a splitting up of the individual ids into non-equivalents.
That in free Nature bud-variations left to themselves can ever become permanent varieties is probably an unlikely assumption, because of the inconstancy of their seeds which only breed true in rare cases; nor is it likely that such variations as the copper-beech, the weeping ash, and so on could hold their own in the struggle for existence with the older species; but there is certainly nothing to prevent our assuming that, in certain circumstances, saltatory variations, when they have a germinal origin, may become persistent varieties and may even lead to a splitting of the species. This may happen, for instance, when the variations remain outside the limits of good and bad, and thus are neither of advantage to the existence of the species nor a drawback thereto. In the next chapter we shall discuss the influence of isolation upon the formation of species, and it will be seen that in certain conditions even indifferent variations may be preserved, and that saltatory variations, as for instance in the evolution of species of land-snails or butterflies, may have materially contributed to bring this about.
I should like to emphasize still more the part played by saltatory variations arising from germinal selection in the origin of secondary sexual characters. As soon as personal selection, whether sexual or ordinary, prefers as useful in any sense a saltatory variation, it is not only preserved and becomes a character of a variety, but it may increase, and we have to ask whether such sudden variations are frequently of a useful kind, especially when not individual characters alone, but whole combinations of them are implicated. If we may judge from the sports of the flowers and the leaves of plants, transformations useful to the species as a whole rarely occur suddenly, that is, they occur only in a few out of very numerous sports; they are much more frequently indifferent, although quite visible and often conspicuous variations.
For this reason I am disposed to attribute to saltatory variations a considerable share in the production of distinctive sexual characters. From saltatory variations in flowers, fruits, and leaves we know that these may be conspicuous enough even on their first appearance, and so we are justified in finding in such variations the first beginnings of many of the decorative distinguishing characters which occur in the males of so many animals, especially butterflies and birds. As soon as it is admitted that variations of considerable amount, which have been slowly prepared in the germ-plasm by means of germinal selection, can suddenly attain to expression, one of the objections against sexual selection is disposed of, for conspicuous variations are necessary for the operation of this kind of selection, since the changes in question must attract the attention of the females if they are to be preferred. Without such preference, even though it be not quite strict and consistent, a long-continued augmentation of the decorative characters is inconceivable.
But as intra-germinal disturbances of the position of equilibrium in the determinant system is at the root of the saltatory variations of our cultivated plants, it must also have played a large share in the evolution of breeds among our domesticated animals, which is therefore by no means wholly due to artificial selection operating upon the variation of individual characters. In all breeds in the formation of which the production of more than a single definite character was concerned, as, for instance, in the broad-nosed breeds of dog—bull-dog and pug-dog—we may refer the peculiar variation of many parts to disturbances of the equilibrium of the determinant system, which bring to light, not suddenly as in the case of saltatory variations, but gradually and increasingly, the curious complex of characters. Darwin referred such transformations of the whole animal facies, where a single varying character is deliberately selected, to correlation, and by this he understood the mutual influence of the parts of an animal upon one another. Such correlation certainly exists, as we have already seen in discussing histonal selection, but here we have rather to do with the correlation of the parts of the germ-plasm, with the effects of germinal selection, which, affected by the artificial selection of particular characters, gradually brings about a more marked disturbance in the whole determinant system.
In the evolution of our breeds of domesticated animals, germinal selection in the negative sense must also have played a part—I mean through the weakening and degeneration of individual determinants. Only in this way, it seems to me, can we explain the tameness of our domestic animals, dogs, cats, horses, &c., in which all the instincts of wildness, fleeing from Man, the inclination to bite, and to attack, have at least partly disappeared. It is, of course, very difficult to estimate how much of this is to be ascribed to acquired habitude during the individual lifetime. The case of the elephant might be cited in evidence of tameness which arises in the individual lifetime, for all tame elephants are caught wild, but it seems that captured young beasts of prey, such as the fox, wolf, and wild cat, not to speak of lions and tigers, never attain to the degree of tameness exhibited by many of our domesticated dogs and cats. The very considerable differences in the degree of tameness of dogs and cats go to show that the case is one of instincts varying in different degree.
If this be so, then the instinct of wildness, if I may express myself so for the sake of brevity, has degenerated in consequence of its superfluity, and through the process of germinal selection, which allowed the determinants of the brain-parts concerned to set out on a path of downward variation upon which they met with no resistance on the part of personal selection.
Herbert Spencer adduced against my position the case of the reduction in the size of the jaws in many breeds of dog, especially in pugs and other lap-dogs, which he regarded as evidence of the inheritance of acquired characters. But this and analogous cases of the degeneration of an organ during a long period in which the animal had been withdrawn from the conditions of natural life is intelligible enough on the assumption of persistent germinal selection aided by panmixia. The jaws and teeth in these spoilt pets no longer require to be maintained at the level of strength and sharpness essential to their ancestors which depended on these characters, and so they fell below it, became smaller and weaker, but could not disappear altogether, for the process of degeneration was brought, or is being brought, to a standstill by the intervention of personal selection.
Even the lower jaw in Man is declared by many authors to be degenerate. Collins found that the lower jaw of the modern Englishman was one-ninth smaller than that of the ancient Briton, and one-half smaller than that of the Australians; Flower showed that we are a microdont race like the Egyptians, while the Chinese, Indians, Malays, and Negroes are mesodont, and the Andamanese, Melanese, Australians, and Tasmanians are macrodont. This does not of itself imply that we exhibit a degeneration of dentition, though this conclusion is hinted at by other facts, such as the variability of the wisdom-teeth. It need not surprise us, indeed, that a retrogressive variation tendency should have started in this case, for, with higher culture and more refined methods of eating, the claims which personal selection was obliged to make on the dentition have been greatly diminished, and germinal selection would thus intervene.
Every one knows how the quality of human teeth has deteriorated with culture, and this not in the higher classes only, but even among the peasantry, as Ammon has observed. The time is past when raw flesh was a dainty, and when bad teeth meant poor nutrition, if not actual starvation. Even nowadays famine plays a terrible and periodically recurrent rôle as an eliminator among some negroid races.
Many other organs in man have been reduced from their former pitch of perfection through culture, and some of them are still in process of dwindling. When I formulated the idea of panmixia and applied it to explain cases which had previously been referred to the inheritance of the results of disuse, I regarded the short-sightedness of civilized Man from this point of view. My opinion aroused lively opposition at the time, especially on the part of oculists, who very emphatically referred the phenomenon to the inheritance of acquired shortsight, and indeed regarded it as a proof of the transmission of functional modifications.
But, apart from the fact that the assumption of this mode of inheritance must now be regarded not only as unproved, but as contradicted by reliable data, panmixia, in conjunction with the ceaseless fluctuations within the germ-plasm—germinal selection—affords a better explanation than the other theory was ever in a position to offer. At that time I pointed out that the survival of the individual among civilized races had not for a very long time depended on the perfection of his eyesight, as it does for instance in the case of a hunting or warlike Indian, or of a beast of prey, or of a herbivore persecuted by the beast of prey. And this is by no means due solely to the invention of spectacles, but in a much greater degree to the fact that every man no longer has to do everything, so that numerous possibilities of gaining a livelihood remain open to the less sharp-sighted; that is, the division of labour in human society has made the survival of the short-sighted quite feasible. As soon as this division of labour reached such a degree that the founding of a family offered no greater difficulty to the short-sighted individual than to one with normal sight, short-sightedness could no longer be eliminated; and partly because of the mingling with normal sight, but partly also because of the never-failing minus-fluctuations of the germ-plasm determinants concerned, a variation in a downward direction was bound to set in, and will continue until a limit is set to it by personal selection. Meantime, we are obviously still in the midst of the process of eye-deterioration; and the resistance to it is somewhat inhibited in its operation, because although individuals with extremely bad sight are for the most part hindered from gaining an independent livelihood and having a family, this is certainly, thanks to our mistaken humanity, not always the case. There are even instances of marriage between two blind persons!
As yet, however, the deterioration of eyes has not advanced very far; not nearly all families are affected by it, and even in Germany, the land of the 'longest school form' and of the greatest number of spectacle-wearers, short-sight is still usually acquired by individuals, although there must frequently be a more or less marked predisposition to it. It is a common objection to this view that in England, France, and Italy the percentage of short-sighted individuals is much lower, and, in point of fact, one sees far fewer people wearing spectacles in those countries. This, however, does not prove that a similar deterioration of eyes has not begun there also, for how could the small inherited beginnings be detected if they were not accentuated by the spoiling of the eyesight in the lifetime of the individual by much reading of bad print, and by writing with bent head, as is still too often the case in many German schools.
That our interpretation, through panmixia on a basis of germinal selection, is the correct one, we infer also from the fact that short-sightedness has been proved to be a frequent character even among our domesticated animals, such as the dog and the horse. These animals receive protection and maintenance from Man, and their survival and reproduction no longer depend on the acuteness of their sight, and thus the eye has fallen from its original perfection, just as in Man, although in this case reading and writing play no part.
A whole series of similar slight deteriorations of individual organs and systems of organs might be enumerated, all of which have appeared in consequence of long and intensive culture in Man. All these must depend upon germinal selection, on a gradually progressive weakening of the determinant-groups concerned, under the conditions of panmixia, that is, in the absence of positive selection.
To these must be added the deterioration of the mammary-glands and breasts, and the inability to suckle the offspring which results chiefly from this. Here we have a variational tendency which could not appear in a people at a lower stage of culture, and it has not become general in the lower classes of society among ourselves.
The muscular weakness of the higher classes is another case in point, and all gymnastics and sports will be of no avail as long as a relative weakness of the muscles is not a hindrance to gaining a livelihood, and having a family. Even universal conscription will do nothing to check this falling off of the bodily strength. Certainly military service strengthens thousands, and hundreds of thousands of individuals, but it does not prevent the weaklings from multiplying, and thus reproducing the race-deterioration. But it would indeed be well if only those who had gone through a term of military service were allowed to beget children.
It is only among the peasantry, inasmuch as they really work and do not merely look on as proprietors of the ground, that such a deterioration of the general muscular strength could not become the permanent variational tendency of the determinants concerned, because among genuine peasants bodily strength is a condition of having and supporting a family—at least on an average.
The diminution in the firmness and thickness of the bones in the higher classes, and many another mark of civilization, must be looked at from the point of view of panmixia and germinal selection; perhaps also the smaller hands and feet which frequently occur along with a more graceful general build in the higher ranks of European peoples. It would certainly not be surprising if in families which usually intermarry, and which in no way depend for their material subsistence on the possession of large and powerful hands and feet or bones generally, a downward variation of the relevant germ-determinants should have developed, but this could never overstep a certain limit, because it would then be prejudicial even in civilized life. That we must be very careful not to regard large hands and feet as the direct result of hard physical toil was brought home to me by an observation of Strasburger's. He was particularly struck by the fact that the peasants of the high Tatra (Carpathians) were distinguished by the smallness of their hands and feet.
But while civilization has excited numerous downward variations in the germ, it has, on the other hand, been the cause of numerous hereditary improvements—variations in an upward direction. This opens up new ground, for hitherto we have been confronted with the alternative of either accepting the inheritance of acquired characters, and on this basis referring the talents and mental endowments of civilized Man to exercise continued throughout many generations, or of admitting an increase of mental powers only in as far as they possess 'selection value,' that is, as they may be decisive in the struggle for existence. To these mental qualities belong cleverness and ingenuity in all directions, courage, endurance, power of combination, inventive power, with its roots in imagination and fertility of ideas, as well as desire for achievement, and industry. Throughout the long history of human civilization these mental qualities must have increased through the struggle for existence, but how have the specific talents such as those exhibited in music, painting, and mathematics come into existence? And how have the moral virtues of civilized Man been evolved, and particularly unselfishness? For it can hardly be maintained of any of these endowments that they possess selection-value for the individual.
It is not my intention to discuss these questions in detail; they are too many-sided and of too much importance to be treated of merely in passing; moreover, I gave expression years ago to my views on this subject by dealing with one example—the musical sense in Man. I do not believe that the musical sense had its beginnings in Man, or that it has materially increased since the days of primitive Man, but in conjunction with the higher psychical life of civilized peoples its expressions and applications have risen to a higher level. It is, so to speak, an instrument which has been transmitted to us from our animal ancestors, and on which we have learnt to play better the more our mind has developed; it is an unintended 'accessory effect' of the extremely fine and highly developed organs of hearing with their nerve-centres which our animal ancestors acquired in the struggle for existence, and which played a much more important rôle in the preservation of life in their case than it does in ours. The musical sense may be compared to the hand, which was developed even among the apes, but which civilized Man in modern times no longer uses merely to perform its original function, grasping, but also for many other purposes, such as writing and playing the piano. And just as the hand did not originate through the necessities of the piano, neither did the extremely delicate sense of hearing of the higher animals develop for the sake of music, but rather that they might recognize their enemies, friends, and prey, in darkness and mist, in the forest, on the heath, and at great distances.
The case is probably the same with the rest of the special psychical endowments or talents. I do not of course maintain that they, like the musical sense, did not at some time play a rôle in the struggle for existence and survival, and therefore could not increase, but the increase was certainly not continuous, but much interrupted, so that it would extend only to small groups of descendants, and therefore could only contribute very slowly to the elevation of the psychic capacities of a whole people. But in certain individuals and families such augmentations would certainly take place through germinal selection, and it seems to me probable that these would never be wholly lost again, even if they appeared to be so, but would be handed on, in id-minorities, through the chain of generations, and would slightly raise the average of the talent in question, and might even, under favourable circumstances, combine in the development of a genius. We know how strongly hereditary such specific talents are; let us suppose that the determinants of, say, the musical sense have, by the intra-germinal chances of nutrition, been started on a path of ascending variation; they will continue in this path until a halt is called from some quarter or other. This can only happen if, in the reducing division, or in amphimixis, the highly developed musical determinants are wholly or partly eliminated, or are reduced to a minority. As long as this does not happen the ascending variation will go on, and then we may have the birth of a Mozart or of a Beethoven. Personal selection will not interfere either in a positive or a negative sense, since high development of the musical sense has no effect either in advancing or retarding the struggle for existence; the increase will therefore go on until the large majority of highly developed musical determinants, which we must assume in the case of a musical genius, is reduced, or even transformed into a minority, through unfavourable reducing divisions of the germ-cells, and by association with the germ-cells of less musical mates.
The fact that highly developed specific talents have never been known to be inherited through more than seven generations is quite in keeping with this view. But even this persistence has been observed only in the case of musical talent, and the long continuance of the inherited talent may well be due, as Francis Galton suggests in his famous statistical investigations into the phenomena of inheritance, to the fact that musical men do not readily choose wives who are absolutely lacking in this talent. It would be easy to rear an exceedingly highly gifted musical group of families within the German nation, if we could secure that only the highly-gifted musically should unite in marriage—that is, if personal selection could play its part. In another more general domain of mental endowment a case of this kind has been recorded, for Galton tells us of three highly gifted English families which intermarried for ten generations, and in that time scarcely produced a descendant who did not deserve to be called a distinguished man in some direction or other.
Of course, such continued persistence, through a long series of generations, of a high general mental level is more possible than the transmission and increase of a specific talent, for in the former case it is a question of a mixture of different high mental endowments, of which not all need be developed in every individual, and yet the individual need not fall to mediocrity if he possesses a combination of other qualities. But in musical talent, on the other hand, the falling from the height once attained takes place as soon as this one character is no longer represented in a sufficiently strong majority of determinants. Of course it would be a mistake to believe that the talent of a Sebastian Bach or a Beethoven depended solely on the highly developed musical sense; in them, as in all great artists, many highly developed mental qualities must have combined with the musical sense; a simpleton could never have written the Mass in B minor or the Passion of St. Matthew even if he had possessed the musical genius of Sebastian Bach. In this fact lies a further reason why genius is seldom found at the same pitch in two successive generations; the combination of mental characters always varies from father to son, and slight displacements may give rise to very great differences in relation to the manifestations of the specific talent. Under certain circumstances, the weak development of a single trait of character, as, for instance, power of action, or the excessive development of another, such as indecision or desultoriness, may so nullify the existing favourable combinations of mental characters, such as, let us say, musical sense, inventive talent, depth of feeling, &c., that they bear no fruit worth mentioning. And since as we have already seen, the different mental qualities of the parents are to a certain extent separately transmitted, that is, since they may appear in the children in the most diverse combinations, we should rather be surprised that pronounced talent in a specific direction can persist in a family for two and a half centuries than that it should do so very rarely. For reducing division is always combining the existing mental qualities anew, and amphimixis is adding fresh ones to them.
Thus germinal selection, that is, the free, spontaneous, but definitely directed variation of individual groups of determinants, is at the root of those striking individual peculiarities which we call specific talents; but it can attain to the highest level only rarely and in isolated cases, because these talents are not favoured by personal selection, and therefore the excessively highly developed determinants upon which they depend may be dispersed in the course of generations; they may sink to smaller majorities, or even to minorities, in which case they will no longer manifest themselves in visible mental qualities.
We deduced the process of germinal selection on the basis of the assumption that the nutrition of all the parts and particles of the body, therefore also of the determinants and biophors of the germ-plasm, is subject to fluctuations. We regarded the resulting variations of these last and smallest units of the germ-plasm as the ultimate source of all hereditary variation, and therefore the basis of all the transformations which the organic world has undergone in the course of ages and is undergoing still.
We have still to inquire whether we can give any more precise account of the nature of these units of the germ-plasm. If I mistake not, we may say at least so much, that all variations are, in ultimate instance, quantitative, and that they depend on the increase or decrease of the vital particles, or their constituents, the molecules. For this reason I have hitherto always spoken of only two directions of variation—a plus or a minus direction from the average. What appears to us a qualitative variation is, in reality, nothing more than a greater or a less, a different mingling of the constituents which make up a higher unit, an unequal increase or decrease of these constituents, the lower units. We speak of the simple growth of a cell when its mass increases without any alteration in its composition, that is, when the proportion of the component parts and chemical combinations remains unchanged; but the cell changes its constitution when this proportion is disturbed, when, for instance, the red pigment-granules which were formerly present but scarcely visible increase so that the cell looks red. If there had previously been no red granules present, they might have arisen through the breaking up of certain other particles—of protoplasm, for instance, in the course of metabolism, so that, among other substances, red granules of uric acid or some other red stuff were produced. In this case also the qualitative change would depend on an increase or decrease of certain simpler molecules and atoms constituting the protoplasm-molecule. Thus, in ultimate instance, all variations depend upon quantitative changes of the constituents of which the varying part is composed.
It might be objected to this argument that chemistry has made us acquainted with isomeric combinations whose qualitative differences do not depend upon a different number of the molecules composing them, but upon their different arrangement; it might be supposed that something similar would occur also in morphological relations. And, in point of fact, this seems to be the case. We may, for instance, imagine one hundred hairs as being at one time equally distributed on the back of a beetle, and at another standing close together and forming a kind of brush, but although this brush would be a new character of the beetle, yet its development would depend upon quantitative differences, namely, on the fact that the same skin-area, which in the first case bore perhaps only one hair, had in the second case a hundred. The quantity of hair cells has notably increased upon this small area. In the same way the characteristic striping of the zebra depends not on a qualitative change in the skin as a whole, but upon an increased deposit of black pigment in particular cells of the skin, therefore on a quantitative change. In relation to the whole animal it is a qualitative variation, as contrasted, for instance, with the horse, but in respect of the constituent parts which give rise to the qualitative variation it is purely quantitative. The character of the whole edifice is changed when the proportion of the stones of which it consists are altered.
Thus the determinants of the germ may not only become larger or smaller as a whole, but some kinds of the biophors of which they are made up may increase more than others, under definite altered conditions, and in that case the determinants themselves will vary qualitatively, so that, from the changing numerical proportions of the different kinds of biophors, a variation of the characters of the determinants can arise, and consequently also qualitative variations of the organs controlled by the determinants—the determinates. But, since nothing living can be thought of as invariable, the biophors themselves may, on account of nutritive fluctuations, grow unequally, and thereby vary in their qualities. To follow this out in greater detail and attempt to guess at the play of forces within the minutest life-complexes would at present only be giving the rein to imagination, but in principle no objection can be made to the assumption that every element of life down to the very lowest and smallest can, by reason of inequalities in its nutrition, be not only started on an ascending or descending movement of uniform growth, but can also be caused to vary qualitatively, that is, in its characters, because its component parts change their proportions.
Of course we know nothing definite or precise with regard to the units of the germ-plasm, and we cannot tell what is necessary in order that a determinant shall determine a part of the developing body in this way or in that; thus we have no definite idea of the relations subsisting between the variations of the determinants and those of their determinates, but we know at least so much, that hereditary variation of a part is only possible when a corresponding particle in the germ-plasm varies; and we may at least assume that these correspond to each other so far, that a greater development of the one implies a greater development of the other, and that a reversal of these relations is impossible. If the determinant X disappears from the germ-plasm the determinate X´ disappears from the soma. It is therefore justifiable to infer from the degree of development of an organ the strength of its determinant, and to assume that plus- and minus-variations in both are correspondingly large.
But in addition to the fluctuations in the equilibrium of the germ-plasm which lie at the root of all hereditary variation, we have to take into account something which we have already touched upon briefly—the correlation of the determinants, the influencing of one determinant by those round about it. I have spoken for the sake of brevity of 'the determinant' of a part, although all the large and more important parts must certainly be thought of as represented by several or many, if not, indeed, by whole groups of determinants. Although it is quite out of our power to follow the complex processes of the mutual influences of the determinants upon each other, we can say this at least, that these influences must exist, and we have here a faint indication of what must occur in the case of spontaneous variations within the germ-plasm. We must, in the first place, think of the individual determinants as arranged in groups, so that, for instance, the determinants of the right and left half of the body lie together, and therefore are frequently affected together by influences which cause variation, so that both vary in the same direction at the same time. In point of fact, analogous deformities, such as polydactylism of both right and left hands, and even of hands and feet at once, do actually occur. That the right and left hands, the fore- and hind-limbs, are represented in the germ by particular determinants, may be inferred from their frequently different phyletic evolution into different forms of hand and foot, e.g. into flipper and rudimentary hind-leg in the whale, as well as from the cases of particulate inheritance, which are rare, but which undoubtedly do occur, such as when, in Man, there is a maternal blue eye on one side of the head and a paternal brown eye on the other. But almost more striking than the differences between these homologous or homotypic parts are their points of resemblance, and these may probably be in part 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.
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.
On the other hand, the sudden first appearance of a saltatory variation takes place quite independently of personal selection, depending on similar variations in a number of ids, which remain latent until they have by the process of reducing division which precedes amphimixis, chanced to attain a majority. In sudden bud-variations we may perhaps suppose that reducing division occurring in some still unverified abnormal manner is the reason why the germinal variation suddenly makes itself visible—a supposition previously suggested as the explanation of the reversion of these sports.
The rarity of bud-variation is thus explained, while the greater frequency of saltatory variations in plants propagated by seed may be accounted for by the regular occurrence of reducing division in sexual reproduction. But that the same or similar variations may occur in several, it may be in many, ids at the same time must depend upon similar general influences which affect the plant as a whole, as happens through cultivation, manuring, and so on. I shall return to this when discussing the influence of the environment.
In some quarters this whole conception of germinal selection has been characterized as the merest figment of imagination, condemned on this ground alone, that it is based on the differences in nutrition between such extremely minute quantities of substance as the chromosomes of nuclear substance within the germ-cell. The quantity of substance is certainly minute, but it needs nutriment none the less, and can we believe that the stream of nourishment for all the invisibly minute vital elements is exactly alike? It may be admitted that the nourishment outside the ids is usually abundant, although undoubtedly fluctuations occur in it also, but it certainly does not follow from this that every vital unit within the id is similarly disposed in relation to the nutritive supply, or has food in equal quantities at its command, or even that each has as much as it can ever need. To make an assertion like this seems to me much the same as if an inhabitant of the moon, looking at this earth through an excellent telescope and clearly descrying the city of Berlin with its thronging crowds and its railways bringing in the necessaries of life from every side, should conclude from this abundant provision that the greatest superfluity prevailed within the town, and that every one of its inhabitants had as much to live upon as he could possibly require.
We certainly ought not to conclude from the fact that we cannot see into the structure and requirements and methods of nutrition of a very minute mass of substance that its nutrition cannot be unequal, and that it cannot, by its inequalities, give rise to very material differences, especially when we are dealing with a substance to which we must attribute an extraordinarily complex organization built up of enormous numbers of extremely minute particles. That this complexity is undeniable is now admitted by many who formerly thought it possible to believe in the simple structure of the germ-substance. How complex not only the germ-substance but every cell of a higher organism is in its structure, and how far below the limits of visibility its differentiations and arrangements reach, is pressed upon our attention by the most recent histological researches, such as those we owe to Heidenhain, Boveri, and many others. The whole scientific world was amazed when it came to know the mysterious nuclear spindle in the seventies, and since then this has been quite thrown into the shade by the discovery of the centrosphere, the centrosome, and more recently even the centriole, and now we believe that these marvellous centres of force may, or must, possess their own dividing apparatus! In the face of discoveries like these no one is likely to be able to persist in recognizing as existing only what is disclosed or even hinted at by the most powerful lenses; no one can any longer doubt that far below the limit of visibility organization is still at the basis of life, and that it is dominated by orderly forces. To me, at least, it seems more cogent to argue from the phenomena of heredity and variation to an enormous mass of minute vital units crowded together in the narrow space of the id, than to argue from the calculated size of atoms and molecules to the number which we are justified in assuming to be present in an id. In my book on the germ-plasm I made a calculation of this kind, and I arrived at figures which seemed rather too small for the requirements of the germ-plasm theory. This has been regarded as a proof that I disregard the facts for the sake of my theory, but it should rather be asked whether the size of the atoms and molecules is a fact, and not rather the very questionable result of an uncertain method of calculation. Undoubtedly modern chemistry has established the relative weight-proportions of the atoms and molecules with admirable precision, but it can make only very uncertain statements in regard to the absolute size of the ultimate particles. It is therefore admissible to assume that these have a still greater degree of minuteness when the facts in another domain of science require this.
We must assume determinants, and consequently the germ-plasm must have room for these; the variations of species can only be explained through variations of the germ-plasm, for these alone give rise to hereditary variation. It is upon this foundation that my germinal selection is built up; whether I have in the main reached the truth the future will show: but that I have not exhausted this new domain, but only opened it up, I am very well aware.