Chapter VIII: Part 8

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.

LECTURE XXVII

THE BIOGENETIC LAW

Fritz Müller's ideas--Development of the Crustaceans--Of the Daphnidæ--Of Sacculina--Of parasitic Copepods--Larvæ of the higher Crustaceans--Change of phyletic stages in Ontogeny--Haeckel's _Fundamental Biogenetic Law_--Palingenesis and Cœnogenesis--Variation of phyletic forms by interpolation in a lengthened Ontogeny--Justification of deductions from Ontogeny to Phylogeny--Würtemberger's series of Ammonites--Phylogeny of the markings in the caterpillars of the Sphingidæ--Condensation of Phylogeny in Ontogeny--Example from the Crustaceans--Disappearance of useless parts--The variation of homologous parts, according to Emery--Germ-plasmic correlations--Harmony with the theory of determinants--Multiplication of the determinants in the course of the phylogeny.

What I propose to discuss in this lecture should have been considered at an earlier stage, if we had pledged ourselves to adhere strictly to the historical sequence of scientific discovery, for the phenomena which we are about to deal with attained recognition shortly after the revival of the evolution idea, and indeed they formed the first important discovery which was made on the basis of the Darwinian Doctrine of Descent. I have introduced them at this stage because they have to do with phenomena of inheritance and modifications of these, the understanding of which--in as far as we can as yet speak of understanding at all--is only possible on the basis of a theory of inheritance. Therefore, in order to examine these phenomena and their causes, it was necessary first to submit a theory of heredity, as I have done in the germ-plasm theory. We have to treat of the connexion between the _development_ of many-celled individuals and the _evolution_ of the species, between germinal history and racial history, or, as we say with Haeckel, between ontogeny and phylogeny.

Long before Darwin's day individual naturalists had observed that certain stages in the development of the higher vertebrates, such as birds and mammals, showed a likeness to fishes, and they had spoken of a fish-like stage of the bird-embryo. The 'Natural Philosophers' of the beginning of the nineteenth century, Oken, Treviranus, Meckel, and others, had, on the basis of the transmutation theory of the time, gone much further, and had professed to recognize in the embryonic history of Man, for example, a repetition of the different animal stages, from polyp and worm up to insect and mollusc. But von Baer afterwards showed that such resemblances are never between different types, but only between representatives of the same general type, e.g. that of Vertebrata; and Johannes Müller maintained, from the standpoint of the old Creation theory, that an 'expression of the most general and simple plan of the Vertebrates' recurred in the development of higher Vertebrates, giving as an instance that, at a certain stage of embryogenesis, even in Man, gill-arches were laid down and were subsequently absorbed. But why this 'plan' should have been carried out where it was afterwards to be departed from remained quite unintelligible.

An answer to this question only became possible with the revival of the Theory of Descent, and the first to throw light in this direction was Fritz Müller, who, in his work _Für Darwin_, published in 1864, interpreted the developmental history of the individual, 'the ontogeny,' as a shortened and simplified repetition, a recapitulation, so to speak, of the racial history of the species, the 'phylogeny.' But at the same time he recognized quite clearly--what indeed was plain to all eyes--that the 'racial history' cannot be simply read out of the 'germinal history,' but that the phylogeny is often 'blurred,' on the one hand by the fusing and shortening of its stages, since development is always 'striking out' a more direct course from the egg to the perfect animal, while, on the other hand, it is frequently 'falsified' by the struggle for existence which the free-living larvæ have to maintain.

For the establishment of these views Fritz Müller relied chiefly upon larvæ, and in particular upon those of Crustaceans, and the facts, which were in part new and in part interpreted in a new manner, were so striking that it was impossible to deny their importance. In particular, he drew attention to the fact that in several of the lower orders of Crustaceans the most diverse species have a similar form when they leave the egg, all of them being small, unsegmented larvæ, with a frontal eye and a helmet-like upper lip, and with three pairs of appendages, the two posterior pairs being two-branched swimming-legs beset with bristles. In the size and form of the body, and especially of the chitinous carapace, these larvæ differ in the various systematic groups; thus, for instance, the larvæ of the Copepods are simply oval, while those of the Cirrhipedes are produced anteriorly into two horn-like processes, and so on, but in essentials they are all alike, and for a long time these larval forms had been distinguished by the special name of 'Nauplius' (Fig. 109).

The development of the perfect animal begins with the longitudinal growth of the Nauplius; the posterior end lengthens and becomes segmented, between the anterior portion and the tail more segments are interpolated, and on these new pairs of limbs may grow. The number of these segments and limbs varies according to the group to which the animal belongs. Thus the body of the perfect animal in the little Cyprids always consists of eight segments, seven of which bear a pair of limbs apiece; in the Branchiopods, on the other hand, the number of segments varies from twenty to sixty, with ten to over forty pairs of legs; in the Daphnids or water-fleas there are about ten segments, with seven to ten pairs of limbs, and in the Copepods about seventeen segments with eleven pairs of limbs. The difference between the orders depends not only upon the differences in the number of segments and limbs, but quite as much upon the form and development of the segments, and above all of the limbs, and in this connexion it is worthy of note that the additional limbs which grow out usually appear at first as biramose swimming-legs, and are subsequently modified in form. Thus the pairs of jaws, three in number, which appear in the Copepods are developed from such swimming-legs, and so also is the second pair of antennæ in the Copepods and the jaws of the Branchiopods, Cirrhipedes, &c.

If then we have before us in the 'germinal history' (ontogeny) a fairly precise repetition of the 'racial history' (phylogeny), we may deduce from this that the primitive forms of the Crustacean race were animals which consisted of few segments, and that from these, in the course of the earth's history, the very diverse modern groups of Crustaceans have arisen, by the addition of new segments, and the adaptation of the limbs upon them, which were at first biramose swimming-legs, to different kinds of functions, one becoming an antenna, another a jaw or a swimming-arm, a third, fourth, fifth, and so on, a jumping-leg, a copulatory organ, an egg-bearer, a gill-bearer, or a tail-fin.

That the development has in general followed those lines is made clear chiefly by the fact that the members of all these different orders of Crustaceans still arise from nauplius larvæ, even in those cases in which the perfect animal possesses a structure differing widely from the usual Crustacean form. _All_ Crustaceans arise from the _nauplius form_, even those of the higher orders, though they may not arise from a nauplius _larva_. But this very circumstance, that in most of the higher and many of the lower Crustaceans, the young animal, when it emerges from the egg, already possesses more numerous segments and limbs than a nauplius larva, again points to the connexion between phylogeny and ontogeny, for in these cases the nauplius stage _is gone through within the ovum_. The whole difference between this and the forms we considered first lies in the fact that, in the latter, the development is greatly shortened, condensed, as we might say, so that the nauplius stage forms a part of the _embryonic_ development, and that new segments and limbs develop in the embryo nauplius within the egg, so that the young animal leaves the egg in a more advanced state, nearer to that of the perfect animal, to which it can, therefore, attain in a shorter time.

We should expect that this shortening of the larval period would be associated with a prolongation of embryogenesis, especially in those Crustaceans which possess a large number of segments and limbs, that is--in the higher forms--and in the main this is the case. But there are exceptions in two directions; in the first place there are some, even among the lower Crustaceans, which leave the egg not as a nauplius but in the perfect form of the adult, and secondly, there are, among the higher Crustaceans, certain species which emerge from the egg not in the more mature form but still in the primitive nauplius form. Fritz Müller was the first to furnish an example of this last case, a Brazilian shrimp, _Peneus potimirim_. Like the lowest Copepods or Branchiopods, this species, which belongs to the highest order of Crustaceans, goes through the whole long development, from the nauplius through a series of higher larval forms up to the perfect animal, and all _outside of the egg_, as an independent free-swimming larva (Fig. 109, _A-E_). This is in sharp contrast to its near relative, the freshwater crayfish, which goes through this whole development within the egg, and emerges perfectly formed.

We see from this example that it is not some inward necessity which thus, in the higher and more complicated organism, contracts the ontogeny into the embryonic state, but that this depends upon external adaptive factors. Here again we have adaptation, mainly to the conditions of larval life. The elimination of the larvæ by enemies, for instance, will, other things being equal, be so much the more incisive the longer the larval development is protracted, but in that case the general ratio of elimination of the species, and the degree of fertility the species must possess in order to hold its own in the struggle for existence, will also play a part in determining the mode of development. For the higher the ratio of elimination the more eggs the female must produce, and the more eggs that have to be produced the smaller will be the quantity of nutritive material for the building up of the young embryo which each egg can be furnished with. I know of no records in regard to the eggs of that Brazilian shrimp in which embryonic development ends with the nauplius stage, but we shall certainly not be wrong in predicting that the eggs in this case will be very small and very numerous, in contrast to those of the freshwater crayfish, which are large and, as compared with others known to us, not very numerous.

It is a point of undeniable theoretical significance which the life-histories of these Crustaceans disclose, that embryogenesis is not condensed according to hidden internal laws when the structure increases in complexity, but that the condensation of the ontogenetic stages depends upon adaptation, and may be quite different in nearly related species. It shows us anew that all biological occurrences are dominated by the process of selection.

I have already mentioned that exceptions to the usual mode of development occur even among the lower Crustaceans, and I was thinking at the time of the Daphnids, which leave the egg as fully formed little animals, already equipped with all their segments and limbs. The nauplius stage is passed through in the egg, and it is an interesting indication that the ancestors of the modern species were in the way of moulting, that this embryo nauplius moults within the egg by forming a fine cuticle which is shed after a time. If it be asked why there should be direct development in the case of these small and not very complex water-fleas, while related species, the Branchiopods, which are much richer in segments and in limbs, should emerge from the egg in the form of a nauplius, and then pass through a longer larval period, we may answer that the reason probably lies in the fact that, in the former case, very few eggs are produced, sometimes only one, often two, seldom more than a dozen, that these eggs can thus be relatively well equipped with yolk, and that the formation of the little body which bears only from seven to nine pairs of limbs can be easily completed within this egg. Other things being equal, the direct development would always be an advantage, because reproduction can begin sooner in the young generation and the number of individuals will thus increase more rapidly. And this is of particular importance in the case of the water-fleas.