Chapter VII: Part 7

Let us take a case of positive variation of the determinants of an organ _N_, which would be more useful to the species if it were more highly developed than it had previously been. The variation in an upward direction is at first purely passive, having arisen from fluctuations in the food-supply, but it soon becomes active, since the determinants that have become stronger will have a stronger affinity for food and will attract more and more of the available supply. The increased food-stream is thus maintained, and its gradual result is such a strengthening of the determinants in the course of generations of germ-cells, that the parts controlled by these determinants--the determinates--must enter on a path of plus-variations. If to this there be added personal selection, either natural or artificial, any fluctuations of this primary constituent towards the minus side will be effectually prevented, the direction of variation will remain positive, and the continued intervention of personal selection may raise its development to its possible maximum, that is, so far that further development in the same direction would not make for greater fitness, and personal selection must call a halt. This will always happen as soon as further increase of the organ would be prejudicial to the living power of the whole, and when the harmony of the bodily parts would thereby be permanently disturbed.

That variation in an upward direction really can persist for a long time is shown by artificial selection as practised by Man in regard to his domesticated animals and cultivated plants. At first general variability, or at least variability in many directions, sets in as a result of the greatly altered conditions of life; the ordinary fluctuations of the determinants are intensified by the greater fluctuations in the nutritive stream, and it becomes possible for Man consciously or unconsciously to select for breeding whatever he prefers among the chance variations that arise in individual parts or in whole complexes of parts, and he may thus give rise to a long-continued, often apparently unlimited, augmentation of variations in the same direction, although he cannot exercise _any direct_ influence upon the germ-plasm or its determinants. When a determinant has assumed a certain variation-direction it will follow it up of itself, and selection can do nothing more than secure it a free course by setting aside variations in other directions by means of the elimination of those that exhibit them.

That artificial selection can cause the increase of a part has long been established, but in what way this is possible, and how it can be theoretically explained has hitherto been very obscure, for even if we take the favourable case that both parents possess the desired variation, it cannot be supposed that the characters of the parents are, so to speak, added together in the child; all we can say is that the probability that the children will also exhibit the character in question--for instance, a long or crooked nose--becomes greater. Certainly an increase of the character may result if in both parents the determinants _K_ are present in excess as compared with the heterodynamous determinants _K´_ and _K´´_, for in that case there is an increased probability that, through reducing divisions and amphimixis, there will again be a preponderance of the determinants _K_ composing the germ-plasm of the child, and further, that these determinants _K_ will dominate strongly as compared with the few _K´_'s. It may thus happen that the long nose of the two parents will give rise to a still longer nose in the child, or that parents of considerable bodily size may have still bigger children, but such increase would be confined to one generation, and would not lead to a permanent increase of the character; permanent increase cannot depend merely on the number of the determinants _K_ and on their supremacy over their converse, the determinants _K´_; it must also depend on their own variation, and this again can depend only on germinal selection and not upon personal selection, although the former can be materially assisted by the latter.

That inheritance from both parents is only a secondary consideration in regard to the increase of a part by artificial selection is made evident by the fact that _many secondary sexual characters_ have been modified, although the breeder selected only in regard to one parent. Nevertheless in this very domain the greatest results have been achieved; witness the Japanese breed of cocks with tail-feathers six feet long. This astonishing result has been reached by the strictest selection of the cocks in which the feathers were a little longer than those of other cocks, and the increase in the length of feathers depended--according to our theory--simply on the fact that, by the selection of the determinants which were already varying in the direction of increased length, this process of increase was guarded from interruption by chance unfavourable conditions of nutrition. The continuance of variation in the upward direction in which it had already started is not effected directly by personal selection, but is so indirectly, for without this constant fresh intervention of selection the increase would be apt to come to a standstill, or the variation might even take a contrary direction. There are two other factors operative to which we have not yet given sufficient attention. They are, the multiplicity of the ids in every germ-plasm, and sexual reproduction.

If--as we must assume--each germ-plasm is made up of several or many ids, there must be several or many determinants of each part of the organism, for each id contains potentially the whole organism, though with some individuality of expression. The child is thus not determined by the determinants of a single id, but by those of many ids, and the variations of any part of the body do not depend on the variations of a single determinant _X_, but on the co-operation of all the determinants _X_ which are contained in the collective ids of the relevant germ-plasm. Thus it is only when a majority of the determinants have varied upwards or downwards that they dominate collectively the development of the part _X´_ and cause it to be larger or smaller.

We have assumed passive fluctuations in nutrition to be the first cause in individual variation, and it is obvious that the action of this first cause of dissimilarity must be greatly restricted by the multiplicity of the ids and the corresponding homologous determinants. For although passive fluctuations in nutrition should occur continually in the case of all determinants, this would not imply that they would follow the same direction in all the determinants _X_ of all ids, for some determinants _X_ might vary upwards, and others downwards, and these might counteract each other in ontogeny; so that in many cases the fluctuations of the individual determinants will not be felt in their products at all. But since there are--as we shall see later--only two directions of variation, upwards and downwards, plus and minus, it must also sometimes happen that a majority take one direction, and this affords the basis on which germinal selection can build further, and on which it is materially supported by reducing division and the subsequent amphimixis.

For reducing division removes half of the ids and thus of the determinants from the mature germ-cell, and according as chance leaves together or separates a majority of _X_-determinants varying in the same direction, this particular germ-cell will contain the primary constituents of a plus- or of a minus-variation of _X_, and it is possible that the presence of a majority or a minority may be entirely due to the reduction. The germ-plasm of the parent may contain, for instance, the determinant _X_ in its twenty ids 12 times in minus-variation form, 8 times in plus-variation form; and the reducing division, according to our view, may separate these into two groups of which one contains eight plus- and two minus-variations, the other ten minus-variations, or the one six plus- and four minus-variations, the other two plus- and eight minus-variations, and so on. Now every germ-cell which contains a majority of plus- or minus-variations--and this must be the case with most of them--may unite, if it attains to amphimixis, with a germ-cell which also contains a majority of plus or minus _X_-determinants, and if similar majorities let us say plus--meet together, the plus-variation of _X_ must be all the more sharply emphasized in the child.

Thus, although the individual determinants _X_ may not be incited to further variation by their co-operation with others varying in the same direction, the collective effect of the plus-determinants will be greater, and adherence to the same direction of variation in the following generation will be assured, for if in the germ-plasm of the parent there be, for instance, sixteen out of twenty determinants possessing the plus-variation, a minus-majority can no longer result from reducing division.

It is upon this that the operation of natural selection, that is, personal selection, must depend--that the germ-plasms in which the favourable variation-direction is in the majority are selected for breeding, for it is this and nothing else that natural selection does when it selects the individuals which possess the preferred variations. The ascending process is thus considerably advanced, because the opposing determinants are more and more eliminated from the germ-plasm, till the preferred variations of _X_ are left, and among these, as ascent in the direction begun continues, the opposing variations are again set aside by germinal selection, and so on. Reducing divisions and amphimixis are thus powerful factors in furthering the transformations of the forms of life, although they are not the ultimate causes of these.

Now that we have made ourselves familiar with the idea of germinal selection we shall attempt to gain clearness as to what it can do, and how far the sphere of its influence extends, and, in particular, whether it can effect lasting transformations of species without the co-operation of personal selection, and what kind of variations we may ascribe to it alone.

First, I must return for a moment to the question we have already briefly discussed--whether the variation of a determinant upwards or downwards must so continue without limit. We might be inclined to think that the great constancy which many species exhibit was a plain contradiction of this, for if every minute variation of a determinant necessarily persisted without limit in the same direction, we should expect to find all the parts of the organism in a state of continual unrest, some varying upwards, some downwards, always ready to break the type of the species. Must there not be some internal self-regulation of the germ-plasm which makes it impossible that every variation which crops up can persist unlimitedly? Must there not be some kind of automatic control on the part of the germ-plasm, which is always striving to re-establish the state of equilibrium that has once been attained by the determinant system whenever it is disturbed?

It is difficult to give any confident answer to this question. We cannot reach clearness on this point through our present knowledge of the germ-plasm, because we possess no insight into its structure; we can only draw conclusions as to the processes in the germ-plasm from the observed phenomena of variation and inheritance. But two facts stand in direct antithesis to one another, first, the high power of adaptation possessed by all species, and the undoubted occurrence of unrestricted persistence in a given direction of variation, as seen in artificial selection, and in the disappearance of parts which have ceased to function; and, secondly, the great constancy of old-established species which do indeed always exhibit a certain degree of individual variability, but without showing marked deviations as a frequent occurrence or in all possible directions, as they certainly would if every determinant favoured by a chance increase in the nutritive stream necessarily and irresistibly went on varying further in the same direction. Or can the constancy of such species be maintained solely by means of personal selection, which is continually setting aside all the determinants which rise above the selection-value by eliminating their possessors? I was for long satisfied that this was the true solution of the difficulty, and even now I do not doubt that personal selection does, in point of fact, maintain the constancy of the species at a certain level, but I do not believe that this is sufficient, but rather that it is necessary to recognize an equalizing influence due to germinal selection, and to attribute to this a share in maintaining the constancy of a species which has long been well adapted. I am led to this assumption chiefly by the phenomena of variation in Man, for we find in him a thousand kinds of minute hereditary individual variations, of which not one is likely to attain to selection value. Of course the constant recurrence of reducing divisions prevents any particular id which contains a varying determinant from being inherited through many generations; for so many ids are being continually removed from the genealogical tree by the constant rejection of the half of all ids of every germ-plasm, that only a small part of the ancestral id remains in the grandchild, great-grandchild, and so on. Certainly some of the ids of the ancestors compose the germ-plasm of the descendants, and if all the determinants of one of these ids had begun to vary persistently upwards or downwards in an ancestor, then all the determinants of the relative id in the descendants would possess the variation in an intensified degree; and however slowly the variation advanced it would attain selection-value in some one or other of the descendants, and would thus break the previously stable type of the most perfectly adapted species. The descendant in question would then succumb in the struggle for existence. But as the number of the determinants in the germ-plasm is probably much greater than that of the descendants of one generation, every descendant would in the course of time deviate unfavourably in some one character from the type of the species, and then either all the descendants would be eliminated or the type would become unstable. But neither of these things happens, and there are undoubtedly species which remain constant for long periods of time, therefore the assumption must be false and every variation of a determinant does not of necessity go on in the same direction without limit.

I therefore suppose that although slight variations are ceaselessly taking place upwards or downwards in all determinants, even in constant species, the majority of these turn again in the other direction before they have attained to any important degree of increase, at least in the germ-plasm of all species which have had a definitely established equilibrium for thousands of generations. In such a germ-plasm, or to speak more precisely, in the id of such a germ-plasm, marked fluctuations in the nutritive stream will not be likely to occur as long as the external conditions are unchanged, but slight fluctuations, which will not be wanting even here, may often alternate and turn in an opposite direction, and thus the upward movement of a determinant may be transformed into a downward one. Every determinant is surrounded by several others, and we can imagine that the regular nutritive stream which we have assumed may be partially dammed up by a slight enlargement of the determinant, and that this will drive the surplus back again. But however we may picture these conditions, which are for all time outside of the sphere of observation, the assumption of a self-regulation of the germ-plasm, up to a certain degree, cannot be regarded as inconceivable or unphysiological.

But there are limits to this self-regulation; as soon as the increase or decrease of a determinant attains a certain degree, as soon as it has got beyond the first slight deviation, it overcomes all obstacles, and goes on increasing in the direction in which it has started. This must happen even in the case of old and constant species, and frequently enough to admit of an apparent capacity for adaptation in all directions. Every part of a species can vary beyond the usual individual fluctuations, and as this is possible only by means of intra-germinal processes, we must assume that even in the case of germ-plasms which have long remained in a state of stable equilibrium there may occasionally be marked fluctuations in the nutritive stream, and thus more than usually pronounced variations of the determinants affected by it will occur. These yield the material for new adaptations if they are in the direction of fitness, or they are eliminated either by the chances of reducing division or by personal selection if new adaptations are not required.

The old-established hereditary equilibrium of the germ-plasm must be most easily disturbed when the species is in some way brought into new conditions of existence, as, for instance, when plants or animals are domesticated, and when in consequence, as we have already assumed, the nutritive currents within the id gradually alter, quantitatively and qualitatively; and on this account alone certain kinds of determinants are favoured, while others are at a disadvantage. In this way there arises the intensified general variability of domesticated animals and cultivated plants which has been known since the time of Darwin. Something analogous to this must occur in natural conditions, though more slowly, when a species is subjected to a change of climatic conditions, but we shall discuss this later on in more detail.

We have thus arrived at the idea that the slight variations of the determinants may be counteracted whether they be directed upwards or downwards, and that in the case of so-called constant species they do frequently equalize themselves; but that more marked variations, produced by more pronounced nutritive fluctuations, may in a sense go on without limit, and then can only be restricted and controlled by personal selection, that is, by the removal of the ids concerned from the genealogical lineage of the species.

In one direction variation can be proved to go on without limit, and that is downwards, as is proved by the fact of the disappearance of _disused organs_, for here we have a variation-direction, which has been followed to its utmost limit, and which is completely independent of personal selection; it proceeds quite _uninterfered with_ by personal selection, and is left entirely to itself. It is a significant fact that the disappearance of the individual parts of a larger organ, according to all the data that are as yet available, proceeds at a very _unequal rate_, so that it evidently depends to a great extent on chance whether a disused part begins to degenerate sooner or later. Thus in one of the Crustaceans living in the darkness of the caves of North America the optic lobes and optic nerves have disappeared, while the retina of the eye, the lens, and the pigment have been retained, and in others the reverse has taken place, and the nerve-centres have persisted while the parts of the eye have been lost (Packard). Variations of the relevant determinants towards the minus direction may thus occur, sometimes sooner, sometimes later; but when once they have started they proceed irresistibly, though with exceeding slowness.

But variation in an upward direction also, when it has once been set a-going, may in many cases go on unchecked until limits are set to it by personal selection, when the excess of the organ would disturb the harmony of the parts, or in any other way lessen the individual's chances of survival in the struggle for existence. This is proved especially by the phenomena of artificial selection, for almost all the parts of fowls and pigeons have been caused to vary to excess by breeding, and must thus have been, so to speak, capable of unlimited increase; and yet, as we have seen, personal selection cannot directly cause progress in any direction of variation; it can only secure a free course by excluding from breeding the bearers of variations with an opposite tendency. The beards of hens, the tail-feathers of the long-tailed domestic cocks, the long and short, straight and curved bills of pigeons, the enormously long ruffled feathers of the Jacobin, the multiplication of the tail-feathers in the fan-tail, and innumerable other breed-characters of these playthings of the breeder, prove that when variation-tendencies of any part are once present, that is, when they have arisen through germinal selection, they apparently go on unchecked until their further development would permanently and irretrievably destroy the harmony of the parts. As soon as this is threatened the breed loses its power of survival, and Darwin in his time cited the case of many extremely short-billed breeds of pigeon, which require the aid of the breeder before they can emerge from the hard-shelled egg, because their short and soft bills no longer allow them to break their way out. Here the correlation between the hardness of the egg-shell and that of the pigeon's bill has been disturbed, and the breed can now only be kept in existence by artificial aid.

There must be a possibility of something similar occurring in natural conditions, and when it does the species concerned must die out. But in the majority of cases the self-regulation which is afforded by personal selection will be enough to force back an organ which is in the act of increasing out of due proportion to within its proper limits. The bearers of such excessively increased determinants succumb in the struggle for existence, and the determinants are thus removed from the genealogical lineage of the species.

Having now established the fact that determinants can continue their direction of variation without limit because of internal, that is intra-germinal, reasons, we have come nearer an understanding of many secondary sexual characters, whose resemblance to the excessive developments artificially produced in our domestic poultry is so very striking. Here, too, we shall have to regard germinal selection as the root of the variations of plumage and other distinguishing characters, which have evolved by intra-germinal augmentation into the magnificently coloured crests, tufts, and collars, into the long or graduated, multiplied or erectile tail-feathers of the birds of Paradise, pheasants, and humming-birds. The conception of sexual selection formulated by Darwin will be so far modified, that we are no longer compelled to regard every minute step in this cumulative process as due to the selection of the males by the females. A preference of the finest males may still take place, and is probably general, since only thus could the distinguishing male characters become common property, that is, be transmitted to all or the majority of the ids of the germ-plasm, but the increase of the individual determinants which are in the act of varying goes on in each individual id, quite independently of this personal selection.

As it is not a single id with its determinant _a_ in ascending variation that controls the organ _A_, but as it always requires a majority of the ids _a_, this must be secured here by personal selection just as it is in ordinary natural selection. If the handsomest males are the successful competitors, then a majority of the transformed ids _a´_ will be transmitted to a number of their descendants, and the oftener this happens the larger will the majority be, and the less becomes the danger that it will be dispersed again by reducing division and amphimixis. Personal selection is thus in no way rendered superfluous by germinal selection, only it does not produce the augmentation of the distinguishing characters, but is chiefly instrumental in fixing them in the germ-plasm; it collects, so to speak, only the favourably varying ids, and, where complex variations depending on the proper variation of many ids are concerned, it combines these. How very great the influence of personal selection may be in this case of secondary sexual characters we see clearly from the soberly coloured mates of the brilliant males, for here natural selection has been operative in conserving the coloration inherited from remote ancestry.

But if the question be asked, how _the first majority_ of determinants varying in the same direction is brought about, there are two possibilities: first, by chance, and secondly, by influences which cause particular determinants of all the ids to vary in almost exactly the same manner. We shall find illustrations of the latter among climatic varieties; but the cases of the first kind are the more important, for they form the foundation and the starting-point for processes of selection of a higher order, for personal selection. It might seem perplexing that processes of such importance should depend ultimately upon chance; but when we remember that there are only two directions of variation, namely a plus direction or a minus direction, we recognize that the chance of a majority in one direction or another is much greater than that of absolute equilibrium between the two, and there is therefore a very strong probability that in many individuals of the species either the upward or the downward movement of a determinant _A_ will preponderate.

Now as such variation movements, when they are of a certain strength, increase automatically, we can easily see that they must gradually attain to a level at which they acquire selection value, and how then, by personal selection, the ids with favourably varying determinants may be collected together.

Of course it is not possible to state positively the time at which in individual cases a variation acquires a biological significance, that is, selection value. We can only say in a general way that, as soon as it attains this, personal selection either in a positive or a negative sense _must_ intervene; an injurious variation tends to the elimination of its possessor, a useful one increases the probability of its survival.

There must, however, be for every variation a stage of development in which it has as yet no decisive biological importance, and this stage need not by any means be so insignificant that we cannot see it, or can hardly do so: in other words, there are characters which have arisen through germinal selection, which are of purely 'morphological importance.'

It has often been disputed whether there can be any such thing as 'purely morphological characters,' which are indifferent as far as the existence of the species is concerned. This question used to be an important one, because the sphere of operation, and therefore the importance of the Darwin-Wallace selection--personal selection--depends on the answer, since this mode of selection only begins when a character has some biological importance. But as soon as we take germinal selection into consideration the question loses its importance, because we now know that every variation is indifferent to begin with, but every one can, under favourable circumstances, be increased to such a pitch that it attains biological importance, and that personal selection then takes over the task of carrying it on, either in a positive or a negative sense. We may therefore leave this disputed point alone just now, for while germinal selection seems still far from being generally recognized, we have to remember that we are not at all in a position to judge with any certainty as to the biological value of a character. What labour and painstaking investigation it has cost to give a verdict as to this even in a few instances! Innumerable characters appear indifferent, and are nevertheless adaptations. Darwin in his day pointed out the need for caution in this matter, referring to the case of animal coloration as an example; very little attention had been directed to it for a long time because it had been believed to be without significance. And how many diverse kinds of characters among animals and plants, which had likewise been regarded as 'purely morphological,' have on more careful investigation shown themselves of very great biological importance. I need only refer to the shape, position, hair-arrangement, colour, and lustre of flowers, and their relation to cross-fertilization by means of insects, or to the thickness and shape of the leaves of tropical trees with their coating of wax and their gutter-like outlets for carrying off the tropical rain which falls in terrible downpour (Haberlandt, Schimper), or to the limp, perpendicular drooping of the tufts of the young and tender leaves of the same trees, which also secures protection from being battered and torn by the rain.

There are even characters the biological use of which is unknown to us, but in regard to which we can affirm that they have a use. Thus Eigenmann described the larva of an American eel, which differs from other so-called 'Leptocephali' in that a row of seven black spots runs along its side. Apparently all these lie upon the side turned towards us, but in reality they are distributed on both sides, three lying on the left and four on the right, and so arranged that they look like a single row of spots at regular intervals, for the flat little fish is absolutely transparent. The habits of this larva are not yet known, but we may conclude that this appearance of a simple row of spots must have some value for the animal, for such a significant asymmetry could not have arisen for purely internal reasons (Fig. 107, _C_). It is possible that the fish is thus made to resemble parts of some marine alga, and that it is thereby protected from many enemies; that there is not a complete row upon each side may depend upon the fact that the two rows would be visible at the same time, and that they would blur each other in the eyes of the swimming enemy, and so destroy the resemblance of the picture to its unknown model.

But it cannot be denied that there are characters which have no special biological significance. There are doubtless many such characters, which stand beyond the threshold of good or bad, and which are therefore not affected by personal selection; it is difficult and often impossible to point these out with certainty. The shape of the human nose and of the human ear, the colour of the hair and of the iris, may be such indifferent characters whose peculiarities are to be referred solely to germinal selection. On the other hand, I would not venture to assert that the gay colouring and the complex markings on the wings of our modern Lepidoptera are always and in all cases unimportant, even when we cannot interpret their details either as protective, or as a sign of nauseousness, or as mimetic. The usually very exact similarity of the colour pattern in the individuals of each species seems to point to the intervention of personal selection in some form or other, for in what other way could such a large majority of variations in the same direction have developed in the germ-plasm as this constancy of the character indicates.

We know, of course, that the colours of butterflies and moths can be caused to vary through external and especially climatic influences, but this would only account for simple modifications of colour, and not for the origin of the complex colour patterns that actually occur. I therefore believe with Darwin that sexual selection has had much to do with this by giving a slight preference to the variations produced by spontaneous germinal selection, and thus preventing the majority of varied ids once acquired from being scattered again, but always collecting more of them, and so securing free play for the increase of the new character through intra-germinal processes. In this way have arisen not only the brilliance of our Lycænidæ and of the large Morphidæ of South America, but also many of the coloured spots, streaks, bands, eyes, and other components which have gradually in the course of time evolved into the complex colour pattern of many of our modern butterflies. I should like to remind any one who doubts this of a fact which corroborates the view that personal selection has co-operated in the production of these colours--I refer to the inconspicuous colouring of the females of many of these brilliant males--while in contradistinction to these cases there are other species in which both sexes are alike brilliant, so that it is impossible that mere spontaneous germinal selection can have determined that the females, because of their femaleness, should vary in a different manner from the males.

But while I believe that sexual selection in particular has had much to do with producing the colours of Lepidoptera, the basis of all these colour variations must still be looked for in germinal selection, and we shall see later on how it is possible to think of the diversified and often relatively abrupt transformations of marking as the resultant of the co-operation of climatic influences with germinal selection.

Of course there must also be unimportant changes in butterfly-markings which depend solely on the internal play of forces in the determinant system, and to this must be referred the markings of many of the 'variable' species whose variations are mere fluctuations in the details of marking, which have therefore caused much trouble to the systematists. Truly unimportant variations will rarely or never combine into a 'constant' form, and the fact that there are species which are 'variable' in such a high degree is enough to make us refer their variations to their lack of importance, for if they possessed any biological value the less valuable among them would gradually be removed by selection. Perhaps the variable species of certain moths like _Arctia caja_, and especially _Arctia plantaginis_, the little 'bear' of the Alps and Apennines, must be reckoned among these. But from the fact that there are such fluctuations in the markings of Lepidoptera, it seems to me that we must conclude that species which show a high degree of constancy in their markings have been influenced by selection, or by climatic influences which turned the play of forces within the determinant system in the same direction in all individuals. All these considerations and conclusions are quite sound and serviceable theoretically, but they are difficult to apply to individual cases, and where this is attempted it must be with the greatest caution, and, if possible, on a basis of investigations specially undertaken for the purpose; for how should we know whether a species which to-day is highly variable may not a geological epoch later become a very constant one? We must in any case assume that marked fluctuations of characters are associated with many transformations.

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.