LECTURE XXV
GERMINAL SELECTION
On what does disappearance after disuse depend, if not on the Lamarckian principle?—Panmixia—Romanes—Fluctuations in the determinant-system of the germ-plasm due to unequal nutrition—Persistence of germinal variations in a definite direction—The disappearance of non-functioning parts—Preponderance of minus germinal variations—Law of the retrogression of useless parts—Variation in an upward direction—Artificial selection—Influence of the multiplicity of ids and of sexual reproduction—Personal selection depends on the removal of certain id-variants—Range of influence of germinal selection—Self-regulation of the germ-plasm, which is striving towards stability—Ascending variation-tendencies may persist to excess—Origin of secondary sexual characters—Significance of purely morphological characters—The markings of butterflies.
Now that we have recognized that the assumption of a transmission of functional modifications is not justifiable, let us discuss some of the many phenomena to explain which many people believe the Lamarckian principle to be indispensable, and let us inquire whether we are in a position to give any other explanation of these. How has it come about that the effects of use and disuse appear to be inherited? Can we find a sufficient explanation in the principle of selection, and in the natural selection of Darwin and Wallace?
The answer to these two questions will be most quickly found if we begin by seeking for an explanation of the disappearance of a part when it ceases to be exercised.
That this cannot lie in the Lamarckian principle we have already learnt from the fact that passively functioning parts, such as superfluous wing-veins, also disappear, and that the loss of the wings and degeneration of the ovaries has taken place in worker ants, which can transmit nothing because they do not reproduce.
We might be inclined to regard this gradual disappearance and ultimate elimination of a disused organ as a direct gain, on the ground that the economy of material and space thus effected may be of decided advantage to the individual animal and thereby also for the maintenance of the species, and that those animals would have an advantage in the struggle for existence in which the superfluous organ was reduced to the smallest expression. But that would be far from supplying us with a sufficient explanation of the phenomenon; the individual variations in the size of an organ which is in process of degenerating are, even in extreme cases, far too slight to have any selection-value, and I cannot call to mind a single case in which the contrary could be assumed with any degree of probability. What advantage can a newt or a crustacean living in darkness derive from the fact that its eye is smaller and more degenerate by one degree of variation than those of its co-partners in the struggle for existence? Or, to use Herbert Spencer's striking illustration, how could the balance between life and death, in the case of a colossus like the Greenland whale, be turned one way or another by the difference of a few inches in the length of the hind-leg, as compared with his fellows, in whom the reduction of the hind-limb may not have gone quite so far? Such a slight economy of material is as nothing compared with the thousands of hundredweights the animal weighs. As long as the limbs protrude beyond the surface of the trunk they may prove an obstacle to rapid swimming, although that could hardly make much difference, but as soon as the phyletic evolution had proceeded so far that they were reduced to the extent of sinking beneath the surface, they would no longer be a hindrance in swimming, and their further reduction to their modern state of great degeneration and absolute concealment within the flesh of the animal cannot be referred even to negative selection.
Years ago I endeavoured to explain the degeneration of disused parts in terms of a process which I called Panmixia. Natural selection not only effects adaptations, it also maintains the organ at the pitch of perfection it has reached by a continual elimination of those individuals in which the organ in question is less perfect. The longer this conservative process of selection continues, the greater must be the constancy of the organ produced by it, and deviations from the perfect organ will be of less and less frequent occurrence as time goes on.
Now if this conservative action of natural selection secures the maintenance of the parts and organs of a species at their maximum of perfection, it follows that these will fall below this maximum as soon as the selection ceases to operate. And it does cease as soon as an organ ceases to be of use to its species, like the eye to the species of crustacean which descends into the dark depths of our lakes, or to the abyssal zones of the ocean, or into a subterranean cave-system. In this case all selection of individuals ceases as far as the eye is concerned; it has no importance in deciding survival in the struggle for existence, because no individual is at a disadvantage through its inferior eyes, for instance, by being in any way hindered in procuring its food. Those with inferior organs of vision will, ceteris paribus, produce as good offspring as those with better eyes, and the consequence of this must be that there will be a general deterioration of eyes, because the bad ones can be transmitted as well as the good, and thus the selection of good eyes is made impossible.
The mixture thus arising may be compared to a fine wine to which a litre of vinegar has been added; the whole cask is ruined because the vinegar mingles with every drop of the wine. As deviations from the normal are always occurring in every part of every species, and among them some that lessen the value of the organ, rarely perhaps at first, but after a time in every generation, a sinking of the organ from the highest point of possible perfection becomes inevitable as soon as the organ becomes superfluous. The functional uselessness of the organ must go on increasing the longer it is disused, as will readily be admitted if it be remembered that only the most perfect adaptation of all the separate parts of an organ can maintain its functional capacity, that all the parts of an organ are subject to variation, and that every deviation from the optimum implies a further deterioration of the whole. An eye, for instance, can no longer vary in the direction of 'better' if it has already reached the highest possible point of perfection; every further variation must deteriorate it.
Romanes gave expression to this idea, that the cessation of natural selection alone must cause the degeneration of a part, a decade before I did, but neither he nor the scientific world of his time attached great importance to it, and it was forgotten again. This was intelligible enough, for, at that time, the validity of the Lamarckian principle had not been called in question, and therefore the need for some other principle to explain the disappearance of disused parts had not begun to be felt.
I found myself in quite a different position. As my doubts regarding the Lamarckian principle grew greater and greater, I was obliged to seek for some other factor in modification, which should be sufficient to effect the degeneration of a disused part, and for a time I thought I had found this in panmixia, that is, in the mingling of all together, well and less well equipped alike. This factor does certainly operate, but the more I thought over it the clearer it became to me that there must be some other factor at work as well, for while panmixia might explain the deterioration of an organ, it could not explain its decrease in size, its gradual wearing away, and ultimate total disappearance. Yet this is the path followed, slowly indeed, but quite surely, by all organs which have become useless. If panmixia alone guided the deterioration of the organ, and it was thus only chance variations which were inherited through panmixia and gradually diffused over the whole species, how could it come about that all the variations were in the direction of smaller size? Yet this is obviously the case. Why should no variations in the direction of larger size occur? And if this were so, why should a useless organ not be maintained at its original size, if it be admitted that an increase in size would be prevented by natural selection? But this never occurs, and diminution in size is so absolutely the rule that the idea of a 'vestigial or rudimentary' organ suggests a 'small organ' almost more than an 'imperfect' one.
There must then be something else at work which causes the minus-variations in a disused organ to preponderate persistently and permanently over the plus-variations, and this something can lie nowhere else than where the roots of all hereditary variations are to be found—in the germ-plasm. This train of thought leads us to the discovery of a process which we must call selection between the elements of the germ-plasm, or, as I have named it shortly, Germinal Selection.
If the substance of the germ-plasm is—as we assumed—composed of heterogeneous living particles, which have dissimilar rôles in the building up of the organism, there must of necessity be among them a definite labile state of equilibrium, which cannot be disturbed without modifying in some way the structure of the organism itself which arises from the germ-plasm. But if our further view be correct, that these individual and different living units of the germ-plasm are 'determinants,' that is, are the primary constituents of particular parts of the organism, in the sense that these parts could not arise if their determinants were absent from the germ-plasm, and that they would be different if the determinants were differently composed, we can draw far-reaching deductions.
It is true that we cannot learn anything directly in regard to the intimate structure of the germ-plasm, and even in regard to the vital processes going on within it we can only guess a very little, but so much we may say—that its living parts are nourished, and that they multiply. But it follows from this that nourishment in a dissolved state must penetrate between its vital particles, and that whether the determinants grow, and at what rate they do so, depends mainly on the amount of nourishment which reaches them. As long as the germ-cells multiply by division the determinants have no other function but to grow; a part of their substance undergoes oxidation and thereby yields the supply of energy necessary to assimilation, that is, to the formation of new living substance.
If each kind of determinant always secured the same quantity of nourishment, all would grow in the same degree, that is, in exact proportion to their power of assimilation. But we know that in less minute conditions which we can observe more directly, there is nowhere absolute equality, that all vital processes are subject to fluctuations; any little obstacles in the current of the nutritive fluid, or in its composition, may cause poorer nutrition of one part, better of another. We may therefore assume that there are similar irregularities and differences in the minute and unobservable conditions of the germ-plasm likewise, and the result must be a slight shifting of the position of equilibrium as regards size and strength in the determinant system; for the less well-nourished determinants will grow more slowly, will fail to attain to the size and strength of their neighbours, and will multiply more slowly.
But the vigour of growth does not depend only on the influence of nourishment; one cell grows quickly, another slowly in the same nutritive fluid; it depends in great part on the cell's power of assimilation. In the same way the assimilating power of the determinants and their affinity for nourishment will vary with their constitution, and a weaker determinant will remain smaller than a stronger one, even when the stream of nourishment is the same.
It seems to me that it is upon the unequal nutrition of the determinants conditioned by the chances of the food-supply that individual hereditary variability ultimately depends. If, for instance, the determinant A receives poorer nourishment at a particular time than the determinant B, it will grow more slowly, remain weaker, and then, when the germ-cells develop into an animal, the part to which it gives rise will be weaker than it usually is in other individuals.
These primary inequalities in the equipment of the determinants which are caused by a passing inequality in the food-stream are, of course, so slight that we are unable to observe their consequences. They must persist for a considerable time before they become observable, but they may persist for a long time, and their effect must then mount up, because every diminution in the strength of the determinant also signifies a lessened power of assimilation, and growth becomes slower for the twofold reason that passive and active nutrition decrease at the same time. In the less minute conditions observable in the histological elements of the body we know that function strengthens the organ, and that disuse weakens it, and we are justified in applying this proposition also to these more intimate conditions and minuter vital units. Thus, in the course of the multiplication of the germ-cells, the less vigorously working determinant, A, will gradually, but very slowly, become weaker, that is, of diminished power of assimilation, presupposing of course that the intra-germinal food-stream does not become stronger again at the same place—a possibility to which I shall subsequently refer. But while one determinant may be slowly becoming weaker, its neighbour, on the other hand, may be varying on an ascending scale, just because the former is, on account of its diminished power of assimilation, no longer able to exhaust completely the food-stream which flows to it.
The determinants are thus in constant motion, here ascending, there descending, and it is in these fluctuations of the equilibrium of the determinant-system that I see the roots of all hereditary variation, while in the fact that the variation-directions of particular determinants must continue the same without limit as long as they meet with no obstacle lies the possibility of the adaptation of the organism to changing conditions, the increase and transformation of one part, the degeneration and disappearance of another, in short, the processes of natural selection. The reason why such variation movements must continue until they meet some resistance is that every chance upward or downward movement—due, that is, to mere passive fluctuation in the food-supply, at the same time strengthens or weakens the determinant, and makes it either more or less capable of attracting nourishment to itself; in the former case an increasingly strong stream of food will be directed towards it, in the latter more and more of the available food-supply will be withdrawn from it by its neighbour-determinants on all sides; in the former the determinant will go on increasing in strength as long as it can go on attracting more nourishment, in the latter it will continue to become weaker until it disappears altogether. To the ascending progression, as is evident, there are limits set, not only by the amount of food which can circulate through the whole id, but also by the neighbour determinants, which will sooner or later resist the withdrawal of nourishment from them; but for the descending progression there are no limits except total disappearance, and this is actually reached in all cases in which the determinants are related to a part which has become useless. But both these movements, the upward and the downward alike, are quite independent of natural selection, i.e. of personal selection; they are processes of a unique kind which run their course purely in accordance with intra-germinal laws. Whether a determinant 'ascends' or 'descends' depends solely upon the play of forces within the germ-plasm, not upon whether the direction of the variation in question is useful or prejudicial, or on whether the organ in question, the determinate, is of value or otherwise. In this fact lies the great importance of this play of forces within the germ-plasm, that it gives rise to variations quite independently of the relations of the organism to the external world. In many cases, of course, personal selection intervenes, but even then it cannot directly effect the rising or falling of the individual determinants—these are processes quite outside of its influence—but it can, by eliminating the bearers of unfavourably varying determinants, set a limit to further advance in such directions. This we shall consider in more detail later on. Personal selection operates by removing unfavourably varying individuals from the genealogical tree of the species, but at the same time the determinants which are varying unfavourably are also removed, and their variation is thus put a stop to for all time.
I have called these processes which are ceaselessly going on within the germ-plasm, Germinal Selection, because they are analogous to those processes of selection which we already know in connexion with the larger vital units, cells, cell-groups and persons. If the germ-plasm be a system of determinants, then the same laws of struggle for existence in regard to food and multiplication must hold sway among its parts which hold sway between all systems of vital units—among the biophors which form the protoplasm of the cell-body, among the cells of a tissue, among the tissues of an organ, among the organs themselves, as well as among the individuals of a species and between species which compete with one another.
If this be the case, we have here ready to hand the explanation of every heritable variation of a part, ascending and descending alike. Let us consider for a little the latter category—that is, the disappearance of functionless or useless organs. It is clear that, from the moment in the life of a species that an organ, N, becomes useless, natural selection withdraws her hand from it; individuals with better or worse organs N are now equally capable of life and struggle, the state of panmixia is entered upon, and the organ N of necessity falls somewhat below its previously attained degree of perfection.
That this must be so will be admitted when it is remembered that each organ of a species is only maintained at its highest level because personal selection keeps ceaseless watch over it, and sets aside all the less favourable variations by eliminating the individuals which exhibit them. But this is no longer the case with a useless organ. When a weaker variant of a disused organ arises through the intra-germinal fluctuations of nutrition, this is transmitted to the descendants just as well as the normally developed organ, and in the course of generations will be inherited by a greater and greater number of individuals, and must ultimately be inherited by all in some degree or other. The objection has been urged from many sides that variations upwards would be quite as likely to arise as those downwards, but this is an error. Even if, at the beginning, the minus-variations were rarer than the plus-variations, in the course of generations the minus ones would preponderate because ascending variations of disused organs are not indifferent for the organism but injurious to it. Perhaps an increase in the size of the organ itself would do no harm, but in that of its determinant it certainly would, because an ascending determinant requires more nourishment than previously, and withdraws it from its surroundings, and thus from the determinants in its immediate neighbourhood; but these are those of functioning and indispensable parts. Individuals in whose germ-plasm the determinants of disused organs ascend, and thereby depress the determinants of organs which are still active, are subject to personal selection, and are eliminated. There thus remain only those with descending determinants; in other words, the chance of variants in the direction of weakness in useless determinants far outweighs that of variants in the direction of increased strength; the latter will soon cease to occur at all, for as soon as a determinant has fallen a little below its normal level, it finds itself upon an inclined plane, along which it glides very slowly but steadily downwards. This might be disputed if it could be maintained that, at every stage of the descent, a change of direction was possible. But this probably takes place rarely and only in the case of individual ids, and will therefore not be permanent because in general the stronger neighbour determinants will possess themselves of the superfluous nourishment, and a lasting ascent will thus be impossible to the weakened determinant. This is precisely what I have called Germinal Selection. The determinant whose assimilating power is weakened by ever so little is continually being robbed by its neighbours of a part of the nourishment which flows towards it, and must consequently become further weakened. As no more help will be given to it by natural selection, since the organ is no longer of any value to the species, the better among the weakened determinants of N are never selected out, and they must gradually give way in the struggle with the neighbouring determinants which are necessary to the species, becoming gradually weaker and ultimately disappearing.
This process can, of course, no more be proved mathematically than any other biological processes. No one who is unwilling to accept germinal selection can be compelled to do so, as he might be to accept the Pythagorean propositions. It is not built up from beneath upon axioms, but is an attempt at an explanation of a fact established by observation—the disappearance of disused parts. But when once the inheritance of functional modifications has been demonstrated to be a fallacy, and when it has been shown that, even with the assumption of such inheritance, the disappearance of parts which are only passively useful, and of any parts whatever in sterile animal forms, remains unexplained, he who rejects germinal selection must renounce all attempt at explanation. It is the same as in the case of personal selection. No one can demonstrate mathematically that any variation possesses selection value, but whoever rejects personal selection gives up hope of explaining adaptations, for these cannot be referred to purely internal forces of development.
The total disappearance of a part which has become useless takes place with exceeding slowness; the whales, which have existed as such since the beginning of the tertiary period, have even now not completely lost their hind-limbs, but carry them about with them as rudiments in the muscular mass of the trunk, and the birds, which are even older, still show in their embryonic primordia the five fingers of their reptilian forefathers, although even their bird-ancestors of the Jurassic period, if we may argue from Archæopteryx, had only three fingers like our modern birds. A long series of similar examples might be given, and modern embryology in particular has contributed much that, like this example of birds' fingers, points to a certain orderliness in the disappearance of the individual parts of an organ which has become superfluous. Parts which, in the complete animal, have disappeared without leaving a trace, appear again in each embryonic primordium, and disappear in the course of the ontogeny. Speaking metaphorically, we might express this on the basis of the determinant theory, by saying that the determinants, as they become weaker, can only control an increasingly short period of the whole ontogeny of the organ, so that ultimately nothing more than its first beginning comes into existence. But this is only a metaphor; we cannot tell what really happens as long as we are ignorant of the physiological rôle of the determinants, and even of the laws governing the degeneration of a useless organ. In respect of the latter, much might still be achieved if comparative anatomy and embryology were studied with this definite end in view, and perhaps we should even be able to draw more definite conclusions in regard to the composition and activity of the determinants in the germ.
In the meantime we must be content with the knowledge that, on the determinant hypothesis, the disappearance of organs which have become useless may be regarded as a process of intra-selection going on between the 'primary constituents' (Anlagen) of the germ, and depending on the same principle of the 'struggle of parts' which William Roux introduced into science with such brilliant results. If a struggle for food and space actually takes place, then every passive weakening must lead to a permanent condition of weakness and a lasting and irretrievable diminution in the size and strength of the primary constituent concerned, unless personal selection intervenes, and choosing out the strongest among these weakened primary constituents, raises them again to their former level. But this never happens when the organ has become useless.
This explains why not only parts with active function, like limbs, muscles, tendons, nerves, and glands, disappear when they cease to function, but also passive parts like the colouring of the external surfaces of animals, the lifeless skeletal parts of Arthropods and the exact adaptation of their thickness to the dwindling function, the disappearance of superfluous wing-veins, and of the hard chitinous covering of the abdomen when it is concealed in a protecting house, as in the case of hermit-crabs, Phryganidæ, and Psychidæ. Here too we find a sufficient explanation of the fact that parts which have become functionless, such as the wings of ants, can disappear even in the case of sterile workers.
The principle of germinal selection, however, can only be understood in its full significance if we take the positive aspect also into consideration. We had reached the conclusion that because of the fluctuations of the food-supply one set of the homologous determinants represented in the various ids may vary in a minus direction, and another set in a plus direction, and that this direction will be adhered to as long as no intra-germinal obstacles come in the way. As long as this does not happen the determinant concerned will pursue the path of variation it has once struck out, and indeed the tendency will be strengthened, because every passive variation, upwards or downwards, results in a strengthening or weakening of the determinant's power of assimilation.
Let us take a case of positive variation of the determinants of an organ N, which would be more useful to the species if it were more highly developed than it had previously been. The variation in an upward direction is at first purely passive, having arisen from fluctuations in the food-supply, but it soon becomes active, since the determinants that have become stronger will have a stronger affinity for food and will attract more and more of the available supply. The increased food-stream is thus maintained, and its gradual result is such a strengthening of the determinants in the course of generations of germ-cells, that the parts controlled by these determinants—the determinates—must enter on a path of plus-variations. If to this there be added personal selection, either natural or artificial, any fluctuations of this primary constituent towards the minus side will be effectually prevented, the direction of variation will remain positive, and the continued intervention of personal selection may raise its development to its possible maximum, that is, so far that further development in the same direction would not make for greater fitness, and personal selection must call a halt. This will always happen as soon as further increase of the organ would be prejudicial to the living power of the whole, and when the harmony of the bodily parts would thereby be permanently disturbed.
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.
Fig. 107, C. Leptocephalus stage of an American Eel, with seven pigment spots, of which three are on the left (l) and four on the right (r) side. After Eigenmann.
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.