APPENDICES AND NOTES
APPENDIX I.
On Panmixia.
There are several points of considerable theoretical importance connected with Panmixia, which were omitted from the text, in order to avoid distracting attention from the main issue which is there under consideration. These side issues may now be appropriately presented in the form in which they were published in Nature, March 13, 1890[140]. After stating, in almost the same words, what has already been said in Chapter X, this paper proceeds, with the exception of a few verbal alterations, as follows.
"There is, however, one respect in which Professor Weismann's statement of the principle of panmixia differs from that which was considered by Mr. Darwin; and it is this difference of statement—which amounts to an important difference of theory—that I now wish to discuss.
"The difference in question is, that while Professor Weismann believes the cessation of selection to be capable of inducing degeneration down to the almost complete disappearance of a rudimentary organ, I have argued that, unless assisted by some other principle, it can at most only reduce the degenerating organ to considerably above one-half its original size—or probably not through so much as one-quarter. The ground of this argument (which is given in detail in the Nature articles of 1873-1874) is, that panmixia depends for its action upon fortuitous variations round an ever-diminishing average—the average thus diminishing because it is no longer sustained by natural selection. But although no longer sustained by natural selection, it does continue to be sustained by heredity; and therefore, as long as the force of heredity persists unimpaired, fortuitous variations alone—or variation which is no longer controlled by natural selection—cannot reduce the dwindling organ to so much as one-half of its original size; indeed, as above foreshadowed, the balance between the positive force of heredity and the negative effects of promiscuous variability will most likely be arrived at above the middle line thus indicated. Only if for any reason the force of heredity begins to fail can the average round which the cessation of selection works become a progressively diminishing average. In other words, so long as the original force of heredity as regards the useless organ remains unimpaired, the mere withdrawal of selection cannot reduce the organ much below the level of efficiency above which it was previously maintained by the presence of selection. If we take this level to be 80 or 90 per cent. of the original size, cessation of selection will reduce the organ through the 10 or 20 per cent., and there leave it fluctuating about this average, unless for any reason the force of heredity begins to fail—in which case, of course, the average will progressively fall in proportion to the progressive weakening of this force.
"Now, according to my views, the force of heredity under such circumstances is always bound to fail, and this for two reasons. In the first place, it must usually happen that when an organ becomes useless, natural selection as regards that organ will not only cease, but become reversed. For the organ is now absorbing nutriment, causing weight, occupying space, and so on, uselessly. Hence, even if it be not also a source of actual danger, 'economy of growth' will determine a reversal of selection against an organ which is now not merely useless, but deleterious. And this degenerating influence of the reversal of selection will throughout be assisted by the cessation of selection, which will now be always acting round a continuously sinking average. Nevertheless, a point of balance will eventually be reached in this case, just as it was in the previous case where the cessation of selection was supposed to be working alone. For, where the reversal of selection has reduced the diminishing organ to so minute a size that its presence is no longer a source of detriment to the organism, the cessation of selection will carry the reduction a small degree further; and then the organ will remain as a 'rudiment.' And so it will remain permanently, unless there be some further reason why the still remaining force of heredity should be abolished. This further (or second) reason I found in the consideration that, however enduring we may suppose the force of heredity to be, we cannot suppose that it is actually everlasting; and, therefore, that we may reasonably attribute the eventual disappearance of rudimentary organs to the eventual failure of heredity itself. In support of this view there is the fact that rudimentary organs, although very persistent, are not everlasting. That they should be very persistent is what we should expect, if the hold which heredity has upon them is great in proportion to the time during which they were originally useful, and thus firmly stamped upon the organization by natural selection causing them to be strongly inherited in the first instance. For example, we might expect that it would be more difficult finally to eradicate the rudiment of a wing than the rudiment of a feather; and accordingly we find it a general rule that long-enduring rudiments are rudiments of organs distinctive of the higher taxonomic divisions—i.e. of organs which were longest in building up, and therefore longest sustained in a state of working efficiency.
"Thus, upon the whole, my view of the facts of degeneration remains the same as it was when first published in these columns seventeen years ago, and may be summarized as follows.
"The cessation of selection when working alone (as it probably does during the first centuries of its action upon structures or colours which do not entail any danger to, or perceptible drain upon, the nutritive resources of the organism) cannot cause degeneration below, probably, some 10 to 20 per cent. But if from the first the cessation of selection has been assisted by the reversal of selection (on account of the degenerating structure having originally been of a size sufficient to entail a perceptible drain on the nutritive resources of the organism, having now become a source of danger, and so forth), the two principles acting together will continue to reduce the ever-diminishing structure down to the point at which its presence is no longer a perceptible disadvantage to the species. When that point is reached, the reversal of selection will terminate, and the cessation of selection will not then be able of itself to reduce the organ through more than at most a very few further percentages of its original size. But, after this point has been reached, the now total absence of selection, either for or against the organ, will sooner or later entail this further and most important consequence, a failure of heredity as regards the organ. So long as the organ was of use, its efficiency was constantly maintained by the presence of selection—which is merely another way of saying that selection was constantly maintaining the force of heredity as regards that organ. But as soon as the organ ceased to be of use, selection ceased to maintain the force of heredity; and thus, sooner or later, that force began to waver or fade. Now it is this wavering or fading of the force of heredity, thus originally due to the cessation of selection, that in turn co-operates with the still continued cessation of selection in reducing the structure below the level where its reduction was left by the actual reversal of selection. So that from that level downwards the cessation of selection, and the consequent failing of heredity, act and react in their common work of causing obsolescence. In the case of newly added characters, the force of heredity will be less than in that of more anciently added characters; and thus we can understand the long endurance of 'vestiges' characteristic of the higher taxonomic divisions, as compared with those characteristic of the lower. But in all cases, if time enough be allowed under the cessation of selection, the force of heredity will eventually fall to zero, when the hitherto obsolescent structure will finally become obsolete. In cases of newly added and comparatively trivial characters, with regard to which reversal of selection is not likely to take place (e.g. slight differences of colour between allied species), cessation of selection is likely to be very soon assisted by a failure in the force of heredity; seeing that such newly added characters will not be so strongly inherited as are the more ancient characters distinctive of higher taxonomic groups.
"Let us now turn to Weismann's view of degeneration. First of all, he has omitted to perceive that 'panmixia' alone (if unassisted either by reversed selection or an inherent diminishing of the force of heredity) cannot reduce a functionless organ to the condition of a rudiment. Therefore he everywhere represents panmixia (or the mere cessation of selection) as of itself sufficient to cause degeneration, say from 100 to 5, instead of from 100 to 90 or 80, which, for the reasons above given, appeared (and still appears) to me about the most that this principle can accomplish, so long as the original force of heredity continues unimpaired. No doubt we have here what must be regarded as a mere oversight on the part of Professor Weismann; but the oversight is rendered remarkable by the fact that he does invoke the aid of reversed selection in order to explain the final disappearance of a rudiment. Yet it is self-evident that the reversal of selection must be much more active during the initial than during the final stages of degeneration, seeing that, ex hypothesi, the greater the degree of reduction which has been attained the less must be the detriment arising from any useless expenditure of nutrition, &c.
"And this leads me to a second oversight in Professor Weismann's statement, which is of more importance than the first. For the place at which he does invoke the assistance of reversed selection is exactly the place at which reversed selection must necessarily have ceased to act. This place, as already explained, is where an obsolescent organ has become rudimentary, or, as above supposed, reduced to 5 per cent. of its original size; and the reason why he invokes the aid of reversed selection at this place is in order to save his doctrine of 'the stability of germ-plasm.' That the force of heredity should finally become exhausted if no longer maintained by the presence of selection, is what Darwin's theory of perishable gemmules would lead us to expect, while such a fact would be fatal to Weismann's theory of an imperishable germ-plasm. Therefore he seeks to explain the eventual failure of heredity (which is certainly a fact) by supposing that after the point at which the cessation of selection alone can no longer act (and which his first oversight has placed some 80 per cent. too low), the reversal of selection will begin to act directly against the force of heredity as regards the diminishing organ, until such direct action of reversed selection will have removed the organ altogether. Or, in his own words, 'The complete disappearance of a rudimentary organ can only take place by the operation of natural selection; this principle will lead to its diminution, inasmuch as the disappearing structure takes the place and the nutriment of other useful and important organs.' That is to say, the rudimentary organ finally disappears, not because the force of heredity is finally exhausted, but because natural selection has begun to utilize this force against the continuance of the organ—always picking out those congenital variations of the organ which are of smallest size, and thus, by its now reversed action, reversing the force of heredity as regards the organ.
"Now the oversight here is in not perceiving that the smaller the disappearing structure becomes, the less hold must 'this principle' of reversed selection retain upon it. As above observed, during the earlier stages of reduction (or while co-operating with the cessation of selection) the reversal of selection will be at its maximum of efficiency; and, as the process of diminution continues, a point must eventually be reached at which the reversal of selection can no longer act. Take the original mass of a now obsolescent organ in relation to that of the entire organism of which it then formed a part to be represented by the ratio 1:100. For the sake of argument we may assume that the mass of the organism has throughout remained constant, and that by 'mass' in both cases is meant capacity for absorbing nutriment, causing weight, occupying space, and so forth. Now, we may further assume that when the mass of the organ stood to that of its organism in the ratio of 1:100, natural selection was strongly reversed with respect to the organ. But when this ratio fell to 1:1000, the activity of such reversal must have become enormously diminished, even if it still continued to exercise any influence at all. For we must remember, on the one hand, that the reversal of selection can only act as long as the presence of a diminishing organ continues to be so injurious that variations in its size are matters of life and death in the struggle for existence; and, on the other hand, that natural selection in the case of the diminishing organ does not have reference to the presence and the absence of the organ, but only to such variations in its mass as any given generation may supply. Now, the process of reduction does not end even at 1:1000. It goes on to 1:10,000, and eventually 1:∞. Consequently, however great our faith in natural selection may be, a point must eventually come for all of us at which we can no longer believe that the reduction of an obsolescent organ is due to reversed selection. And I cannot doubt that if Professor Weismann had sufficiently considered the matter, he would not have committed himself to the statement that 'the complete disappearance of a rudimentary organ can only take place by the operation of natural selection.'
"According to my view, the complete disappearance of a rudimentary organ can only take place by the cessation of natural selection, which permits the eventual exhaustion of heredity, when heredity is thus simply left to itself. During all the earlier stages of reduction, the cessation of selection was assisted in its work by the reversal of selection; but when the rudiment became too small for such assistance any longer to be supplied, the rudiment persisted in that greatly reduced condition until the force of heredity with regard to it was eventually worn out. This appears to me, as it appeared in 1873, the only reasonable conclusion that can be drawn from the facts. And it is because this conclusion is fatal to Professor Weismann's doctrine of the permanent 'stability' of germ-plasm, while quite in accordance with all theories which belong to the family of pangenesis, that I deem the facts of degeneration of great importance as tests between these rival interpretations of the facts of heredity. It is on this account that I have occupied so much space with the foregoing discussion; and I shall be glad to ascertain whether any of the followers of Professor Weismann are able to controvert these views.
"George J. Romanes."
"P.S.—Since the above article was sent in, Professor Weismann has published in these columns (February 6) his reply to a criticism by Professor Vines (October 24, 1889). In this reply he appears to have considerably modified his views on the theory of degeneration; for while in his Essays he says (as in the passage above quoted) that 'the complete disappearance of a rudimentary organ can only take place by the operation of natural selection'—i.e. only by the reversal of selection,—in his reply to Professor Vines he says, 'I believe that I have proved that organs no longer in use become rudimentary, and must finally disappear, solely by 'panmixia'; not through the direct action of disuse, but because natural selection no longer sustains their standard structure'—i.e. solely by the cessation of selection. Obviously, there is here a flat contradiction. If Professor Weismann now believes that a rudimentary organ 'must finally disappear solely' through the withdrawal of selection, he has abandoned his previous belief that 'the complete disappearance of a rudimentary organ can only take place by the operation of selection.' And this change of belief on his part is a matter of the highest importance to his system of theories as a whole, since it betokens a surrender of his doctrine of the 'stability' of germ-plasm—or of the virtually everlasting persistence of the force of heredity, and the consequent necessity for a reversal of this force itself (by natural selection placing its premium on minus instead of on plus variations), in order that a rudimentary organ should finally disappear. In other words, it now seems he no longer believes that the force of heredity in one direction (that of sustaining a rudimentary organ) can only be abolished by the active influence of natural selection determining this force in the opposite direction (that of removing a rudimentary organ). It seems he now believes that the force of heredity, if merely left to itself by the withdrawal of natural selection altogether, will sooner or later become exhausted through the mere lapse of time. This, of course, is my own theory of the matter as originally published in these columns; but I do not see how it is to be reconciled with Professor Weismann's doctrine of so high a degree of stability on the part of germ-plasm, that we must look to the Protozoa and the Protophyta for the original source of congenital variations as now exhibited by the Metazoa and Metaphyta. Nevertheless, and so far as the philosophy of degeneration is concerned, I shall be very glad if (as it now appears) Professor Weismann's more recent contemplation has brought his principle of panmixia into exact coincidence with that of my cessation of selection."
Before passing on it may here be noted that, to any one who believes in the inheritance of acquired characters, there is open yet another hypothetical cause of degeneration, and one to which the final disappearance of vestigial organs may be attributed. Roux has shown in his work on The Struggle for Existence between Parts of an Organism that the principle of selection must operate in every constituent tissue, and as between every constituent cell of which an organism is composed. Now, if an organ falls into disuse, its constituent cells become worsted in their struggles with other cells in the organism. Hence, degeneration of the disused organ may progressively increase, quite independently of any struggle for existence on the part of the organism as a whole. Consequently, degeneration may proceed without any reference to the principle of "economized nutrition"; and, if it does so, and if the effects of its doing so are transmitted from generation to generation, the disused organ will finally disappear by means of Roux's principle.
The long communication above quoted led to a still longer correspondence in the pages of Nature. For Professor Ray Lankester wrote[141] to impugn the doctrine of panmixia, or cessation of selection, in toto, arguing with much insistence that "cessation of selection must be supplemented by economy of growth in order to produce the results attributed to panmixia." In other words, he denied that panmixia alone can cause degeneration in any degree at all; at most, he said, it can be but "a condition," or "a state," which occurs when an organ or part ceases to be useful, and therefore falls under the degenerating influence of active causes, such as economy of nutrition. Or, in yet other words, he refused to recognize that any degenerative process can be due to natural selection as merely withdrawn: only when, besides being withdrawn, natural selection is reversed, did he regard a degenerative process as possible. As a result of the correspondence, however, he eventually[142] agreed that, if the "birth-mean" of an organ, in respect either of size or complexity of structure, be lower than the "selection-mean" while the organ is useful (a fact which he does not dispute); then, if the organ ceases to be useful, it will degenerate by the withdrawal of selection alone. Which, of course, is merely a re-statement of the doctrine of panmixia, or cessation of selection, in somewhat varied terminology—provided that the birth-mean be taken over a number of generations, or not only over a few following the selection-mean of the structure while still in its highest state of efficiency. For the sake of brevity I will hereafter speak of these "few following" generations by the term of "first generations."
It remains to consider the views of Professor Lloyd Morgan upon the subject. In my opinion he is the shrewdest, as well as the most logical critic that we have in the field of Darwinian speculation; therefore, if possible, I should like to arrive at a full agreement with him upon this matter. His latest utterance with regard to it is as follows:—
"To account for the diminution of organs or structures no longer of use, apart from any inherited effects of disuse, Mr. Romanes has invoked the Cessation of Selection; and Mr. Francis Galton has, in another connexion, summarized the effects of this cessation of selection in the convenient phrase 'Regression to Mediocrity.' This is the Panmixia of Professor Weismann and his followers; but the phrase regression to mediocrity through the cessation of selection appears to me preferable. It is clear that so long as any organ or structure is subject to natural selection through elimination, it is, if not actually undergoing improvement, kept at a high standard of efficiency through the elimination of all those individuals in which the organ in question falls below the required standard. But if, from change in the environment or any other cause, the character in question ceases to be subject to selection, elimination no longer takes place, and the high standard will no longer be maintained. There will be reversion to mediocrity. The probable amount of this reversion is at present a matter under discussion[143]."
So far, then, Professor Lloyd Morgan is in complete agreement with previous writers upon the subject. He does not doubt that the cessation of selection must always be a cause of degeneration: the only question is as to the potency of this cause, or the amount of degeneration which it is capable of effecting.
Taking, first, the case of bulk or size of an organ, as distinguished from its organization or complexity, we have seen that Weismann represents the cessation of selection—even if working quite alone, or without any assistance from the reversal of selection—to be capable of reducing a fully developed organ to the state of a rudiment, or even, if we take his most recent view, of abolishing the organ in toto.
Professor Lloyd Morgan, on the other hand, does not think that the cessation of selection alone can cause reduction further than the level of "mediocrity" in the first generations—or, which is much the same thing, further than the difference between the "birth-mean" and the "selection-mean" of the first generations. This amount of reduction he puts at 5 per cent., as "a very liberal estimate."
Here, then, we have three estimates of the amount of degeneration which can be produced by panmixia alone, where mere size or bulk of an organ is concerned—say, 3 to 5 per cent., 10 to 20 per cent., and 95 per cent. to 0. At first sight, these differences appear simply ludicrous; but on seeking for the reasons of them, we find that they are due to different views touching the manner in which panmixia operates. The oversights which have led to Weismann's extremely high estimate have already been stated. The reason of the difference between the extremely low estimate of Professor Lloyd Morgan, as compared with my own intermediate one, is, that he supposes the power of panmixia to become exhausted as soon as the level of mediocrity of the first generations has become the general level in succeeding generations. In my view, however, the level of mediocrity is itself a sinking level in successive generations, with the result that there is no reason why the reducing power of panmixia should ever become exhausted, save that the more reduction it effects the greater is the force of heredity which remains to be overcome, as previously explained. Thus the only question between Professor Lloyd Morgan and myself is—Does the level of mediocrity fall in successive generations under the cessation of selection, or does it remain permanently where it used to be under the presence of selection? Does the "birth-mean" remain constant throughout any number of generations, notwithstanding that the sustaining influence of selection has been withdrawn; or does it progressively sink as a consequence of such withdrawal?
In order to answer this question we had better begin by considering now the case of organization of structure, as distinguished from mere size of structure. Take any case where a complex organ—such as a compound eye—has been slowly elaborated by natural selection, and is it not self-evident that, when natural selection is withdrawn, the complex structure will deteriorate? In other words, the level of mediocrity, say in the hundred thousandth generation after the sustaining influence of natural selection has been withdrawn, will not be so high as it was in the first generations. For, by hypothesis, there is now no longer any elimination of unfavourable variations, which may therefore perpetuate themselves as regards any of the parts of this highly complex mechanism; so that it is only a matter of time when the mechanism must become disintegrated. I can scarcely suppose that any one who considers the subject will question this statement, and therefore I will not say anything that might be said in the way of substantiating it. But, if the statement be assented to, it follows that there is no need to look for any cause of deterioration, further than the withdrawal of selection—or cessation of the principle which (as we are supposing) had hitherto been the sole means of maintaining efficient harmony among all the independently variable parts of the highly complex structure.
Now, I hold that the same thing is true, though in a lesser degree, as regards degeneration of size. That there is no difference in kind between the two cases, Professor Lloyd Morgan implicitly allows; for what he says is—
"In any long-established character, such as wing-power in birds, brain-development, the eyes of crustacea, &c., no shortcomer in these respects would have been permitted by natural selection to transmit his shortcomings for hundreds of generations. All tendency to such shortcomings would, one would suppose, have been bred out of the race. If after this long process of selection there still remains a strong tendency to deterioration, this tendency demands an explanation[144]."
Here, then, deterioration as to size of structure (wings of birds), and deterioration as to complexity of structure (brain and eyes) are expressly put upon the same footing. Therefore, if in the latter case the "tendency to deterioration" does not "demand an explanation," beyond the fact that the hitherto maintaining influence has been withdrawn, neither is any such further explanation demanded in the former case. Which is exactly my own view of the matter. It is also Mr. Galton's view. For although, in the passage formerly quoted, Professor Lloyd Morgan appears to think that by the phrase "Regression to Mediocrity" Mr. Galton means to indicate that panmixia can cause degeneration only as far as the mediocrity level of the first generations, this, in point of fact, is not what Galton means, nor is it what he says. The phrase in question occurs "in another connexion," and, indeed, in a different publication. But where he expressly alludes to the cessation of selection, this is what he says. The italics are mine.
"A special cause may be assigned for the effects of use in causing hereditary atrophy of disused parts. It has already been shown that all exceptionally developed organs tend to deteriorate: consequently, those that are not protected by selection will dwindle. The level of muscular efficiency in the wing of a strongly flying bird [curiously enough, the same case that is chosen by Professor Lloyd Morgan to illustrate his opposite view], is like the level of water in the leaky vessel of a Danaid, only secured to the race by constant effort, so to speak. Let the effort be relaxed ever so little, and the level immediately falls[145]."
I take it, then, that the burden of proof lies with Professor Lloyd Morgan to show why the withdrawal of selection is not sufficient to account for degeneration any further than the mediocrity-level in the former presence of selection. Why does "the strong tendency[146] to deterioration demand an explanation," further than the fact that when all variations below the average in every generation are allowed to survive, they must gradually lower the average itself through a series of generations? To answer that any such tendency "would have been bred out of the race" by the previous action of selection, is to suppose that the function of selection is at an end when once it has built up a structure to the highest point of working efficiency,—that the presence of selection is no longer required to maintain the structure at that point. But it is enough to ask in reply—Why, under the cessation of selection, does complexity of structure degenerate so much more rapidly than size of structure? Why is it, for instance, that "the eyes of crustacea" in dark caves have entirely disappeared, while their foot-stalks (when originally present) still remain? Can it be maintained that "for hundreds of generations" natural selection was more intent on developing the foot-stalks than the eyes which were mounted upon them—so that while the latter were left by selection with "a strong tendency to deterioration," the former have had this tendency "bred out in the race"[147]?
To sum up. There is now no question in any quarter touching the fact that panmixia, or the cessation of selection, is a true cause of degeneration. The only question is as to the amount of degeneration which it is able to effect when not assisted by the reversal of selection, or any other cause of degeneration. Moreover, even with regard to this question of amount, there is no doubt on any side that panmixia alone causes degeneration more rapidly where it has to do with complexity of organization, than it does where it is concerned with a mere reduction of mass.
The question as to the amount of degeneration that is caused by the cessation of selection alone is without any practical importance where species in a state of nature are concerned, because here the cessation of selection is probably always associated more or less with the reversal of it; and it is as impossible as it is immaterial to determine the relative shares which these two co-operating principles take in bringing about the observed results. But where organisms in a state of domestication are concerned, the importance of the question before us is very great. For if the cessation of selection alone is capable of reducing an organ through 10 or 12 per cent. of its original size, nearly all the direct evidence on which Darwin relied in favour of use-inheritance is destroyed. On the other hand, if reduction through 5 per cent. be deemed a "very liberal estimate" of what this principle can accomplish, the whole body of Darwin's direct evidence remains as he left it. I have now given my reasons for rejecting this lower estimate on the one band, and what seems to me the extravagant estimate of Weismann on the other. But my own intermediate estimate is enough to destroy the apparent proof of use-inheritance that was given by Darwin. Therefore it remains for those who deny Lamarckian principles, either to accept some such estimate, or else to acknowledge the incompatibility of any lower one with the opinion that there is no evidence in favour of these principles.
APPENDIX II.
On Characters as Adaptive and Specific.
It is the object of this Appendix to state, more fully than in the text, the opinions with regard to this subject which have been published by the two highest authorities on the theory of natural selection—Darwin and Professor Huxley. I will take first the opinion of Professor Huxley, quoted in extenso, and then consider it somewhat more carefully than seemed necessary in the text.
As far as I am aware, the only occasion on which Professor Huxley has alluded to the subject in question, is in his obituary notice of Darwin in the Proceedings of the Royal Society, Vol. XLIV, No. 269, p. xviii. The allusion is to my paper on Physiological Selection, in the Journal of the Linnæan Society, Zool. Vol. XIX, pp. 337-411. But it will be observed that the criticism has no reference to the theory which it is the object of that paper to set forth. It refers only to my definition of the theory of natural selection as primarily a theory of the origin, or cumulative development, of adaptations. This criticism, together with my answer thereto at the time, is conveyed in the following words.
"Every variety which is selected into a species is favoured and preserved in consequence of being, in some one or more respects, better adapted to its surroundings than its rivals. In other words, every species which exists, exists in virtue of adaptation, and whatever accounts for that adaptation accounts for the existence of the species. To say that Darwin has put forward a theory of the adaptation of species, but not of their origin, is therefore to misunderstand the first principles of the theory. For, as has been pointed out, it is a necessary consequence of the theory of selection that every species must have some one or more structural or functional peculiarities, in virtue of the advantage conferred by which it has fought through the crowd of its competitors, and achieved a certain duration. In this sense, it is true that every species has been 'originated' by selection."
Now, in the first place, I have nowhere said that "Darwin has put forward a theory of the adaptation of species, but not of their origin." I said, and continue to say, that he has put forward a theory of adaptations in general, and that where such adaptations appertain to species only (i.e. are peculiar to particular species), the theory becomes "also a theory of the origin of the species which present them." The only possible misunderstanding, therefore, which can here be alleged against me is, that I fail to perceive it as a "necessary consequence of the theory of selection that every species must have some one or more structural or functional peculiarities" of an adaptive or utilitarian kind. Now, if this is a misunderstanding, I must confess to not having had it removed by Mr. Huxley's exposition.
The whole criticism is tersely conveyed in the form of two sequent propositions—namely, "Every species which exists, exists in virtue of adaptation; and whatever accounts for that adaptation accounts for the existence of the species." My answer is likewise two-fold. First, I do not accept the premiss; and next, even if I did, I can show that the resulting conclusion would not overturn my definition. Let us consider these two points separately, beginning with the latter, as the one which may be most briefly disposed of.
I. Provisionally conceding that "every species which exists, exists in virtue of adaptation," I maintain that my definition of the theory of natural selection still holds good. For even on the basis of this concession, or on the ground of this assumption, the theory of natural selection is not shown to be "primarily" a theory of the origin of species. It follows, indeed, from the assumption—is, in fact, part and parcel of the assumption—that all species have been originated by natural selection; but why? Only because natural selection has originated those particular adaptive features in virtue of which (by the hypothesis) species exist as species. It is only in virtue of having created these features that natural selection has created the species presenting them—just as it has created genera, families, orders, &c., in virtue of other adaptive features extending through progressively wider areas of taxonomic division. Everywhere and equally this principle has been "primarily" engaged in the evolution of adaptations, and if one result of its work has been that of enabling the systematist to trace lines of genetic descent under his divisions of species, genera, and the rest, such a result is but "secondary" or "incidental."
In short, it is "primarily" a theory of adaptations wherever these occur, and only becomes "also" or "incidentally" a theory of species in cases where adaptations happen to be restricted in their occurrence to organic types of a certain order of taxonomic division.
II. Hitherto, for the sake of argument, I have conceded that, in the words of my critic, "it is a necessary consequence of the theory of selection that every species must have some one or more structural or functional peculiarities" of an adaptive kind. But now I will endeavour to show that this statement does not "follow as a necessary consequence" from "the theory of selection."
Most obviously "it follows" from the theory of selection that "every variety which is selected into a species is favoured and preserved in consequence of being, in some one or more respects, better adapted to its surroundings than its rivals." This, in fact, is no more than a re-statement of the theory itself. But it does not follow that "every species which exists, exists in virtue of adaptation" peculiar to that species; i.e. that every species which exists, exists in virtue of having been "selected." This may or may not be true as a matter of fact: as a matter of logic, the inference is not deducible from the selection theory. Every variety which is "selected into" a species must, indeed, present some such peculiar advantage; but this is by no means equivalent to saying, "in other words," that every variety which becomes a species must do so. For the latter statement imports a completely new assumption—namely, that every variety which becomes a species must do so because it has been "selected into" a species. In short, what we are here told is, that if we believe the selection principle to have given origin to some species, we must further believe, "as a necessary consequence," that it has given origin to all species.
The above reply, which is here quoted verbatim from Nature, Vol. 38, p. 616-18, proceeded to show that it does not belong to "the first principles of the theory of natural selection" to deny that no other cause than natural selection can possibly be concerned in the origin of species; and facts were given to prove that such unquestionably has been the case as regards the origin of "local" or "permanent" varieties. Yet such varieties are what Darwin correctly terms "incipient" species, or species in process of taking origin. Therefore, if Professor Huxley's criticism is to stand at all, we must accept it "as a necessary consequence of the theory of selection," that every such variety "which exists, exists in virtue of adaptation"—a statement which is proved to be untrue by the particular cases forthwith cited. But as this point has been dealt with much more fully in the text of the present treatise, I shall sum up the main points in a few words.
The criticism is all embodied in two propositions—namely, (a) that the theory of natural selection carries with it, as a "necessary consequence," the doctrine that survival of the fittest has been the cause of the origin of all species; and (b) that therefore it amounts to one and the same thing whether we define the theory as a theory of species or as a theory of adaptations. Now, as a mere matter of logical statement, it appears to me that both these propositions are unsound. As regards the first, if we hold with Darwin that other causes have co-operated with natural selection in the origination of some (i. e. many) species, it is clearly no part of the theory of natural selection to assume that none of these causes can ever have acted independently. In point of fact, as we have seen in the foregoing chapters, such has probably and frequently been the case under the influences of isolation, climate, food, sexual selection, and laws of growth; but I may here adduce some further remarks with regard to yet another possible cause. If the Lamarckian principles are valid at all, no reason can be shown why in some cases they may not have been competent of themselves to induce morphological changes of type by successive increments, until a transmutation of species is effected by their action alone—as, indeed, Weismann believes to have been the case with all the species of Protozoa[148]. That such actually has often been the case also with numberless species of Metozoa, is the belief of the neo-Lamarckians; and whether they are right or wrong in holding this belief, it is equally certain that, as a matter of logical reasoning, they are not compelled by it to profess any disbelief in the agency of natural selection. They may be mistaken as to the facts, as Darwin in a lesser degree may have been similarly mistaken; but just as Darwin has nowhere committed himself to the statement that all species must necessarily have been originated by natural selection, so these neo-Lamarckians are perfectly logical in holding that some species may have been wholly caused by the inheritance of acquired characters, as other species may have been wholly caused by the natural selection of congenital characters. In short, unless we begin by assuming (with Wallace and against Darwin) that there can be no other cause of the origin of species than that which is furnished by natural selection, we have no basis for Professor Huxley's statement "that every species has been originated by selection"; while, if we do set out with this assumption, we end in a mere tautology. What ought to be done is to prove the validity of this assumption; but, as Professor Huxley makes no attempt to do this, his criticism amounts to mere begging of the question.
And now, as regards the second point (b), even if we grant the assumption that natural selection is the only possible cause of the origin of species—or, which is the same thing, that every species has been originated by natural selection,—is it likewise the same thing whether we define the theory of natural selection as a theory of species or as a theory of adaptations? Professor Huxley's criticism endeavours to show that it is; but a little consideration is enough to show that it is not. What does follow from the assumption is, that, so far as specific characters are concerned, it is one and the same thing to say that the theory is a theory of species, and to say that it is a theory of adaptations. But specific characters are not conterminous with adaptive characters; for innumerable adaptive characters are not distinctive of species, but of genera, families, orders, classes, and sub-kingdoms. Therefore, if it is believed (as, of course, Professor Huxley believes) that the theory in question explains the evolution of all adaptive characters, obviously it is not one and the same thing to define it indifferently as a theory of species or as a theory of adaptations.
Now, all this is not merely a matter of logic chopping. On the contrary, the question whether we are to accept or to reject the deduction that all species must necessarily have owed their origin to natural selection, is a question of no small importance to the general theory of evolution. And our answer to this question must be determined by that which we give to the ulterior question—Is the theory of natural selection to be defined as a theory of species, or as a theory of adaptations?
We now pass on to our consideration of Darwin's opinion touching the question, as stated by himself,—"The doctrine of utility, how far true?" As I cannot ascertain that Darwin has anywhere expressed an opinion as to whether natural selection has been necessarily concerned in the origin of all species, the issue here is as to whether he held this with regard to all specific characters. It will be remembered that while opposing this doctrine as erroneous both in logic and in fact, I have represented that it is not a doctrine which Darwin sanctioned; but, on the contrary, that it is one which he expressly failed to sanction, by recognizing the frequent inutility of specific characters. Mr. Wallace, on the other hand, alleges that Darwin did believe in the universal—as distinguished from the general—utility of such characters. And he adds that he has "looked in vain in Mr. Darwin's works" for any justification of my statements to the contrary[149]. Therefore I will endeavour to show that Mr. Wallace's search has not been a very careful one.
We must remember, however, that it was not until the appearance of my paper on Physiological Selection, four years after Darwin's death, that the question now in debate was raised. Consequently, he never had occasion to deal expressly with this particular question—viz. whether "the doctrine of utility" has any peculiar reference to specific characters—as he surely would have done had he entertained the important distinction between specific and all other characters which Mr. Wallace now alleges that he did entertain. But, be this as it may, we cannot expect to find in Darwin's writings any express allusion to a question which had not been raised until 1886. The most we can expect to find are scattered sentences which prove that the distinction in question was never so much as present to his mind,—i. e. never occurred to him as even a possible distinction.
I will first take the passages which Mr. Wallace himself supplies from among those which I had previously indicated.
"But when, from the nature of the organism and of the conditions, modifications have been induced which are unimportant for the welfare of the species, they may be, and apparently often have been, transmitted in nearly the same state to numerous, otherwise modified, descendants[150]."
On this passage Mr. Wallace remarks that the last five words "clearly show that such characters are usually not 'specific,' in the sense that they are such as distinguish species from one another, but are found in numerous allied species." But I cannot see that the passage shows anything of the sort. What to my mind it does show is, (a) that Mr. Darwin repudiated Mr. Wallace's doctrine touching the necessary utility of all specific characters: (b) that he takes for granted the contrary doctrine touching the inutility of some specific characters: (c) that without in this place alluding to the proportional number of useless specific characters, he refers their origin in some cases to "the nature of the organism" (i.e. "spontaneous variability" due to internal causes), and in other cases to "the conditions" (i.e. variability induced by external causes): (d) that when established as a specific character by heredity, such a useless character was held by him not to tend to become obsolete by the influence of natural selection or any other cause; but, on the contrary, to be "transmitted in nearly the same state to numerous, otherwise modified, descendants"—or progeny of the species in genera, families, &c.: (e) and, therefore, that useless characters which are now distinctive of genera, families, &c., were held by him frequently, if not usually, to point to uselessness of origin, when first they arose as merely specific characters. Even the meaning which Mr. Wallace reads into this passage must imply every one of these points; and therefore I do not see that he gains much by apparently seeking to add this further meaning—viz. that in Darwin's opinion there must have been some unassignable reason preventing the occurrence of useless specific characters in cases where species are not destined to become the parents of genera.
Moreover, any such meaning is out of accordance with the context from which the passage is taken. For, after a long consideration of the question of utility, Darwin sums up,—"We thus see that with plants many morphological changes may be attributed to the laws of growth and the interaction of parts, independently of natural selection." And then he adds,—"From the fact of the above characters being unimportant for the welfare of the species, any slight variations which occurred in them would not have been augmented through natural selection." Again, still within the same passage, he says, while alluding to the causes other than natural selection which lead to changes of specific characters,—"If the unknown cause were to act almost uniformly for a length of time, we may infer that the result would be almost uniform; and in this case all the individuals of the species would be modified in the same manner." For my own part I do not understand how Mr. Wallace can have overlooked these various references to species, all of which occur on the very page from which he is quoting. The whole argument is to show that "many morphological changes may be attributed to the laws of growth and the inter-action of parts [plus external conditions of life], independently of natural selection"; that such non-adaptive changes, when they occur as "specific characters," may, if the species should afterwards give rise to genera, families, &c., become distinctive of these higher divisions. But there is nothing here, or in any other part of Darwin's writings, to countenance the inconsistent notion which Mr. Wallace appears to entertain,—viz. that species which present useless characters are more apt to give rise to genera, families, &c., than are species which do not present such characters.
The next passage which Mr. Wallace quotes, with his comments thereon, is as follows. The italics are his.
"'Thus a large yet undefined extension may safely be given to the direct and indirect results of natural selection; but I now admit, after reading the essay of Nägeli on plants, and the remarks by various authors with respect to animals, more especially those recently made by Professor Broca, that in the earlier editions of my Origin of Species I perhaps attributed too much to the action of natural selection, or the survival of the fittest. I have altered the fifth edition of the Origin so as to confine my remarks to adaptive changes of structure; but I am convinced, from the light gained during even the last few years, that very many structures which now appear to be useless, will hereafter be proved to be useful, and will therefore come within the range of natural selection. Nevertheless I did not formerly consider sufficiently the existence of structures which, as far as we can at present judge, are neither beneficial nor injurious; and this I believe to be one of the greatest oversights as yet detected in my work.'
Now it is to be remarked that neither in these passages nor in any of the other less distinct expressions of opinion on this question, does Darwin ever admit that "specific characters"—that is, the particular characters which serve to distinguish one species from another—are ever useless, much less that "a large proportion of them" are so, as Mr. Romanes makes him "freely acknowledge." On the other hand, in the passage which I have italicised he strongly expresses his view that much of what we suppose to be useless is due to our ignorance; and as I hold myself that, as regards many of the supposed useless characters, this is the true explanation, it may be well to give a brief sketch of the progress of knowledge in transferring characters from the one category to the other[151]."
It is needless to continue this quotation, because of course no one is disputing that an enormous number of specific characters whose utility is unknown are nevertheless useful, and therefore due to natural selection. In other words, the question is not—Are there not many useful specific characters whose utility is unknown? but—Does it follow from the theory of natural selection that all specific characters must necessarily be useful? Well, it appears to me that without going further than the above passage, which Mr. Wallace has quoted, we can see clearly enough what was Darwin's opinion upon the subject. He did not believe that it followed deductively from his theory that all specific characters must necessarily be useful; and therefore he regarded it as a question of fact—to be determined by induction as distinguished from deduction—in what proportional number of cases they are so. Moreover he gives it as his more matured opinion, that, "as far as we can at present judge" (i.e. from the present state of observation upon the subject: if, with Mr. Wallace, his judgement were a priori, why this qualification?), he had not previously sufficiently considered the existence of non-adaptive characters—and this he ended by believing was one of the greatest oversights as yet detected in his work. To me it has always seemed that this passage is one of the greatest exhibitions of candour, combined with solidity of judgement, that is to be met with even in the writings of Darwin. There is no talk about any deductive "necessity"; but a perfect readiness to allow that causes other than natural selection may have been at work in evoking non-adaptive characters, so that the fifth edition of the Origin of Species was altered in order to confine the theory of natural selection to "adaptive changes"—i.e. to constitute it, as I have said in other words, "a theory of the origin, or cumulative development, of adaptations."
If to this it be said that in the above passage there is no special mention of species, the quibble would admit of a three-fold reply. In the first place, the quibble in question had never been raised. As already stated, it is only since the appearance of my own paper on Physiological Selection that anybody ever thought of drawing a distinction between species and genera, such that while all specific characters must be held necessarily useful, no such necessity extends to generic characters. In the second place, that Darwin must have had specific characters (as well as generic) in his mind when writing the above passage, is rendered unquestionable by the fact that many of the instances of inutility adduced by Nägeli and Broca have reference to specific characters. Lastly, as shown in the passages previously quoted from the sixth edition of the Origin of Species, Darwin attributed the origin of useless generic characters to useless specific characters; so that Mr. Wallace really gains nothing by his remark that specific characters are not specially mentioned in the present passage.
Once more:—
"Darwin's latest expression of opinion on this question is interesting, since it shows he was inclined to return to his earlier view of the general, or universal, utility of specific characters[152]."
This "latest expression of opinion," as I shall immediately prove, shows nothing of the kind—being, in fact, a mere re-statement of the opinion everywhere and at all times expressed by Darwin, touching the caution that must be observed in deciding, with respect to individual cases, whether an apparently useless specific character is to be regarded as really useless. Moreover, at no time and in no place did Darwin entertain any "view of the general, or universal, utility of specific characters." But the point now is, that if (as was the case) Darwin "inclined" to depart more and more from his earlier view of the highly general utility of specific characters; and if (as was not the case) he ended by showing an inclination "to return" to this earlier view; what becomes of the whole of Mr. Wallace's contention against which this Appendix is directed, namely, that Darwin never entertained any other view than that of the "general, or universal, utility of specific characters"?
The "latest expression of opinion" which Mr. Wallace quotes, occurs in a letter written to Professor Semper in 1878. It is as follows:—
"As our knowledge advances, very slight differences, considered by systematists as of no importance in structure, are continually found to be functionally important; and I have been especially struck with this fact in the case of plants, to which my observations have of late years been confined. Therefore it seems to me rather rash to consider the slight differences between representative species, for instance those inhabiting the different islands of the same archipelago, as of no functional importance, and as not in any way due to natural selection[153]."
Now, with regard to this passage it is to be observed, as already remarked, that it refers to the formation of final judgements touching particular cases: there is nothing to show that the writer is contemplating general principles, or advocating on deductive grounds the dogma that specific characters must be necessarily and universally adaptive characters. Therefore, what he here says is neither more nor less than I have said. For I have always held that it would be "rather rash" to conclude that any given cases of apparent inutility are certainly cases of real inutility, merely on the ground that utility is not perceived. But this is clearly quite a distinct matter from resisting the a priori generalization that all cases of apparent inutility must certainly be cases of real utility. And, I maintain, in every part of his writings, without any exception, where Darwin alludes to this matter of general principle, it is in terms which directly contradict the deduction in question. As the whole of this Appendix has been directed to proving that such is the case, it will now, I think, be sufficient to supply but one further quotation, in order to show that the above "latest expression of opinion," far from indicating that in his later years Darwin "inclined" to Mr. Wallace's views upon this matter, is quite compatible with a distinct "expression of opinion" to the contrary, in a letter written less than six years before his death.
"In my opinion the greatest error which I have committed, has been not allowing sufficient weight to the direct action of the environment, i.e. food, climate, &c., independently of natural selection. Modifications thus caused, which are neither of advantage nor disadvantage to the modified organisms, would be especially favoured, as I can now see chiefly through your observations, by isolation in a small area, where only a few individuals lived under nearly uniform conditions[154]."
I will now proceed to quote further passages from Darwin's works, which appear to have escaped the notice of Mr. Wallace, inasmuch as they admit of no doubt regarding the allusions being to specific characters.
"We may easily err in attributing importance to characters, and in believing that they have been developed through natural selection. We must by no means overlook the effects of the definite action of changed conditions of life,—of so-called spontaneous variations, which seem to depend in a quite subordinate degree on the nature of the conditions,—of the tendency to reversion to long-lost characters,—of the complex laws of growth, such as of correlation[155], compensation, of pressure of one part on another, &c., and finally of sexual selection, by which characters of use to one sex are often gained and then transmitted more or less perfectly to the other sex, though of no use to this sex. But structures thus indirectly gained, although at first of no advantage to a species, may subsequently have been taken advantage of by its modified descendants, under new conditions of life and newly acquired habits[156]."
It appeared—and still appears—to me, that where so many causes are expressly assigned as producing useless specific characters, and that some of them (such as climatic influences and independent variability) must be highly general in their action, I was justified in representing it as Darwin's opinion that "a large proportional number of specific characters" are useless to the species presenting them, although afterwards they may sometimes become of use to genera, families, &c. Moreover, this passage goes on to point out that specific characters which at first sight appear to be obviously useful, are sometimes found by fuller knowledge to be really useless—a consideration which is the exact inverse of the argument from ignorance as used by Mr. Wallace, and serves still further to show that in Darwin's opinion utility is by no means an invariable, still less a "necessary," mark of specific character. The following are some of the instances which he gives.
"The sutures in the skulls of young mammals have been advanced as a beautiful adaptation for aiding parturition, and no doubt they may facilitate, or be indispensable for this act; but as sutures occur in the skulls of young birds and reptiles, which have only to escape from a broken egg, we may infer that this structure has arisen from the laws of growth, and has been taken advantage of in the parturition of the higher animals[157]."
"The naked skin on the head of a vulture is generally considered as a direct adaptation for wallowing in putridity; and so it may be, or it may possibly be due to the direct action of the putrid matter; but we should be very cautious in drawing any such inference [i.e. as to utility] when we see the skin on the head of the clean-feeding male Turkey is likewise naked[158]."
Similarly, in the Descent of Man it is said:—
"Variations of the same general nature have often been taken advantage of and accumulated through sexual selection in relation to the propagation of the species, and through natural selection in relation to the general purposes of life. Hence, secondary sexual characters, when equally transmitted to both sexes, can be distinguished from ordinary specific characters, only by the light of analogy. The modifications acquired through sexual selection are often so strongly pronounced that the two sexes have frequently been ranked as distinct species, or even as distinct genera[159]."
As Mr. Wallace does not recognize sexual selection, he incurs the burden of proving utility (in the life-preserving sense) in all these "frequently" occurring cases where there are such "strongly pronounced modifications," and we have already seen in the text his manner of dealing with this burden. But the point here is, that whether or not we accept the theory of sexual selection, we must accept it as Darwin's opinion—first, that in their beginnings, as specific characters, these sexual modifications were often of a merely "general nature" (or without reference to utility even in the life-embellishing sense), and only afterwards "have often been taken advantage of and accumulated through sexual selection": and, secondly, that "we know they have been acquired in some instances at the cost not only of inconvenience, but of exposure to actual dangers[160]."
We may now pass on to some further, and even stronger, expressions of opinion with regard to the frequent inutility of specific characters.
"I have made these remarks only to show that, if we are unable to account for the characteristic differences of our several domestic breeds, which nevertheless are generally admitted to have arisen through ordinary generation from one or a few parent stocks, we ought not to lay too much stress on our ignorance of the precise cause [i.e. whether natural selection or some other cause] of the slight analogous differences between true species.... I fully admit that many structures are now of no use to their possessors, and may never have been of any use to their progenitors; but this does not prove that they were formed solely for beauty or variety. No doubt the definite action of changed conditions, and the various causes of modification, lately specified, have all produced an effect, probably a great effect, independently of any advantage thus gained.... It is scarcely possible to decide how much allowance ought to be made for such causes of change, as the definite action of external conditions, so-called spontaneous variations, and the complex laws of growth; but, with these important exceptions, we may conclude that the structure of every living creature either now is, or formerly was, of some direct or indirect use to its possessor[161]."
Here again, if we remember how "important" these "exceptions" are, I cannot understand any one doubting Darwin's opinion to have been that a large proportional number of specific characters are useless. For that it is "species" which he here has mainly in his mind is evident from what he says when again alluding to the subject in his "Summary of the Chapter"—namely, "In many other cases [i.e. in cases where natural selection has not been concerned] modifications are probably the direct result of the laws of variation or of growth, independently of any good having been thus gained." Now, not only do these "laws" apply as much to species as they do to genera; "but," the passage goes on to say, "even such structures have often, we may feel assured, been subsequently taken advantage of, and still further modified, for the good of species under new conditions of life." Obviously, therefore, the inutility in such cases is taken to have been prior to any utility subsequently acquired; and genera are not historically prior to the species in which they originate.
Here is another quotation:—
"Thus, as I am inclined to believe, morphological differences, which we consider as important—such as the arrangement of the leaves, the divisions of the flower or of the ovarium, the position of the ovules, &c.—first appeared in many cases as fluctuating variations, which sooner or later became constant through the nature of the organism and of the surrounding conditions, as well as through the intercrossing of distinct individuals, but not through natural selection; for as these morphological characters do not affect the welfare of the species, any slight deviations in them could not have been governed or accumulated through this latter agency. It is a strange result which we thus arrive at, namely, that characters of slight vital importance to the species, are the most important to the systematist; but, as we shall hereafter see when we treat of the genetic principle of classification, this is by no means so paradoxical as it may at first appear[162]."
Clearly the view here expressed is that characters which are now distinctive of higher taxonomic divisions "first appeared" in the parent species of such divisions; for not only would it be unreasonable to attribute the rise and preservation of useless characters to "fluctuating variations" affecting a number of species or genera similarly and simultaneously; but it would be impossible that, if such were the case, they could be rendered "constant through the nature of the organism and of the surrounding conditions, as well as through the intercrossing of distinct individuals[163]."
Here is another passage to the same general effect. In alluding to the objection from inutility as advanced by Bronn, Broca, and Nägeli, Mr. Darwin says:—"There is much force in the above objection"; and, after again pointing out the important possibility in any particular cases of hidden or former use, and the action of the laws of growth, he goes on to say,—"In the third place, we have to allow for the direct and definite action of changed conditions of life, and for so-called spontaneous variations, in which the nature of the conditions plays quite a subordinate part[164]." Elsewhere he says,—"It appears that I formerly underrated the frequency and value of these latter forms of variation as leading to permanent modifications of structure independently of natural selection[165]." The "forms of variation" to which he here alludes are "variations which seem to us in our ignorance to arise spontaneously"; and it is evident that such variations cannot well "arise" in two or more species of a genus similarly and simultaneously, so as independently to lead "to permanent modifications of structure" in two or more parallel lines. It is further evident that by "spontaneous variations" Darwin alludes to extreme cases of spontaneous departure from the general average of specific characters; and therefore that lesser or more ordinary departures must be of still greater "frequency."
Again, speaking of the principles of classification, Darwin writes:—
"We care not how trifling a character may be—let it be the mere inflection of the angle of the jaw, the manner in which an insect's wing is folded, whether the skin be covered by hair or feathers—if it prevail throughout many and different species, especially those having very different habits of life, it assumes high value [i.e. for purposes of classification]; for we can account for its presence in so many forms with such different habits, only by inheritance from a common parent. We may err in this respect in regard to single points of structure, but when several characters, let them be ever so trifling, concur throughout a large group of beings having different habits, we may feel almost sure, on the theory of descent, that these characters have been inherited from a common ancestor; and we know that such aggregated characters have especial value in classification[166]."
Now it is evident that this argument for the general theory of evolution would be destroyed, if Wallace's assumption of utility of specific characters as universal were to be entertained. And the fact of apparently "trifling" characters occurring throughout a large group of beings "having different habits" is proof that they are really trifling, or without utilitarian significance.
It is needless to multiply these quotations, for it appears to me that the above are amply sufficient to establish the only point with which we are here concerned, namely, that Darwin's opinion on the subject of utility in relation to specific characters was substantially identical with my own. And this is established, not merely by the literal meaning of the sundry passages here gathered together from different parts of his writings; but likewise, and perhaps still more, from the tone of thought which pervades these writings as a whole. It requires no words of mine to show that the literal meaning of the above quotations is entirely opposed to Mr. Wallace's view touching the necessary utility of all specific characters; but upon the other point—or the general tone of Mr. Darwin's thought regarding such topics—it may be well to add two remarks.
In the first place, it must be evident that so soon as we cease to be bound by any a priori deduction as to natural selection being "the exclusive means of modifications," it ceases to be a matter of much concern to the theory of natural selection in what proportion other means of modification have been at work—especially when non-adaptive modifications are concerned, and where these have reference to merely "specific characters," or modifications of the most incipient kind, least generally diffused among organic types, and representing the incidence of causes of less importance than any others in the process of organic evolution considered as a whole. Consequently, in the second place, we find that Darwin nowhere displays any solicitude touching the proportional number of specific characters that may eventually prove to be due to causes other than natural selection. He takes a much wider and deeper view of organic evolution, and, having entirely emancipated himself from the former conception of species as the organic units, sees virtually no significance in specific characters, except in so far as they are also adaptive characters.
Such, at all events, appears to me the obvious interpretation of his writings when these are carefully read with a view to ascertaining his ideas upon "Utilitarian doctrine: how far true." And I make these remarks because it has been laid to my charge, that in quoting such passages as the above I have been putting "a strained interpretation" upon Darwin's utterances: "such admissions," it is said, "Mr. Romanes appears to me to treat as if wrung from a hostile witness[167]." But, from what has gone before, it ought to be apparent that I take precisely the opposite view to that here imputed. Far from deeming these and similar passages as "admissions wrung from a hostile witness," and far from seeking to put any "strained interpretation" upon them, I believe that they are but the plain and unequivocal expressions of an opinion which I have always understood that Darwin held. And if any one has been led to think otherwise, I throw back this charge of "strained interpretation," by challenging such a person to adduce a single quotation from any part of Darwin's works, which can possibly be held to indicate that he regarded passages like those above quoted as in any way out of conformity with his theory of natural selection—or as put forward merely to "admit the possibility of explanations, to which really, however, he did not attach much importance." To the best of my judgement it is only some bias in favour of Mr. Wallace's views that can lead a naturalist to view in this way the clear and consistent expression of Darwin's.
That Mr. Wallace himself should be biassed in this matter might, perhaps, be expected. After rendering the following very unequivocal passage from the Origin of Species (p. 72)—"There can be little doubt that the tendency to vary in the same manner has often been so strong, that all individuals of the same species have been similarly modified without the aid of any form of selection"—Mr. Wallace says, "But no proof whatever is offered of this statement, and it is so entirely opposed to all we know of the facts of variation as given by Darwin himself, that the important word 'all' is probably an oversight." But, if Mr. Wallace had read the very next sentence he would have seen that here the important word "all" could not possibly have been "an oversight." For the passage continues,—"Or only a third, fifth, or tenth part of the individuals may have been thus affected, of which fact several instances could be given. Thus Graba estimates that about one-fifth of the guillemots in the Faroe Islands consist of a variety so well marked, that it was formerly ranked as a distinct species under the name of Uria lacrymans." And even if this passage had not been thus specially concerned with the question of the proportion in which "individuals of the same species have been similarly modified without the aid of any form of selection" the oversight with respect to "the important word 'all'" would still have remained an oversight of a recurrent character, as the following additional quotations from other parts of Darwin's writings may perhaps render apparent.
"There must be some efficient cause for each slight individual difference, as well as for more strongly marked variations which occasionally arise; and if the unknown cause were to act persistently, it is almost certain that all the individuals of the species would be similarly modified[168]."
"The acquisition of a useless part can hardly be said to raise an organism in the natural scale.... We are so ignorant of the exciting cause of the above specified modifications; but if the unknown cause were to act almost uniformly for a length of time, we may infer that the result would be almost uniform; and in this case all the individuals of the species would be modified in the same manner[169]."
Moreover, when dealing even with such comparatively slight changes as occur between our domesticated varieties—and which, a fortiori, are less likely to become "stable" through the uniform operation of causes other than selection, seeing that they are not only smaller in amount than occurs among natural species, but also have had but a comparatively short time in which to accumulate—Darwin is emphatic in his assertion of the same principles. For instance, in the twenty-third chapter of the Variation of Plants and Animals under Domestication, he repeatedly uses the term "definite action of external conditions," and begins the chapter by explaining his use of the term thus:—
"By the term definite action, as used in this chapter, I mean an action of such a nature that, when many individuals of the same variety are exposed during several generations to any change in their physical conditions of life, all, or nearly all, the individuals are modified in the same manner. A new sub-variety would thus be produced without the aid of selection[170]."
As an example of the special instances that he gives, I may quote the following from the same work:—
"Each of the endless variations which we see in the plumage of our fowls must have had some efficient cause; and if the same cause were to act uniformly during a long series of generations on many individuals, all probably would be modified in the same manner."
And, as instances of his more general statements in Chapter XXIII, these may suffice:—
"The direct action of the conditions of life, whether leading to definite or indefinite results, is a totally distinct consideration from the effects of natural selection.... The direct and definite action of changed conditions, in contradistinction to the accumulation of indefinite variations, seems to me so important that I will give a large additional body of miscellaneous facts[171]."
Then, after giving these facts, and showing how in the case of species in a state of nature it is often impossible to decide how much we are to attribute to natural selection and how much to the definite action of changed conditions, he begins his general summary of the chapter thus:—
"There can be no doubt, from the facts given in the early part of this chapter, that extremely slight changes in the conditions of life sometimes act in a definite manner on our already variable domesticated productions [productions, therefore, with regard to which uniformity and 'stability' of modification are least likely to arise]; and, as the action Of changed conditions in causing general or indefinite variability is accumulative, so it may be with their definite action. Hence it is possible that great and definite modifications of structure may result from altered conditions acting during a long series of generations. In some few instances a marked effect has been produced quickly on all, or nearly all, the individuals which have been exposed to some considerable change of climate, food, or other circumstance[172]."
Once more, in order to show that he retained these views to the end of his life, I may quote a passage from the second edition of the Descent of Man, which is the latest expression of his opinion upon these points:—
"Each of the endless diversities in plumage, which we see in our domesticated birds, is, of course, the result of some definite cause; and under natural and more uniform conditions, some one tint, assuming that it was in no way injurious, would almost certainly sooner or later prevail. The free-intercrossing of the many individuals belonging to the same species would ultimately tend to make any change of colour thus induced uniform in character.... Can we believe that the very slight differences in tints and markings between, for instance, the female black-grouse and red-grouse serve as a protection? Are partridges as they are now coloured, better protected than if they had resembled quails? Do the slight differences between the females of the common pheasant, the Japan and golden pheasants, serve as a protection, or might not their plumage have been interchanged with impunity? From what Mr. Wallace has observed of the habits of certain gallinaceous birds in the East, he thinks that such slight differences are beneficial. For myself, I will only say, I am not convinced[173]."
Yet "convinced" he certainly must have been on merely a priori grounds, had he countenanced Mr. Wallace's reasoning from the general theory of natural selection; and the fact that he here fails to be convinced even by "what Mr. Wallace has observed of the habits of certain gallinaceous birds," appears to indicate that he had considered the question of utility with special reference to Mr. Wallace's opinion. That opinion was then, as now, the avowed result of a theoretical prepossession; and this prepossession, as the above quotations sufficiently show, was expressly repudiated by Darwin.
Lastly, this is not the only occasion on which Darwin expressly repudiates Mr. Wallace's opinion on the point in question. For it is notorious that these co-authors of the theory of natural selection have expressed divergent opinions concerning the origin by natural selection of the most general of all specific characters—cross-sterility. Although allowing that cross-sterility between allied species may be of adaptive value in "keeping incipient species from blending," Darwin persistently refused to be influenced by Wallace's belief that it is due to natural selection; i.e. the belief on which alone can be founded the "necessary deduction" with which we have been throughout concerned.