Chapter XLV: Appendix: B (1)

THE INADEQUACY OF NATURAL SELECTION, ETC., ETC.

[_In this Appendix are included four essays originally published in the_ Contemporary Review _and subsequently republished as pamphlets. The first appeared under the above title in February and March, 1893; the second in May of that year under the title "Prof. Weismann's Theories;" the third in December of that year under the title "A Rejoinder to Prof. Weismann;" and the fourth in October, 1894, under the title "Weismannism Once More." As these successive essays practically form parts of one whole, I have thought it needless to keep them separate by repeating their titles, and have simply marked them off from one another by the numbers I, II, III, IV. Of course, as they are components of a controversy, some incompleteness arises from the absence of the essays to which portions of them were replies; but in each the course of the argument sufficiently indicates the counter-arguments which were met._]

I.

Students of psychology are familiar with the experiments of Weber on the sense of touch. He found that different parts of the surface differ widely in their ability to give information concerning the things touched. Some parts, which yielded vivid sensations, yielded little or no knowledge of the sizes or forms of the things exciting them; whereas other parts, from which there came sensations much less acute, furnished clear impressions respecting the tangible characters, even of relatively small objects. These unlikenesses of tactual discriminativeness he ingeniously expressed by actual measurements. Taking a pair of compasses, he found that if they were closed so nearly that the points were less than one-twelfth of an inch apart, the end of the forefinger could not perceive that there were two points: the two points seemed one. But when the compasses were opened so that the points were one-twelfth of an inch apart, then the end of the forefinger distinguished the two points. At the same time, he found that the compasses must be opened to the extent of two and a half inches, before the middle of the back could distinguish between two points and one. That is to say, as thus measured, the end of the forefinger has thirty times the tactual discriminativeness which the middle of the back has.

Between these extremes he found gradations. The inner surfaces of the second joints of the fingers can distinguish separateness of positions only half as well as the tip of the forefinger. The innermost joints are still less discriminating, but have powers of discrimination equal to that of the tip of the nose. The end of the great toe, the palm of the hand, and the cheek, have alike one-fifth of the perceptiveness which the tip of the forefinger has; and the lower part of the forehead has but one-half that possessed by the cheek. The back of the hand and the crown of the head are nearly alike in having but a fourteenth or a fifteenth of the ability to perceive positions as distinct, which is possessed by the finger-end. The thigh, near the knee, has rather less, and the breast less still; so that the compasses must be opened more than an inch and a half before the breast distinguishes the two points from one another.

What is the meaning of these differences? How, in the course of evolution, have they been established? If "natural selection," or survival of the fittest, is the assigned cause, then it is required to show in what way each of these degrees of endowment has advantaged the possessor to such extent that not infrequently life has been directly or indirectly preserved by it. We might reasonably assume that in the absence of some differentiating process, all parts of the surface would have like powers of perceiving relative positions. They cannot have become widely unlike in perceptiveness without some cause. And if the cause alleged is natural selection, then it is necessary to show that the greater degree of the power possessed by this part than by that, has not only conduced to the maintenance of life, but has conduced so much that an individual in whom a variation has produced better adjustment to needs, thereby maintained life when some others lost it; and that among the descendants inheriting this variation, there was a derived advantage such as enabled them to multiply more than the descendants of individuals not possessing it. Can this, or anything like this, be shown?

That the superior perceptiveness of the forefinger-tip has thus arisen, might be contended with some apparent reason. Such perceptiveness is an important aid to manipulation, and may have sometimes given a life-saving advantage. In making arrows or fish-hooks, a savage possessing some extra amount of it may have been thereby enabled to get food where another failed. In civilized life, too, a sempstress with well-endowed finger-ends might be expected to gain a better livelihood than one with finger-ends which were obtuse; though this advantage would not be so great as appears. I have found that two ladies whose finger-ends were covered with glove-tips, reducing their sensitiveness from one-twelfth of an inch between compass-points to one-seventh, lost nothing appreciable of their quickness and goodness in sewing. An experience of my own here comes in evidence. Towards the close of my salmon-fishing days I used to observe what a bungler I had become in putting on and taking off artificial flies. As the tactual discriminativeness of my finger-ends, recently tested, comes up to the standard specified by Weber, it is clear that this decrease of manipulative power, accompanying increase of age, was due to decrease in the delicacy of muscular co-ordination and sense of pressure--not to decrease of tactual discriminativeness. But not making much of these criticisms, let us admit the conclusion that this high perceptive power possessed by the forefinger-end may have arisen by survival of the fittest; and let us limit the argument to the other differences.

How about the back of the trunk and its face? Is any advantage derived from possession of greater tactual discriminativeness by the last than the first? The tip of the nose has more than three times the power of distinguishing relative positions which the lower part of the forehead has. Can this greater power be shown to have any advantage? The back of the hand has scarcely more discriminative ability than the crown of the head, and has only one-fourteenth of that which the finger-tip has. Why is this? Advantage might occasionally be derived if the back of the hand could tell us more than it does about the shapes of the surfaces touched. Why should the thigh near the knee be twice as perceptive as the middle of the thigh? And, last of all, why should the middle of the forearm, middle of the thigh, middle of the back of the neck, and middle of the back, all stand on the lowest level, as having but one-thirtieth of the perceptive power which the tip of the forefinger has? To prove that these differences have arisen by natural selection, it has to be shown that such small variation in one of the parts as might occur in a generation--say one-tenth extra amount--has yielded an appreciably greater power of self-preservation; and that those inheriting it have continued to be so far advantaged as to multiply more than those who, in other respects equal, were less endowed with this trait. Does any one think he can show this?

But if this distribution of tactual perceptiveness cannot be explained by survival of the fittest, how can it be explained? The reply is that, if there has been in operation a cause which it is now the fashion among biologists to ignore or deny, these various differences are at once accounted for. This cause is the inheritance of acquired characters. As a preliminary to setting forth the argument showing this, I have made some experiments.

It is a current belief that the fingers of the blind, more practised in tactual exploration than the fingers of those who can see, acquire greater discriminativeness: especially the fingers of those blind who have been taught to read from raised letters. Not wishing to trust to this current belief, I recently tested two youths, one of fifteen and the other younger, at the School for the Blind in Upper Avenue Road, and found the belief to be correct. I found that instead of being unable to distinguish between points of the compasses until they were opened to one-twelfth of an inch apart, both of them could distinguish between points when only one-fourteenth of an inch apart. They had thick and coarse skins; and doubtless, had the intervening obstacle, so produced, been less, the discriminative power would have been greater. It afterwards occurred to me that a better test would be furnished by those whose finger-ends are exercised in tactual perceptions, not occasionally, as by the blind in reading, but all day long in pursuit of their occupations. The facts answered expectation. Two skilled compositors, on whom I experimented, were both able to distinguish between points when they were only one-seventeenth of an inch apart. Thus we have clear proof that constant exercise of the tactual nervous structure leads to further development.[102]

Now if acquired structural traits are inheritable, the various contrasts above set down are obvious consequences; for the gradations in tactual perceptiveness correspond with the gradations in the tactual exercises of the parts. Save by contact with clothes, which present only broad surfaces having but slight and indefinite contrast, the trunk has scarcely any converse with external bodies, and it has but small discriminative power; but what discriminative power it has is greater on its face than on its back, corresponding to the fact that the chest and abdomen are much more frequently explored by the hands: this difference being probably in part inherited from inferior creatures; for, as we may see in dogs and cats, the belly is far more accessible to feet and tongue than the back. No less obtuse than the back are the middle of the back of the neck, the middle of the forearm, and the middle of the thigh; and these parts have but rare experiences of irregular foreign bodies. The crown of the head is occasionally felt by the fingers, as also the back of one hand by the fingers of the other; but neither of these surfaces, which are only twice as perceptive as the back, is used with any frequency for touching objects, much less for examining them. The lower part of the forehead, though more perceptive than the crown of the head, in correspondence with a somewhat greater converse with the hands, is less than one-third as perceptive as the tip of the nose; and manifestly, both in virtue of its relative prominence, in virtue of its contacts with things smelt at, and in virtue of its frequent acquaintance with the handkerchief, the tip of the nose has far greater tactual experience. Passing to the inner surfaces of the hands, which, taken as wholes, are more constantly occupied in touching than are the back, breast, thigh, forearm, forehead, or back of the hand, Weber's scale shows that they are much more perceptive, and that the degrees of perceptiveness of different parts correspond with their tactual activities. The palms have but one-fifth the perceptiveness possessed by the forefinger-ends; the inner surfaces of the finger-joints next the palms have but one-third; while the inner surfaces of the second joints have but one-half. These abilities correspond with the facts that whereas the inner parts of the hand are used only in grasping things, the tips of the fingers come into play not only when things are grasped, but when such things, as well as smaller things, are felt at or manipulated. It needs but to observe the relative actions of these parts in writing, in sewing, in judging textures, &c., to see that above all other parts the finger-ends, and especially the forefinger-ends, have the most multiplied experiences. If, then, it be that the extra perceptiveness acquired from actual tactual activities, as in a compositor, is inheritable, these gradations of tactual perceptiveness are explained.

Doubtless some of those who remember Weber's results, have had on the tip of the tongue the argument derived from the tip of the tongue. This part exceeds all other parts in power of tactual discrimination: doubling, in that respect, the power of the forefinger-tip. It can distinguish points that are only one-twenty-fourth of an inch apart. Why this unparalleled perceptiveness? If survival of the fittest be the ascribed cause, then it has to be shown what the advantages achieved have been; and, further, that those advantages have been sufficiently great to have had effects on the maintenance of life.

Besides tasting, there are two functions conducive to life, which the tongue performs. It enables us to move about food during mastication, and it enables us to make many of the articulations constituting speech. But how does the extreme discriminativeness of the tongue-tip aid these functions? The food is moved about, not by the tongue-tip, but by the body of the tongue; and even were the tip largely employed in this process, it would still have to be shown that its ability to distinguish between points one-twenty-fourth of an inch apart, is of service to that end, which cannot be shown. It may, indeed, be said that the tactual perceptiveness of the tongue-tip serves for detection of foreign bodies in the food, as plum-stones or as fish-bones. But such extreme perceptiveness is needless for the purpose. A perceptiveness equal to that of the finger-ends would suffice. And further, even were such extreme perceptiveness useful, it could not have caused survival of individuals who possessed it in slightly higher degrees than others. It needs but to observe a dog crunching small bones, and swallowing with impunity the sharp-angled pieces, to see that but a very small amount of mortality would be prevented.

But what about speech? Well, neither here can there be shown any advantage derived from this extreme perceptiveness. For making the _s_ and _z_, the tongue has to be partially applied to a portion of the palate next the teeth. Not only, however, must the contact be incomplete, but its place is indefinite--may be half an inch further back. To make the _sh_ and _zh_, the contact has to be made, not with the tip, but with the upper surface of the tongue; and must be an incomplete contact. Though, for making the liquids, the tip of the tongue and the sides of the tongue are used, yet the requisite is not any exact adjustment of the tip, but an imperfect contact with the palate. For the _th_, the tip is used along with the edges of the tongue; but no perfect adjustment is required, either to the edges of the teeth, or to the junction of the teeth with the palate, where the sound may equally well be made. Though for the _t_ and _d_ complete contact of the tip and edges of the tongue with the palate is required, yet the place of contact is not definite, and the tip takes no more important share in the action than the sides. Any one who observes the movements of his tongue in speaking, will find that there occur no cases in which the adjustments must have an exactness corresponding to the extreme power of discrimination which the tip possesses: for speech, this endowment is useless. Even were it useful, it could not be shown that it has been developed by survival of the fittest; for though perfect articulation is an aid, yet imperfect articulation has rarely such an effect as to impede a man in the maintenance of his life. If he is a good workman, a German's interchanges of _b's_ and _p's_ do not disadvantage him. A Frenchman who, in place of the sound of _th_, always makes the sound of _z_, succeeds as a teacher of music or dancing, no less than if he achieved the English pronunciation. Nay, even such an imperfection of speech as that which arises from cleft palate, does not prevent a man from getting on if he is capable. True, it may go against him as a candidate for Parliament, or as an "orator" of the unemployed (mostly not worth employing). But in the struggle for life he is not hindered by the effect to the extent of being less able than others to maintain himself and his offspring. Clearly, then, even if this unparalleled perceptiveness of the tongue-tip is required for perfect speech, such use is not sufficiently important to have been developed by natural selection.

How, then, is this remarkable trait of the tongue-tip to be accounted for? Without difficulty, if there is inheritance of acquired characters. For the tongue-tip has, above all other parts of the body, unceasing experiences of small irregularities of surface. It is in contact with the teeth, and either consciously or unconsciously is continually exploring them. There is hardly a moment in which impressions of adjacent but different positions are not being yielded to it by either the surfaces of the teeth or their edges; and it is continually being moved about from some of them to others. No advantage is gained. It is simply that the tongue's position renders perpetual exploration almost inevitable; and by perpetual exploration is developed this unique power of discrimination. Thus the law holds throughout, from this highest degree of perceptiveness of the tongue-tip to its lowest degree on the back of the trunk; and no other explanation of the facts seems possible.

"Yes, there is another explanation," I hear some one say: "they may be explained by _panmixia_." Well, in the first place, as the explanation by _panmixia_ implies that these gradations of perceptiveness have been arrived at by the dwindling of nervous structures, there lies at the basis of the explanation an unproved and improbable assumption; and, in the second place, even were there no such difficulty, it may with certainty be denied that _panmixia_ can furnish an explanation. Let us look at its pretensions.

* * * * *

It was not without good reason that Bentham protested against metaphors. Figures of speech in general, valuable as they are in poetry and rhetoric, cannot be used without danger in science and philosophy. The title of Mr. Darwin's great work furnishes us with an instance of the misleading effects produced by them. It runs:--_The Origin of Species by means of Natural Selection, or the Preservation of favoured Races in the Struggle for Life_. Here are two figures of speech which conspire to produce an impression more or less erroneous. The expression "natural selection" was chosen as serving to indicate some parallelism with artificial selection--the selection exercised by breeders. Now selection connotes volition, and thus gives to the thoughts of readers a wrong bias. Some increase of this bias is produced by the words in the second title, "favoured races;" for anything which is favoured implies the existence of some agent conferring a favour. I do not mean that Mr. Darwin himself failed to recognize the misleading connotations of his words, or that he did not avoid being misled by them. In chapter iv of the _Origin of Species_, he says that, considered literally, "natural selection is a false term," and that the personification of Nature is objectionable; but he thinks that readers, and those who adopt his views, will soon learn to guard themselves against the wrong implications. Here I venture to think that he was mistaken. For thinking this, there is the reason that even his disciple, Mr. Wallace--no, not his disciple, but his co-discoverer, ever to be honoured--has apparently been influenced by them. When, for example, in combating a view of mine, he says that "the very thing said to be impossible by variation and natural selection has been again and again effected, by variation and artificial selection," he seems clearly to imply that the processes are analogous, and operate in the same way. Now this is untrue. They are analogous only within certain narrow limits; and, in the great majority of cases, natural selection is utterly incapable of doing that which artificial selection does.

To see this it needs only to de-personalise Nature, and to remember that, as Mr. Darwin says, Nature is "only the aggregate action and product of many natural laws [forces]." Observe its relative shortcomings. Artificial selection can pick out a particular trait, and, regardless of other traits of the individuals displaying it, can increase it by selective breeding in successive generations. For, to the breeder or fancier, it matters little whether such individuals are otherwise well constituted. They may be in this or that way so unfit for carrying on the struggle for life, that were they without human care, they would disappear forthwith. On the other hand, if we regard Nature as that which it is, an assemblage of various forces, inorganic and organic, some favourable to the maintenance of life and many at variance with its maintenance--forces which operate blindly--we see that there is no such selection of this or that trait; but that there is a selection only of individuals which are, by the aggregate of their traits, best fitted for living. And here I may note an advantage possessed by the expression "survival of the fittest;" since this does not tend to raise the thought of any one character which, more than others, is to be maintained or increased; but tends rather to raise the thought of a general adaptation for all purposes. It implies the process which Nature can alone carry on--the leaving alive of those which are best able to utilize surrounding aids to life, and best able to combat or avoid surrounding dangers. And while this phrase covers the great mass of cases in which there are preserved well-constituted individuals, it also covers those special cases which are suggested by the phrase "natural selection," in which individuals succeed beyond others in the struggle for life, by the help of particular characters which conduce in important ways to prosperity and multiplication. For now observe the fact which here chiefly concerns us, that survival of the fittest can increase any serviceable trait, only if that trait conduces to prosperity of the individual, or of posterity, or of both, _in an important degree_. There can be no increase of any structure by natural selection unless, amid all the slightly varying structures constituting the organism, increase of this particular one is so advantageous as to cause greater multiplication of the family in which it arises than of other families. Variations which, though advantageous, fail to do this, must disappear again. Let us take a case.

Keenness of scent in a deer, by giving early notice of approaching enemies, subserves life so greatly that, other things equal, an individual having it in an unusual degree is more likely than others to survive; and, among descendants, to leave some similarly endowed or more endowed, who again transmit the variation with, in some cases, increase. Clearly this highly useful power may be developed by natural selection. So also, for like reasons, may quickness of vision and delicacy of hearing; though it may be remarked in passing that since this extra sense-endowment, serving to give early alarm, profits the herd as a whole, which takes the alarm from one individual, selection of it is not so easy, unless it occurs in a conquering stag. But now suppose that one member of the herd--perhaps because of more efficient teeth, perhaps by greater muscularity of stomach, perhaps by secretion of more appropriate gastric juices--is enabled to eat and digest a not uncommon plant which the others refuse. This peculiarity may, if food is scarce, conduce to better self-maintenance, and better fostering of young if the individual is a hind. But unless this plant is abundant, and the advantage consequently great, the advantages which other members of the herd gain from other slight variations may be equivalent. This one has unusual agility, and leaps a chasm which others balk at. That one develops longer hair in winter, and resists the cold better. Another has a skin less irritated by flies, and can graze without so much interruption. Here is one which has an unusual power of detecting food under the snow; and there is one which shows extra sagacity in the choice of a shelter from wind and rain. That the variation giving ability to eat a plant before unutilized, may become a trait of the herd, and eventually of a variety, it is needful that the individual in which it occurs shall have more descendants, or better descendants, or both, than have the various other individuals severally having their small superiorities. If these other individuals severally profit by their small superiorities, and transmit them to equally large numbers of offspring, no increase of the variation in question can take place: it must soon be cancelled. Whether in the _Origin of Species_ Mr. Darwin has recognized this fact, I do not remember, but he has certainly done it by implication in his _Animals and Plants under Domestication_. Speaking of variations in domestic animals, he there says that "any particular variation would generally be lost by crossing, reversion, and the accidental destruction of the varying individuals, unless carefully preserved by man." (Vol. II, p. 292.) That which survival of the fittest does in cases like the one I have instanced, is to keep all faculties up to the mark, by destroying such individuals as have faculties in some respect below the mark; and it can produce development of some one faculty only if that faculty is predominantly important. It seems to me that many naturalists have practically lost sight of this, and assume that natural selection will increase _any_ advantageous trait. Certainly a view now held by some assumes as much.

The consideration of this view, to which the foregoing paragraph is introductory, may now be entered upon. This view concerns, not direct selection, but what has been called, in questionable logic, "reversed selection"--the selection which effects, not increase of an organ, but decrease of it. For as, under some conditions, it is of advantage to an individual and its descendants to have some structure of larger size, it may be, under other conditions--namely, when the organ becomes useless--of advantage to have it of smaller size; since, even if it is not in the way, its weight and the cost of its nutrition are injurious taxes on the organism. But now comes the truth to be emphasized. Just as direct selection can increase an organ only in certain cases, so can reversed selection decrease it only in certain cases. Like the increase produced by a variation, the decrease produced by one must be such as will sensibly conduce to preservation and multiplication. It is, for instance, conceivable that were the long and massive tail of the kangaroo to become useless (say by the forcing of the species into a mountainous and rocky habitat filled with brushwood), a variation which considerably reduced the tail might sensibly profit the individual in which it occurred; and, in seasons when food was scarce, might cause survival when individuals with large tails died. But the economy of nutrition must be considerable before any such result could occur. Suppose that in this new habitat the kangaroo had no enemies; and suppose that, consequently, quickness of hearing not being called for, large ears gave no greater advantage than small ones. Would an individual with smaller ears than usual, survive and propagate better than other individuals, in consequence of the economy of nutrition achieved? To suppose this is to suppose that the saving of a grain or two of protein per day would determine the kangaroo's fate.

Long ago I discussed this matter in the _Principles of Biology_ (§ 166), taking as an instance the decrease of the jaw implied by the crowding of the teeth, and now proved by measurement to have taken place. Here is the passage:--

"No functional superiority possessed by a small jaw over a large jaw, in
civilized life, can be named as having caused the more frequent survival
of small-jawed individuals. The only advantage which smallness of jaw
might be supposed to give, is the advantage of economized nutrition; and
this could not be great enough to further the preservation of men
possessing it. The decrease of weight in the jaw and co-operative parts
that has arisen in the course of many thousands of years, does not amount
to more than a few ounces. This decrease has to be divided among the many
generations that have lived and died in the interval. Let us admit that
the weight of these parts diminished to the extent of an ounce in a
single generation (which is a large admission); it still cannot be
contended that the having to carry an ounce less in weight, or having to
keep in repair an ounce less of tissue, could sensibly affect any man's
fate. And if it never did this--nay, if it did not cause a _frequent_
survival of small-jawed individuals where large-jawed individuals died,
natural selection could neither cause nor aid diminution of the jaw and
its appendages."

When writing this passage in 1864, I never dreamt that a quarter of a century later, the supposable cause of degeneration here examined and excluded as impossible, would be enunciated as an actual cause and named "reversed selection."

One of the arguments used to show the adequacy of natural selection under its direct or indirect form consists of a counter-argument to the effect that inheritance of functionally-wrought changes, supposing it to be operative, does not explain certain of the facts. This is alleged by Prof. Weismann as a part justification for his doctrine of Panmixia. Concerning the "blind fish and amphibia" found in dark places, which have but rudimentary eyes "hidden under the skin," he argues that "it is difficult to reconcile the facts of the case with the ordinary theory that the eyes of these animals have simply degenerated through disuse." After giving instances of rapid degeneration of disused organs, he argues that if "the effects of disuse are so striking in a single life, we should certainly expect, if such effects can be transmitted, that all traces of an eye would soon disappear from a species which lives in the dark." Doubtless this is a reasonable conclusion. To explain the facts on the hypothesis that acquired characters are inheritable, seems very difficult. One possible explanation may, indeed, be named. It appears to be a general law of organization that structures are stable in proportion to their antiquity--that while organs of relatively modern origin have but a comparatively superficial root in the constitution, and readily disappear if the conditions do not favour their maintenance, organs of ancient origin have deep-seated roots in the constitution, and do not readily disappear. Having been early elements in the type, and having continued to be reproduced as parts of it during a period extending throughout many geological epochs, they are comparatively persistent. Now the eye answers to this description as being a very early organ. But waiving possible explanations, let us take the particular instance cited by Prof. Weismann and see what is to be made of it. He writes:--

"The caverns in Carniola and Carinthia, in which the blind _Proteus_ and
so many other blind animals live, belong geologically to the Jurassic
formation; and although we do not exactly know when for example the
_Proteus_ first entered them, the low organization of this amphibian
certainly indicates that it has been sheltered there for a very long
period of time, and that thousands of generations of this species have
succeeded one another in the caves.

"Hence there is no reason to wonder at the extent to which the
degeneration of the eye has been already carried in the _Proteus_; even
if we assume that it is merely due to the cessation of the conserving
influence of natural selection."[103]

Let me first note a strange oversight on the part of Prof. Weismann. He points out that the caverns in question belong to the Jurassic formation: apparently intending to imply that they have an antiquity related to that of the formation. But there is no such relation, except that the caverns cannot be older than the formation. They may have originated at any period since the containing strata were deposited; and they may be therefore relatively modern. But passing over this, and admitting that the _Proteus_ has inhabited the caverns for an enormous period, what is to be said of the fact that their eyes have not disappeared entirely, as Prof. Weismann contends they should have done had the inheritance of the effects of disuse been all along operative? There is a very sufficient answer--the rudimentary eyes are not entirely useless. It seems that when the underground streams it inhabits are unusually swollen, some individuals of the species are carried out of the caverns into the open (being then sometimes captured). It is also said that the creatures shun the light; this trait being, I presume, observed when it is in captivity. Now obviously, among individuals carried out into the open, those which remain visible are apt to be carried off by enemies; whereas, those which, appreciating the difference between light and darkness, shelter themselves in dark places, survive. Hence the tendency of natural selection is to prevent the decrease of the eyes beyond that point at which they can distinguish between light and darkness. Thus the apparent anomaly is explained.

Let me suggest, as another possible reason for persistence of rudimentary organs, that the principle of economy of growth will cause diminution of them only in proportion as their constituents are of value for other uses in the organism; and that in many cases their constituents are practically valueless. Hence probably the reason why, in the case of stalk-eyed crustaceans, the eye is gone but the pedicle remains, or to use Mr. Darwin's simile, the telescope has disappeared but not its stand.

* * * * *

Along with that inadequacy of natural selection to explain changes of structure which do not aid life in important ways, alleged in § 166 of _The Principles of Biology_, a further inadequacy was alleged. It was contended that the relative powers of co-operative parts cannot be adjusted solely by survival of the fittest; and especially where the parts are numerous and the co-operation complex. In illustration it was pointed out that immensely developed horns, such as those of the extinct Irish elk, weighing over a hundred-weight, could not, with the massive skull bearing them, be carried at the extremity of the outstretched neck without many and great modifications of adjacent bones and muscles of the neck and thorax; and that without strengthening of the fore-legs, too, there would be failure alike in fighting and in locomotion. And it was argued that while we cannot assume spontaneous increase of all these parts proportionate to the additional strains, we cannot suppose them to increase by variations, one at once, without supposing the creature to be disadvantaged by the weight and nutrition of parts that were for the time useless--parts, moreover, which would revert to their original sizes before the other needful variations occurred.

When, in reply to me, it was contended that co-operative parts vary together, I named facts conflicting with this assertion--the fact that the blind cray-fish of the Kentucky caves have lost their eyes but not the foot-stalks carrying them; the fact that the normal proportion between tongue and beak in certain selected varieties of pigeons is lost; the fact that lack of concomitance in decrease of jaws and teeth in sundry kinds of pet dogs, has caused great crowding of the teeth ("The Factors of Organic Evolution," _Essays_, i, 401-402). And I then argued that if co-operative parts, small in number and so closely associated as these are, do not vary together, it is unwarrantable to allege that co-operative parts which are very numerous and remote from one another vary together. After making this rejoinder I enforced my argument by a further example--that of the giraffe. Tacitly recognizing the truth that the unusual structure of this creature must have been, in its most conspicuous traits, the result of survival of the fittest (since it is absurd to suppose that efforts to reach high branches could lengthen the legs), I illustrated afresh the obstacles to co-adaptation. Not dwelling on the objection that increase of any components of the fore-quarters out of adjustment to the others, would cause evil rather than good, I went on to argue that the co-adaptation of parts required to make the giraffe's structure useful, is much greater than at first appears. This animal has a grotesque gallop, necessitated by the great difference in length between the fore and the hind limbs. I pointed out that the mode of action of the hind limbs shows that the bones and muscles have all been changed in their proportions and adjustments; and I contended that, difficult as it is to believe that all parts of the fore-quarters have been co-adapted by the appropriate variations, now of this part now of that, it becomes impossible to believe that all the parts in the hind-quarters have been simultaneously co-adapted to one another and to all the parts of the fore-quarters: adding that want of co-adaptation, even in a single muscle, would cause fatal results when high speed had to be maintained while escaping from an enemy.

Since this argument, repeated with this fresh illustration, was published in 1886, I have met with nothing to be called a reply; and might, I think, if convictions usually followed proofs, leave the matter as it stands. It is true that, in his _Darwinism_, Mr. Wallace has adverted to my renewed objection, and, as already said, contended that changes such as those instanced can be effected by natural selection, since such changes can be effected by artificial selection: a contention which, as I have pointed out, assumes a parallelism that does not exist. But now, instead of pursuing the argument further along the same line, let me take a somewhat different line.

If there occurs some change in an organ, say by increase of its size, which adapts it better to the creature's needs, it is admitted that when, as commonly happens, the use of the organ demands the co-operation of other organs, the change in it will generally be of no service unless the co-operative organs are changed. If, for instance, there takes place such a modification of a rodent's tail as that which, by successive increases, produces the trowel-shaped tail of the beaver, no advantage will be derived unless there also take place certain modifications in the bulks and shapes of the adjacent vertebræ and their attached muscles, as well as, probably, in the hind limbs; enabling them to withstand the reactions of the blows given by the tail. And the question is, by what process these many parts, changed in different degrees, are co-adapted to the new requirements--whether variation and natural selection alone can effect the readjustment. There are three conceivable ways in which the parts may simultaneously change:--(1) they may all increase or decrease together in like degree; (2) they may all simultaneously increase or decrease independently, so as not to maintain their previous proportions, or assume any other special proportions; (3) they may vary in such ways and degrees as to make them jointly serviceable for the new end. Let us consider closely these several conceivabilities.

And first of all, what are we to understand by co-operative parts? In a general sense, all the organs of the body are co-operative parts, and are respectively liable to be more or less changed by change in any one. In a narrower sense, more directly relevant to the argument, we may, if we choose to multiply difficulties, take the entire framework of bones and muscles as formed of co-operative parts; for these are so related that any considerable change in the actions of some entails change in the actions of most others. It needs only to observe how, when putting out an effort, there goes, along with a deep breath, an expansion of the chest and a bracing up of the abdomen, to see that various muscles beyond those directly concerned are strained along with them. Or, when suffering from lumbago, an effort to lift a chair will cause an acute consciousness that not the arms only are brought into action, but also the muscles of the back. These cases show how the motor organs are so tied together that altered actions of some implicate others quite remote from them.

But without using the advantage which this interpretation of the words would give, let us take, as co-operative organs, those which are obviously such--the organs of locomotion. What, then, shall we say of the fore limbs and hind limbs of terrestrial mammals, which co-operate closely and perpetually? Do they vary together? If so, how have there been produced such contrasted structures as that of the kangaroo, with its large hind limbs and small fore limbs, and that of the giraffe, in which the hind limbs are small and the fore limbs large--how does it happen that, descending from the same primitive mammal, these creatures have diverged in the proportions of their limbs in opposite directions? Take, again, the articulate animals. Compare one of the lower types, with its rows of almost equal-sized limbs, and one of the higher types, as a crab or a lobster, with limbs some very small and some very large. How came this contrast to arise in the course of evolution, if there was the equality of variation supposed?

But now let us narrow the meaning of the phrase still further, giving it a more favourable interpretation. Instead of considering separate limbs as co-operative, let us consider the component parts of the same limb as co-operative, and ask what would result, from varying together. It would in that case happen that, though the fore and hind limbs of a mammal might become different in their sizes, they would not become different in their structures. If so, how have there arisen the unlikenesses between the hind legs of the kangaroo and those of the elephant? Or if this comparison is objected to, because the creatures belong to the widely different divisions of implacental and placental mammals, take the cases of the rabbit and the elephant, both belonging to the last division. On the hypothesis of evolution these are both derived from the same original form; but the proportions of the parts have become so widely unlike that the corresponding joints are scarcely recognized as such by the unobservant: at what seem corresponding places the legs bend in opposite ways. Equally marked, or more marked, is the parallel fact among the _Articulata_. Take that limb of the lobster which bears the claw and compare it with the corresponding limb in an inferior articulate animal, or the corresponding limb of its near ally, the rock lobster, and it becomes obvious that the component segments of the limb have come to bear to one another in the one case, proportions immensely different from those they bear in the other case. Undeniably, then, on contemplating the general facts of organic structure, we see that the concomitant variations in the parts of limbs, have not been of a kind to produce equal amounts of change in them, but quite the opposite--have been everywhere producing inequalities. Moreover, we are reminded that this production of inequalities among co-operative parts, is an essential principle of development. Had it not been so, there could not have been that progress from homogeneity of structure to heterogeneity of structure which constitutes evolution.

We pass now to the second supposition:--that the variations in co-operative parts occur irregularly, or in such independent ways that they bear no definite relations to one another--miscellaneously, let us say. This is the supposition which best corresponds with the facts. Glances at the faces around yield conspicuous proofs. Many of the muscles of the face and some of the bones, are distinctly co-operative; and these respectively vary in such ways as to produce in each person a different combination. What we see in the face we have reason to believe holds in the limbs and in all other parts. Indeed, it needs but to compare people whose arms are of the same lengths, and observe how stumpy are the fingers of one and how slender those of another; or it needs but to note the unlikenesses of gait of passers-by, implying small unlikenesses of structure; to be convinced that the relations among the variations of co-operative parts are anything but fixed. And now, confining our attention to limbs, let us consider what must happen if, by variations taking place miscellaneously, limbs have to be partially changed from fitness for one function to fitness for another function--have to be re-adapted. That the reader may fully comprehend the argument, he must here have patience while a good many anatomical details are set down.

Let us suppose a species of quadruped of which the members have, for immense past periods, been accustomed to locomotion over a relatively even surface, as, for instance, the "prairie-dogs" of North America; and let us suppose that increase of numbers has driven part of them into a region full of obstacles to easy locomotion--covered, say, by the decaying stems of fallen trees, such as one sees in portions of primeval forest. Ability to leap must then become a useful trait; and, according to the hypothesis we are considering, this ability will be produced by the selection of favourable variations. What are the variations required? A leap is effected chiefly by the bending of the hind limbs so as to make sharp angles at the joints, and then suddenly straightening them; as any one may see on watching a cat leap on to the table. The first required change, then, is increase of the large extensor muscles, by which the hind limbs are straightened. Their increases must be duly proportioned; for if those which straightened one joint become much stronger than those which straightened the other joint, the result must be collapse of the other joint when the muscles are contracted together. But let us make a large admission, and suppose these muscles to vary together; what further muscular change is next required? In a plantigrade mammal the metatarsal bones chiefly bear the reaction of the leap, though the toes may have a share. In a digitigrade mammal, however, the toes form almost exclusively the fulcrum, and if they are to bear the reaction of a higher leap, the flexor muscles which depress and bend them must be proportionately enlarged: if not, the leap will fail from want of a firm _point d'appui_. Tendons as well as muscles must be modified; and, among others, the many tendons which go to the digits and their phalanges. Stronger muscles and tendons imply greater strains on the joints; and unless these are strengthened, one or other, dislocation will be caused by a more vigorous spring. Not only the articulations themselves must be so modified as to bear greater stress, but also the numerous ligaments which hold the parts of each in place. Nor can the bodies of the bones remain unstrengthened; for if they have no more than the strengths needed for previous movements they will fail to bear more violent movements. Thus, saying nothing of the required changes in the pelvis, as well as in the nerves and blood-vessels, there are, counting bones, muscles, tendons, ligaments, at least fifty different parts in each hind leg which have to be enlarged. Moreover they have to be enlarged in unlike degrees. The muscles and tendons of the outer toes, for example, need not be added to so much as those of the median toes. Now, throughout their successive stages of growth, all these parts have to be kept fairly well balanced; as any one may infer on remembering sundry of the accidents he has known. Among my own friends I could name one who, when playing lawn-tennis, snapped the Achilles tendon; another who, while swinging his children, tore some of the muscular fibres in the calf of his leg; another who, in getting over a fence, tore a ligament of one knee. Such facts, joined with every one's experience of sprains, show that during the extreme exertions to which limbs are now and then subject, there is a giving way of parts not quite up to the required level of strength. How, then, is this balance to be maintained? Suppose the extensor muscles have all varied appropriately; their variations are useless unless the other co-operative parts have also varied appropriately. Worse than this. Saying nothing of the disadvantage caused by extra weight and cost of nutrition, they will be causes of mischief--causes of derangement to the rest by contracting with undue force. And then, how long will it take for the rest to be brought into adjustment? As Mr. Darwin says concerning domestic animals:--"Any particular variation would generally be lost by crossing, reversion, &c. ... unless carefully preserved by man." In a state of nature, then, favourable variations of these muscles would disappear again long before one or a few of the co-operative parts could be appropriately varied, much more before all of them could.

With this insurmountable difficulty goes a difficulty still more insurmountable--if the expression may be allowed. It is not a question of increased sizes of parts only, but of altered shapes of parts, too. A glance at the skeletons of mammals shows how unlike are the forms of the corresponding bones of their limbs; and shows that they have been severally re-moulded in each species to the different requirements entailed by its different habits. The change from the structures of hind limbs fitted only for walking and trotting to hind limbs fitted also for leaping, implies, therefore, that, along with strengthenings of bones there must go alterations in their forms. Now the fortuitous alterations of form which may take place in any bone are countless. How long, then, will it be before there takes place that particular alteration which will make the bone fitter for its new action? And what is the probability that the many required changes of shape, as well as of size, in bones will each of them be effected before all the others are lost again? If the probabilities against success are incalculable, when we take account only of changes in the sizes of parts, what shall we say of their incalculableness when differences of form also are taken into account?

"Surely this piling up of difficulties has gone far enough"; the reader will be inclined to say. By no means. There is a difficulty immeasurably transcending those named. We have thus far omitted the second half of the leap, and the provisions to be made for it. After ascent of the animal's body comes descent; and the greater the force with which it is projected up, the greater is the force with which it comes down. Hence, if the supposed creature has undergone such changes in the hind limbs as will enable them to propel it to a greater height, without having undergone any changes in the fore limbs, the result will be that on its descent the fore limbs will give way, and it will come down on its nose. The fore limbs, then, have to be changed simultaneously with the hind. How changed? Contrast the markedly bent hind limbs of a cat with its almost straight fore limbs, or contrast the silence of the spring on to the table with the thud which the fore paws make as it jumps off the table. See how unlike the actions of the hind and fore limbs are, and how unlike their structures. In what way, then, is the required co-adaptation to be effected? Even were it a question of relative sizes only, there would be no answer; for facts already given show that we may not assume simultaneous increases of size to take place in the hind and fore limbs; and, indeed, a glance at the various human races, which differ considerably in the ratios of their legs to their arms, shows us this. But it is not simply a question of sizes. To bear the increased shock of descent the fore limbs must be changed throughout in their structures. Like those in the hind limbs, the changes must be of many parts in many proportions; and they must be both in sizes and in shapes. More than this. The scapular arch and its attached muscles must also be strengthened and re-moulded. See, then, the total requirements. We must suppose that by natural selection of miscellaneous variations, the parts of the hind limbs will be co-adapted to one another, in sizes, shapes, and ratios; that those of the fore limbs will undergo co-adaptation similar in their complexity, but dissimilar in their kinds; and that the two sets of co-adaptations will be effected _pari passu_. If, as may be held, the probabilities are millions to one against the first set of changes being achieved, then it may be held that the probabilities are billions to one against the second being simultaneously achieved, in progressive adjustment to the first.