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Lecture 14. Directive Factors in Evolution: Selection.

§ 1. Selection the Central Idea in Darwinism. § 2. Logical Objections to Darwinism. § 3. Sentimental Recoil from Darwinism. § 4. Changes in Selection Theory since Darwin's Day. § 5. Scientific Critique of Selection Theory. § 6. Subtlety of Selection Theory.§ 7. Sexual Selection § 8. Selection and Progressiveness. § 9. Selectionist Interpretations and the Argument from Design.

§ 1. Selection the Central Idea in Darwinism.

THE central idea in Darwinism is the natural selection of the relatively fitter variants in the struggle for existence. Our understanding of Darwinism must therefore depend on our appreciation of what is implied in variation, in the struggle for existence, and in selection. A rough and ready-understanding of it is easy, but when we are dealing with living creatures that is apt to mean misunderstanding, and so it has been. By the hasty-minded, and by those more anxious to score points than to get at the truth, Darwinism has been persistently misunderstood. This has been largely due to trusting to second-hand impressions instead of going to Darwin's own works. Natural Selection may be described as the process by which, in the struggle for existence, certain variants of a species, marked from their fellows by the presence or absence of some innate character, are on that very account favoured with longer life or with more successful families than their neighbours, who are on that account sooner or later eliminated. Darwin stated his theory in a couple of sentences: “As many more individuals of each species are born than can possibly survive, and as, consequently, there is frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance any selected variety will tend to propagate its new and modified form.”

§ 2. Logical Objections to Darwinism.

From Darwinism there have been several hasty recoils, some logical and others sentimental, but both due to misunderstanding. Let us take first the logical, and second the emotional or sentimental recoils. At a later stage we shall consider the emendations of Darwinism which further investigation has necessitated—a very different matter.

(a) It is a misunderstanding of Darwinism to dwell on the fact that Natural Selection is not originative, only directive; that it is comparable to the action of pruning shears or of a sieve, not to the welling forth of a spring; that it corresponds to Siva, the destroyer, rather than to Brahma, the creator. That is quite true, but while Darwin sometimes spoke for brevity's sake of the creative work of Natural Selection, he made it quite clear that the sifting process could only operate on the raw materials which the variability of organisms brought within its scope. When we say that the strange shape of an evergreen in the garden is due to the gardener's shears, we do not forget the growing living plant. So when we say that the wing of a bird is the outcome of selection, we do not forget the varying organism, strong in endeavour. One forgives much to Samuel Butler in admiration of his genius, one forgives even the jibe that “Darwinism tries to explain how I am here by showing how my uncles, cousins, and aunts have gone away”. But it seems to us to promote misunderstanding when an expert writes in cold blood—“Darwin…left the question of variability open, a course which reduced his doctrine to the self-evident proposition that what was not capable of existence could not exist”…“Darwinism…explained how by throwing stones one could build houses of typical style” (Driesch, History and Theory of Vitalism. Trans. London, 1914).

(b) It also promotes misunderstanding to make very much of the fact that Natural Elimination is often a more accurate phrase than Natural Selection. A wonder-working gardener like Mr. Luther Burbank actively selects and fosters variants that catch his eye and seem to him to be promiseful; what happens in Nature is in great part a weeding-out of the relatively less fit to given conditions. But it is familiar Darwinian doctrine to distinguish between ‘lethal selection’ which works by the discriminate elimination of the relatively less fit, and ‘reproductive selection’ which works through the increased and more effective multiplication of the relatively more fit. As a matter of fact the weeding out of the relatively less fit must always to some extent involve the fostering of the relatively more fit which survive.

§ 3. Sentimental Recoil from Darwinism.

(A) The sentimental recoil from Darwinism may be illustrated by those who shudder at the so-called automatism of the selective process. The raw material of novelties passes over an unending sieve which never ceases to sift; an uncertain fraction of the variants pass through the meshes and are ground to powder 'twixt the upper and the lower millstone; another uncertain fraction escapes and continues its kind. These are no mills of God, but of Moloch, and all is dread automatism. But it was to remove this misunderstanding that we lingered in a previous lecture over the struggle for existence, and saw that it included all the individual endeavours and answers-back which creatures with a will to live and abundance of resource make to their environing limitations and difficulties. After allowing a little for chance, the relatively best candidates will come to the top in a number of wisely and accurately conducted examinations. This is not mechanical or automatic; neither is Natural Selection. We must recognise that Natural Selection includes all the subtlety of endeavour, all the patient perseverance, all the indomitable insurgence, of living creatures. They share in their own evolution; they often help to make the sieves by which they are sifted.

(B) Another sentimental reason for recoil is because of the supposed grimness of the selection-method. “Contention is the vital force”; rank individualism is the order of Nature; “Each for himself” is the cry from every corner, and extinction take the hindmost. It is a vast gladiatorial show, said Huxley, this Nature,—a dismal cockpit. But, as we have seen, this is a travesty. The struggle for existence is a metaphor, it includes every new endeavour after well-being; it is rarely very intense between near kin; it is often not competitive at all. One organism survives, indeed, by sharpening its claws and whetting its teeth, but another by increasing maternal care or mutual aid.

Speaking of the Darwinian conception of the way in which evolution has chiefly been brought about, Prof. Arthur O. Lovejoy (1909, p. 93) writes: “The doctrine of natural selection represents Nature as a scene of monstrous waste and of universal conflict, a veritable bellum omnium contra omnes. It pictures the teeming Universal Mother as reckless in the production of aspirants for life, but strangely parsimonious in her provision of the means of maintaining life,—leaving to every one of the hungry children at her board only the privilege of snatching the food of his neighbours, only the grim alternative of destroying or being destroyed.”…We quote this as typical of common caricatures, not as representing Professor Lovejoy's own picture of Natural Selection. There is a tendency to exaggerate the destructiveness and instability of wild nature. Apart from man's interference, which is quite per se, cases of rapid disappearance of species, as in the Passenger Pigeon, are rare, and are very puzzling. What is impressive is the Live-and-let-Live equilibrium, the stability of species. Mr. F. C. S. Schiller writes that “Every species is in constant danger of extinction”, but one would like to have the evidence for such a statement. The fact is, that many species have attained to positions of extraordinary stability and security.

§ 4. Changes in Selection Theory since Darwin's Day.

It would be ominous if the theory of Natural Selection stood to-day as it did in Darwin's lifetime. Emendations have been made and saving-clauses have been added, and while extreme critics hold that the theory has been discredited, this conclusion is largely due, we think, to taking the theory in a wooden way and failing to realise its full significance. Before we consider typical criticisms, it will be convenient to discuss some of the positive changes in the theory.

There has been in a few cases a welcome demonstration of Natural Selection at work. The theory is not merely a hypothesis as to what might have happened long ago; it is a statement of what does happen now. There is some actual proof of discriminate selection, where the survivors are shown to survive in virtue of the possession of particular qualities. Let us take a well-known diagrammatic instance, (Cesnola, Biometrika, 1904). The praying Mantis, Mantis religiosa, occurs in Italy in a green and a brown variety, the former usually on the grass, the latter usually on the withered herbage. The Italian naturalist Cesnola tethered twenty green Mantises among green herbage and a similar number of brown ones among withered grass. After seventeen days they were all alive, having escaped the notice of their enemies. He tethered twenty-five of the green variety among brown herbage; all were dead after eleven days. In the converse experiment, of forty-five brown insects exposed on green grass, only ten survived at the end of seventeen days. Most of the Mantises were killed by birds; five of the green ones were killed by ants. The experiment should be extended, but it proved the selective value of the coloration. If green and brown Mantises were exposed in a green country, the green ones would survive, the brown ones would be eliminated, and the selective death-rate would have reference to the particular quality of coloration. Similar experiments have been recorded by Professors Poulton, Crampton, Bumpus, and Weldon—all proving discriminate elimination. Prof. Karl Pearson has also demonstrated the occurrence of a selective death-rate in man.

These demonstrations require more exposition than is here possible, but, as we are dealing with one of the most important of biological theories, with the question of the directive factors in evolution, we may cite from a previous discussion two simple observations which illustrate discriminate elimination picturesquely (Thomson, Darwinism and Human Life, 1911). Prof. C. B. Davenport, of the Carnegie Institution for Experimental Evolution, had 300 chickens in a field, eighty per cent, white or black and conspicuous, twenty per cent, spotted and inconspicuous. In a short time twenty-four were killed by crows, and it was interesting to observe that only one of the killed was spotted. The elimination seemed to be discriminate, and in wild conditions it would doubtless have led to the elimination of the conspicuous variants. It will be understood that we are not attaching great importance to any individual case, such as this, for criticism and corroboration are required all round; we are giving an illustration merely.

In a heavy snowstorm at Johannesburg in August, 1909, many hundreds of trees were destroyed by the weight of snow on the branches. In many places the roads were blocked by the fallen trees. It was interesting, after the storm, to notice that the elimination was in a marked degree discriminate. The trees that suffered most were the imported Australian trees, such as the Blue Gums and Black Wattles, quickly growing, with soft wood, and with abundant foliage that caught the snow. On the other hand, the Deodars from the Himalaya mountains, constitutionally adapted to let the snow slide from their pendulous branches and acicular leaves, had hardly a twig broken.

In the second place, the position of the selection theory has been strengthened by a recognition of its manifoldness. It takes several different forms, the logic of which is the same. When Darwin says “Natural selection acts by life and death…by the survival of the fittest and by the destruction of the less well-fitted individuals”, he describes lethal selection. Insects with reduced wings or none at all abound in wind-swept islands like Madeira, the flying insects having been blown out to sea and destroyed. When Weismann points out that the animals best adapted to the colour of their surroundings will secure the most abundant food and multiply most prolifically, and will thus increase the numerical proportion of others like themselves, he is describing reproductive selection. If an advantageous character is linked to an increase of fertility it will tend to persist apart from lethal lopping off. In the cultivation of a lawn one may eliminate the weeds by direct lethal selection; but one may also stimulate the multiplication of the grass by giving it a specific food which is not profitable for the weeds. There is a special form of selection in the sometimes fatal combats of rival males, and in preferential mating when there is evidence of discrimination on the female's part. There is social selection between rival anthills, where community sometimes competes with community, and, at the other pole, there may be selection between potential egg-cells, the ovarian struggle sometimes ending in the survival of one out of many, and selection between the hundreds of sperm-cells in their race towards the ovum. Allowing a wide margin for chance, the most vigorous and perhaps the most sensitive spermatozoon will tend to succeed, and the elimination of the others by the blocking of the entrance to the egg will be for the advantage of the species. As Weismann suggested, it is also possible that fluctuations in the nutritive supply of the germ-cells, and inequalities in the vigour and assimilating power of the hereditary constituents or determinants, may result in an intra-germinal struggle and selection. But we need not go further, since our point is simply that the selective processes are probably more manifold than even Darwin realised.

Whenever we turn from expositors of Darwin to Darwin himself we discover afresh how subtle was his idea of the process of Natural Selection. We realise, for instance, that the selection need not imply a sudden elimination of the relatively less fit, for a persistently shortened life and a consistently unsuccessful family will work to the same result in the long run as lopping off heads. As Professor Punnett puts it: “If a population contains .001 per cent of a new variety, and if that variety has even a 5 per cent. Selection advantage over the original form, the latter will almost completely disappear in less than a hundred generations.” In human affairs we may be thus encouraged in patience. It has also to be realised that the web of life has so fine a texture that apparently trivial differences in organisms may be of critical moment in determining the survival of those who possess them. And just as in animal courtship what determines the female's preference for one suitor out of many is very probably an irresistible tout ensemble of gifts and graces, rather than excellence in one particular decoration or quality, so in natural selection it may be that what gives survival value is often a general stability of. constitution and efficiency of behaviour. In a well-known instance when 136 storm-spent sparrows were brought into shelter, 72 revived and 64 died. Careful measurements showed Professor Bumpus that the eliminated birds were less near the normal than those which survived. Except in one measured character, the range of variation was greater in those that succumbed. Thus while natural selection may operate with great delicacy in reference to a sieve with fine meshes, it may also rough-hew and cut off in a crisis a large number of organisms which are in a general way less fit than their fellows. As Professor Bumpus said, general stability of structure was the essential characteristic of the surviving sparrows (Bumpus, 1898).

An interesting corollary to the selection proposition is that a relaxation of sifting may admit of exuberance. When organisms reach a position of relative security, as many species do, then, the criticism of circumstances being removed, there may be extraordinary abandon in the way of coloration and decoration. The limit is the stability of the constitution; the risk is that some environmental change may involve a heavy tax on the exuberance which the conditions of relaxed selection tolerated. It may be one reason of the diversified brilliance of humming birds that they have few enemies. A clearer case is to be found in the coral-fishes, whose exuberance of coloration beggars description (see Reighard). It may be that the gorgeousness has been made possible by the safety of the labyrinthine reefs, and by the agility of the swimmers. Prof. J. P. Lotsy (1916) speaks of the bewildering diversity exhibited by a series of about 200 specimens of the Common Buzzard (Buteo buteo) in the Leiden Museum, “hardly two of which are alike”. “The reason probably is that here no selection has been at work, because this bird of prey is so strong that it has practically no enemies in the regions in which it occurs.”

Of great importance is the change that has been involved in our appreciation of Natural Selection by an increased knowledge of the raw materials supplied to the sieve by variability. As we have seen, discontinuous variations or mutations are not of rare occurrence; there is a brusque passage from one position of equilibrium to another; the Proteus leaps as well as creeps. An advance marketed from the first by a certain measure of perfectness is made at a stride, not by minute steps generation after generation. A copper-beech, a laciniate celandine, a hornless calf, a calculating boy, or the like, just appears—out of the inexhaustible conjurer's box. Now it is plain that as the list of these mutations or saltations grows in length, the lighter will be the burden that has to be laid on the shoulders of Natural Selection. Apart from the palæontological record it is only by analogy from the present that we can argue back to what occurred in the distant past, but it looks as if mutations were much more frequent, than has been till recently supposed, and the more frequent mutations were in the past, the less work would there be for Natural Selection to do in the way of fostering small increments in a particular direction.

It is quite premature, however, to think of abandoning the idea—so characteristically Darwinian—of the cumulative importance of minute advances. Many palæontologists insist on the origin of new characters “by excessively fine gradations which appear to be continuous” (Osborn), and also on the frequent occurrence of orthogenesis, i.e., change in a definite direction without marked divagations. As Prof. H. F. Osborn says (1919), the palæontological record often confirms the prophetic judgment of Aristotle: “Nature produces those things which, being continuously moved by a certain principle contained in themselves, arrive at a certain end.”

We must be on our guard, however, against the possible fallacy of concluding, from the apparent orthogenesis in fossilised and surviving stages along an evolutionary line, that there was no zigzagness and pruning in the process. Types may have their waywardness gradually sifted out of them. The uniformity of the flow of cartridges from a testing machine gives a fallacious impression unless we discover that they have passed through three siftings which reject the too heavy and the too light, the too long and the too short, and those whose calibre is too broad or too narrow.

On the other hand, one of the impressions that we get from Prof. D'Arcy Thompson's magistral work on Growth and Form is that the variability of organisms runs on lines laid down by the conditions of the inorganic. Variations must conform to the trammels of surface-tension, minimal areas, stability, and so on; there is not an indefinite number of ways in which an aggregate of cells can be arranged; one skull or leaf often differs from a related form in a way which might be described as a general deformation—due, for instance, to a tilting of axes. The same general impression of definiteness we get from considering what we have alluded to as temporal variations: one species often seems to differ from another in rate or tempo, and this fits in with Prof. D'Arcy Thompson's morphological illustrations, for differences of form depend in great part on different rates of growth in different directions.

But even mutations and definite orthogenetic variations cannot dispense with the criticism of Natural Selection. There is ever a risk that they may go too far. It is easy to have too much of a good thing. The antlers of the Irish Elk which hastened the doom of their possessors are diagrams of the evolutionary adage Nequid nimis. If we accept De Vries's view that evolution is often effected by mutation, by sudden considerable jumps, this is contrary to the idea of Natural Selection working as an accumulator of small gains. But De Vries does not propose to dispense with the theory of Natural Selection. He attaches less importance to intra-specific selection, but not. less to the sifting of species by one another and by the environment. Speaking of the Mutation-Theory, Prof. G. H. Parker writes (1913, p. 263): “Organic evolution, then, is accomplished by occasional strides rather than by many oft-repeated short steps. This theory is in no sense antagonistic to natural selection. In fact, it works effectively only in conjunction with natural selection, for, after all, what determines whether a race showing a trait produced as a mutation will survive or not is natural selection…As De Vries himself rightly maintains, the mutation theory is significant only in connection with natural selection.”

§ 5. Scientific Critique of Selection Theory.

As our whole view of Animate Nature is coloured by our position in regard to the scope and importance of the processes of selection, we must consider some of the most serious objections to the theory. We select three. One of the criticisms is thus clearly stated by Prof. G. H. Parker (1913, p. 256): “The chief objection that has been raised against natural selection is one which was well known to Darwin himself, but which has been gathering strength for some years past. It is to the effect that the initial phases of a favourable variation, as conceived by Darwin, are too slight to be of use to the organism, and consequently they cannot come under the influence of the selective process. When the slight individual differences that Darwin laid so much stress upon are closely scrutinised, it seems scarcely conceivable that they could be, even in the long run, of life-and-death importance to an organism; in other words, that they could afford a starting-point for the formation of a new species. And when closely related species in nature are examined, such as the different kinds of warblers, or of sedges, it seems impossible that the slight differences separating them should represent gaps produced by natural selection through an elimination of intermediate forms. Thus an inspection of nature reveals a state of affairs which many investigators have come to believe to be much too refined to be a product of natural selection,” Some who admit that natural selection is “capable of rough-hewing a species” doubt its ability to put on “the polishing touches”. The answers to this objection are three. (1) The idea that established differences between species are too refined to be the work of natural selection, shows a lack of appreciation of the fact that the selection is often in relation to a very intricate and subtle web of life, where the shibboleth that decides survival or failure may be a very refined criterion indeed. (2) Variations are sometimes correlated, and a minor variation which is not itself of sufficient magnitude to have survival value may be carried in the wake of one that has. (3) Some variations are not minute fluctuations, but are brusque mutations, springing fully formed into existence and therefore at once of a magnitude to be sifted in the sieves of natural selection.

A second objection, also familiar to Darwin, is that individuals possessing an advantageous variation would have to pair with others not possessing it, and that the new departure would be swamped by the inter-crossing. To this there are three answers: that similar variations often occur about the same time in several individuals; that many factors of isolation operate towards reducing the range of inter-crossing and bringing similar forms together; and, thirdly, that many variations are of the nature called Mendelian, which do not blend, but are handed on in intactness to a certain proportion of the descendants.

A third even more serious criticism has arisen out of the recent selection-experiments of the Danish biologist Johannsen, the Dutch botanist De Vries, the American zoologists Jennings and Pearl, and others, which are to some extent at variance with the Darwinian view, that the average of a stock can be improved as regards a particular character by always breeding from those that show most of it. If the descendants of an individual high-class bean are kept apart, forming what is called “a pure line”, there are observable fluctuations of characters. Some are tall plants, others are short, and so on. But if the talls are selected out and bred from, or the shorts, there is no establishment of a tall race, getting gradually taller, or of a short race getting gradually shorter, nor is there anything to choose between the descendants of the talls and the descendants of the shorts. There is no departure from the average of the original pure line. From a mixed wild stock a selection may be made of particular types which start pure lines or distinct races, but when the pure line has been started there is no further progress, select as one may. There is no getting beyond the mean of the inbred line. The reason for this seems to be that the fluctuations within the pure or inbred line are modifications or indents, and not transmissible.

If selection of the best of a pure line does not improve the stock, how do the breeders succeed? The answer is that their success is due to making a good start with a good line; beyond the level of this they cannot pass without the introduction of fresh blood from another line. There are obvious reasons, however, why these facts from artificial selection must not be used hurriedly in depreciation of the rôle of selection in natural wild conditions, (a) Pure or inbred lines are not typical of wild stocks, in which cross-fertilisation is of frequent occurrence. (b) It is dangerous to argue from very short-lived experiments to the age-long processes of Nature, (c) It is premature to deny the possibility of stable germinal variations occurring in a pure or inbred line. If one did, it might be the starting-point of a new advance. In any ease there remains a great, rôle for Natural Selection in eliminating certain lines or races and favouring others in its ceaseless sifting.

§ 6. Subtlety of Selection Theory.

Natural Selection is a technical expression for a manifold and almost ubiquitous process of sifting, which discriminates in life and in death between the relatively more fit to the given conditions and the relatively less fit. It must always be thought of in the Here and Now, i.e., in reference to particular conditions of space and time. There are three reasons why it is important to keep this obvious fact in view. (a) It is a frequent and pernicious error to suppose that there is any sort of ceaseless winnowing towards an ideal of fitness, except perhaps self-consistency. The only common character of surviving variants is that they survive,—they must have consistent viable constitutions suited to particular conditions, which may be those of parasitism or putridity. The fallacy of supposing that Natural Selection necessarily works towards ‘fitness’ in the colloquial sense is largely due to thinking of the process abstractly and hypostatising it, and to misunderstanding the word ‘fit’, which means merely relatively advantageous in given conditions, making for survival in short. But the error is also due to a shrewd perception of the big fact that, after all, life has teen slowly creeping upwards as the ages have come and gone. We shall consider this fact later on, but meanwhile it is necessary to be perfectly clear that being selected does not necessarily confer on the creature any dignity or approval. It means wholly and solely survivability in certain conditions, which may be those of parasitism or sloth. The value of survival, as judged by any human standard, depends altogether on the conditions under which survival is secured. Survival may be to a type that does not work for its living, but is an unpaying boarder inside another creature, or to a mere drifter in the stream of things, or to a rough egoistic combative type, much less desirable, when judged by æsthetic or ethical standards, than a gentle, altruistic, fine-brained type for which the times were too stern. Survivability means little in itself: one has to know the regional conditions and the price paid.

(b) It is important, for a second reason, to remember that Natural Selection operates in great part with an external reference to an established system of inter-relations which we call the web of life. For it is this reference to an intricate sieve that enables us to understand how minute and rather subtle advances might have survival-value, or might turn the scale between success and failure. A nuance—a shibboleth—may be decisive. There are some kinds of fresh-water mussel which cannot continue their kind without the unconscious co-operation of a particular species of fresh-water fish. The parasite which causes the disease of liver-rot in sheep cannot in Britain continue its race unless the free-swimming larva find entrance to a particular species of fresh-water snail Limnæa truncatula, for other species do not seem to serve. There are some flowers which cannot be pollinated except by a particular kind of insect-visitor. We miss the significance of Natural Selection unless we realise its frequent specificity. Meredith speaks of Nature winnowing “roughly”, and that may sometimes be; but it is also a fact that she often winnows with a meticulous nicety.

To sum up. In variation and selection we have, so far as we know, the chief factors of Animate Evolution. The method is theoretically very simple. A move is made and it is tested; a new idea occurs and it is criticised. But this kind of formal summary of the tactics is quite fallacious. It conceals the heart of the matter, that living creatures with a will to live, with an insurgent self-assertiveness, with a spirit of adventure, with an endeavour after well-being—it is impossible to exaggerate the personal aspect of the facts, even if the words which we use in our ignorance may be too metaphorical—do trade with time and have commerce with circumstance, as genuine agents, sharing in their own evolution. There is abundant room for sympathetic admiration of the tactics of Animate Nature, though the strategy may—and, for science, must—remain obscure.

§ 7. Sexual Selection.

(a) To illustrate still further the subtlety of the process of Selection we shall now consider how it works in the case of preferential mating. It was primarily in reference to secondary sex-characters that Darwin suggested his theory of sexual selection. Certain variations, e.g., in the improvement of weapons and food-catching apparatus, are favoured by natural selection in the course of the everyday struggle for existence; in the same way, variations which are advantageous in securing mates and consummating sexual reproduction will be favoured by sexual selection. Darwin began with instances of the importance of masculine vigour and equipment when rival males compete for the possession of the females. “The strongest and, with some species, the best-armed of the males drive away the weaker; and the former would then unite with the more vigorous and better-nourished females, because they are the first to breed. Such vigorous pairs would surely rear a larger number of offspring than the retarded females, which would be compelled to unite with the conquered and less powerful males, supposing the sexes to be numerically equal; and this is all that is wanted to add, in the course of successive generations, to the size, strength, and courage of the males, or to improve their weapons” (Descent of Man, 2nd Ed., 1888, Vol. I., p. 329). Now it is plain that forceful competition among rival males for the possession of a female or of several females, does not differ in kind from the ordinary struggle for food and foothold, except that it is strictly intra-specific. Darwin pointed out indeed (p. 349) that sexual selection is less rigorous than natural selection; that it is less of a life-and-death affair; that it operates through the unsuccessful males having fewer, less vigorous, or no offspring; and that it is not limited by the general conditions of life; but there is in all this no departure from the natural selection position. This part of the theory, therefore, remains valid to those who regard natural selection as a vera causa.

(b) Darwin went on to those characters that are useful in the recognition and capture of the females. When a male excels his neighbours in his capacities for finding, pursuing, and catching the female, sexual selection, he said, again comes into action. (Descent of Man, p. 324.) The male moth often finds his mate by the olfactory acuteness of his large antennæ some small crustaceans recognise the other sex almost instantaneously when there is chance contact in the water; in some fishes, recognition depends on colour and on behaviour; many experiments led Goltz to believe that the male frog distinguishes the female by touch; in birds, visual and auditory impressions count for most; in mammals, the scent is often of chief importance (see S. J. Holmes, Studies in Animal Behaviour, Boston, 1916, pp. 219-328). Since correct recognition of the one sex by the other is often of essential importance to the race, it is not surprising to find Darwin saying (Descent of Man, p. 324): “But in most cases of this kind it is impossible to distinguish between the effects of natural and sexual selection.” This part of the theory also remains valid, if one believes in selection at all.

(c) Darwin primarily used the term sexual selection for all cases where sifting occurs in relation, not to ordinary nutrition and self-preservation, but to pairing. It was only secondarily that he laid emphasis on the ‘choice’ that the female is supposed to exercise in reference to rival suitors. An interesting confusion, which has misled some biologists, has arisen by a double use of the word selection. Darwin spoke of the female's selection, but it is perfectly clear that he recognised a large field of selection in which there was no question of selection or choice on the part of the female. (See Descent of Man, 2nd Ed., 1888, Vol. I., p. 323, foot-note.) Sexual selection meant, for Darwin, sifting in connection with mating, whether the female held the sieve or not.

(d) In his next step Darwin used the word selection in a non-metaphorical sense:—“Just as man can give beauty, according to his standard of taste, to his male poultry, or more strictly can modify the beauty originally acquired by the parent species…so it appears that female birds in a state of nature have, by a long selection of the more attractive males, added to their beauty or other attractive qualities” (Descent of Man, 2nd Ed., 1888, Vol. I., p. 326). In many animals, at diverse levels of organisation, there is an elaborate courtship-ceremonial, allied, according to Groos, to play. It is sometimes on both sides; it is usually for the most part on the male's side. It includes a manifold display of decorations, colours, agility, and vocal powers. Darwin's theory in this connection was simply this: if there are rival males, and if they are unequally endowed with structural and emotional equipment, or with the capacity of using this to advantage, there will be preferential mating on the female's part, and, other things equal, there will be a selection of the type of male most successful as a suitor. It is the female who sifts, but the logic of the process is the same as in natural selection.

(e) It is conceivable that pronounced and persistent differential mating might lead not merely to the establishment and augmentation of characters determining the result of the contest or the courtship, but also to a process of physiological and psychological ‘isolation’ (narrowing of the range of inter-crossing), and thus to an accentuation of the apartness of a species as regards crossing with related neighbour-species (see Karl Pearson, Grammar of Science, 2nd Ed., 1900, p. 418).

(f) At this point attention may be directed to the important contributions to the natural history of mating to be found in H. Eliot Howard's monumental British Warblers (1907-1915). We venture to think that this acute and sympathetic observer exaggerates the instinctive at the expense of the intelligent element in the behaviour of birds, and that he is unnecessarily antagonistic to Darwin's theory of sexual selection, but his work is a rich treasure-house of reliable data. It is of great interest, for instance, to discover how much competition there is among the male warblers, before the females arrive on the scene, in the way of discovering and securely holding the most advantageous territories for nesting. Not less important is the evidence that the soberly coloured warblers do not fall behind brilliantly coloured birds in the elaborateness and abandon of their display attitudes and poses.

(g) Darwin was well aware of many of the difficulties besetting his theory. With his wonted candour he anticipated various objections, e.g., that the theory “implies powers of discrimination and taste on the part of the female which at first appear extremely improbable” (Descent of Man, p. 326). The first very serious criticism came from Wallace in 1871, and was restated in his Darwinism in 1889. The most elaborate criticism as yet is surely to be found in T. H. Morgan's Evolution and Adaptation (1903), where no fewer than 24 reasons are given for rejecting the theory. Within our narrow limits we must confine our attention to the three criticisms which seem most important.

There is, in the first place, an admitted difficulty in the scarcity of direct evidence that some of the males are actually disqualified and left unmated. If all the males get mates sooner or later, then no discriminate elimination is effected. Prof. Karl Pearson has given statistical evidence of preferential mating in mankind, but this is hardly procurable in the animal world. Darwin met the objection in various ways. He pointed out that in some species the males outnumber the females, and that in some other species there is polygamy. If the more attractive males have in such cases an advantage in mating, the direction of evolutionary movement will be determined by them, and not by the handicapped residue of the unattractive. He also pointed out that the more vigorous and more attractive males would be accepted by the more vigorous females which are the first to breed, and this would imply a cumulative preponderance of the more vigorous and more attractive types. Even earlier hatching of the young birds might be of critical moment. As a matter of fact, definite information as to the elimination of some of the males is by no means wholly lacking. Thus in diagrammatic illustration we may refer to some spiders where, as the Peckhams and others have shown, the female sometimes kills a suitor who does not adequately please her. That she may also kill a successful suitor is immaterial, since the mating has been accomplished. (See G. W. and E. G. Peckham, Observations on Sexual Selection in Spiders of the Family Attidæ, Milwaukee, 1889, p. 60.)

In the second place, many critics have objected to crediting the female organism—whether bird or butterfly—with the power of ‘choice’, and while comparative psychology has not advanced far enough to admit of many definite statements as to the subjective aspect of animal courtship, it may be granted that there is not in the, ‘choice’ of any female animal much that would correspond to a human weighing of pros and cons. But the point of importance is whether the mating is in any real way selective, preferential, discriminative. It has been proved experimentally that insects as well as birds may be selective in their eating: is the same true as regards their mating? It appears to us that the phenomena of mating recorded by Darwin, by Groos (Play of Animals, 1898), by Cunningham (Sexual Dimorphism, 1900), by Pycraft (Courtship of Animals, 1913), and so on, place the reality of some measure of preferential mating beyond doubt. Even if one adopts the modern, view that the female does not choose the ‘best’ out of a bunch of suitors, but rather remains unresponsive to the solicitations of males who do not raise her emotional interest to the requisite pitch, that is quite enough for the purposes of the theory; and it is in agreement with Darwin's own remark about the female bird: “it is not probable that she consciously deliberates: but she is most excited or attracted by the most beautiful, or melodious, or gallant males”.

A third objection is more serious. It is one thing to admit the reality of a somewhat vague preferential mating, it is quite another thing to credit the female animal with a capacity for appreciating slight differences in decorativeness or musical talent or lithesomeness. Wallace's statement of this objection is well known. Referring to Darwin's four chapters in The Descent of Man, he says: “Any one who reads these most interesting chapters will admit that the fact of display is demonstrated; and it may also be admitted, as highly probable, that the female is pleased or excited by the display. But it by no means follows that slight differences in the shape, pattern, or colours of the ornamental plumes are what lead a female to give the preference to one male over another; still less that all the females of a species, or the great majority of them, over a wide area of country, and for many successive generations, prefer exactly the same modification of the colour or ornament (Darwinism, 1899, p. 285).

But the edge has been taken off this objection by Lloyd Morgan and others, who point out the gratuitousness of crediting the hen bird with a standard of taste or capacity for æsthetic valuation. “The chick selects the worm that excites the strongest impulse to pick it up and eat it. So, too, the hen selects that mate which by his song or otherwise excites in greatest degree the mating impulse. Stripped of all its unnecessary aesthetic surplusage, the hypothesis of sexual selection suggests that the accepted mate is the one that most strongly evokes the pairing instinct” (Habit and Instinct, 1896, p. 217).

It may be insisted, however, that if individual excellence in attractive characters (such as plumes, singing power, dancing agility) does not appeal to the female, it cannot he determinative in preferential mating, and therefore its establishment cannot be effected by any process of sexual selection. Unless the female is somehow aware of the individual variation in question, the theory breaks down, and yet it is difficult to believe that the female is so meticulous in fastidiousness, so detailed in her preferential excitability.

The answer, probably sound, is that the details count, not as such, but as contributory to a general impression. Each has its effect, but synthetically, not analytically. “Even when the female seems to choose some slight improvement in colour or song or dance, the probability is that she is simply surrendering herself to the male whose tout ensemble has most successfully excited her sexual interest” (Geddes and Thomson, Evolution, 1911, p. 172).

(h) If one provisionally accepts the theory that a secondary sex-character may have been established and augmented because it contributed to a decision in preferential mating, one has to face the further question of the significance or racial justification of the courtship-habits—often so prolonged, elaborate, and exhausting. The sifting probably works well in keeping up a standard of racial fitness, for the most persuasive male is likely to be, among animals, the fittest all round. But there is surely more than this.

To Groos and to Julian S. Huxley we owe two luminous suggestions. In his Play of Animals (Eng. Trans., 1898, p. 242), Groos suggests that “in order to preserve the species the discharge of the sexual function must be rendered difficult, since the impulse to it is so powerful that without some such arrest it might easily become prejudicial to that end”. “This very strength of impulse is itself necessary to the preservation of the species; but, on the other hand, dams must be opposed to the impetuous stream, lest the impulse expend itself before it is made effectual, or the mothers of the race be robbed of their strength, to the detriment of their offspring.”…“The most important factor in maintaining this necessary check is the coyness of the female; coquetry is the conflict between natural impulse and coyness, and the male's part is to overcome the latter” (op. cit., p. 243).

Not less interesting is the suggestion developed by Julian S. Huxley in his remarkable study of the courtship-habits of the Great Crested Grebe, Podiceps cristatus (Proc. Zool. Soc. London, 1914, pp. 491-562). In the Great Crested Grebe the two sexes are practically alike in plumage, colour, and habits; but the courtship is extraordinarily elaborate—a self-exhausting ritual, “not leading up to or connected with coition”. Mr. Huxley believes that “the court-ship ceremonies serve to keep the two birds of a pair together, and to keep them constant to each other”. “Birds have obviously got to a pitch where their psychological states play an important part in their lives. Thus, if a method is to be devised for keeping two birds together, provision will have to be made for an interplay of consciousness or emotion between them” The courtship is justified by the strength of the emotional bond it establishes. There is a “Mutual Selection” which is in a way “a blend between Sexual and Natural Selection”.

(i) A survey of recent observations on mating, as in Mr. W. P. Pycraft's Courtship of Animals (1913), leaves an impression of an intricacy and subtlety that baffles description. We agree with this distinguished expert as to the need for psychological as well as physiological interpretation. It is probable that no naturalist has studied a courtship with the thoroughness that Mr. Huxley shows in his account of the Great Crested Grebe, and what is his verdict? Display and ornament do not act on the æsthetic sense of the female, but on her emotional state; they are—using the words in no narrow or unpleasant sense—excitants, aphrodisiacs, serving to raise the female into that state of exaltation and emotion when alone she will be ready to pair…But the element of choice does, in another form, remain. In animals such as Birds, where there is a regular pairing-up season, and where, too, the mental processes are already of considerable complexity, it is impossible to doubt but that mating may be, and in some species is, guided by impulse, unanalysable fancies, individual predilection.”

(j) In his Studies in Animal Behaviour (1916) Mr. S. J. Holmes has an interesting chapter on “the rôle of sex in the evolution of mind”. Let us take one illustration. “The primary function of the vocal apparatus of the Vertebrates was probably to furnish a sex call, as is now its exclusive function in the Amphibia. Only later and secondarily did the voice come to be employed in protecting and fostering the young, and as a means of social communication. And the evolution of the voice in Vertebrates doubtless influenced to a marked degree the evolution of the sense of hearing. It is not improbable, therefore, that the evolution of the voice, with all its tremendous consequences in regard to the evolution of mind, is an outgrowth of the differentiation of sex.” There can be little doubt that the biology of the future will attach not less but more importance to sexual selection. For it seems likely that characters and qualities originally established in this way have often influenced both body and behaviour in reaches now more or less remote from the tides of sex-impulses.

§ 8. Selection and Progressiveness.

There is a very important reason why we should keep in mind the relation of Natural Selection to the Systema Naturæ which has been gradually evolved, which is continually becoming more complex, which is made up of numerous components, mostly stable and beautiful, often intelligent and purposeful. The reason is that we have here part of the explanation of the progressiveness of evolution. For, while there are blind alleys and other paths that turn back on themselves, the large fact is that on the whole evolution has been in the direction of increased differentiation and integration, of growing mastery and freedom. In this way Nature has led up to Man, her minister and interpreter. But how was it effected?

It may be that part of the secret is insoluble, that it is wrapped up with a tendency to complexify which may be seen even in the inorganic, where corpuscles form atoms and these molecules, where small molecules form large ones, and large molecules colloid masses, and so on. A fortiori it may be inherent in the very nature of an organism to complexify, to differentiate.

We could suggest, however, that part of the riddle is solved when we carefully observe the process of Natural Selection, which operates in relation to an external Systema Nature, the building-up of which is the work of æons. As organisms evolved there was a pari passu complexifying of the web of life, and this extra-organismal registration worked towards conservation and towards further advance. For it is in relation to the external system that selection works.

In the evolution of human societies much has always depended on the external registration of ideas and ideals. They form a framework of institutions and organisations, as stable as folk-ways and traditions; they become immortal in literature and art; and this extra-organismal registration works both towards conservation and towards further advance. For it is in relation to the external system that selection works. It may be urged, however, that the social system is often unsound, that it may give fixed expression to the vicious as well as to what is noble, and that the result is to help degeneration not progressive evolution. The answer is sadly familiar, that this does occur; and that nationalities and their monuments alike are then swept from the stage.

The difference in the realm of organisms is that we have there to deal with an external system which is the product of many millions of years, that the disintegrate elements which entered into it have for inherent reasons failed to stand the test of time. Like rotten stones in a building they have crumbled away. But they have been replaced by others more enduring.

What we mean may be made clearer by a concrete instance. It was probably in the Carboniferous age that various insects became flower-visitors, that inter-relations began to be established between insects and flowers, between flowers and insects. The flowers evolving in their own way came to have flower-visiting insects (likewise evolving) as part of their environment, as part of the system in relation to which they were naturally selected. Similarly with the insects in relation to ‘entomophilous’ flowers. And as the inter-relations became more and more intricate, more and more precise, they would tend to make the selection progressive.

There may be a sort of momentum in the organism itself, for nothing succeeds like success. As Walt Whitman wisely said, “It is provided in the essence of things that from any fruition of success, no matter what, shall come forth something to make a greater struggle necessary.” As was said of old time: “For to every one who has shall more be given and richly given; but from him who has nothing, even that which he has shall be taken.” Organisms run on a compound interest principle. But our present point is that the external web of life becoming ever more complex will tend to secure progressiveness.

Whether or not our idea means as much as we think it does, its consideration should in any case put an end to the notion that Natural Selection is capricious. Both as regards the raw materials and the sieve, evolution is very far removed from being ‘a chapter of accidents’.

§ 9. Selectionist Interpretations and the Argument from Design

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This seems the appropriate place for a consideration of what has been called the Argument from Design. Discovering some of the thousand-and-one ways in which the structure and function of organisms are fit for the conditions of life, many keen-sighted and reverent naturalists of older days argued directly from the adaptations to the agency of a Divine Adapter. It was in a way a wholesome attitude, for the abundance of adaptations is a prominent fact in the realm of organisms; they have, as Mr. Balfour says, “exquisite nicety and amazing complexity” they are not easily accounted for; and some of them make for the continuance of what has for Man great value. But it can hardly be maintained that the argument in its old form was logically sound. As Professor Lovejoy puts it (1909), “from knowing, through experience, that certain effects are caused only by purposive human agency, we have no ground whatever for concluding that certain other effects, of whose causation we have no experience at all, must be due to non-human purposive agency”. It has been called by logicians the fallacy pf transcendent inference, but perhaps there is a truth of transcendent inference in the idea behind the argument.

Many naturalists know and admire three monumental volumes by the late Prof. Bell Pettigrew entitled Design in Nature (1908). They form a magnificent, generously illustrated treasury of adaptations. But not the least interesting thing about these volumes is the fact that the author, with the thousand-and-one fitnesses before him, found himself forced, like Darwin, to abandon the position of the Bridge-water Treatises, that one may find in adaptations the evidence of Divine Design. There is no doubt as to the reality of the thousand-and-one adaptations: Why is the Bridge-water Treatise position untenable?

(1) It is a curious characteristic of some minds that they cannot give a living creature credit for doing anything very wonderful. They refuse to contemplate the possibility that what the creature does may be accounted for in terms of itself. They insist on helping the organism on by some extraneous introduction—an Entelechy, a Purpose in Nature, an élan vital, a Directive Intelligence, and so forth. What the older Naturalists should have done before concluding their argument was to inquire how far the intelligence, which adaptations certainly suggest, may be resident as intelligence or some analogous form in the creatures themselves. Modern study shows that many animals work out their own salvation.

(2) The second reason why modern naturalists do not occupy the old position is because their outlook is evolutionist. When they scrutinise the magnificent series of adaptations more closely they discern less perfect stages of them in antecedent forms of life. The eye of a fly is an extraordinary instrument, but there is a long ladder of eyes approximating to it. The community of hive-bees or of social wasps amazes us—at first almost bewilders us,—with its complexity and subtlety, but there is a long series of gradations connecting it with the life of solitary bees and wasps. Moreover, as we look around, we see that many adaptations are still in progress, and very far from perfect.

(3) The third reason is, that, given a sufficient crop of variations, plenty of time, and a process of sifting, the Darwinian can give a plausible and approximate—we do not say an easy or complete—account of the way in which most of the wonderful adaptations have been evolved. The hard-shelled Darwinian says: These effective adaptations you so justly admire are the outcome of natural tentatives and natural siftings. We assume that the forms of life are restlessly but not inconsistently variable, that they are continually offering new qualities and characters to the sieve of selection, and that the conditions of life are such that they eliminate in a very discriminating fashion the relatively less fit. If these assumptions are granted, we can account for adaptations. The immediate operation of a Divine Adapter is a hypothesis of which, we say it with the utmost reverence, we cannot scientifically make any use.

The idea of a Divine Designer is outside the scientific mode of formulation, to which it is an impiety not to be loyal, but it is not outside the right of interpretation which we claim as rational beings. It is a religious idea—this of the Divine Designer; the question is whether it is inconsistent with securely established scientific thinking. In our judgment it is not inconsistent.

The old form of the Argument from Design has no longer more than a historical interest, but it may be reasonably maintained, it seems to us, that the general idea behind the argument remains. For if we. free ourselves, as we think we must, from a purely mechanical evolutionism, and recognise organisms as genuine agents, we may see in the factors of evolution the relatively, though, of course, not absolutely self-sufficient, means of working out a purpose, or thought, or idea which was involved by the Creator in the origination of the first organisms, or wherever it seems clearest to begin. We must not forget the problem of the origin of the conditions that made Organic Evolution possible. That He—the Unmoved Prime Mover—has made things to make themselves and to go on perfecting themselves—albeit they may be never separable in thought from Him—seems a finer kind of creation than Paley pictures. As Professor Pettigrew said in his Design in Nature (p. 820), “Natural Selection may be regarded merely as a process of so-called evolution by which the Creator works and accomplishes His purpose. Indeed the Creator, by conferring upon living matter in its simplest and lowest forms the power of appropriating the elements and building them up by endless elaboration and gradation from a monad to a man, proves Himself to be an infinitely more wonderful Designer than was ever dreamt of by even the most ardent teleologist.” This surely strikes the true note.

But it must be noted that it would not occur to scientific investigators, as such, to speak of the factors of evolution as means to an end. That is a point of view beyond science, though naturally taken by those who feel the extraordinary value and significance of certain results of evolution,—such as the beauty of Nature, or the moving equilibrium of things, or the progressiveness of organisation, or the emancipation of mind, or the incomparable worth of a noble human life.

SUMMARY.

The central idea in Darwinism is the selection of the relatively fitter variants in the struggle for existence.

An immediate logical recoil from Darwinism has been based on the fact that Natural Selection is not originative, only directive; and that it is rather eliminative than selective. But these points are freely admitted by Darwinians; the recoil is due to a misunderstanding of insufficiently criticised phraseology.

A sentimental recoil from Darwinism has been based on the supposed mechanical character of the selective process (but many organisms share as agents in their own evolution), and on the supposed grimness of the eliminative methods (but this is a very partial view).

Since Darwin's day the theory of Selection has undergone some modification. Its position has been strengthened by the demonstration of several cases of Natural Selection at work, by actual proof of a differential death-rate. It is not a mere interpretative hypothesis. Its position has been strengthened by a recognition of the manifoldness of the selective processes, e.g., lethal and reproductive. There has also been a clearer view of the probable consequences, e.g., exuberant decorativeness, that may ensue in situations where the elimination has been greatly relaxed. The estimate of the scope of Natural Selection is affected by the view taken in regard to the raw materials supplied. If these reach by mutational abruptness to some degree of perfectness, there is little for Natural Selection to do in the way of accumulating minutiae. If they are in large measure definite, then Natural Selection bas not to sift out the serviceable from a large casual crop. It has been shown by Johannsen, do Vries, Jennings, Pearl, and others that selection does not count for much within pure-lines or inbred stocks. The abundant ‘fluctuations’ that occur there cannot be used as a basis for selection, for they are not transmissible, and are probably for the most part of the nature of modifications.

As our whole view of Animate Nature is coloured by our estimate of the validity and importance of the Selection-Theory, it is useful to consider some of the more serious criticisms, e.g., that slight initial changes could not have survival value, that they would be swamped or levelled down by inter-crossing…It does not seem too much to say that the theory survives these criticisms and has been the better for them.

It is very important to recognise that Natural Selection is a technical expression for a manifold, almost ubiquitous, and often subtle process of sifting, which has, in most cases, a particular reference to particular conditions in time and space. It does not work consistently towards an ideal of fitness, but it eliminates inconsistent non-viable constitutions; it often operates in reference to an intricate web of life, and thus a nuance—a shibboleth—may have survival value; it operates, generally speaking, in relation to a Systema Naturæ which has been increasingly elaborated through the ages, in which even ideas and affection get embodied, and this is part of the explanation of the progressiveness of evolution. Another part of the explanation of the progressiveness, which has always been a puzzle except to teleological interpretation, is what may be called organismal momentum. Organisms run on a compound interest principle.

The question rises again whether the operative factors in organic evolution are more than complications or compositions of factors which operate in inorganic genesis. The answer is, much more. Natural Selection operates on what is not accounted for mechanically, and the sifting process itself is more than mechanical. What Ward has shown in regard to Subjective Selection is vitally important to an accurate view of the facts. The same conclusion may be reached from a different set of data, the phenomena of preferential mating.

In variation and selection we have, so far as we know, the chief tactics of Animate Evolution, A move is made and it is tested; a new idea occurs and it is criticised. But a formal statement of the tactics is fallacious. It conceals the heart of the matter, that living creatures with a will to live, with an insurgent self-assertiveness, with a spirit of adventure, with an endeavour after well-being—it is impossible to exaggerate the facts, even if the verbal suggestion is in our ignorance too metaphorical—do trade with time and have commerce with circumstance as genuine agents, sharing in their own evolution. This should at least increase our sympathetic admiration of the tactics of Animate Nature, though the strategy remain obscure.

Science has to do with description and formulation—not with interpretation. Thus the selectionist account of the evolution of adaptations does not conflict with the general idea behind the old ‘argument from design’.