Charles Darwin

CHAPTER IX

THE THEORY OF COURTSHIP

In the same volumes with the 'Descent of Man' Darwin included his admirable treatise on sexual selection. This form of selection he had already dealt with briefly in the 'Origin of Species;' but as in his opinion it was largely instrumental in producing the minor differences which separate one race of men from another, he found it necessary to enlarge and expand it in connection with his account of the rise and progress of the human species.

Among many animals, and especially in the higher classes of animals, the males and females do not mate together casually; there is a certain amount of selection or of courtship. In some cases, as with deer and antelopes, the males fight with one another for the possession of the females. In other cases, as with the peacock and the humming-birds, the males display their beauty and their skill before the eyes of the assembled females. In the first instance, the victor obtains the mates; in the second instance, the mates themselves select from the group the handsomest and most personally pleasing competitor. Sexual selection, of which these are special cases, depends on the advantage possessed by certain individuals over others of the same sex and species solely in respect to the question of mating. In all such instances, the males have acquired their weapons of offence and defence or their ornamental decorations, not from being better fitted to survive in the struggle for existence, but from having gained an advantage over other males of the same kind, and from having transmitted this advantage to offspring of their own sex alone.

Just as man can improve the breed of his game-cocks by the selection of those birds which are victorious in the cockpit, so the strongest and most vigorous males, or those provided with the best weapons, have prevailed in the state of nature over their feebler and more cowardly competitors. Just as man can give beauty, according to his own standard of taste, to his male poultry, by selecting special birds for their plumage, their port, their wattles, or their hackles, so female birds in a state of nature have by a long-continued choice of the more attractive males added to their beauty and their ornamental adjuncts. In these two ways, Darwin believed, a limited selection has slowly developed weapons like the horns of buffaloes, the antlers of stags, the tusks of boars, and the spurs of game-birds, together with the courage, strength, and pugnacity always associated with such special organs. It has also developed the ornamental plumage of the peacock, the argus pheasant, and the birds of paradise; the song of the lark, the thrush, and the nightingale; the brilliant hues on the face of the mandrill; and the attractive perfume of the musk-deer, the snakes, and the scented butterflies. Wherever one sex possesses any decorative or alluring adjunct not equally shared by the other, Darwin attributed this special gift either to the law of battle, or to the long and slowly exerted selective action of their fastidious mates.

The germ of the doctrine of sexual selection is to be found, like so many other of Charles Darwin's theories, in a prophetic passage of his grandfather's 'Zoonomia.' Stags, the Lichfield physician tells us, are provided with antlers 'for the purpose of combating other stags for the exclusive possession of the females, who are observed, like the ladies in the time of chivalry, to attend the car of the victor. The birds which do not carry food to their young, and do not therefore marry, are armed with spurs for the purpose of fighting for the exclusive possession of the females, as cocks and quails. It is certain that these weapons are not provided for their defence against other adversaries, because the females of these species are without this armour. The final cause of this contest among the males seems to be that the strongest and most active animal should propagate the species, which should thence become improved.'

It must be noticed, however, that Erasmus Darwin here imports into the question the metaphysical and teleological notion of the final cause, implying that the struggle of the males was ordained from without, for this express and preconceived purpose; whereas Charles Darwin, never transcending the world of phenomena, more logically regards the struggle itself as an efficient cause, having for its result the survival of the strongest or the handsomest as the case may be. This distinction is fundamental; it marks the gulf between the essentially teleological spirit of the eighteenth century and the essentially positive spirit of philosophy and science at the present day.

Here again, too, the immense logical superiority of Charles Darwin's rigorous and exhaustive inductive method over the loose suggestiveness of his grandfather Erasmus may easily be observed. For while Erasmus merely throws out a clever and interesting hint as to the supposed method and intention of nature, Charles Darwin proves his thesis, point by point, with almost mathematical exactitude, leaving no objection unmet behind him, but giving statistical and inductive warrant for every step in his cumulative argument. He goes carefully into the numerical proportion of the two sexes in various species; into the relative dates of arrival in any particular country of the males and females of migratory birds; into the question whether any individuals ever remain in the long run unpaired; into the chances of the earliest-mated or most vigorous couples leaving behind more numerous or stronger offspring to represent them in the next generation. He collects from every quarter and from all sources whatever available evidence can be obtained as to the courtship and rivalry of birds and butterflies, of deer and antelopes, of fish and lizards. He shows by numerous examples and quotations how even flies coquet together in their pretty rhythmical aerial dances; how wasps battle eagerly with one another to secure possession of their unconcerned mates; how cicadas strive to win their 'voiceless brides' with stridulating music; how sphinx-moths endeavour to allure their partners with the musky odour of their pencilled wings; and how emperors and orange-tips display their gorgeous spots and bands in the broad sunshine before the admiring and attentive eyes of their observant dames. He traces up the same spirit of rivalry and ostentation to the cock-pheasant strutting about before the attendant hen, and to the meeting-places of the blackcock, where all the males of the district fight with one another and undertake long love-dances in regular tournaments, while the females stand by and watch the chances and changes of the contest with affected indifference. Finally, he points out how similar effects are produced by like causes among the higher animals, especially among our near relations the monkeys; and then he proceeds to apply the principles thus firmly grounded to the particular instance of the human race itself, the primary object of his entire treatise.

Some of the most interesting of the modifications due to this particular form of selective action are to be found amongst the insects and other low types of animal life. The crickets, the locusts, and the grasshoppers, for example, are all famous for their musical powers; but the sounds themselves are produced in the different families by very different and quaintly varied organs. The song of the crickets is evoked by the scraping of minute teeth on the under side of either wing-cover; in the case of the locusts, the left wing, which acts as a bow, overlies the right wing, which serves as a fiddle; while with the grasshoppers, the leg does duty as the musical instrument, and has a row of lancet-shaped elastic knobs along its outer surface, which the insect rubs across the nerves of the wing-covers when it wishes to charm the ears and rouse the affection of its silent mate. In a South African species of the same family, the whole body of the male is fairly converted into a musical instrument, being immensely inflated, hollow, and distended like a pellucid air-bladder in order to act as an efficient sounding-board. Among the beetles, taste seems generally to have specialised itself rather on form than on music or colour, and the males are here usually remarkable for their singular and very complicated horns, often compared in various species to those of stags or rhinoceroses, and entirely absent in the females of most kinds. But it is among the butterflies and moths that insect æstheticism has produced its greatest artistic triumphs; for here the beautiful eye-spots and delicate markings on the expanded wing-membranes are almost certainly due to sexual selection.

The higher animals display like evidence of the same slow selective action. The courtship of the stickleback, who dances 'mad with delight' around the mate he has allured into the nest he prepares for her, has been observed by dozens of observers both before and since in the domestic aquarium. The gem-like colours of the male dragonet, the butterfly wings of certain gurnards, and the decorated tails of some exotic carps all point in the same direction. Our own larger newt is adorned during the breeding season with a serrated crest edged with orange; while in the smaller kind the colours of the body acquire at the same critical period of love-making a vivid brilliancy. The strange horns and luridly coloured throat-pouches of tropical lizards are familiar to all visitors in equatorial climates, and they are confined exclusively to the male sex. Among birds, the superior beauty of the male plumage is known to everybody; and their greatest glory invariably coincides with the special season for the selection of mates. In the spring, as even our poets have told us, the wanton lapwing gets himself another crest. The law of battle produces the spur of the game-birds and the still stranger wing-spurs of certain species of the plover kind. Æsthetic rivalry is answerable rather for vocal music, and for the plumage of the umbrella-bird, the lyre-bird, the humming-birds, and the cock of the rocks. Among mammals, strength rather than beauty seems to have carried the day; horns, and tusks, and spikes, and antlers are here the special guerdon of the victorious males. Yet even mammals show occasional signs of distinctly æsthetic and artistic preferences, as in the gracefully twisted horns of the koodoo, the scent-glands of the musk-deer or of certain antelopes, the brilliant hues of the male mandrill, and the tufts and moustaches of so many monkeys.

It must be frankly conceded that the reception accorded to Darwin's doctrine of sexual selection, even among the biological public, was far less unanimous, enthusiastic, and full than that which had been granted to his more extensive theory of survival of the fittest. Many eminent naturalists declined from the very outset to accept the conclusions thus definitely set before them, and others who at first seemed disposed to bow to the immense weight of Darwin's supreme authority gradually withdrew their grudging assent from the new doctrine, as they found their relapse backed up by others, and refused to believe that the theory of courtship had been fairly proven before the final tribunal of science. Several critics began by objecting that the whole theory was a mere afterthought. Darwin, they said, finding that natural selection did not suffice by itself to explain all the details of structure in man, had invented sexual selection as a supplementary principle to help it over the hard places. Those who wrote and spoke in this thoughtless fashion could have had but a very inadequate idea of Darwin's close experimental methods of enquiry. As a matter of fact, indeed, they were entirely wrong; the doctrine of sexual selection itself, already faintly foreshadowed by Erasmus Darwin in the 'Zoonomia,' had been distinctly developed in the first edition of the 'Origin of Species' with at least as much provisional elaboration as any other equally important factor in the biological drama as set forth in that confessedly introductory work. Nay, Haeckel had caught gladly at the luminous conception there expressed, even before the appearance of the 'Descent of Man,' and had worked it out in his 'Generelle Morphologie,' with great insight, to its legitimate conclusions in many directions. Indeed, the sole reason why so much space was devoted to the subject in Darwin's work on human development was simply because there for the first time an opportunity arose of utilising his vast store of collected information on this single aspect of the evolutionary process. It was no afterthought, but a necessary and inevitable component element of the fully-developed evolutionary concept.

Still, it cannot be denied that naturalists generally did not accept with effusion the new clause in the evolutionary creed. Many of them hesitated; a few acquiesced; the majority more or less openly dissented. But Darwin's belief remained firm as a rock. 'I am glad you defend sexual selection,' he wrote a few years later in a private letter; 'I have no fear about its ultimate fate, though it is now at a discount;' and in the preface to the second edition of the 'Descent of Man,' he remarks acutely, 'I have been struck with the likeness of many of the half-favourable criticisms on sexual selection with those which appeared at first on natural selection; such as that it would explain some few details, but certainly was not applicable to the extent to which I have employed it. My conviction of the power of sexual selection remains unshaken… When naturalists have become familiar with the idea, it will, as I believe, be much more largely accepted; and it has already been fully and favourably received by several capable judges.'

In spite of the still continued demurrer of not a few among the leading evolutionists, it is probable, I think, that Darwin's prophecy on this matter will yet be justified by the verdict of time. For the opposition to the doctrine of sexual selection proceeds almost invariably, as it seems to me, from those persons who still desire to erect an efficient barrier of one sort or another between the human and animal worlds; while on the contrary the theory in question is almost if not quite universally accepted by just those rigorously evolutionary biologists who are freest from preconceptions or special a priori teleological objections of any kind whatever. The half of the doctrine which deals with the law of battle, indeed, can hardly be doubted by any competent naturalist; the other half, which deals with the supposed æsthetic preferences of the females, is, no doubt, distasteful to certain thinkers because it seems to imply the existence in the lower animals of a sense of beauty which many among us are not even now prepared generously to admit. The desire to arrogate to mankind alone all the higher faculties either of sense or intellect has probably much to do with the current disinclination towards the Darwinian idea of sexual selection. Thinkers who allow themselves to be emotionally swayed by such extraneous considerations forget that the beautiful is merely that which pleases; that beauty has no external objective existence; and that the range of taste, both among ourselves and among animals at large, is practically infinite. The greatest blow ever aimed at the Darwinian theory of sexual selection was undoubtedly that dealt out by Mr. Alfred Russel Wallace (et tu, Brute!) in his able and subtle article on the Colours of Animals in 'Macmillan's Magazine,' since reprinted in his delightful work on 'Tropical Nature.' Wallace there urges with his usual acuteness, ingenuity, and skill several fundamental objections to the Darwinian hypothesis of no little importance and weight. But it must always be remembered (with all due respect to the joint discoverer of natural selection) that Mr. Wallace himself, after publishing his own admirable essay on the development of man, drew back aghast in the end from the full consequences of his own admission, and uttered his partial recantation in the singular words, 'Natural selection could only have endowed the savage with a brain a little superior to that of an ape.' It seems probable that in every case an analogous desire to erect a firm barrier between man and brute by positing the faculty for perceiving beauty as a special quasi-divine differentia of the human race has been at the bottom of the still faintly surviving dislike amongst a section of scientific men to sexual selection. Nevertheless, a candid and impartial critic would be compelled frankly to admit that Darwin's admirable theory of courtship has not on the whole proved so generally acceptable to the biological world up to the present time as his greater and far more comprehensive theory of survival of the fittest. It still waits for its final recognition, towards which it is progressing more rapidly and surely every day it lives.

CHAPTER X

VICTORY AND REST

The last eleven years of Darwin's life were spent in enforcing and developing the principles already reached, and in enjoying the almost unchequered progress of the revolution he had so unconsciously to himself succeeded in inaugurating.

Only one year elapsed between the publication of the 'Descent of Man' and that of its next important successor, the 'Expression of the Emotions.' The occasion of this learned and bulky treatise in itself stands as an immortal proof of the conscientious way in which Darwin went to work to anticipate the slightest and most comparatively impertinent possible objections to his main theories. Sir Charles Bell, in one of the quaintly antiquated Bridgwater treatises – those marvellous monuments of sadly misplaced teleological ingenuity – had maintained that man was endowed with sundry small facial muscles solely for the sake of expressing his emotions. This view was so obviously opposed to the belief in the descent of man from some lower form, 'that,' says Darwin, 'it was necessary for me to consider it;' and so he did, in a lengthy work, where the whole subject is exhaustively treated, and Bell's idea is completely pulverised by the apt allegation of analogous expressions in the animal world. In his old age Darwin grew, in fact, only the more ceaselessly and wonderfully industrious. In 1875, after three years of comparative silence, came the 'Insectivorous Plants,' a work full of minute observation on the habits and manners of the sundew, the butterwort, the Venus's fly-catcher, and the various heterogeneous bog-haunting species known by the common name of pitcher plants. The bare mass and weight of the facts which Darwin had collected for the 'Origin of Species' might well-nigh have stifled the very existence of that marvellous book: it was lucky that the premature publication of Wallace's paper compelled him to hurry on his 'brief abstract,' for if he had waited to select and arrange the whole series of observations that he finally published in his various later justificatory volumes, we might have looked in vain for the great systematic and organising work, which would no doubt have been 'surcharged with its own weight, and strangled with its waste fertility.' But the task that he himself best loved was to watch in minute detail the principles whose secret he had penetrated, and whose reserve he had broken, working themselves out before his very eyes, naked and not ashamed – to catch Actæon-like the undraped form of nature herself in the actual process of her inmost being. He could patiently observe the red and slimy hair-glands of the drosera closing slowly and remorselessly round the insect prey, and sucking from their bodies with sensitive tentacles the protoplasmic juices denied to its leaves by the poor and boggy soil, on which alone its scanty rootlets can properly thrive. He could watch the butterwort curving round the edges of its wan green foliage upon the captured limbs of fly or aphis. He could note how the serried mass of finger-like processes in the utricles of the bladderwort slowly absorb organic matter from the larva of a gnat, or the minute water-insects entangled within its living and almost animated lobster-pot. He could track the long line of treacherous honey-glands by which the sarracenia entices flies into the festering manure-wells of its sticky pitchers. The minuteness and skill of all his observations on these lesser problems of natural selection inevitably inspired faith among outsiders in the cautious judgment of the observer and experimenter; and day by day throughout his later years the evidence of the popular acceptance of his doctrine, and of the dying away of the general ridicule with which it was first received by the unlearned public, was very gratifying to the great naturalist.

A year later, in 1876, came the 'Effects of Cross and Self Fertilisation in the Vegetable Kingdom.' So far as regarded the world of plants, especially with respect to its higher divisions, this work was of immense theoretical importance; and it also cast a wonderful side-light upon the nature of that strange distinction of sex which occurs both in the vegetable and animal kingdom, and in each is the concomitant – one might almost say the necessary concomitant – of high development and complex organisation. The great result attained by Darwin in his long and toilsome series of experiments on this interesting subject was the splendid proof of the law that cross-fertilisation produces finer and healthier offspring, while continuous self-fertilisation tends in the long run to degradation, degeneration, and final extinction.

Here as elsewhere, however, Darwin's principle does not spring spontaneous, like Athene from the head of Zeus, a goddess full-formed, uncaused, inexplicable: it arises gradually by a slow process of development and modification from the previous investigations of earlier biologists. At the close of the last century, in the terrible year of upheaval 1793, a quiet German botanist, Christian Konrad Sprengel by name, published at Berlin his long unheeded but intensely interesting work on the 'Fertilisation of Flowers.' In the summer of 1789, while all Europe was ablaze with the news that the Bastille had been stormed, and a new era of humanity begun, the calm and peaceful Pomeranian observer was noting in his own garden the curious fact that many flowers are incapable of being fertilised without the assistance of flying insects, which carry pollen from the stamens of one blossom to the sensitive surface or ovary of the next. Hence he concluded that the secretion of honey or nectar in flowers, the contrivances by which it is protected from rain, the bright hues or lines of the corolla, and the sweet perfume distilled by the blossoms, are all so many cunning devices of nature to ensure fertilisation by the insect-visitors. Moreover, Sprengel observed that many flowers are of one sex only, and that in several others the sexes do not mature simultaneously; 'so that,' said he, 'nature seems to intend that no flower shall be fertilised by means of its own pollen.' Indeed, in some instances, as he showed by experiments upon the yellow day lily, plants impregnated from their own stamens cannot be made to set seed at all. 'So near,' says his able successor, Hermann Müller, 'was Sprengel to the distinct recognition of the fact that self-fertilisation leads to worse results than cross-fertilisation, and that all the arrangements which favour insect-visits are of value to the plant itself, simply because the insect-visitors effect cross-fertilisation!' As in most other anticipatory cases, however, it must be here remarked that Sprengel's idea was wholly teleological: he conceived of nature as animated by a direct informing principle, which deliberately aimed at a particular result; whereas Darwin rather came to the conclusion that cross-fertilisation as a matter of fact does actually produce beneficial results, and that therefore those plants which varied most in the direction of arrangements for favouring insect-visits were likely to be exceptionally fortunate in the struggle for existence against competitors otherwise arranged. It is just the usual Darwinian substitution of an efficient for a final cause.

Even before Sprengel, Kölreuter had recognised, in 1761, that self-fertilisation was avoided in nature; and his observations and experiments on intercrossing and on hybridism were largely relied upon by Darwin himself, to whom they suggested at an early period many fruitful lines of original investigation. In 1799, again, Andrew Knight, following up the same line of thought in England as Sprengel in Germany, declared as the result of his close experiments upon the garden pea, that no plant ever fertilises itself for a perpetuity of generations. But Knight's law, not being brought into causal connection with any great fundamental principle of nature, was almost entirely overlooked by the scientific world until the publication of Darwin's 'Origin of Species,' half a century later. The same neglect also overtook Sprengel's immensely interesting and curious work on fertilisation of flowers. The world, in fact, was not yet ready for the separate treatment of functional problems connected with the interrelations of organic beings; so Knight and Sprengel were laid aside unnoticed on the dusty top bookshelves of public libraries, while the dry classificatory and systematic biology of the moment had it ail its own way for the time being on the centre reading-tables. So many separate and independent strands of thought does it ultimately require to make up the grand final generalisation which the outer world attributes in its totality to the one supreme organising intelligence.