Kitabı oku: «Charles Darwin», sayfa 8
CHAPTER VIII
THE DESCENT OF MAN
In 1871, nearly twelve years after the 'Origin of Species,' Darwin published his 'Descent of Man.'
We have seen already that he would fain have avoided the treatment of this difficult and dangerous topic a little longer, so as to let his main theory be fairly judged on its own merits, without the obtrusion of theological or personal feelings into so purely biological a question; but the current was too strong for him, and at last he yielded. On the one hand, the adversaries had drawn for themselves the conclusion of man's purely animal origin, and held it up to ridicule under false forms in the most absurd and odious light. On the other hand, imprudent allies had put forth under the evolutionary ægis their somewhat hypothetical and extravagant speculations on this involved subject, which Darwin was naturally anxious to correct and modify by his own more sober and guarded inferences. The result was the second great finishing work of the complete Darwinian system of things.
Ever since evolutionism had begun to be at all it had been observed that a natural corollary from the doctrine of descent with modification was the belief in man's common ancestry with the anthropoid apes. As early as the middle of the last century, indeed, Lord Monboddo, a whimsical Scotch eccentric, had suggested in his famous book on the origin of language the idea that men were merely developed monkeys. But this crude and unorganised statement of a great truth, being ultimately based upon no distinct physical grounds, deserved scarcely to be classed higher than the childish evolutionism of 'Telliamed' De Maillet, which makes birds descend from flying-fish and men the offspring of the hypothetical tritons. On this point as on most others the earliest definite scientific views are those of Buffon, who ventured to hint with extreme caution the possibility of a common ancestry for man and all other vertebrate animals. Goethe the all-sided had caught a passing glimpse of the same profound conception about the date of the Reign of Terror; and Erasmus Darwin had openly announced it, though without much elaboration, in his precocious and premature 'Zoonomia.' Still more specifically, in a note to the 'Temple of Nature,' the English evolutionist says: 'It has been supposed by some that mankind were formerly quadrupeds… These philosophers, with Buffon and Helvetius, seem to imagine that mankind arose from one family of monkeys on the banks of the Mediterranean;' and in the third canto of that fantastic poem, he enlarges upon the great part performed by the hand, with its opposable thumb, in the development and progress of the human species. Lamarck, in his 'Philosophie Zoologique,' distinctly lays down the doctrine that man is descended from an ape-like ancestor, which gradually acquired the upright position, not even now wholly natural to the human race, and maintained only by the most constant watchfulness. The orang-outang was then the highest known anthropoid ape; and it was from the orang-outang, therefore, that the fancy of Lyell and other objectors in the pre-Darwinian days continually derived the Lamarckian Adam.
The introduction of the chimpanzee into our European Zoological Gardens gave a fresh type of anthropoid to the crude speculators of the middle decades of the century; and in 1859, Paul du Chaillu, the explorer and hunter of the Gaboon country, brought over to America and Europe the first specimens of the true gorilla ever seen by civilised men. There can be little doubt that the general interest excited by his narrative of his adventures (published in London in 1861) and by the well-known stuffed specimen of the huge African anthropoid ape so long conspicuous in the rooms of the British Museum, and now surviving (somewhat the worse for wear) in the natural history collection at South Kensington, did much to kindle public curiosity as to the nature of our relations with the lower animals. It is no mere accidental circumstance, indeed, that Huxley should have brought out 'Man's Place in Nature' just two years after Du Chaillu's 'Explorations and Adventures in Equatorial Africa' had made the whole world, lay and learned, familiar with the name and features of the most human in outer aspect among the anthropoid family. Thenceforth the gorilla, and not the orang-outang, was popularly hit upon by scoffer and caricaturist as the imaginary type of our primitive ancestors.
On the other hand, during the twelve intervening years immense strides had been made in every department of anthropological science, and the whole tenor of modern speculation had been clearing the ground for the 'Descent of Man,' In 1865, Rolle in Germany had published his work on 'Man Viewed by the Light of the Darwinian Theory.' Two years later, Canestrini in Italy read before the Naturalists' Society of Modena his interesting paper on rudimentary characters as bearing on the origin of the human species. In 1868, Büchner brought out his rudely materialistic sledge-hammer lectures on the Darwinian principle; and in 1869, Barrago flung straight at the head of the Roman clericals his offensive work on man and the anthropoid apes. Most of these foreign publications were unhappily marked by that coarse and almost vituperative opposition to received views which too often disfigures French and German controversial literature. In England, on the contrary, under our milder and gentler ecclesiastical yoke, the contest had been conducted with greater decorum and with far better results. Wallace had broken ground tentatively and reverently in his essay on the 'Origin of Human Races,' where he endeavoured to show that man is the co-descendant with the anthropoid apes of some ancient lower and extinct form. Lubbock's 'Prehistoric Times' (1865) and 'Origin of Civilisation' (1870) helped to clear the way in the opposite direction by demolishing the old belief, firmly upheld by Whately and others, that savages represent a degraded type, and that the civilised state is natural and, so to speak, congenital to man. Tylor's 'Early History of Mankind' (1865) did still more eminent service in the same direction. Colenso's 'Pentateuch and Book of Joshua Critically Examined,' the publication of which began in 1862, had already shaken the foundations of the Mosaic cosmogony, and incidentally discredited the received view of the direct creation of the first human family. McLennan's 'Primitive Marriage' (1865) and Herbert Spencer's articles on the origin of religion had kept speculation alive along other paths, all tending ultimately towards the same conclusion. Darwin's own cousin, Hensleigh Wedgwood, and Canon Farrar, had independently endeavoured to prove that language, instead of being a divine gift, might have arisen in a purely natural manner from instinctive cries and the imitation of external sounds. The Duke of Argyll and Professor Max Müller, by the obvious feebleness of their half-hearted replies, had unconsciously aided in disseminating and enforcing the very views they attempted to combat. Bagehot and Flower, Maudsley and Jevons, Vogt and Lindsay, Galton and Brown-Séquard had each in his way contributed facts and arguments ultimately utilised by the great master architect in building up his consistent and harmonious edifice. Finally, in 1868, Haeckel had published his 'Natural History of Creation,' in which he discussed with surprising and perhaps excessive boldness the various stages in the genealogy of man. These various works, following so close upon Huxley's 'Man's Place in Nature' and Lyell's conclusive 'Antiquity of Man,' left Darwin no choice but to set forth his own reasoned opinions on the subject of the origin and development of the human species.
The evidence of the descent of man from some lower form, collected and marshalled together by Darwin, consists chiefly of minute inferential proofs which hardly admit of deliberate condensation. In his bodily structure man is formed on the same underlying type or model as all the other mammals, bone answering throughout to bone, as, for example, in the fore limb, where homologous parts have been modified in the dog into toes, in the bat into wing-supports, in the seal into flippers, and in man himself into fingers and thumb, while still retaining in every case their essential fundamental likeness of construction. Even the brain of man resembles closely the brain of the higher monkeys; the differences which separate him in this respect from the orang or the gorilla are far slighter than the differences which separate those apes themselves from the inferior monkeys. Indeed, as Huxley conclusively showed, on anatomical grounds alone, man must be classed in the order Primates as only one among the many divergent forms which that order includes within its wide limits.
In his embryonic development man closely resembles the lower animals, the human creature being almost indistinguishable in certain stages from the dog, the bat, the seal, and especially the monkeys. At a very early age he possesses a slight projecting tail; at another, the great toe is shorter than its neighbours, and projects like the thumb at a slight angle; and at a third, the convolutions of the brain reach a point of development about equivalent to that of the adult baboon. In his first stages man himself stands far more closely related to the apes than the apes in turn stand to cats or hyænas.
Rudiments of muscles not normally found in man occur in many aberrant human individuals. Some people possess the power of moving their scalps and wagging their ears like dogs and monkeys; others can twitch the skin of their bodies, as horses do when worried by flies. Mr. Woolner, the sculptor, pointed out to Darwin a certain little projecting point or knob on the margin of the ear, observed by him in the course of modelling, which comparison shows to be the last folded remnant or rudiment of the once erect and pointed monkey-like ear-tip. The nictitating membrane, or third eyelid, once more, which in birds can be drawn so rapidly across the ball of the eye, and which gives the familiar glazed or murky appearance, is fairly well developed in the ornithorhyncus and the kangaroo, as well as in a few higher mammals, like the walrus; but in man, as in the monkey group, it survives only under the degenerate form of a practically useless rudiment, the semilunar fold. Man differs from the other Primates in his apparently hairless condition; but the hair, though short and downy, still remains on close inspection, and in some races, such as the Ainos of Japan, forms a shaggy coat like an orang's or a gibbon's. A few long rough hairs sometimes project from the short smooth down of the eyebrows; and these peculiar bristles, occasional only in the human species, are habitual in the chimpanzee and in many baboons. Internal organs show similar rudiments, of less enthralling interest, it must be candidly confessed, to the unscientific outside intelligence. Even the bony skeleton contributes its share of confirmatory evidence; for in the lower monkeys and in many other mammals a certain main trunk nerve passes through a special perforation in the shoulder-blade, and this perforation, though now almost obsolete, sometimes recurs in man, in which case the nerve in question invariably passes through it, as in the inferior monkeys. What is still more remarkable is the fact that the perforation occurs far more frequently (in proportion) among the skeletons of very ancient races than among those of our own time. One chief cause why in this and other cases ancient races often present structures resembling those of the lower animals seems to be that they stand nearer in the long line of descent to their remote animal-like progenitors.
The conclusion at which, after fully examining all the evidence, Darwin finally arrives is somewhat as follows:
The early ancestors of man must have been more or less monkey-like animals, belonging to the great anthropoid group, and related to the progenitors of the orang-outang, the chimpanzee, and the gorilla. They must have been once covered with hair, both sexes possessing beards. Their ears were probably pointed and capable of movement, and their bodies were provided with a movable tail. The foot had a great toe somewhat thumb-like in its action, with which they could grasp the branches of trees. They were probably arboreal in their habits, fruit-eaters by choice, and inhabitants of some warm forest-clad land. The males had great canine teeth, with which they fought one another for the possession of the females. At a much earlier period, the internal anatomical peculiarities approached those of the lowest mammals, and the eye was provided with a third eyelid. Peering still further back into the dim abyss of the ages, Darwin vaguely describes the ancestors of humanity as aquatic animals, allied to the mudfish; for our lungs are known to consist of modified swim-bladders, which must once have served our remote progenitors in the office of a float. The gill-clefts on the neck of the human embryo still point to the spot where the branchiæ once, no doubt, existed. Our primordial birthplace appears to have been a shore washed twice a day by the recurrent tides. The heart then took the shape merely of a simple pulsating vessel; and a long undivided spinal cord usurped the place of the vertebral column. These extremely primitive ancestors of man, thus dimly beheld across the gulf of ages, must have been at least as simply and humbly organised as that very lowest and earliest of existing vertebrates, the worm-like lancelet.
From such a rude and indefinite beginning natural selection, aided by the various concomitant principles, has slowly built up the pedigree of man. Starting from these remote half-invertebrate forms, whose vague shape is still perhaps in part preserved for us by the soft and jelly-like larva of the modern ascidian, we rise by long stages to a group of early fishes, like the lancelet itself. From these the ganoids and then the lung-bearing mudfish must have been gradually developed. From such fish a very small advance would carry us on to the newts and other amphibians. The duck-billed platypus helps us slightly to bridge over the gap between the reptiles and the lower mammals, such as the kangaroo and the wombat, though the connection with the amphibians is still, as when Darwin wrote, highly problematical. From marsupials, such as the kangaroo, we ascend gradually to the insectivorous type represented by the shrews and hedgehogs, and thence once more by very well-marked intermediate stages to the lemurs of Madagascar, a group linked on the one hand to the insectivores, and on the other to the true monkeys. The monkeys, again, 'branched off into two great stems – the New World and Old World monkeys; and from the latter, at a remote period, man, the wonder and glory of the universe, proceeded.'
The word was spoken; the secret was out. The world might well have been excused for treating it scornfully. But as a matter of fact, the storm which followed the 'Descent of Man' was as nothing compared with the torrent of abuse that had pursued the author of the 'Origin of Species.' In twelve years society had grown slowly accustomed to the once startling idea, and it listened now with comparatively languid interest to the final utterance of the great biologist on the question of its own origin and destinies. In 1859 it cried in horror, 'How very shocking!' in 1871, it murmured complacently, 'Is that all? Why, everybody knew that much already!'
Nevertheless, on the moral and social side, the ultimate importance of the 'Descent of Man' upon the world's history can hardly be overrated by a philosophic investigator. Vast as was the revolution effected in biology by the 'Origin of Species,' it was as nothing compared with the still wider, deeper, and more subtly-working revolution inaugurated by the announcement of man's purely animal origin. The main discovery, strange to say, affected a single branch of thought alone; the minor corollary drawn from it to a single species has already affected, and is destined in the future still more profoundedly to affect, every possible sphere of human energy. Not only has it completely reversed our entire conception of history generally, by teaching us that man has slowly risen from a very low and humble beginning, but it has also revolutionised our whole ideas of our own position and our own destiny, it has permeated the sciences of language and of medicine, it has introduced new conceptions of ethics and of religion, and it threatens in the future to produce immense effects upon the theory and practice of education, of politics, and of economic and social science. These wide-reaching and deep-seated results began to be felt from the first moment when the Darwinian principle was definitely promulgated in the 'Origin of Species,' but their final development and general acceptance was immensely accelerated by Darwin's own authoritative statement in the 'Descent of Man.'
To some among us still, as to Lyell before us, this new belief in the animal origin of man seems far less beautiful, noble, and inspiriting than the older faith in his special and separate divine creation. Such thinkers find it somehow more pleasant and comfortable to suppose that man has fallen than that man has risen; the doctrine of the universal degradation of humanity paradoxically appears to them more full of promise and aspiration for the times to come than the doctrine of its universal elevation. To Darwin himself, however, it seemed otherwise. 'Man,' he says, 'may be excused for feeling some pride at having risen, though not through his own exertions, to the very summit of the organic scale; and the fact of his having thus risen, instead of having been aboriginally placed there, may give him hope for a still higher destiny in the distant future.' Surely this is the truer and manlier way of looking at the reversed and improved attitude of man. Surely it is better to climb to the top than to have been placed there – and fallen – at the very outset. Surely it is a nobler view of life that we may yet by our own strenuous exertions raise our race some places higher in the endless and limitless hierarchy of nature than that we are the miserable and hopelessly degenerate descendants of a ruined and degraded angelic progenitor. Surely it is well, while we boast with Glaucus that we indeed are far braver and better than our ancestors, to pray at the same time, in the words of Hector, that our sons may be yet braver and better than ourselves.
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.
