Kitabı oku: «Charles Darwin», sayfa 5

During the fifteen years from 1844 to 1859, however, Darwin's pen was by no means idle. In the first-named year he published his 'Geological Observations on Volcanic Islands' – part of the 'Beagle' exploration series; in 1846 he followed this up by his 'Geological Observations on South America;' in 1851 he gave to the world his monograph on 'Recent Barnacles;' and in 1853, his treatise on the fossil species of the same family. But all these works of restricted interest remained always subsidiary to the one great central task of his entire lifetime, the preparation of his projected volume on the Origin of Species.

All through the middle decades of the century Darwin continued to labour at his vast accumulation of illustrative facts; and side by side with his continuous toil, outside opinion kept paving the way for the final acceptance of his lucid ideas. The public was buying and reading all the time its ten editions of the 'Vestiges of Creation.' It was slowly digesting Lyell's 'Principles of Geology,' in which the old cataclysmic theories were featly demolished, and the uniformitarian conception of a past gradually and insensibly merging into the present was conclusively established. It was getting accustomed to statements like those of the younger St. Hilaire, in 1850, that specific characters may be modified by changes in the environing conditions, and that the modifications thus produced may often be of generic value – may make a difference so great that we must regard the product not merely as belonging to a distinct species, but even to a distinct genus or higher kind. In 1852 Herbert Spencer published in the 'Leader' his remarkable essay, contrasting the theories of creation and evolution, as applied to organic beings, with all the biting force of his profound intelligence; and in 1855, the same encyclopædic philosopher put forth the first rough sketch of his 'Principles of Psychology,' in which he took the lead in treating the phenomena of mind from the point of view of gradual development. In that extraordinary work, the philosopher of evolution traced the origin of all mental powers and faculties by slow gradations from the very simplest subjective elements. The 'Principles of Psychology' preceded the 'Origin of Species' by nearly five years; the first collected volume of Mr. Spencer's essays preceded Darwin's work by some twelve months. Baden-Powell's essay on the 'Philosophy of Creation' (much debated and condemned in ecclesiastical circles), and Professor Owen's somewhat contradictory utterances on the nature of types and archetypal ideas, also helped to keep alive interest in the problem of origins up to the very moment of the final appearance of Darwin's great and splendid solution.

It is interesting during these intermediate years to watch from time to time the occasional side-hints of Darwin's activity and of the interest it aroused among his scientific contemporaries. In 1854, for example, Sir Charles Lyell notes, after an evening at Darwin's, how Sir Joseph Hooker astonished him with an account of that strange orchid, Catasetum, which bears three totally distinct kinds of flowers on the same plant; 'It will figure,' he says, 'in C. Darwin's book on species, with many other "ugly facts," as Hooker, clinging like me to the orthodox faith, calls these and other abnormal vagaries.' On a similar occasion, a little later, Lyell asks, after meeting 'Huxley, Hooker, and Wollaston at Darwin's,' 'After all, did we not come from an ourang?' Last of all, in 1857, Darwin himself writes an anticipatory letter to his American friend, Asa Gray, in which he mentions 'six points' – the cardinal conceptions of the 'Origin of Species.' His book is now fairly under weigh; he speaks of it himself to acquaintance and correspondents as an acknowledged project.

Events were growing ripe for the birth. A lucky accident precipitated its parturition in the course of the year 1858.

CHAPTER VI
'THE ORIGIN OF SPECIES.'

The accident came in this wise.

Alfred Russel Wallace, a young Welsh biologist, went out at twenty-four, in 1848, to the Amazons River, in company with Bates (the author of 'The Naturalist on the Amazons'), to collect birds and butterflies, and to study tropical life in the richest region of equatorial America. Like all other higher zoologists of their time, the two young explorers were deeply interested in the profound questions of origin and metamorphosis, and of geographical distribution, and in the letters that passed between them before they started they avowed to one another that the object of their quest was a solution of the pressing biological enigma of creation or evolution. Starting with fresh hopes and a few pounds in pocket, on an old, worn-out, and unseaworthy slave-trader, they often discussed these deep problems of life and nature together upon the Sargasso sea, or among the palms and lianas of the Brazilian woodlands. The air was thick with whiffs and foretastes of evolutionism, and the two budding naturalists of the Amazons expedition had inhaled them eagerly with every breath. They saw among the mimicking organisms of that equatorial zone strange puzzles to engage their deepest attention; they recognised in the veins and spots that diversified the filmy membranes of insects' wings the hieroglyphs of nature, writing as on a tablet for them to decipher the story of the slow modification of species. In 1852 – the year when Herbert Spencer in England published his essay on the 'Development Hypothesis,' and when Naudin in France put forth his bold and able paper on the 'Origin of Species' – Wallace once more returned to Europe, and gave to the world his interesting 'Travels on the Amazons and the Rio Negro.' Two years later the indefatigable traveller set out a second time on a voyage of tropical exploration, among the islands of the Malay archipelago, and for eight years he wandered about in Malay huts and remote islets, gathering in solitude and isolation the enormous store of minute facts which he afterwards lavished with so prodigal a hand upon 'Tropical Nature,' and the 'Geographical Distribution of Animals.'

While Wallace was still at Amboyna, he sent home in 1858 a striking memoir, addressed to Darwin, with a request that he would forward it to Sir Charles Lyell, for presentation to the Linnean Society. Darwin opened and read his brother naturalist's paper, and found to his surprise that it contained his own theory of natural selection, not worked out in detail, as he himself was working it out, but still complete in spirit and essence, with no important portion of the central idea lacking to its full rotundity of conception. A jealous man would have thrown obstacles in the way of publication; but both Darwin and Wallace were born superior to the meannesses of jealousy. The elder naturalist commended his young rival's paper at once to Sir Charles Lyell, who sent it on immediately to the Linnean Society.

But Sir Charles Lyell and Sir Joseph Hooker, both of whom knew of Darwin's work, thought it advisable that he should publish, in the 'Journal' of the Society, a few extracts from his own manuscripts, side by side with Wallace's paper. Darwin, therefore, selected some essential passages for the purpose from his own long-gathered and voluminous notes, and the two contributions were read together before the Society on July the 1st, 1858. That double communication marks the date of birth of modern evolutionism. It is to the eternal credit of both thinkers that each accepted his own true position with regard to the great discovery in perfect sincerity. The elder naturalist never strove for a moment to press his own claim to priority against the younger: the younger, with singular generosity and courtesy, waived his own claim to divide the honours of discovery in favour of the elder. Not one word save words of fraternal admiration and cordial appreciation ever passed the lips of either with regard to the other.

The distinctive notion of natural selection, indeed, like all true and fruitful ideas, had more than once flashed for a moment across the penetrating mind of more than one independent investigator. As early as 1813, Dr. Wells, the famous author of the theory of dew, applied that particular conception to the single case of the production of special races among mankind.

'Of the accidental varieties of man, which would occur among the first few and scattered inhabitants of the middle regions of Africa,' he wrote, 'some one would be better fitted than the others to bear the diseases of the country. This race would consequently multiply, while the others would decrease; not only from their inability to sustain the attacks of disease, but from their incapacity of contending with their more vigorous neighbours… The same disposition to form varieties still existing, a darker and a darker race would in the course of time occur; and as the darkest would be the best fitted for the climate, this would at last become the most prevalent, if not the only race in the country.' Here we have not merely the radical concept of natural selection, but also the subordinate idea of its exertion upon what Darwin calls 'spontaneous variations.' What is wanting in the paper is the application of the faintly descried law to the facts and circumstances of general biology: Wells saw only a particular instance, where Darwin and Wallace more vividly perceived a universal principle. Again, in 1831, Mr. Patrick Matthew in that singular appendix to his book on naval timber actually enunciates the same idea, applied this time to the whole of nature, in words sometimes almost identical with Darwin's own. 'As nature in all her modifications of life,' says this unconscious discoverer, 'has a power of increase far beyond what is needed to supply the place of what falls by Time's decay, those individuals who possess not the requisite strength, swiftness, hardihood, or cunning, fall prematurely without reproducing – either a prey to their natural devourers, or sinking under disease, generally induced by want of nourishment, their place being occupied by the more perfect of their own kind, who are pressing on the means of existence… The self-regulating adaptive disposition of organised life may, in part, be traced to the extreme fecundity of nature, who, as before stated, has in all the varieties of her offspring a prolific power much beyond (in many cases a thousandfold) what is necessary to fill up the vacancies caused by senile decay. As the field of existence is limited and preoccupied, it is only the hardier, more robust, better-suited-to-circumstance individuals, who are able to struggle forward to maturity, these inhabiting only the situations to which they have superior adaptation and greater power of occupancy than any other kind; the weaker and less circumstance-suited being prematurely destroyed. This principle is in constant action; it regulates the colour, the figure, the capacities, and instincts; those individuals in each species whose colour and covering are best suited to concealment or protection from enemies, or defence from inclemencies and vicissitudes of climate, whose figure is best accommodated to health, strength, defence, and support; whose capacities and instincts can best regulate the physical energies to self-advantage according to circumstances – in such immense waste of primary and youthful life those only come forward to maturity from the strict ordeal by which nature tests their adaptation to her standard of perfection and fitness to continue their kind by reproduction.' Of the ideas expressed in these paragraphs, and others which preceded them, Darwin himself rightly observes, 'He gives precisely the same view on the origin of species as that propounded by Mr. Wallace and myself. He clearly saw the full force of the principle of natural selection.'

In 1852, once more, so eminent and confirmed an evolutionist as Mr. Herbert Spencer himself had hit upon a glimpse of the same great truth, strange to say without perceiving the width and scope of its implications. 'All mankind,' he wrote in that year in an essay on population in the 'Westminster Review,' 'in turn subject themselves more or less to the discipline described; they either may or may not advance under it; but, in the nature of things, only those who do advance under it eventually survive. For, necessarily, families and races whom this increasing difficulty of getting a living which excess of fertility entails does not stimulate to improvements in production… are on the high road to extinction; and must ultimately be supplanted by those whom the pressure does so stimulate… And here, indeed, without further illustration, it will be seen that premature death, under all its forms, and from all its causes, cannot fail to work in the same direction. For as those prematurely carried off must, in the average of cases, be those in whom the power of self-preservation is the least, it unavoidably follows that those left behind to continue the race must be those in whom the power of self-preservation is the greatest, must be the select of their generation.' In this striking pre-Darwinian passage we have a partial perception of what Mr. Spencer afterwards described as the survival of the fittest; but, as our great philosopher himself remarks, it 'shows how near one may be to a great generalisation without seeing it.' For not only does Mr. Spencer, like Wells before him, limit the application of the principle to the case of humanity; but, unlike Wells, he overlooks the all-important factor of spontaneous variation, and the power of natural selection, acting upon such, to produce specific and generic divergences of structure. In short, in his own words, the paragraph 'contains merely a passing recognition of the selective process, and indicates no suspicion of the enormous range of its effects, or of the conditions under which a large part of its effects are produced.' On the other hand, it must be noted that both Spencer and Matthew, like Darwin himself, based their ideas largely upon the Malthusian principle, and thus held the two true keys of the situation fairly within their unconscious hands.

Frankly to recognise these various foreshadowings of the distinctive Darwinian theory of natural selection is not in any way to undermine the foundations of Charles Darwin's own real and exceptional greatness. On the contrary, the mere fact that his views were so far anticipated by Wells, Matthew, Spencer, and others, and were simultaneously arrived at across half the globe by the independent intellect of Alfred Russel Wallace, is in itself the very best proof and finest criterion of Charles Darwin's genuine apostleship. No truly grand and fruitful idea was ever yet the sole property of a single originator. Great discoveries, says an acute critic, must always be concerned with some problem of the time which many of the world's foremost minds are just then cudgelling their active brains about. It was so with the discovery of the differential calculus, and of the planet Neptune; with the interpretation of the Egyptian hieroglyphics, and of the cuneiform inscriptions; with the undulatory theory of light, with the mechanical equivalent of heat, with the doctrine of the correlation and conservation of energies, with the invention of the steam engine, the locomotive, the telegraph and the telephone; with the nebular hypothesis, and with spectrum analysis. It was so, too, with the evolutionary movement. The fertile upturning of virgin sod in the biological field which produced Darwin's forerunners, as regards the idea of descent with modification, in the persons of Buffon, Lamarck, and Erasmus Darwin, necessarily produced a little later, under the fresh impetus of the Malthusian conception, his forerunners or coadjutors, as regards the idea of natural selection, in the persons of Wells, Matthew, and Wallace. It was Darwin's task to recognise the universal, where Wells and Spencer had seen only the particular; to build up a vast and irresistible inductive system, where Matthew and Wallace had but thrown out a pregnant hint of wonderful a priori interest and suggestiveness. It is one thing to draw out the idea of a campaign, another thing to carry it to a successful conclusion; one thing rudely to sketch a ground-plan, another thing finally to pile aloft to the sky the front of an august and imposing fabric.

As soon as the papers at the Linnean had been read and printed, Darwin set to work in real earnest to bring out the first instalment of his great work. That instalment was the 'Origin of Species.' The first edition was ready for the public on November the 24th, 1859.

In his own mind Darwin regarded that immortal work merely in the light of an abstract of his projected volumes. So immense were his collections and so voluminous his notes that the 'Origin of Species' itself seemed to him like a mere small portion of the contemplated publication. And indeed he did ultimately work out several other portions of his original plan in his detailed treatises on the Variation of Animals and Plants under Domestication, on the Effects of Cross and Self-Fertilisation, and on the Descent of Man and Sexual Selection. But the immense and unexpected vogue of his first volume, the almost immediate revolution which it caused in biological and general opinion, and the all but universal adhesion to his views of all the greatest and most rising naturalists, to a great extent saved him the trouble of carrying out in full the task he had originally contemplated as necessary. Younger and less occupied labourers took part of the work off their leader's hands; the great chief was left to prosecute his special researches in some special lines, and was relieved from the necessity of further proving in minuter detail what he had already proved with sufficient cogency to convince all but the wilfully blind or the hopelessly stupid.

The extraordinary and unprecedented success of the 'Origin of Species' is the truest test of the advance it made upon all previous evolutionary theorising. Those who had never been convinced before were now convinced by sheer force of reasoning; those who believed and those who wavered had their faith confirmed into something like the reposeful calm of absolute certitude.

Let us consider, therefore, what exactly were the additions which Charles Darwin offered in his epoch-making work to the pre-existing conceptions of evolutionists.

In 1852, seven years before the publication of Darwin's masterpiece, Mr. Herbert Spencer wrote as follows in an essay in the 'Leader' on creation and evolution. The expressions of so profound and philosophical a biologist may be regarded as the high-water mark of evolutionary thinking up to the date of the appearance of Wallace and Darwin's theory: —

'Even could the supporters of the development hypothesis merely show that the production of species by the process of modification is conceivable, they would be in a better position than their opponents. But they can do much more than this; they can show that the process of modification has effected and is effecting great changes in all organisms, subject to modifying influences … they can show that any existing species – animal or vegetable – when placed under conditions different from its previous ones, immediately begins to undergo certain changes of structure fitting it for the new conditions. They can show that in successive generations these changes continue until ultimately the new conditions become the natural ones. They can show that in cultivated plants and domesticated animals, and in the several races of men, these changes have uniformly taken place. They can show that the degrees of difference, so produced, are often, as in dogs, greater than those on which distinctions of species are in other cases founded. They can show that it is a matter of dispute whether some of these modified forms are varieties or modified species. They can show too that the changes daily taking place in ourselves; the facility that attends long practice, and the loss of aptitude that begins when practice ceases; the development of every faculty, bodily, moral or intellectual, according to the use made of it, are all explicable on this same principle. And thus they can show that throughout all organic nature there is at work a modifying influence of the kind they assign as the cause of these specific differences, an influence which, though slow in its action, does in time, if the circumstances demand it, produce marked changes; an influence which, to all appearance, would produce in the millions of years, and under the great varieties of condition which geological records imply, any amount of change.'

This admirable passage, written seven years before the publication of the 'Origin of Species,' contains explicitly almost every idea that ordinary people, not specially biological in their interests, now associate with the name of Darwin. That is to say, it contains, in a very philosophical and abstract form, the theory of 'descent with modification' without the distinctive Darwinian adjunct of 'natural selection' or 'survival of the fittest.' Yet it was just that particular lever, dexterously applied, and carefully weighted with the whole weight of his endlessly accumulated inductive instances, that finally enabled our modern Archimedes in so short a time to move the world. The public, that was deaf to the high philosophy of Herbert Spencer, listened at once to the practical wisdom of Charles Darwin. They did not care at all for the a priori proof, but they believed forthwith as soon as a cautious and careful investigator laid bare before their eyes in minute detail the modus operandi of nature herself.

The main argument of Darwin's chief work runs somewhat after the following fashion²: —

Variation, to a greater or less degree, is a common and well-known fact in nature. More especially, animals and plants under domestication tend to vary from one another far more than do the individuals of any one species in the wild state. Rabbits in a warren are all alike in shape, size, colour, and features: rabbits in a hutch vary indefinitely in the hue of their fur, the length of their ears, the character of their coat, and half a dozen other minor particulars, well known to the observant souls of boys and fanciers. This great variability, though partly perhaps referable to excess of food, is probably due on the whole to their having been raised under conditions of life not so uniform as, and somewhat different from, those to which the parent species is commonly exposed in a state of nature. In other words, variability is one result of altered and more varied surrounding circumstances.

Again, this variability is usually indefinite. You cannot say what direction it will take, or to what particular results it is likely in any special instance to lead. Marked differences sometimes occur even between the young of the same litter, or between the seedlings sown from the same capsule. As a rule, the variations exhibit themselves in connection with sexual reproduction; but sometimes, as in the case of 'sporting plants,' a new bud suddenly produces leaves or flowers of a different character from the rest of those on the self-same stem, thus showing that the tendency to vary is inherent, as it were, in the organism itself. Upon this fundamental fact of the existence in nature of numerous and indefinite variations, the whole theory of natural selection is ultimately built up. In illustrating by example the immense variability of domesticated creatures, Darwin lays great stress upon the case of pigeons, with which he was familiar from his long experience as a breeder and fancier in his own home at Down. Naturalists are almost universally of opinion that all the breeds of domestic pigeons, from the carrier to the tumbler, from the runt to the fantail, are alike descended from the wild rock pigeon of the European coasts. The immense amount of variation which this original species has undergone in domestication may be seen by comparing the numberless breeds of pigeon now exhibited at all our poultry shows with one another.

But variation gives us only half the elements of the ultimate problem, even in the case of domestic kinds. For the other half, we must have recourse to human selection, which, by picking out for seed or breeding purposes certain specially favoured varieties, has produced at last all the purposive or intentional diversity between the different existing stocks or breeds. In these artificially produced domestic races we see everywhere special adaptations to man's particular use or fancy. The dray-horse has been fashioned for purposes of strength and sure-footedness in draught, the race-horse for purposes of fleetness in running. In the fox-hound, man has encouraged the special properties that tend to produce a good day's hunting; in the sheepdog, those that make for the better maintenance and safety of a herd. The cauliflower is a cabbage, with specialised and somewhat abortive flower-heads; the fuller's teasel is a sport of the wild form, with curved hooks specially adapted by a freak of nature for the teasing of wool. So in every case man, by deliberately picking out for breeding or seeding purposes the accidental variations which happened best to suit his own needs, has succeeded at last in producing races admirably fitted in the minutest particulars for the special functions to which they are applied. There appears indeed to be hardly any limit to the almost infinite plasticity and modifiability of domestic animals. 'It would seem,' said a great sheep-breeder, speaking of sheep, 'as if farmers had chalked out upon a wall a form perfect in itself, and then proceeded to give it existence.'

Now, what is thus true within narrow limits, and in a short space of time about the deliberate action of man, Darwin showed to be also true within wider limits and spread over longer geological epochs about the unconscious action of nature. And herein consisted his great advance upon the earlier evolutionism of Lamarck, Goethe, and Erasmus Darwin. For while these instinctive pioneers of the evolutionary spirit saw clearly that animals and plants betrayed signs of common descent from one or a few original ancestors, they did not see what was the mechanism by which such organisms had been differentiated into so many distinct genera and species. They caught, indeed, at the analogy of variation under domestication and in the wild state, but they missed the subtler and deeper analogy between human and natural selection. Now, variation alone would give us a world consisting not of definite kinds fairly well demarcated one from the other, but of innumerable unclassified and unorganisable individuals, all shading off indefinitely one into the other, and incapable of being reduced by human ingenuity to any orderly hierarchical system. Furthermore, it would give us creatures without special adaptation of any kind to the peculiar circumstances of their own environment. To account for adaptation, for the almost perfect fitness of every plant and every animal to its position in life, for the existence (in other words) of definitely correlated parts and organs, we must call in the aid of survival of the fittest. Without that potent selective agent, our conception of the becoming of life is a mere chaos; order and organisation are utterly inexplicable save by the brilliant illuminating ray of the Darwinian principle. That is why Darwin destroyed at one blow the specious arguments of the early teleologists; he showed that where Chambers and even Erasmus Darwin had seen the working of a final cause, we ought rather to recognise the working of an efficient cause, whose outcome necessarily but fallaciously simulates the supposed features of an a priori finality.

From art, then, Darwin harks back once more to nature. He proceeds to show that variability occurs among all wild plants and animals, though not so frequently under ordinary circumstances as in the case of domesticated species. Individual differences everywhere occur between plant and plant, between animal and animal. Sometimes these differences are so very numerous that it is impossible to divide the individuals at all into well-marked kinds; for example, among British wild-roses, brambles, hawkweeds and epilobes, with a few other very variable families, Babington makes as many as 251 distinct species, where Bentham gives only 112 – a margin of 139 doubtful forms of shadowy indefiniteness. Varieties, in fact, are always arising, and dominant species in particular always tend to vary most in every direction. The reason why variation is not so marked in the wild state as under domestication is of course because the conditions are there less diverse; but where the conditions of wild things are most diverse, as in the case of dominant kinds, which range over a wide space of country or of ocean, abundant individual variations habitually occur. Local varieties thus produced are regarded by Darwin as incipient species: they are the raw material on which natural selection gradually exerts itself in the struggle for existence.

Granting individual variability, then, how do species arise in nature? And how are all the exquisite adaptations of part to whole, and of whole to environment, gradually initiated, improved, and perfected?

Here Malthus and the struggle for life come in to help us.

For the world is perpetually over-populated. It is not, as many good people fearfully imagine, on a half-comprehension of the Malthusian principle, shortly going to be over-populated; it is now, it has always been, and it will always be, pressed close up to the utmost possible limit of population. Reproduction is everywhere and in all species for ever outrunning means of subsistence; and starvation or competition is for ever keeping down the number of the offspring to the level of the average or normal supply of raw material. A single red campion produces in a year three thousand seeds; but there are not this year three thousand times as many red campions as there were last summer, nor will there be three thousand times as many more in the succeeding season. The roe of a cod contains sometimes nearly ten million eggs; but supposing each of these produced a young fish which arrived at maturity, the whole sea would immediately become a solid mass of closely packed codfish. Linnæus reckoned that if an annual plant had two seeds, each of which produced two seedlings in the succeeding season, and so on continually, in twenty years their progeny would amount to a million plants. A struggle for existence necessarily results from this universal tendency of animals and plants to increase faster than the means of subsistence, whether those means be food, as in the first case, or carbonic acid, water, and sunshine as in the second. Animals are all perpetually battling with one another for the food-supply of the moment; plants are perpetually battling with one another for their share of the soil, the rainfall, and the sunshine.