The case of the plant is a very important one to understand in this connection, because it is probable that most people greatly misunderstand the biological meaning of the phrase 'struggle for existence.' They imagine that the struggle is chiefly conducted between different species, whereas in reality it is chiefly conducted between members of the same species. It is not so much the battle between the tiger and the antelope, between the wolf and the bison, between the snake and the bird, that ultimately results in natural selection or survival of the fittest, as the struggle between tiger and tiger, between bison and bison, between snake and snake, between antelope and antelope. A human analogy may help to make this difficult principle a little clearer. The baker does not fear the competition of the butcher in the struggle for life: it is the competition of the other bakers that sometimes inexorably crushes him out of existence. The lawyer does not press hard upon the doctor, nor the architect upon the journeyman painter. A war in the Soudan or in South Africa is far less fatal to the workman in our great towns than the ceaseless competition of his fellow-workmen. It is not the soldier that kills the artisan, but the number of other artisans who undersell him and crowd to fill up every vacant position. In this way the great enemies of the individual herbivore are not the carnivores, but the other herbivores. The lion eats the antelope, to be sure; but the real struggle lies between lion and lion for a fair share of meat, or between antelope and antelope for a fair share of pasturage. Homo homini lupus, says the old proverb, and so, we may add, in a wider sense, lupus lupo lupus, also. Of course, the carnivore plays a great part in the selective process; but he is the selector only; the real competition is between the selected. Now, let us take the case of the plant. A thousand seedlings occupy the space where few alone can ultimately grow; and between these seedlings the struggle is fierce, the strongest and best adapted ultimately surviving. To take Darwin's own example, the mistletoe, which is a parasite, cannot truly be said to struggle with the apple tree on which it fastens; for if too many parasites cover a tree, it perishes, and so they kill themselves as well as their host, all alike dying together. But several seedling mistletoes growing together on the same branch may fairly be said to struggle with one another for light and air; and since mistletoe seeds are disseminated by birds and dropped by them in the angles of branches, the mistletoe may also be said to compete with other berry-bearing bushes, like cornel and hawthorn, for the ministrations of the fruit-eating birds. The struggle is fierce between allied kinds, and fiercest of all between individual members of the same species.
Owing to this constant struggle, variations, however slight, and from whatever cause arising, if in any degree profitable to the individual which presents them, will tend to the preservation of the particular organism, and, being on the average inherited by its offspring, will similarly tend to increase and multiply in the world at large. This is the principle of natural selection or survival of the fittest – the great principle which Darwin and Wallace added to the evolutionism of Lamarck and his successors.
Let us take a single concrete example. In the desert, with its monotonous sandy colouring, a black insect or a white insect, still more a red insect or a blue insect, would be immediately detected and promptly devoured by its natural enemies, the birds and lizards. But any greyish or yellowish insects would be less likely to attract attention at first sight, and would be overlooked as long as there were any more conspicuous individuals of their own kind about for the birds and lizards to feed on at their leisure. Hence, in a very short time, the desert would be depopulated of all but the greyest and yellowest insects; and among these the birds would pick out those which differed most markedly in hue or shade from the sand around them. But those which happened to vary most in the direction of a sandy or spotty colour would be most likely to survive, and to become the parents of future generations. Thus, in the course of long ages, all the insects which inhabit deserts have become sand-coloured; because the least sandy were perpetually picked out for destruction by their ever-watchful foes, while the most sandy escaped and multiplied and replenished the earth with their own likes.
Conversely, the birds and the lizards again would probably begin by being black, and white, and blue, and green, like most other birds and lizards in the world generally. But the insect would have ample warning of the near approach of such conspicuous self-advertising enemies, and would avoid them accordingly whenever they appeared within range of his limited vision, either by lying close, or by shamming death, or by retreating precipitately to holes and crannies. Therefore, whatever individual birds or lizards happened to vary most in the direction of grey or sand-colour, and so to creep unobserved upon the unguarded insects, would succeed best on the average in catching beetles or desert grasshoppers. Hence, by the slow dying out of the more highly coloured and distinctive insect-eaters, before the severe competition of the greyest and sandiest, all the birds and lizards of the desert have become at last as absolutely sand-coloured as the insects themselves. Only the greyest insect could escape the bird; only the greyest bird, en revanche, could surprise and devour the unwary insect.
Sir Charles Lyell and the elder De Candolle had already seen the great importance of the struggle for existence in the organic world, but neither of them had observed the magnificent corollary of natural selection, which flows from it almost as a mathematical necessity when once suggested; for, given indefinite variability, and a geometrical, rate of increase, it must needs follow that some varieties will be better suited to the circumstances than others, and therefore that they will survive on the average in increased proportions. A passage from one of Lyell's early letters will show how near he too went to this great luminous generalisation, and yet how utterly he missed the true implications of his own vague and chaotic idea. He writes thus to Sir John Herschel in 1836, while Darwin was still but homeward bound on the voyage of the 'Beagle': —
'In regard to the origination of new species, I am very glad to find that you think it probable that it may be carried on through the intervention of intermediate causes… An insect may be made in one of its transformations to resemble a dead stick, or a leaf, or a lichen, or a stone, so as to be somewhat less easily found by its enemies; or if this would make it too strong, an occasional variety of the species may have this advantage conferred on it; or if this would be still too much, one sex of a certain variety. Probably there is scarcely a dash of colour on the wing or body of which the choice would be quite arbitrary, or which might not affect its duration for thousands of years.'
Now, this comes in some ways perilously near to Darwin indeed; but in the most important point of all it is wide apart from him as the pole is from the equator. For Lyell thought of all this as a matter of external teleological arrangement; he imagined a deliberate power from outside settling it all by design beforehand, and granting to varieties or species these special peculiarities in a manner that was at bottom essentially supernatural, or in other words miraculous; whereas Darwin thinks of it as the necessary result of the circumstances themselves, an inevitable outcome of indefinite variability plus the geometrical rate of increase. Where Lyell sees a final cause, Darwin sees an efficient cause; and this distinction is fundamental. It marks Darwin's position as that of a great philosophical thinker, who can dash aside at once all metaphysical cobwebs, and penetrate to the inmost recesses of things, unswerved by the vain but specious allurements of obvious and misleading teleological fallacies.
Darwin also laid great stress on the immense complexity of the relations which animals and plants bear to one another, in the struggle for existence. For example, on the heathy uplands near Farnham in Surrey, large spaces were at one time enclosed, on which, within ten years, self-grown fir-trees from the wind-borne seeds of distant clumps sprang up so thickly as actually to choke one another with their tiny branches. All over the heaths outside, when Darwin looked for them, he could not find a single fir, except the old clumps on the hilltops, from which the seedlings themselves had originally sprung. But, on looking closer among the stems of the heath, he descried a number of very tiny firs, which had been perpetually browsed down by the cattle on the commons; and one of them, with twenty-six rings of growth, had during many years endeavoured unsuccessfully to raise its head above the surrounding heather. Hence, as soon as the land was enclosed, and the cattle excluded, it became covered at once with a thick growth of vigorous young fir-trees. Yet who would ever have supposed beforehand that the mere presence or absence of cattle would absolutely have determined the very existence of the Scotch fir throughout a wide range of well-adapted sandy English upland?
To take another curious instance mentioned by Darwin. In Paraguay, unlike the greater part of neighbouring South America, neither horses nor cattle have ever run wild. This is due to the presence of a parasitic fly, which lays its eggs in their bodies when first born, the maggots killing off the tender young in their first stages. But if any cause were to alter the number of the dangerous flies, then cattle and wild horses would abound; and this would alter the vegetation, as Darwin himself observed in other parts of America; and the change in the vegetation would affect the insects; and that again the insectivorous birds; and so on in ever widening circles of incalculable complexity. Once more, to quote the most famous instance of all, the visits of humble-bees are absolutely necessary in order to place the pollen in the right position for setting the seeds of purple clover. Heads from which Darwin excluded the bees produced no seeds at all. Hence, if humble-bees became extinct in England, the red clover, too, would die off: and indeed, in New Zealand, where there are no humble-bees, and where the efforts to introduce them for this very purpose have been uniformly unsuccessful, the clover never sets its seed at all, and fresh stocks have to be imported at great expense every year from Europe. But the number of humble-bees in any district largely depends upon the number of field-mice, which destroy the combs and nests in immense quantities. The number of mice, again, is greatly affected by the proportion of cats in the neighbourhood; so that Colonel Newman, who paid much attention to this subject, found humble-bees most numerous in the neighbourhood of villages and small towns, an effect which he attributed to the abundance of cats, and the consequent scarcity of the destructive field-mice. Yet here once more, who could suppose beforehand that the degree to which the purple clover set its seeds was in part determined by the number of cats kept in houses in the surrounding district?
One of Darwin's own favourite examples of the action of natural selection, which he afterwards expanded largely in his work on Orchids and in several other volumes, is that which relates to the origin of conspicuous flowers. Many plants have a sweet excretion, which is eliminated sometimes even by the leaves, as in the case of the common laurel. This juice, though small in quantity, is eagerly sought and eaten by insects. Now let us suppose that, in some variety of an inconspicuous flower, similar nectar was produced in the neighbourhood of the petals and stamens. Insects, in seeking the nectar, would dust their bodies over with the pollen, and would carry it away with them to the next flower visited. This would result in an act of crossing; and that act, as Darwin afterwards abundantly proved in a separate and very laborious treatise, gives rise to exceptionally vigorous seedlings, which would therefore have the best chance of flourishing and surviving in the struggle for existence. The flowers which produced most honey would oftenest be visited, and oftenest crossed; so that they would finally form a new species. The more brightly coloured among them, again, would be more readily discriminated than the less brightly coloured; and this would give them such an advantage that in the long run, as we actually see, almost all habitually insect-fertilised flowers would come to have brilliant petals. The germ of this luminous idea, once more, is to be found in Sprengel's remarkable work on the fertilisation of flowers – a work far in advance of its time in many ways, and to which Darwin always expressed his deep obligations; but, as in so many other instances, while Sprengel looked upon all the little modifications and adaptations of flower and insect to one another as the result of distinct creative design, Darwin looked upon them as the result of natural selection, working upon the basis of indeterminate spontaneous variations.
How do these variations arise? Not by chance, of course (for in the strict scientific sense nothing on earth can be considered as really fortuitous), but as the outcome for the most part of very minute organic causes, whose particular action it is impossible for us to predict with our present knowledge. Some physical cause in each case there must necessarily be; and indeed it is often possible to show that certain changes of condition in the parent do result in variations in the offspring, though what special direction the variation will take can never be foretold with any accuracy. In short, our ignorance of the laws of variation is profound, but our knowledge of the fact is clear and certain. The fact alone is essential to the principle of natural selection; the cause, though in itself an interesting subject of inquiry, may be safely laid aside for the present as comparatively unimportant. What we have actually given to us in the concrete universe is, organisms varying perpetually in minute points, and a rapid rate of increase causing every minute point of advantage to be exceptionally favoured in the struggle for existence.
But Darwin is remarkable among all broachers of new theories for the extraordinary candour and openness of his method. He acknowledged beforehand all the difficulties in the way of his theory, and though he himself confessed that some of them were serious (a statement which subsequent research has often rendered unnecessary), he met many of them with cogent arguments by anticipation, and demolished objections before they could even be raised against him by hostile critics. Of these objections, only two need here be mentioned. The first is the question, why is not all nature even now a confused mass of transitional forms? Why do genera and species exist as we see them at present in broad distinction one from the other? To this Darwin answers rightly that, where the process of species-making is still going on, we do actually find fine gradations and transitional forms existing between genera, varieties, and species.3 But, furthermore, as natural selection acts solely by the preservation of useful modifications, each better-adapted new form will always tend in a fully stocked country to oust and exterminate its own unimproved parent type, as well as all other competing but less perfect varieties. Thus natural selection and extinction of intermediates go for ever hand in hand. The more perfect the new variety, the more absolutely will it kill off the intermediate forms. The second great difficulty lies in the question of the origin of instinct, which, as Darwin shows, by careful inductive instances, may have arisen by the slow and gradual accumulation of numerous slight yet profitable variations.
I have dwelt at some length upon those portions of the 'Origin of Species' which deal in detail with the theory of natural selection, the chief contribution which Darwin made to the evolutionary movement, because it is impossible otherwise fully to understand the great gulf which separates his evolutionism from the earlier evolutionism of Lamarck and his followers. But it is impracticable here to give any idea of the immense wealth of example and illustration which Darwin brought to the elucidation of every part of his complex problem. In order to gain a full conception of this side of his nature, we must turn to the original treatise itself, and still more to the subsequent volumes in which the ground-work of observations and experiments on which he based his theory was more fully detailed for the specialist public.
The remainder of Darwin's epoch-making work deals, strictly speaking, rather with the general theory of 'descent with modification' than with the special doctrine of natural selection. It restates and reinforces, by the light of the new additional concept, and with fuller facts and later knowledge, the four great arguments already known in favour of organic evolution as a whole, the argument from Geological Succession, the argument from Geographical Distribution, the argument from Embryological Development, and the argument from Classificatory Affinities. Each of these we may briefly summarise.
The geological record is confessedly imperfect. At the time when Darwin first published the 'Origin of Species,' it had disclosed to our view comparatively few intermediate or transitional forms between the chief great classes of plants or animals; since that time, in singular confirmation of the Darwinian hypothesis, it has disclosed an immense number of such connecting types, amongst which may be more particularly noticed the 'missing links' between the birds and reptiles, the ancestors of the horses, the camels, and the pigs, and the common progenitor of the ruminants and the pachyderms, two great groups classed by Cuvier as distinct orders – all of which instances were incorporated by Darwin in later editions of his 'Origin of Species.' But, apart from these special and newly discovered cases, the whole general course of geological history 'agrees admirably with the theory of descent with modification through variation and natural selection.' The simpler animals of early times are followed by the more complex and more specialised animals of later geological periods. As each main group of animals appears upon the stage of life, it appears in a very central and 'generalised' form; as time goes on, we find its various members differing more and more widely from one another, and assuming more and more specialised adaptive forms. And in each country it is found, as a rule, that the extinct animals of the later formations bear a close general resemblance and relationship to the animals which now inhabit the same regions. For example, the fossil mammals from the Australian caves are nearly allied to the modern kangaroos, phalangers, and wombats; and the gigantic extinct sloths and armadillos of South America are reproduced in their smaller representatives at the present day. So, too, the moa of New Zealand was a huge apteryx; and the birds disentombed from the bone-caves of Brazil show close affinities to the toucans and jacanars that still scream and flit in countless flocks among Brazilian forests. The obvious implication is that the animals now inhabiting any given area are the modified descendants of those that formerly inhabited it. 'On the theory of descent with modification, the great law of the succession of the same types within the same areas is at once explained.'
This last consideration leads us up to the argument from Geographical Distribution. In considering the various local faunas and floras on the face of the globe, no point strikes one more forcibly than the fact that neither their similarities nor their dissimilarities can be accounted for by climate or physical conditions. The animals of South Africa do not in the least resemble the animals of the corresponding belt of South America; the Australian beasts and birds and trees are utterly unlike those of France and Germany; the fishes and crustaceans of the Pacific at Panama are widely different from those of the Caribbean at the same point, separated from them only by the narrow belt of intervening isthmus. On the other hand, within the same continuous areas of sea or land, however great the differences of physical conditions, we find everywhere closely related types in possession of the most distinct and varied situations. On the burning plains of La Plata we get the agouti and the bizcacha as the chief rodents; we ascend the Cordillera, and close to the eternal snows we discover, not hares and rabbits like those of Europe, but a specialised chilly mountain form of the same distinctly South American type. We turn to the rivers, and we see no musk-rat or beaver, but the coypu and capybara, slightly altered varieties of the original bizcacha ancestor. Australia has no wolf, but it has instead fierce and active carnivorous marsupials; it has no mice, but some of its tiny kangaroo-like creatures fulfil analogous functions in its animal economy. Everywhere the evidence points to the conclusion that local species have been locally evolved from pre-existing similar species. The oceanic isles, of which Darwin had had so large an experience, and especially his old friends the Galapagos, come in usefully for this stage of the question. They are invariably inhabited, as Darwin pointed out, and as Wallace has since abundantly shown in the minutest detail, by waifs and strays from neighbouring continents, altered and specialised by natural selection in accordance with the conditions of their new habitat. As a rule, they point back to the districts whence blow the strongest and most prevalent winds; and the modifications they have undergone are largely dependent upon the nature of the other species with which they have to compete, or to whose habits they must needs accommodate themselves. In such cases it is easy to see how far Darwin's special conception of natural selection helps to explain and account for facts not easily explicable by the older evolutionism of mere descent with modification.
Embryology, the study of early development in the individual animal or plant, also throws much side light upon the nature and ancestry of each species or family. For example, gorse, which is a member of the pea-flower tribe, has in its adult stage solid, spiny, thorn-like leaves, none of which in the least resemble the foliage of the clover, to which it is closely related; but the young seedling in its earliest stages has trefoil leaves, which only slowly pass by infinitesimal gradations into flat blades and finally into the familiar defensive prickles. Here, natural selection under stress of herbivorous animals on open heaths and commons has spared only those particular gorse-bushes which varied in the direction of the stiffest and most inedible foliage; but the young plant in its first days still preserves for us the trefoil leaf which it shared originally with a vast group of clover-like congeners. The adult barnacle, once more, presents a certain fallacious external resemblance to a mollusk, and was actually so classed even by the penetrating and systematic intellect of Cuvier; but a glance at the larva shows an instructed eye at once that it is really a shell-making and abnormal crustacean. On a wider scale, the embryos of mammals are at first indistinguishable from those of birds or reptiles; the feet of lizards, the hoofs of horses, the hands of man, the wings of the bat, the pinions of birds, all arise from the same fundamental shapeless bud, in the same spot of an almost identical embryo. Even the human foetus, at a certain stage of its development, is provided with gill-slits, which point dimly back to the remote ages when its ancestor was something very like a fish. The embryo is a picture, more or less obscured and blurred in its outline, of the common progenitor of a whole great class of plants or animals.
Finally, classification points in the same way to the affiliation of all existing genera and species upon certain early divergent ancestors. The whole scheme of the biological system, as initiated by Linnæus and improved by Cuvier, Jussieu, De Candolle, and their successors, is essentially that of a genealogical tree. The prime central vertebrate ancestor – to take the case of the creatures most familiar to the general reader – appears to have been an animal not unlike the existing lancelet, a mud-haunting, cartilaginous, undeveloped fish, whose main lineaments are also embryologically preserved for us in the ascidian larva and the common tadpole. From this early common centre have been developed, apparently, in one direction the fishes, and in another the amphibian tribes of frogs, newts, salamanders, and axolotls. From an early amphibian, again, the common ancestor of birds, reptiles, and mammals seems to have diverged: the intermediate links between bird and reptile being faintly traced among the extinct deinosaurians and the archæopteryx, some years subsequently to the first appearance of the 'Origin of Species;' while the ornithorhyncus, which to some extent connects the mammals, and especially the marsupials, with the lower egg-laying types of vertebrate, was already well-known and thoroughly studied before the publication of Darwin's great work. Throughout, the indications given by all the chief tribes of animals and plants point back to slow descent and divergence from common ancestors; and all the subsequent course of palæontological research has supplied us rapidly, one after another, with the remains of just such undifferentiated family starting-points.
Stress has mainly been laid, in this brief and necessarily imperfect abstract, on the essentially Darwinian principle of natural selection. But Darwin did not himself attribute everything to this potent factor in the moulding of species. 'I am convinced,' he wrote pointedly in the introduction to his first edition, 'that natural selection has been the main but not the exclusive means of modification.' He attributed considerable importance as well to the Lamarckian principle of use and disuse, already so fully insisted upon before him by Mr. Herbert Spencer. The chief factors in his compound theory, as given in his own words at the end of his work, areas follows: 'Growth with Reproduction; Inheritance, which is almost implied by reproduction; Variability, from the indirect and direct action of the conditions of life, and from use and disuse; a Ratio of Increase, so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character, and the Extinction of the less improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows.'
Such was the simple and inoffensive-looking bombshell which Darwin launched from his quiet home at Down into the very midst of the teleological camp in the peaceful year 1859. Subsequent generations will remember the date as a crisis and turning-point in the history of mankind.