Charles Darwin

by Grant Allen

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.

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.