The tendency of organic life, whether vegetable or animal, being to propagate itself enormously,—and the life-sustaining capacity of the earth being limited,—it necessarily follows that only a fraction of the creatures which are born can survive to maturity, and that while those best fitted to live will live, those less well fitted will die. Thus, there is set up a constant struggle for existence, in which every advantage, however slight, must tell, so that those possessing such advantages in one generation will be the parents of the next. But in the course of propagation, the offspring never exactly reproduce the parent form, from which they vary, some in one way some in another, and as some of these variations cannot help being advantageous to their possessors in the struggle, we have here the required factor for the production of new forms. Any thus beneficially equipped, (although the variation, and consequently the advantage, must in each instance be exceedingly slight,) will have the chances on their side against their less favoured fellows, whom in the long run they will supplant. And as their offspring, or some of them, will carry the profitable variation somewhat further, the stream of life will thus be set in such a direction as will ultimately bring about what might at first appear impossible metamorphoses.
Thus, to take a simple and favourite illustration,[180][{154}] winged insects inhabiting an island far from other land, are liable to be blown out to sea and drowned. It is in consequence, an advantage to them to have their power of flight curtailed, or taken away, and consequently in such situations their wings are generally found to be so reduced as to permit little or even nothing in the way of flying. Or to take an example of another kind,[181] the extraordinary length of neck which characterizes the giraffe enables it to browse on the higher branches of trees inaccessible to other vegetable feeders, and thus gives it an advantage over them in times of drought and scarcity of fodder. It can accordingly be easily understood, how its present structure has resulted from gradual elongations of the neck, each conferring on its possessor a slight advantage.
The work attributed to Natural Selection in such instances, though no doubt highly important, is comparatively facile, and it would be difficult to say that it could not be accomplished. But Mr. Darwin ascribes to the same factor, not merely such modification of existing structures, but the creation of entirely new mechanisms for specific purposes. We have, for instance, heard his description of the eye and its manifold "inimitable contrivances:" yet all these, he persuaded himself, might be thus accounted for. The idea, he confessed,[182] seems at first sight preposterous; yet, though not without much difficulty,[183] he succeeded in convincing himself,[{155}] that given the rudest and most rudimentary form of eye to start with—no more than a nerve sensitive to light but incapable of forming an image—Natural Selection might develop therefrom, through an infinite series of gradations the inconceivably complex machine that is now found in the higher vertebrates,[184] and the totally different but equally marvellous organs of sight possessed by insects, crustaceans, and other creatures.
In like manner, Mr. Darwin contended, might the most complex and wonderful instincts be[{156}] generated. As an example may be cited that by which the hive-bee constructs its combs—of which he thus speaks:[185]
He must be a dull man who can examine the exquisite structure of a comb, so beautifully adapted to its end, without enthusiastic admiration. We hear from mathematicians that bees have practically solved a recondite problem, and have made their cells of the proper shape to hold the greatest possible amount of honey, with the least possible consumption of precious wax in their construction. It has been remarked that a skilful workman with fitting tools and measures, would find it very difficult to make cells of wax of the true form, though this is perfectly effected by a crowd of bees working in a dark hive.[186] Granting whatever instincts you please, it seems at first sight quite inconceivable how they can make all the necessary angles and planes, or even perceive when they are correctly made. But the difficulty is not nearly so great as it at first appears: all this beautiful work can be shown, I think, to follow from a few simple instincts.
He accordingly proceeds to argue, that beginning with circular cells, like those of Humble Bees, and progressing through an intermediate form, circular where free, but with flat partition walls where two[{157}] or more cells touch one another, it is quite possible to suppose that Natural Selection has effected the whole improvement, those insects which accomplished any advance towards more scientific workmanship, and thus made materials go further, having been able to secure a livelihood better than their competitors.
Such in brief outline is the Darwinian system, which undertakes to account for all the alleged facts of Organic Evolution by means of the above factor, variously described as "Natural Selection," or the "Survival of the fittest in the Struggle for Existence." It should be remembered, though it is constantly forgotten, that it is this particular theory as to the working-cause of evolutionary transformations which is the essence of Darwinism. Mr. Darwin did not originate the idea of genetic transformism, which is almost necessarily suggested by the systematic development of life-forms to which Geology bears witness. Consequently, long before he came on the scene, the doctrine of transformation had been propounded, especially by Lamarck, and if it had met with no general acceptance, this was chiefly because no force was indicated which seemed to offer a satisfactory account of the mode in which the required changes could have been wrought. Such a force Mr. Darwin's "Natural Selection" was widely taken to furnish, and his theory was eagerly welcomed and adopted by those who only required such a basis on which to ground beliefs to which they were already predisposed, and Darwinism thus obtained that[{158}] pre-eminent position which it still retains, at least in popular estimation.
Two special arguments may here be mentioned, which, although they really apply to all systems of Organic Evolution, have obtained a prescriptive right to be quoted particularly in favour of Darwinism, their bearing on which is easily seen.
The first is based on the frequent occurrence of "rudimentary," "fragmentary," or "vestigial" structures in animals and plants, which, although now seemingly useless, or even harmful, to their possessors, may be assumed to have been of service to their ancestors, but under changed conditions to have been thrown out of work by Natural Selection, and atrophied by disuse. Such are—the splint-bones of the horse, representing lost digits,—the rudimentary legs of some whales and serpents,—the mammae and mammary glands of male mammals; and in the vegetable kingdom,—the aborted pistil in male florets of some compositae,—the useless corolla of certain wind-fertilized flowers, as plantago, and indeed the whole floral apparatus of plants which, like Wordsworth's pet the Lesser Celandine,[187] seldom ripen their seeds, but depend on other methods of propagation. The other fact cited on behalf of Darwinism is unquestionably very striking. In the course of their embryonic development, and even in the initial stages of their life after birth, higher animals pass through various phases in which they[{159}] exhibit the characteristics of lower forms. Thus all life starts from a cell, in which there is nothing to shew whether it is ever to be anything more than a cell, or is to evolve a plant or animal,—nor, in this latter case, what sort of animal it is to be—a mollusc, for instance, a frog, or a mammal. At a later stage[188] it is impossible to distinguish the embryos of lizards, birds, and mammals except by size. Even the human fetus at an early period bears vestiges of gill-clefts or arches, pointing to an aquatic existence. When the extremities come to be developed,[189] "The feet of lizards and mammals, the wings and feet of birds, no less than the hands and feet of man, all arise from the same fundamental form." The young of flat-fish such as soles and turbots, when they leave the egg are not flat, but shaped like ordinary fish, and they wear their eyes in the normal fashion, one on each side of their head, not both on the same side like their parents—whose form however they presently by degrees assume. Young lions and black birds are spotted, showing their affinity respectively to panthers and thrushes—and so on in numberless instances. All such features, it is assumed, indicate the phylogeny of each animal, or the history of the race to which it belongs. As Professor Milnes Marshall succinctly put the matter:[190][{160}]
The phases through which an animal passes in its progress from the egg to the adult are no accidental freaks, no mere matters of developmental convenience, but represent more or less closely ... the successive ancestral stages through which the present condition has been acquired. Evolution tells us that each animal has had a pedigree in the past. Embryology reveals to us this ancestry, because every animal in its own development repeats this history, climbs up its own genealogical tree.