Darwin's feathered metaphor: How the domestic pigeon became the cornerstone of evolution theory

“ Darwin calls attention to 'the great changes introduced into various animals by artificial or accidental cultivation,' a subject afterwards fully elucidated by his greater grandson in his work on 'The Variation of Animals and Plants under Domestication.' He specially notes 'the immense changes of shape and colour' produced by man in rabbits and pigeons, the very species on which Charles Darwin subsequently made some of his most remarkable and interesting observations. ”

“ The details will often be tediously minute; but no one who really wants to understand the progress of change in domestic animals will regret this; and no one who has kept pigeons and has marked the great difference between the breeds and the trueness with which most of them propagate their kind, will think this care superfluous. Notwithstanding the clear evidence that all the breeds are the descendants of a single species, I could not persuade myself until some years had passed that the whole amount of difference between them had arisen since man first domesticated the wild rock-pigeon. ”

“ Darwin showed, by most interesting experiments with pigeons, that the various breeds come from the wild rock-pigeon; that dogs are descended, probably, from the wolf; that different varieties can be produced and perpetuated under changing conditions of life; that species are only well marked and permanent varieties. ”

“ 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. ”

“ Darwin explains this by- 80 - the changed conditions of their life, excess or changed quality of food, climate, changed habits, &c. Thus man has effected remarkable changes in many species by consciously or unconsciously selecting particular qualities in the animals or plants kept for use or beauty. Domestic productions seem in fact to have become plastic in man’s hands, and the inheritance of acquired qualities by offspring is reckoned on as almost certain. The breeds of cattle, poultry, dogs, and pigeons, are striking examples. ”

“ Darwin's masterly volume on the 'Origin of Species,' by the law of 'natural selection,' which now substantiates on undeniable grounds the very principle so long denounced by the first naturalists—the origination of new species by natural causes: a work which must soon bring about an entire revolution of opinion in favour of the grand principle of the self-evolving powers of nature."—Prof. ”

“ How will the struggle for existence, briefly discussed in the last chapter, act in regard to variation? Can the principle of selection, which we have seen is so potent in the hands of man, apply under nature? I think we shall see that it can act most efficiently. Let the endless number of slight variations and individual differences occurring in our domestic productions, and, in a lesser degree, in those under nature, be borne in mind; as well as the strength of the hereditary tendency. ”

“ Charles Darwin, the result of twenty years of observations and experiments in Zoology, Botany, and Geology, by which he had been led to the conclusion that those powers of nature which give rise to races and permanent varieties in animals and plants, are the same as those which in much longer periods produce species, and in a still longer series of ages give rise to differences of generic rank. ”

“ Fanciers use exactly the same language with respect to domestic breeds; thus, an author of an excellent treatise on pigeons says he will never allow that the carrier and fantail are the descendants of the wild rock-pigeon, until the transitions have “actually been observed, and can be repeated whenever man chooses to set about the task.” No doubt it is difficult to realise that slight changes added up during long centuries can produce such great results; but he who wishes to understand the origin of domestic breeds or of natural species must overcome this difficulty. ”

“ UNCONSCIOUS SELECTION is that which follows from men naturally preserving the most valued and destroying the less valued individuals, without any thought of altering the breed; and undoubtedly this process slowly works great changes. Unconscious selection graduates into methodical, and only extreme cases can be distinctly separated; for he who preserves a useful or perfect animal will generally breed from it with the hope of getting offspring of the same character; but as long as he has not a predetermined purpose to improve the breed, he may be said to be selecting unconsciously. ”

“ He shows how an analogous divergence takes place under domestication where an originally uniform stock of horses has been split up into race-horses, dray-horses, etc., and then goes on to explain how the same principle applies to natural species. "From the simple circumstance that the more diversified the descendants from any one species become in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in numbers." ”

“ On page 464 we read—
“Here are the phenomena of Hybridism staring you in the face, and you cannot say, ‘I can, by selective modification, produce these same results.’ Now, it is admitted on all hands, at present, so far as experiments have gone, it has not been found possible to produce this complete physiological divergence by selective breeding.... If we were shewn that this must be the necessary and inevitable results of all experiments, I hold that Mr. Darwin’s hypothesis would be utterly shattered.” ”

“ After commenting on these purely imaginary causes, I pointed out in 1832, as the two great flaws in Lamarck's attempt to explain the origin of species, first, that he had failed to adduce a single instance of the initiation of a new organ in any species of animal or plant; and secondly, that variation, whether taking place in the course of nature or assisted artificially by the breeder and horticulturist, had never yet gone so far as to produce two races sufficiently remote from each other in physiological constitution as to be sterile when intermarried, or, if fertile, only capable of producing sterile hybrids, &c. ”

“ Darwin firmly connected Variation to the Origin by devoting its introduction to an overview of his theory of evolution by natural selection. In particular, the two volumes of Variation, cumbersomely organized and packed with zoological and botanical detail, addressed some of the difficulties inherent in the attractive but paradoxical analogy between natural selection and artificial selection. For selection of any sort to operate, diversity already had to exist. With wild populations living under natural conditions, however, diversity was difficult to discern. ”

“ Passing the sand grouse of Africa and Russia, we come to the pigeons, represented in a bewildering variety of forms in every part of the world. The United States has several species—the common wood dove, or mourning dove, the extinct "wild pigeon," once here in millions, the banded pigeon of the Pacific Coast, and several kinds of ground doves in the southwest. The rock dove, which is the original of the domestic varieties, is still wild in Europe, together with several other species ”

“ We have therefore good reason to believe that all the domestic races of the pigeon are descended either from some one or from several species which both roosted and built their nests on rocks, and were social in disposition. As only five or six wild species have these habits, and make any near approach in structure to the domesticated pigeon, I will enumerate them. ”

“ In India, as Captain Hutton informs me, the wild rock-pigeon is easily tamed, and breeds readily with the domestic kind; and Mr. Blyth [331] asserts that wild birds come frequently to the dovecots and mingle freely with their inhabitants. In the ancient 'Ayeen Akbery' it is written that, if a few wild pigeons be taken, "they are speedily joined by a thousand others of their kind."
Dovecot-pigeons are those which are kept in dovecots in a semi-domesticated state; for no special care is taken of them, and they procure their own food, except during the severest weather. ”

“ With animals bred for amusement, the same principle is even more powerful; for fashion, as we see in our dress, always runs to extremes. This view has been expressly admitted by fanciers. Instances were given in the chapters on the pigeon, but here is another: Mr. Eaton, after describing a comparatively new variety, namely, the Archangel, remarks, “What fanciers intend doing with this bird I am at a loss to know, whether they intend to breed it down to the tumbler’s head and beak, or carry it out to the carrier’s head and beak ”

“ Tendency in Man to carry the practice of Selection to an extreme point.
It is an important principle that in the process of selection man almost invariably wishes to go to an extreme point. Thus, there is no limit to his desire to breed certain kinds of horses and dogs as fleet as possible, and others as strong as possible; certain kinds of sheep for extreme fineness, and others for extreme length of wool; and he wishes to produce fruit, grain, tubers, and other useful parts of plants, as large and excellent as possible. ”

“ In order to demonstrate that such populations spontaneously produced sufficient variation to support artificial selection, Darwin devoted most of the first volume of Variation to a species-by-species survey of domesticated plants and animals. He began with the dog, the breeds of which differed so greatly in size, shape, disposition, talents, and every other characteristic that Darwin attributed its exemplary plasticity to its derivation from several different species of wild canines. ”

“ If organic beings had not possessed an inherent tendency to vary, man could have done nothing. (Introduction/2. M. Pouchet has recently ('Plurality of Races' English Translation 1864 page 83 etc.) insisted that variation under domestication throws no light on the natural modification of species. I cannot perceive the force of his arguments, or, to speak more accurately, of his assertions to this effect.) He unintentionally exposes his animals and plants to various conditions of life, and variability supervenes, which he cannot even prevent or check. ”

“ Summing up this Division. If variation be admitted to occur occasionally in some wild animals, and how can we doubt it, when we see [all] thousands «of» organisms, for whatever use taken by man, do vary. If we admit such variations tend to be hereditary, and how can we doubt it when we «remember» resemblances of features and character,—disease and monstrosities inherited and endless races produced (1200 cabbages) . If we admit selection is steadily at work, and who will doubt it, when he considers amount of food on an average fixed and reproductive powers act in geometrical ratio. ”

“ Causes of Variability — Effects of Habit and the use or disuse of Parts — Correlated Variation — Inheritance — Character of Domestic Varieties — Difficulty of distinguishing between Varieties and Species — Origin of Domestic Varieties from one or more Species — Domestic Pigeons, their Differences and Origin — Principles of Selection, anciently followed, their Effects — Methodical and Unconscious Selection — Unknown Origin of our Domestic Productions — Circumstances favourable to Man's power of Selection. ”

“ I further believe that these slow, intermittent results accord well with what geology tells us of the rate and manner at which the inhabitants of the world have changed.
Slow though the process of selection may be, if feeble man can do much by artificial selection, I can see no limit to the amount of change, to the beauty and complexity of the coadaptations between all organic beings, one with another and with their physical conditions of life, which may have been effected in the long course of time through nature's power of selection, that is by the survival of the fittest. ”

“ What limit can be put to this power, acting during long ages and rigidly scrutinising the whole constitution, structure, and habits of each creature, — favouring the good and rejecting the bad? I can see no limit to this power, in slowly and beautifully adapting each form to the most complex relations of life. The theory of natural selection, even if we look no farther than this, seems to be in the highest degree probable. ”

“ The more I work the more I feel convinced that it is by the accumulation of such extremely slight variations that new species arise. I do not plead guilty to the Duke's charge that I forget that natural selection means only the preservation of variations which independently arise. ("Strictly speaking, therefore, Mr. Darwin's theory is not a theory on the Origin of Species at all, but only a theory on the causes which lead to the relative success and failure of such new forms as may be born into the world."—"Scotsman", December 6, 1864.) ”

“ We must suppose each new state of the instrument to be multiplied by the million; and each to be preserved till a better be produced, and then the old ones to be destroyed. In living bodies, variation will cause the slight alterations, generation will multiply them almost infinitely, and natural selection will pick out with unerring skill each improvement. ”

“ Can we wonder, then, that nature’s productions should be far ‘truer’ in character than man’s productions; that they should be infinitely better adapted to the most complex conditions of life, and should plainly bear the stamp of far higher workmanship?” The words in italics certainly are a good answer to those who think Darwin had any tendency to depreciate the marvels of nature by bringing them under the law of natural selection. ”

“ But as Darwin acknowledged, there were some fairly obvious reasons why the two processes might diverge. The superior power of natural selection—“Man can act only on external and visible characters: nature . . . can act on . . . the whole machinery of life. Man selects only for his own good; Nature only for that of the being which she tends” (Origin, chap. 5) —might constitute a difference of kind rather than of degree, as might the much greater stretches of time available for natural selection. ”

“ Darwin acknowledges that the belief that species were immutable productions was almost unavoidable, as long as the history of the world was thought to be of short duration: which affords another proof how profoundly Lyell’s views on the long duration of the past history of the globe, and its modification by the slow operation of existing causes, influenced- 71 - Darwin, and led him to comprehend how species might be modified. ”

“ The more I study nature, the more I become impressed, with ever-increasing force, that the contrivances and beautiful adaptations slowly23 acquired through each part occasionally varying in a slight degree but in many ways, with the preservation of those variations which were beneficial to the organism under complex and ever-varying conditions of life, transcend in an incomparable manner the contrivances and adaptations which the most fertile imagination of man could invent.
The use of each trifling detail of structure is far from a barren search to those who believe in natural selection. ”

“ Nor do I suppose that the most divergent varieties are invariably preserved: a medium form may often long endure, and may or may not produce more than one modified descendant; for natural selection will always act according to the nature of the places which are either unoccupied or not perfectly occupied by other beings; and this will depend on infinitely complex relations. But as a general rule, the more diversified in structure the descendants from any one species can be rendered, the more places they will be enabled to seize on, and the more their modified progeny will increase. ”

“ When we hear a scientist speak of Darwinism as being dead, this is what he means. He does not think evolution false, but believes that Natural Selection is not sufficient to account for evolution. There are three main difficulties involved in Darwin's theory. The chief defect lies in the fact that selection cannot originate varieties. In all his earlier works Darwin simply accepted variations as he found them. He was content to say that all species varied constantly, and in every direction. He gave no theory to account for variation. ”

“ On the following page (page 243) a passage (not marked by Mr. Darwin) emphasises the limitation which Prichard ascribed to the results of variation and inheritance:—
"Even those physiologists who contend for what is termed the indefinite nature of species admit that they have limits at present and under ordinary circumstances. Whatever diversities take place happen without breaking in upon the characteristic type of the species. This is transmitted from generation to generation: goats produce goats, and sheep, sheep." ”

“ The special difficulty of accounting for the sudden emergence of monstrosities, or even less dramatically novel traits, led Darwin, in later editions of the Origin as well as in Variation, to become increasingly receptive to the notion that characteristics acquired by one generation might he inherited by the next. ”

“ As to the limits of the truth of Darwin’s theory, Professor Huxley, writing on “Evolution in Biology,” in “The Encyclopædia Britannica,” says: “How far natural selection suffices for the production of species remains to be seen. ”

“ In like manner, Darwin, while unable to say what the operation of variation and natural selection in every individual case will be, demonstrates that in the long run they will adapt animals to their circumstances. Whether or not existing animal forms are due to such action, or what position the theory ought to take, forms the subject of a discussion in which questions of fact and questions of logic are curiously interlaced. ”