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Sketch on Natural Selection (1842)

by Charles Darwin

I. On Variation Under Domestication, And On the Principles of Selection.

n individual organism placed under new conditions sometimes varies in a small degree and in very trifling respects such as stature, fatness, sometimes colour, health, habits in animals and probably disposition. Also habits of life develop certain parts. Disuse atrophies.

When the individual is multiplied for long periods by buds the variation is yet small, though greater and occasionally a single bud or individual departs widely from its type (example) and continues steadily to propagate, by buds, such new kind.

When the organism is bred for several generations under new or varying conditions, the variation is greater in amount and endless in kind [especially holds good when individuals have long been exposed to new conditions]. The nature of the external conditions tends to effect some definite change in all or greater part of offspring-little food, small size—certain foods harmless, etc., organs affected and diseases—extent unknown. A certain degree of variation (Muller's twins) seems inevitable effect of process of reproduction. But more important is that simple generation, especially under new conditions [when no crossing] causes infinite variation and not direct effect of external conditions, but only in as much as it affects the reproductive functions. There seems to be no part (beau ideal of liver) of body, internal or external, or mind or habits, or instincts which does not vary in some small degree and [often] some to a great amount.

[All such] variations [being congenital] or those very slowly acquired of all kinds [decidedly evince a tendency to become hereditary], when not so become simple variety, when it does a race. Each parent transmits its peculiarities, therefore if varieties allowed freely to cross, except by the chance of two characterized by same peculiarity happening to marry, such varieties will be constantly demolished. If individuals of two widely different varieties be allowed to [often] some to a great amount. [All such] variations [being congenital] or those very slowly acquired of all kinds [decidedly evince a tendency to become hereditary], when not so become simple variety, when it does a race. Each parent transmits its peculiarities, therefore if varieties allowed freely to cross, except by the chance of two characterized by same peculiarity happening to marry, such varieties will be constantly demolished. If individuals of two widely different varieties be allowed to cross, a third race will be formed—a most fertile source of the variation in domesticated animals. If freely allowed, the characters of pure parents will be lost, number of races thus…but differences besides the….But if varieties differing in very slight respects be allowed to cross, such small variation will be destroyed, at least to our senses—a variation [clearly] just to be, distinguished by long legs will have offspring not to be so distinguished. Free crossing great agent in producing uniformity in any breed. Introduce tendency to revert to parent form.

All bisexual animals must cross, hermaphrodite plants do cross, it seems very possible that hermaphrodite animals do cross-conclusion strengthened: ill effects of breeding in and in, good effects of crossing possibly analogous to good effects of change in condition.

Therefore if in any country or district all animals of one species be allowed freely to cross, any small tendency in them to vary will be constantly counteracted. Secondly reversion to parent form—analogue of vis medicatrix. But if man selects then new races rapidly formed—of late years systematically followed—in most ancient times often practically followed. By such selection make race-horse, dray-horse—one cow good for tallow, another for eating, etc.—one plant's good'lay…in leaves another in fruit, etc.: the same plant to supply his wants at different times of year. By former means animals become adapted, as a direct effect to a cause, to external conditions, as size of body to amount of food. By this latter means they may also be so adapted, but further they may be adapted to ends and pursuits, which by no possibility can affect growth, as existence of tadlow—chandler cannot tend to make fat. In such selected races, if not removed to new conditions, and if preserved from all cross, after several generations become very true, like each other and not varying. But man selects only what is useful and curious—has bad judgment, is capricious—grudges to destroy those that do not come up to his pattern—has no [knowledge] power of selecting according to internal variations—can hardly keep his conditions uniform—[cannot] does not select those best adapted to the conditions under which the form lives, but those most useful to him. This might all be otherwise.

II. On Variation in a State of Nature and on the Natural Means of Selection.

Let us see how far above principles of variation apply to wild animals. Wild animals vary exceedingly little—yet they are known as individuals. British Plants in many genera number quite uncertain of varieties and species: in shells chiefly external conditions. Primrose and cowslip. Wild animals from different [countries can be recognized]. Specific character gives some organs as varying. Variations analogous in kind, but less in degree with domesticated animals—chiefly external and less important parts.

Our experience would lead us to expect that any and every one of these organisms would vary if the organism were taken away and placed under new conditions. Geology proclaims a constant round of change, bringing into play, by every possible change of climate and the death of pre-existing inhabitants, endless variations of new conditions. These generally very slow, doubtful though…how far the slowness would produce tendency to vary. But geologists show change in configuration which, together with the accidents of air and water and the means of transportal which every being possesses, must occasionally bring rather suddenly, organism to new conditions and expose it for several generations. Hence we should expect every now and then a wild form to vary; possibly this may be cause of some species varying more than others.

According to nature of new conditions, so we might expect all or majority of organisms born under them to vary in some definite way. Further we might expect that the mould in which they are cast would likewise vary in some small degree. But is there any means of selecting those offspring which vary in the same manner, crossing them and keeping their offspring separate and thus producing selected races: otherwise as the wild animals freely cross, so must such small heterogeneous varieties be constantly counter-balanced and lost, and a uniformity of character preserved. The former variation as the direct and necessary effects of causes, which we can see can act on them, as size of body from amount of food, effect of certain kinds of food on certain parts of bodies, etc.; such new varieties may then become adapted to those external [natural] agencies which act on them. But can varieties be produced adapted to end, which cannot possibly influence their structure and which it is absurd to look at as effects of chance.

Can varieties like some vars of domesticated animals, like almost all wild species be produced adapted by exquisite means to prey on one animal or to escape from another—or rather, as it puts out of question effects of intelligence and habits, can a plant become adapted to animals, as a plant which cannot be impregnated without agency of insect; or hooked seeds depending on animals' existence: woolly animals cannot have any direct effect on seeds of plant. This point which all theories about climate adapting woodpecker to crawl up trees,…mistletoe,…But if every part of a plant or animal was to vary…, and if a being infinitely more sagacious than man (not an omniscient creator) during thousands and thousands of years were to select all the variations which tended towards certain ends ([or were to produce causes which tended to the same end]), for instance, if he foresaw a canine animal would be better off, owing to the country producing more hares, if he were longer legged and keener sight-greyhound produced. If he saw that aquatic animal-skinned toes. If for some unknown cause he found it would advantage a plant, which like most plants is occasionally visited by bees, etc.: if that plant's seed were occasionally eaten by birds and were then carried on to rotten trees, he might select trees with fruit more agreeable to such birds as perched, to ensure their being carried to trees; if he perceived those birds more often dropped the seeds, he might well have selected a bird who would…rotten trees or [gradually select plants which he had proved to live on less and less rotten trees]. Who, seeing how plants vary in garden, what blind foolish man has done in a few years, will deny an all-seeing being in thousands of years could effect (if the Creator chose to do so), either by his own direct foresight or by intermediate means—which will represent the creator of this universe. Seems usual means. Be it remembered I have nothing to say about life and mind and all forms descending from one common type. (Good place to introduce, saying reasons hereafter to be given, how far I extend theory, say to all mammalia—reasons growing weaker and weaker.) I speak of the variation of the existing great divisions of the organized kingdom, how far I would go, hereafter to be seen.

Before considering whether there be any natural means of selection, and secondly (which forms the second part of this sketch) the far more important point whether the characters and relations of animated beings are such as favour the idea of wild species being races descended from a common stock, as the varieties of potato or dahlia or cattle having so descended, let us consider probable character of [selected races] wild varieties.

Natural selection. De Candolle's war of nature—seeing contented face of nature may be well at first doubted; we see it on borders of perpetual cold. But considering the enormous geometrical power of increase in every organism and as every country, in ordinary cases, must be stocked to full extent, reflection will show that this is the case. Malthus on man—in animals no moral [check] restraint—they breed in time of year when provision most abundant, or season most favourable, every country has its season—calculate robins—oscillating from years of destruction. If proof were wanted let any singular change of climate occur here, how astoundingly some tribes increase, also introduced animals, the pressure is always ready capacity of alpine plants to endure other climates—think of endless seeds scattered abroad—forests regaining their percentage—a thousand wedges are being forced into the economy of nature. This requires much reflection; study Malthus and calculate rates of increase and remember the resistance—only periodical.

The unavoidable effect of this is that many of every species are destroyed either in egg or [young or mature (the former state the more common)]. In the course of a thousand generations infinitesimally small differences must inevitably tell, when unusually cold winter, or hot or dry summer comes, then out of the whole body of individuals of any species, if there be the smallest differences in their structure, habits, instincts [senses], health, etc., it will on an average tell; as conditions change a rather larger proportion will be preserved: so if the chief check to increase falls on seeds or eggs, so will, in the course of 1000 generations or ten thousand, those seeds (like one with down to fly) which fly furthest and get scattered most ultimately rear most plants, and such small differences tend to be hereditary like shades of expression in human countenance. So if one parent fish deposits its egg in infinitesimally different circumstances, as in rather shallower or deeper water etc., it will then tell.

Let hares increase very slowly from change of climate affecting peculiar plants, and some other…rabbit decrease in same proportion [let this unsettle organization of], a canine animal, who formerly derived its chief sustenance by springing on rabbits or running them by scent, must decrease too and might thus readily become exterminated. But if its form varied very slightly, the long-legged fleet ones, during a thousand years being selected, and the less fleet rigidly destroyed must, if no law of nature be opposed to it, alter forms.

Remember how soon Bakewell on the same principle altered cattle and Western, sheep carefully avoiding a cross (pigeons) with any breed. We cannot suppose that one plant tends to vary in fruit and another in flower, and another in flower and foliage—some have been selected for both fruit and flower: that one animal varies in its covering and another not—another in its milk. Take any organism and ask what is it useful for and on that point it will be found to vary—cabbages in their leaf—corn in size and quality of grain, both in times of year—kidney beans for young pod and cotton for envelope of seeds, etc.: dogs in intellect, courage, fleetness and smell: pigeons in peculiarities approaching to monsters. This requires consideration—should be introduced in first chapter if it holds, I believe it does. It is hypothetical at best.

Nature's variation far less, but such selection far more rigid and scrutinizing. Man's races not only not better adapted to conditions than other races, but often not one race adapted to its conditions, as man keeps and propagates some alpine plants in garden. Nature lets an animal live, till on actual proof it is found less able to do the required work to serve the desired end, man judges solely by his eye, and knows not whether nerves, muscles, arteries, are developed in proportion to the change of external form.

Besides selection by death, in bisexual animals…the selection in time of fullest vigour, namely struggle of males; even in animals which pair there seems a surplus and a battle, possibly as in man more males produced than females, struggle of war or charms. Hence that male which at that time is in fullest vigour, or best armed with arms or ornaments of its species, will gain in hundreds of generations some small advantage and transmit such characters to its offspring. So in female rearing its young, the most vigorous and skilful and industrious, instincts best developed, will rear more young, probably possessing her good qualities, and a greater number will thus be prepared for the struggle of nature. Compared to man using a male alone of good breed. This latter section only of limited application, applies to variation of sexual characters. Introduce here contrast with Lamarck—absurdity of habit, or chance ?? or external conditions, making a woodpecker adapted to tree.

Before considering difficulties of theory of selection let us consider character of the races produced, as now explained, by nature. Conditions have varied slowly and the organisms best adapted in their whole course of life to the changed conditions have always been selected—man selects small dog and afterwards gives it profusion of food—selects a long-backed and short-legged breed and gives it no particular exercise to suit this function, etc. In ordinary cases nature has not allowed her race to be contaminated with a cross of another race, and agriculturists know how difficult they find always to prevent this effect would be trueness. This character and sterility when crossed, and generally a greater amount of difference are two main features, which distinguish domestic races from species.

[Sterility not universal admitted by all. Gladiolus, Crinum, Calceolaria must be species if there be such a thing. Races of dogs and oxen: but certainly very general; indeed a gradation of sterility most perfect very general. Some nearest species will not cross (crocus, some heath), some genera cross readily (fowls and grouse, peacock, etc.). Hybrids no ways monstrous quite perfect except secretions hence even the mule has bred—character of sterility especially a few years ago thought very much more universal than it now is, has been thought the distinguishing character; indeed it is obvious if all forms freely crossed, nature would be a chaos. But the very gradation of the character, even if it always existed in some degree which it does not, renders it impossible as marks those suppose distinct as species.] Will analogy throw any light on the fact of the supposed races of nature being sterile, though none of the domestic ones are? Mr Herbert and Kolreuter have shown external differences will not guide one in knowing whether hybrids will be fertile or not, but the chief circumstance is constitutional differences, such as being adapted to different climate or soil, differences which [must] probably affect the whole body of the organism and not any one part. Now wild animals, taken out of their natural conditions, seldom breed. I do not refer to shows or to Zoological Societies where many animals unite, but do not breed, and others will never unite, but to wild animals caught and kept quite tame left loose and well fed about houses and living many years. Hybrids produced almost as readily as pure breds. St Hilaire great distinction of tame and domestic elephants—ferrets. Reproductive organs not subject to disease in Zoological Garden. Dissection and microscope show that hybrid is in exactly same condition as another animal in the intervals of breeding season, or those animals which taken wild and not bred in domesticity remain without breeding their whole lives. It should be observed that so far from domesticity being unfavourable in itself it makes more fertile: [when animal is domesticated and breeds, productive power increased from more food and selection of fertile races]. As far as animals go might be thought an effect on their mind and a special case.

But turning to plants we find same class of facts. I do not refer to seeds not ripening, perhaps the commonest cause, but to plants not setting, which either is owing to some imperfection of ovule or pollen. Lindley says sterility is the [curse] bane of all propagators—Linnaeus about alpine plants. American bog plants—pollen in exactly same state as in hybrids—same in geraniums. Persian and Chinese lilac will not seed in Italy and England. Probably double plants and all fruits owe their developed parts primarily to sterility and extra food thus applied. There is here gradatiion in sterility and then parts, like diseases, are transmitted hereditarily. We cannot assign any cause why the Pontic azalea produces plenty of pollen and not American, why common lilac seeds and not Persian, we see no difference in healthiness. We know not on what circumstances these facts depend, why ferret breeds, and cheetah, elephant and pig in India will not.

Now in crossing it is certain every peculiarity in form and constitution is transmitted: an alpine plant transmits its alpine tendency to its offspring, an American plant American—bog constitution, and animals, those peculiarities, on which when placed out of their natural conditions they are incapable of breeding; and moreover they transmit every part of their constitution, their respiration, their pulse, their instinct, which are all suddenly modified, can it be wondered at that they are incapable of breeding? I think it may be truly said it would be more wonderful if they did. But it may be asked why have not the recognized varieties, supposed to have been produced through the means of man, [not refused to breed] have all bred. Variation depends on change of condition and selection, as far as man's systematic or unsystematic selection has gone; he takes external form, has little power from ignorance over internal invisible constitutional diferences. Races which have long been domesticated, and have much varied are precisely those which were capable of bearing great changes, whose constitutions were adapted to a diversity of climates. Nature changes slowly and by degrees. According to many authors probably breeds of dogs are another case of modified species freely crossing. There is no variety which…has been…adapted to peculiar soil or situation for a thousand years and another rigorously adapted to another, till such can be produced, the question is not tried. Man in past ages, could transport into different climates, animals and plants which would freely propagate in such new climates. Nature could effect, with selection, such changes slowly, so that precisely those animals which are adapted to submit to great changes have given rise to diverse races—and indeed great doubt on this head.

Before leaving this subject well to observe that it was shown that a certain amount of variation is consequent on mere act of reproduction both by buds and sexually—is vastly increased when parents exposed for some generations to new conditions, and we now find that many animals when exposed for first time to very new conditions, are as incapable of breeding as hybrids. It [probably] bears also on supposed fact of crossed animals when not infertile, as in mongrels, tending to vary much, as likewise seems to be the case, when true hybrids possess just sufficient fertility to propagate with the parent breeds and inter se for some generations. This is Kolreuter's belief. These facts throw light on each other and support the truth of each other, we see throughout a connection between the reproductive faculties and exposure to changed conditions of life whether by crossing or exposure of the individuals.

Difficulties on theory of selection. It may be objected such perfect organs as eye and ear, could never be formed, in latter less difficulty as gradations more perfect; at first appears monstrous and to the end appears difficulty. But think of gradation, even now manifest, (tibia and fibula). Everyone will allow if every fossil preserved, gradation infinitely more perfect; for possibility of selection a perfect gradation is required. Different groups of structure, slight gradation in each group—every analogy renders it probable that intermediate forms have existed. Be it remembered what strange metamorphoses; part of eye, not directly connected with vision, might come to be [thus used] gradually worked in for this end—swimming bladder by gradation of structure is admitted to belong to the ear system—rattlesnake. [Woodpecker best adapted to climb.] In some cases gradation not possible—as vertebrae—actually vary in domestic animals—less difficult if growth followed. Looking to whole animals, a bat formed not for flight. Suppose we had flying fish and not one of our now called flying fish preserved, who would have guessed intermediate habits. Woodpeckers and tree-frogs both live in countries where no trees.

The gradations by which each individual organ has arrived at its present state, and each individual animal with its aggregate of organs has arrived, probably never could be known, and all present great difficulties. I merely wish to show that the proposition is not so monstrous as it at first appears, and that if good reason can be advanced for believing the species have descended from common parents, the difficulty of imagining intermediate forms of structure not sufficient to make one at once reject the theory.

III. On Variation in Instincts and other Mental Attributes.

The mental powers of different animals in wild and tame state [present still greater difficulties] require a separate section. Be it remembered I have nothing to do with origin of memory, attention, and the different faculties of the mind, but merely with their differences in each of the great divisions of nature. Disposition, courage, pertinacity, suspicion, restlessness, ill-temper, sagacity and the reverse unquestionably vary in animals and are inherited (Cuba wildness dogs, rabbits, fear against particular object as man Galapagos). Habits purely corporeal, breeding season, etc., time of going to rest, etc., vary and are hereditary, like the analogous habits of plants which vary and are inherited. Habits of body, as manner of movement ditto and ditto. Habits, as pointing and setting on certain occasions ditto. Taste for hunting certain objects and manner of doing so—sheep-dog. These are shown clearly by crossing and their analogy with true instinct thus shown—retriever. Do not know objects for which they do it. Lord Brougham's definition. Origin partly habit, but the amount necessarily unknown, partly selection. Young pointers pointing stones and sheep—tumbling pigeons—sheep going back to place where born. Instinct aided by reason, as in the taylor-bird. Taught by parents, cows choosing food, birds singing. Instincts vary in wild state (birds get wilder) often lost; more perfect—nest without roof. These facts [only clear way] show how incomprehensibly brain has power of transmitting intellectual operations.

Faculties distinct from true instincts—finding [way]. It must I think be admitted that habits whether congenital or acquired by practice [sometimes] often become inherited; instincts, influence, equally with structure, the preservation of animals; therefore selection must, with changing conditions tend to modify the inherited habits of animals. If this be admitted it will be found possible that many of the strangest instincts may be thus acquired. I may observe, without attempting definition, that an inherited habit or trick (trick because may be born) fulfils closely what we mean by instinct. A habit is often performed unconsciously, the strangest habits become associated, ditto tricks, going in certain spots etc., even against will, is excited by external agencies, and looks not to the end—a person playing a pianoforte. If such a habit were transmitted it would make a marvellous instinct. Let us consider some of the most difficult cases of instincts, whether they could be possibly acquired. I do not say probably, for that belongs to our third part, I beg this may be remembered, nor do I mean to attempt to show exact method. I want only to show that whole theory ought not at once to be rejected on this score.

Every instinct must, by my theory, have been acquired gradually by slight changes…of former instinct, each change being useful to its then species. Shamming death struck me at first as remarkable objection. I found none really sham death, and that there is gradation; now no one doubts that those insects which do it either more or less, do it for some good, if then any species was led to do it more, and then escaped etc.

Take migratory instincts, faculty distinct from instinct, animals have notion of time like savages. Ordinary finding way by memory, but how does savage find way across country—as incomprehensible to us, as animal to them—geological changes—fishes in river—case of sheep in Spain. Architectural instincts—a manufacturer's employee in making single articles extraordinary skill—often said seem to make it almost…child born with such a notion of playing—we can fancy tailoring acquired in same perfection—mixture of reason—water-ouzel—taylor-bird—gradation of simple nest to most complicated.

Bees again, distinction of faculty—how they make a hexagon—Waterhouse's theory—the impulse to use whatever faculty they possess—the taylor-bird has the faculty of sewing with beak, instinct impels him to do it.

Last case of parent feeding young with different food (take case of Galapagos birds, gradation from hawfinch to Sylvia) selection and habit might lead old birds to vary taste and form, leaving their instinct of feeding their young with same food—or I see no difficulty in parents being forced or induced to vary the food brought, and selection adapting the young ones to it, and thus by degree any amount of diversity might be arrived at. Although we can never hope to see the course revealed by which different instincts have been acquired, for we have only present animals (not well known) to judge of the course of gradation, yet once grant the principle of habits, whether congenital or acquired by experience, being inherited and I can see no limit to the [amount of variation] extraordinariness of the habits thus acquired.

Summing up this division. If variation be admitted to occur occasionally in some wild animals, and how can we doubt it, when we see 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. If we admit that external conditions vary, as all geology proclaims, they have done and are now doing—then, if no law of nature be opposed, there must occasionally be formed races [slightly] differing from the parent races. So then any such law, none is known, but in all works it is assumed, in flat contradiction to all known facts, that the amount of possible variation is soon acquired. Are not all the most varied species, the oldest domesticated: who would think that horses or corn could be produced? Take Dahlia and potato, who will pretend in 5000 years that great changes might not be effected: perfectly adapted to conditions and then again brought into varying conditions. Think what has been done in few last years, look at pigeons, and cattle. With the amount of food man can produce he may have arrived at limit of fatness or size, or thickness of wool, but these are the most trivial points, but even in these I conclude it is impossible to say we know the limit of variation. And therefore with the selecting power of nature, infinitely wise compared to those of man, (I conclude) that it is impossible to say we know the limit of races, which would be true to their kind; if of different constitutions would probably be infertile one with another, and which might be adapted in the most singular and admirable manner, according to their wants, to external nature and to other surrounding organisms—such races would be species. But is there any evidence that species have been thus produced, this is a question wholly independent of all previous points, and which on examination of the kingdom of nature we ought to answer one way or another.

[ Charles Darwin, "1842 Sketch on Natural Selection," Reprinted in Skeptic 1(3): 68-73. ]

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