The origin of species E-Book

Charles Darwin.

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• Sprache: Englisch

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Published in 1859, Charles Darwin's The Origin of Species became known as ''The Book that Shook the World''. Its first edition sold out on the first day, and the same happened with six subsequent versions. To this day, the naturalist's theory of evolution is controversial. According to Darwin, species compete for survival, and those that survive give rise to the next generation, which in turn incorporates favorable natural variations and passes them on hereditarily. Until his death, Darwin tirelessly rewrote his theory, and new research was included in each of the six editions. This is what can be considered his definitive work. Still considered one of the most innovative and challenging biological treatises ever written, The Origin of Species, with its approach to evolutionary processes, shocked much of the Western world when it was released in 1859. In this new edition, readers will come into contact with the most important work on Biology ever written.

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The Origin of Species

EISBN: 9781723465239

Original English title: On the Origin of Species by Means of Natural Selection , or The Preservation of Favored Races in the Struggle for Life

This edition was made from an old translation that is in the public domain, therefore, it may contain words and expressions that are not used as frequently in current Portuguese.

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DARWIN, Charles (1859). On the Origin of Species by Means of Natural Selection, or The Preservation of Favoured Races in the Struggle for Life

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CHAPTER I - VARIATION OF SPECIES IN THE DOMESTIC STATE

CAUSES OF VARIABILITY

When individuals belonging to the same variety or subvariety of our long-cultivated plants and our oldest domestic animals are compared, it is immediately noted that they ordinarily differ more from each other than individuals belonging to any one species or variety in the wild state. Now, if we think of the immense diversity of our cultivated plants and domestic animals, which have varied throughout time, as soon as they are exposed to the most diverse climates and treatments, we come to the conclusion that this great variability comes from the fact that our domestic productions were produced under living conditions that were less uniform, or even somewhat different from those to which the parent species was subjected in nature. There is also some truth in the view held by Andrew Kniglit , namely, that variability may in part arise from over-nutrition. It seems evident that organized beings must, for many generations, be exposed to new conditions of existence, before any appreciable variation can be produced in them; but it is also evident that, once an organism has begun to vary, it ordinarily continues to do so during numerous generations. No example could be given of a variable organism which has ceased to vary in the domestic state. Our long-cultivated plants, such as wheat, still produce new varieties; Animals that have long been reduced to a domestic state are still susceptible to very rapid modifications or improvements.

So that I can judge, after having studied this subject for a long time, that the conditions of life seem to act in two distinct ways: directly on the whole organism, or on certain parts only, and indirectly by affecting the reproductive system. As to the direct action, we must remember that in all cases, as Professor Weismann has lately remarked , and as I have incidentally shown in my work on the Variation à l'Êtat Domestique, we must remember, I said, that this action is subject to two factors: the nature of the organism and the nature of the conditions. The first of these factors seems to be much the more important, because, so far as we can judge, almost similar variations sometimes occur under different conditions, and, on the other hand, different variations occur under conditions which seem almost uniform. The effects on the offspring are either definite or indefinite. They may be considered as definite when all, or nearly all, the descendants of individuals subjected to certain conditions of existence for many generations are modified in the same way. It is extremely difficult to specify the extent of the changes which have been definitely produced in this way. There can, however, be no doubt as to the many very slight modifications, such as: modifications in shape arising from the quantity of nourishment; modifications in colour arising from the nature of the food; modifications in the thickness of the skin and its productions arising from the nature of the climate, etc. Each of the indefinite variations which we find in the plumage of our poultry-fowl must be the result of an effective cause; therefore, if the same cause acted uniformly, during a long series of generations, upon a large number of individuals , they would probably all change in the same manner. Facts such as the extraordinary and complicated excrescences, which are the invariable consequence of the deposit of a microscopic drop of poison supplied by the insect, prove to us that singular modifications may, among plants, result from a chemical alteration in the nature of the sap.

Changing conditions produce much more often an indefinite than a definite variability, and the former probably plays a much more important part than the latter in the formation of our domestic breeds. This indefinite variability is translated into countless small particularities that cannot be attributed, due to heredity, to either the father, the mother, or another distant relative. Considerable differences sometimes appear even in offspring of the same brood, or in plants born from grains originating from the same capsule. At long intervals, strongly pronounced deviations of formation are seen to arise to merit the qualification of monstrosities; these deviations affect a few individuals, among millions of others born in the same country and fed in much the same way; However, an absolute boundary cannot be established between monstrosities and simple variations. All these changes in conformation, whether they are slightly or very pronounced, which manifest themselves in a large number of individuals living together, can be considered as indefinite effects of the conditions of existence on each individual organism. These undefined effects could be compared to the effects of a cold, which affects different people in undefined ways, according to their state of health or constitution, resulting in some people having bronchitis, others having a runny nose, this one having rheumatism, and that one having inflammation. of various bodies.

Let us now pass to what I have called the indirect action of the change in the conditions of existence, that is, the changes resulting from modifications affecting the reproductive system. Two principal causes allow us to admit the existence of these variations: the extreme sensitivity of the reproductive system to any change in external conditions; the great analogy, proved by Kölreuter and other naturalists, between the variability resulting from the crossing of distinct species and that which can be observed in plants and animals bred under new or artificial conditions. Many facts testify to the excessive sensitivity of the reproductive system to this change, however insignificant, in the environmental conditions. Nothing is easier than to domesticate an animal; but nothing is more difficult than to induce it to reproduce in captivity, even if the union of the two sexes is easily effected. How many animals do not reproduce, even if left almost free in their native country! This fact is usually attributed, although without reason, to a corruption of instincts. Many cultivated plants sprout with all their vigor, and yet produce little or no grain. It has been found in some cases that a slight change, a little too much or too little water, for example, at a particular time of growth, will or will not affect the production of grain in a plant. I cannot go into the details of the facts I have collected and published elsewhere on this curious subject here; but to show how singular are the laws governing the reproduction of captive animals, I can state that carnivorous animals, even those from tropical countries, reproduce quite easily in our countries, except, however, animals belonging to the plantigrade family; and I can also note that carnivorous birds almost always do not lay fertile eggs. Many exotic plants produce only pollen of no value, like that of the most sterile hybrids. We see, therefore, on the one hand, that animals and plants reduced to the domestic state reproduce easily in captivity, although they are often stunted and diseased; and on the other hand, individuals taken very young from their forests and perfectly enduring captivity, admirably domesticated in the strength of their age, and healthy (I could cite numerous examples), of which the reproductive system, being seriously compromised by unknown causes, has ceased to function. In the presence of these two orders of facts, is it surprising that the reproductive system acts so irregularly when it functions in captivity, and that the offspring are somewhat different from their parents? I may add that, just as certain animals reproduce easily under less natural conditions (for example, rabbits and ferrets confined in cages), which proves that their reproductive system has not been affected by captivity, so, too, certain animals and certain plants endure domestication or cultivation without changing much.

Some naturalists maintain that all variations are linked to the act of sexual reproduction; is certainly a mistake. I have, in fact, cited in another work an extensive list of plants that gardeners call crazy plants, that is, plants in which a shoot suddenly appears presenting some new character and, sometimes, completely different from the other shoots in the same plant. These variations of buds, if this expression can be used, can in turn be propagated by grafting or layering, etc. , or sometimes even by seeding. Such variations are rarely reproduced in nature, but are quite frequent in cultivated plants. We may conclude, therefore, that the nature of the organism plays the chief part in producing the particular form of each variation, and that the nature of the conditions is subordinate to it; in fact, we often see in the same tree, subjected to uniform conditions, a single bud, among thousands of others produced annually, suddenly present new characteristics; we see, moreover, shoots belonging to different trees, placed in different conditions, producing almost the same variety; peach shoots, for example, producing red peaches, and common rose shoots producing moss roses. The nature of the conditions, therefore, is perhaps of no more importance in this case than the nature of the spark, communicating the fire to a mass of fuel, in determining the nature of the flame.

EFFECTS OF HABITS AND USE OR NON-USE OF PARTS; VARIATION BY CORRELATION; HEREDITY

Changing habits produces hereditary effects; one could mention, for example, the flowering season of plants transported from one climate to another. In animals, the use or non-use of parts has an even more considerable influence. Thus, in proportion to the rest of the skeleton, the wing bones weigh less and the thigh bones weigh more in the domestic canary than in the wild canary. Now, this change can undoubtedly be attributed to the fact that the domestic canary flies less and walks more than the wild canary. We can also mention, as one of the effects of the use of the parts, the considerable development, transmissible by heredity, of the udders of cows and goats in countries where there is a habit of milking these animals, compared to the state of these organs in other countries. In some countries, all domestic animals have hanging ears; This peculiarity is attributed to the fact that these animals, having fewer causes for alarm, end up not using the muscles of the ear, and this opinion seems well founded.

Variability is subject to many laws; some are imperfectly known, which I will discuss shortly. I wish to deal here only with variation by correlation. Important changes that occur in the embryo or larva almost always bring about similar changes in the adult animal. In monstrosities, the effects of correlation between completely distinct parts are very curious; Isidore Geoffroy de Saint-Hilaire cites numerous examples in his great work on this subject. Keepers admit that when the limbs are long, the head is almost always long too. Some cases of correlation are extremely singular: thus, completely white cats, which have blue eyes, are ordinarily deaf; However, M. Talt has recently proven that the costume is limited to males. Certain colors and certain constitutional particularities ordinarily go together; I could cite many remarkable examples in this regard in animals and plants. According to a large number of fantasies collected by Heusinger , it seems that certain plants annoy white sheep and pigs, while individuals of a color laden with them feed with impunity. Professor Wyman recently communicated to me an excellent proof of what I say. He asked some Virginia farmers why they only had black pigs; and they answered that the pigs ate the root of the lachnanthes , which stains their bones pink and makes their hooves fall off; This occurs in all varieties except the black variety. One of them added: “We choose all the black individuals from a litter to treat them, because they are the only ones that are fit to live.” Hairless dogs have imperfect teeth; Animals with long, rough hair are said to be predisposed to having long, numerous horns; feathered-legged pigeons have webbing between their front toes; short-billed pigeons have small feet; long-billed pigeons have large feet. It follows, therefore, that man, while always continuing to choose, and consequently to develop, any particularity, unintentionally modifies other parts of the organism, by virtue of the mysterious laws of correlation.

The various laws, absolutely ignored or imperfectly understood, which govern variation, have extremely complex effects. It is interesting to study the different treatises relating to some of our long-cultivated plants, such as the hyacinth, the potato, or even the dahlia, etc. ; It is truly surprising to see why in countless points of conformation and constitution the varieties and subvarieties differ slightly from each other. Its organization seems to become completely plastic and deviate slightly from that of the original type.

All non-hereditary variation is of no interest to us, but the number and diversity of deviations of conformation transmissible by heredity, whether they be insignificant or of considerable physiological importance, are almost infinite. The best and most complete work we have on the subject is that of Dr. Prosper Lucas. No keeper has questioned the great energy of hereditary tendencies; Everyone has as a fundamental axiom that like produces like, and only a few theorists question the value of this principle. When a division of structure is reproduced many times, when we look for it in the father and the son, it is very difficult to say whether or not this deviation comes from something that acted on both of them. But, on the other hand, when among individuals, evidently exposed to the same conditions, any very rare deviation, due to some extraordinary concurrence of circumstances, appears in a single individual, in the midst of millions of others who are not affected, we see this deviation appear in the descendant, the simple theory of probabilities forces us almost to attribute this appearance to heredity. Who hasn't heard of cases of albinism, spiny skin, hairy skin, etc. , hereditary in many members of the same family? Now, if rare and extraordinary deviations can really be transmitted by heredity, it can be argued with even stronger reason that less extraordinary and more common deviations can also be transmitted. The best way to summarize the issue would perhaps be to consider that, as a general rule, every character, whatever it may be, is transmitted by heredity and that non-transmission is the exception.

The laws regulating heredity are for the most part unknown. What is the reason why , for example, the same characteristic, appearing in different individuals of the same species or different species, is sometimes transmitted and sometimes not transmitted through heredity? Why do certain characteristics of the grandfather or grandmother, or of more distant ancestors, reappear in the individual? Why is a particularity often transmitted from one sex, either to both sexes or to one, but more commonly to one, though not exclusively to the similar sex? The peculiarities which appear in the males of our domestic species are often transmitted, either exclusively or to a much more considerable degree, in the male alone; Now, this is a fact that has extraordinary importance for us. A much more important rule, and one which, I believe, suffers few exceptions, is that in any period of life in which a particularity appears initially, it tends to reappear in descendants at a corresponding age, sometimes even a little earlier. In many cases, it cannot be otherwise; in fact, the hereditary peculiarities that the horns of the great bull present can only manifest themselves in its descendants in adulthood, more or less; The particularities that silkworms present also do not appear except at the corresponding age in which the worm exists in the form of a larva or chrysalis. But hereditary diseases, and some other facts, lead me to believe that this rule is susceptible of greater extension; in fact, even if there is no apparent reason for a particularity to reappear at a given age, it tends , however, to be represented in the descendant of the same age as the ancestor. This rule seems to me to have great value in explaining the laws of embryology. The present notes apply, of course, only to the first appearance of the particularity, and not to the primary cause which may have acted upon the ova or upon the male element; thus, in the offspring of a bare-horned cow and a long-horned bull, their development, although it manifests itself only very late, is evidently due to the influence of the male element.

Since I have alluded to the return of primitive characters, I may now deal with an observation often made by naturalists; that is, that our domestic varieties, returning to wild life, gradually but invariably resume the characters of the original type. It has been concluded from this fact that no deduction applicable to the knowledge of wild species can be drawn from the study of domestic breeds. I try in vain to discover on what decisive facts this assertion, so frequently and so cunningly renewed, can be supported; It would be very difficult, in fact, to prove its accuracy, because we can affirm, without fear of being mistaken, that the majority of our domestic varieties, the most strongly characterized, could not live in the wild. In many cases, we really do not know what their primitive origin is; therefore, it is almost impossible to say whether the return to this origin is more or less perfect. Furthermore, it would be essential, to prevent the effects of crossbreeding, that a single variety be released. However, as it is certain that our varieties may accidentally return to the ancestral type by some of their characters, it seems to me quite probable that, if we could manage to acclimatize, or even cultivate for many generations, the different races of cabbage, for example , on very poor soil (in this case, however, it would be necessary to attribute any influence to the definite action of the poverty of the soil), would return, more or less completely, to the primitive wild type. Whether the experience gives this result or not is of little importance from the point of view of our argument, because the conditions of existence would be completely modified by the experience itself. If it could be demonstrated that our domestic varieties have a great tendency to regress, that is, if it could be established that they tend to lose their acquired characters, even when they are subjected to the same conditions and are kept in considerable numbers, in such a way that the crosses could stop, confusing them, the small deviations of conformation, I recognize, in this case, that we could not conclude from the domestic varieties to the species. But this way of seeing finds no evidence in its favor. To assert that we could not perpetuate our draft horses and race horses, our long-horned and short-horned cattle, our poultry of various breeds, our vegetables, for an infinite number of generations, would be contrary to what everyday experience teaches us. .

CHARACTERISTICS OF DOMESTIC VARIETIES; DIFFICULTY IN DISTINGUISHING BETWEEN VARIETIES AND SPECIES; ORIGIN OF DOMESTIC VARIETIES ATTRIBUTED TO ONE OR MANY SPECIES

When we examine the hereditary varieties or breeds of our domestic animals and cultivated plants, and compare them with closely related species, we usually notice, as we have said, that in each domestic breed there are less uniform characters than in the true species. Domestic breeds often present a somewhat monstrous character; by this I mean that, although they differ from each other and from neighboring species of the same genus in some slight character, they often differ in a great degree in one particular point, whether we compare them with each other, or, more importantly, with the wild species to which they most closely resemble. Besides this (and excepting the perfect fecundity of varieties crossed among themselves, a subject which we shall discuss later), domestic breeds of the same species differ from each other in the same way as neighboring species of the same genus in the wild state; but the differences are, in most cases, less considerable. It must be admitted that this point has been proved, for competent judges have pointed out that the domestic races of many animals and plants are derived from distinct original species, while others, no less competent, regard them as mere varieties. Now, if there were a very clear distinction between domestic races and species, this doubt would not arise so frequently. It has often been repeated that domestic races do not differ from each other in characters of generic value. It may be shown that this assertion is not correct; but naturalists have very different opinions as to what constitutes a generic character, and consequently all current judgments on this point are purely empirical. When I shall explain the origin of natural genera, it will be seen that we must by no means expect to find differences of generic order in domestic races.

We are reduced to chances since we attempt to estimate the value of the differences in conformation which separate our most closely related domestic breeds; we do not know, in fact, whether they derive from one or many mother species. It would, therefore, be a very interesting point to elucidate. If, for example, it could be proved that the Greyhound, the Hound, the Hunting Dog, the Spanish Mastiff, and the Bulldog, animals whose breed, as we know, is so purely propagated, are all derived from the same species, we would evidently be entitled to doubt the immutability of of a large number of closely related wild species, such as foxes, for example, that inhabit different parts of the globe. I do not believe, as we shall soon see, that the sum of the differences which we find among our several breeds of dogs, has been produced entirely in the state of domesticity; I believe, on the contrary, that some of these differences come from the offspring of different species. Despite the very characteristic breeds of some other domestic species, there is strong presumption, or even absolute proof, that they all descend from a common wild origin.

It has often been claimed that, in order to reduce them to domesticity, man chose animals and plants which present an exceptional inherent tendency to variation, and which possessed the faculty of withstanding the most different climates. I do not dispute that these aptitudes have greatly increased the value of most of our domestic products; but how could a savage know, when he had imprisoned an animal, whether that animal was likely to vary in future generations and withstand changes of climate? Did the low variability of the donkey and the duck, the reindeer's lack of disposition to heat or the camel's lack of disposition to cold, prevent their domestication? I am persuaded that if animals and plants were taken from the wild in numbers equal to our domestic products, and belonging to a great number of classes and countries, and if they were reproduced in the domestic state, for an equal number of generations, they would vary on the average as much as the parent species of our present domestic breeds have varied. It is impossible to decide, with respect to the greater part of our oldest cultivated plants and of the animals reduced to domesticity for many centuries, whether they derive from one or more wild species. The main argument of those who believe in the multiple origin of domestic animals rests on the fact that we find, from the most remote times, in the monuments of Egypt and in the lake dwellings of Switzerland, a great diversity of breeds. Many of them have a striking similarity, or are even identical to those that exist today. But this only pushes back the origin of civilization, and proves that animals were reduced to domesticity at a much earlier period than we currently think. The inhabitants of the lakeside towns of Switzerland cultivated many species of wheat and oats, peas and poppies to extract oil and hemp; They had many domestic animals and were in commercial relations with other nations. All this clearly proves, as Heer pointed out, that they had progressed considerably; This, however, also implies a long preceding period of less advanced civilization, during which domestic animals, kept in different regions, could, varyingly, give rise to distinct breeds. After the discovery of flint tools in the surface layers of many parts of the world, all geologists believed that barbarian man existed at an extraordinarily remote period, and we know today that there is no tribe, however barbarous, that has not domesticated the dog. .

The origin of most domestic animals will remain in doubt forever. But I must add that, after laboriously collecting all the known facts concerning the domestic dogs of the whole world, I have been led to conclude that many wild species of Canidae must have been kept in captivity, and that their blood runs more or less mixed in their veins. of our natural domestic breeds. I could not come to any precise conclusion regarding sheep and goats. From the facts which M. Blyth has communicated to me concerning the habits, voice, constitution, and formation of the Indian humped bull, it is almost certain that he descended from a primitive stock different from that which produced our European bull. Some competent critics believe that the latter derives from two or three wild origins, without pretending to affirm whether or not these origins are considered as species. This conclusion, as well as the specific distinction which exists between the humped bull and the ordinary ox, has been almost definitely established by the admirable studies of Professor Rütimeyer . As for horses, I hesitate to believe, for reasons which I cannot develop here, and which are contrary to the opinion of many wise men, that all breeds derive from a single species. I have treated nearly all the English breeds of our poultry, I have crossed them, I have studied their skeletons, and I have come to the conclusion that they all descend from a wild Indian species, Gallus bankiva ; This is also the opinion of M. Blyth and other naturalists who studied this bird in India. As to ducks and rabbits, of which some breeds differ considerably from each other, it is evident that they are all derived from the common wild duck and the wild rabbit.

Some authors have carried to an extreme the doctrine that our domestic breeds are derived from many wild origins. They believe that every race that reproduces purely, however slight its distinctive characters, had its wild prototype. Thus there must be at least twenty species of wild bulls, as many species of sheep, and many species of goats in Europe, many of which are in Great Britain alone. One author claims that there must have been eleven species of wild sheep in Britain that were unique to it! When we remember that this country does not have today a mammal that is peculiar to it, that France has only a few, very few, that are distinct from those of Germany, and that the same is true of Hungary and Spain, etc., but that Each of these countries has many particular species of bulls, sheep, etc., it is necessary then to admit that a large number of domestic breeds originated in Europe, because where could they come from? And it takes place in India. It is true that hereditary variations have played an important part in the formation of the numerous breeds of domestic dogs, for which I admit, however, many distinct origins. Who could believe, indeed, that many animals resembling the Italian Greyhound, the Mongrel, the Bulldog, the French Bulldog, and the Blenheim Spaniel , types so different from the wild canids, had existed in the primitive state? It has often been asserted, without certain proof, that all our breeds of dogs come from the crossing of a small number of primitive species. However, only intermediate forms between the parents are obtained through crossing ; Now, if we want to explain the existence of our different domestic breeds in this way, it is necessary to admit the previous existence of the most extreme forms, such as the Italian Greyhound, the Rafeiro, the Bulldog, etc., in the wild state. Furthermore, the possibility of forming distinct races through crossbreeding has been greatly exaggerated. It has been proven that a breed can be modified by accidental crosses, assuming, however, that the individuals representing the desired type are carefully selected: but it would be very difficult to obtain a breed intermediate between two completely distinct breeds. Sir J. Sebrigth tried numerous experiments to this end, but could obtain no results. The products of the first cross between two pure breeds are quite uniform, sometimes even perfectly identical, as I have observed in pigeons. Nothing, therefore, seems simpler; when, however, these crossbreeds are crossed among themselves for many generations, two similar products are no longer obtained and the difficulties of operation become evident.

DOMESTIC PIGEON BREEDS, THEIR DIFFERENCES AND THEIR ORIGIN

Convinced that it is always worth studying a special group, I decided, after mature reflection, on domestic pigeons. I have treated every breed I could obtain by purchase or otherwise; Furthermore, skins have been sent to me from almost every part of the world; I am chiefly indebted for these remittances to the Honorable W. Elliot, who brought to my attention specimens from India, and to the Honorable C. Murray, who sent me specimens from Persia. Treatises on pigeons have been published in every language; Some of these works are very important, as they date back to the most remote antiquity. I have joined many leading breeders and am a member of both London Pigeon Clubs. The diversity of pigeon breeds is truly admirable. If one compares the English Courier with the Short-faced Tumbler, one is struck by the enormous difference in the beak, which is matched by corresponding differences in the skull. The Courier, and more particularly the male, presents a pronounced development of the carbuncular membrane of the head, accompanied by a great elongation of the eyelids, wide nasal openings and a large opening of the beak. The Short-faced Tumbler's beak resembles that of a sparrow; The Ordinary Somersault has the singular habit of rising to a great height in a disorderly fashion, and then performing a complete somersault in the air. The Runt (Roman Hen Pigeon) is a large bird with a long, massive beak and large feet; Some sub-races have long necks, others long wings and a long tail. Barbado is allied with the carrier pigeon; but the beak, instead of being long, is wide and very short. The Sparrow Pigeon has an elongated body, wings and legs; the enormous crop, which swells with pride, gives it a bizarre and comical appearance. The bower pigeon has a short, conical bill and an array of ruffled feathers on its breast; has the habit of slightly dilating the upper part of the esophagus. The Cabeleira has feathers that are so ruffled on the dorsal part of its neck that they form a kind of hood; proportionally to its size, it has very elongated wing and neck feathers. The Trumpet, or Drum Pigeon, and the Laughing Pigeon, as their name suggests, make a cooing sound that is very different from that of other breeds. The fantail pigeon has thirty or even forty feathers in its tail, instead of twelve or fourteen, a normal number in all members of the pigeon family; It has such ostentatious and ruffled feathers that, in purebred birds, the head and tail touch; but the oil gland is completely atrophied. We could also indicate some other less distinct breeds.

The development of the bones of the face differs greatly, both in length and in width and curvature, in the skeleton of different races. The shape, as well as the dimensions of the lower jaw, vary very markedly. The number of caudal vertebrae and sacral vertebrae also varies in the same way as the number of ribs and processes, as well as their relative width. The shape and size of the sternal openings, the degree of divergence and the dimensions of the fork branches are also very varied. The proportional width of the nozzle opening; the relative length of the eyelids; the dimensions of the nostril opening and those of the tongue, which are not always in absolutely exact correlation with the length of the beak; the development of the crop and the upper part of the esophagus; the development or atrophy of the oil gland; the number of primary wing and tail feathers; the relative length of the wings and tail, either relative to each other or in relation to the body; the relative length of the leg and foot; the number of finger scales; the development of the interdigital membrane has so many essentially variable parts. The time at which the new birds acquire their perfect plumage, as well as the nature of the plumage with which the young are clothed on hatching, also vary; and also the shape and size of the eggs. Flight, and in certain breeds, voice and instincts, present remarkable diversities. Finally, in certain varieties, males and females can differ somewhat from each other.

One could easily collect a score of pigeons such that, if they were shown to an ornithologist, and told him that they were wild birds, he would certainly classify them as so many distinct species. I do not believe that any ornithologist would agree to place in the same genus the English Courier, the Short-faced Tumbler, the Runt , the Bearded Pigeon, the Sparrow Pigeon and the Fan-tailed Pigeon; he would do so all the less if one could show him, for each of these races, many sub-varieties of pure descent, that is, of species, as he would certainly call them.

However considerable the difference observed between the various breeds of pigeons, I am entirely of the opinion of the common naturalists that they are descendants of the Wood Pigeon ( Columba livia ), comprising under this term many geographical races, or subspecies, which differ from each other only in insignificant points. I shall briefly state many of the reasons which lead me to adopt this opinion, because they are, to some extent, applicable to other cases. If our various breeds of pigeons are not varieties, if, in a word, they do not derive from the Torcaz, they must derive from at least seven or eight original types, because it would be impossible to produce our present domestic breeds by reciprocal crossings of a smaller number. How, for example, can a Crop Pigeon be produced by crossing two breeds, unless one of the parent breeds has the characteristic enormous crop? The supposed original types must all have been rock dwellers like the Woodcreeper, that is, species that do not voluntarily roost or nest in trees. But beyond Columba livia and its geographic subspecies, only two or three other species of rock pigeons are known and they do not present any of the characteristics typical of domestic breeds. The primitive species must therefore either still exist in the countries where they were originally reduced to domesticity, and in this case they escaped the attention of ornithologists, which, considering their size, habits and remarkable character, seems impossible; or have become extinct in the wild. It is, however, difficult to exterminate birds that nest on the edge of cliffs and have powerful flight. Furthermore, the common woodpecker, which has the same habits as the domestic breeds, has not been exterminated either on the small islands surrounding Great Britain or on the coasts of the Mediterranean. It would therefore be a false assumption to admit the extinction of such a large number of species having the same customs as the Torcaz. Furthermore, the domestic breeds, of which we have spoken above, were transported to all parts of the world; some, therefore, must have been taken to their country of origin; none, however, has returned to the wild state, although the common pigeon, which is none other than the Wood Pigeon in a very little modified form, has become wild in many places. Ultimately, experience shows us how difficult it is to force a wild animal to reproduce regularly in captivity; However, admitting the multiple origin of our pigeons, it is also necessary to admit that at least seven or eight species were imprisoned by man in a semi-wild state to make them perfectly fertile in the captive state.

There is another argument that seems to me to have great value and that can be applied to many other cases: the breeds we have spoken of, although generally resembling the wild Woodpecker in constitution, habits, voice, color and for the most part of their conformation, they differ from it, however, in many other points. It would be in vain to look, in the whole great family of the Columbidae, for a beak similar to that of the English Postman, the Short-faced Tumbler or the Bearded One; ruffled feathers similar to those of the Cabeleira; crop compared to that of the Crop Pigeon; tail feathers comparable to those of the peacock pigeon. It would be necessary, therefore, to admit, not only that semi-savage men have completely imprisoned many species, but even, by chance or intention, they have chosen the most extraordinary and most abnormal species; It was also necessary to admit that all these species subsequently became extinct or became unknown. Such a concurrence of circumstances is improbable in the highest degree.

Some facts concerning the colour of pigeons are worthy of mention. The Wood Pigeon is slate-blue with white flanks; in the indica subspecies, Strickland's Columba intermedia, the flanks are bluish; the tail has a heavily edged terminally, and the feathers on the sides are externally bordered with white at the base; the wings have two black bars. In some semi-domestic breeds , as well as in some absolutely wild ones, the wings, in addition to the two black edges, are dotted with black. These various marks are not found together in any other species of the family. Now, all the marks which we have just indicated are sometimes perfectly developed up to the white edge of the outer feathers of the tail, in pure-bred birds belonging to all our domestic breeds. Furthermore, when pigeons belonging to two or more distinct breeds are crossed, and do not have either the blue colour or any of the other signs just mentioned, the offspring of these crosses are very likely to acquire these characters very quickly. I will limit myself to mentioning one example, among many others, which I have observed. I crossed some white peacock pigeons of the purest breed with some black Barbados (the blue varieties of the Barbado are so rare that I do not know of a single specimen in England); the birds I obtained were black, grey and spotted. I also crossed a Barbado with a Spot pigeon, which is a white bird with a red tail and a red spot on the top of the head, and which reproduces faithfully; I obtained grey and spotted crossbreeds. I then crossed one of the bearded-peacock crossbreeds with a bearded-spot crossbreed, and obtained a bird of such a beautiful blue as any wild pigeon, with white flanks, a double black border on the wings, and the outer feathers of the tail edged with black and bordered with white! If all breeds of domestic pigeons are descended from the Wood Pigeon, these facts are easily explained by the well-known principle of reversion to the characters of the ancestors; but if this origin is disputed, it is necessary to admit one of the following two hypotheses, hypotheses as improbable as possible: either all the different original types were colored and marked like the Wood Pigeon, since no other existing species presents these same characters, so that in each separate breed there is a tendency to reversion of colors and characteristics; or else each breed, even the purest, has been crossed with the Woodcock within an interval of ten or even more than twenty generations—I say twenty generations, because no example is known of the products of a cross having returned to an ancestor of foreign blood removed from them by a more considerable number of generations.—In a breed which has been crossed only once, the tendency to reversion to one of these characters due to this cross naturally diminishes, each successive generation containing an ever smaller quantity of foreign blood. But when there has been no cross, and there is a tendency in a breed to return to a character lost during many generations, this tendency, after what has been said, may be transmitted without weakening for an indefinite number of generations. Authors who have written on heredity have often confused these two quite distinct cases of reversion.

Finally, as I have been able to verify through the observations I have made expressly on the most distinct breeds, the hybrids or crossbreeds coming from all domestic breeds of pigeon are perfectly fertile. Now it is difficult, if not impossible, to cite a well-established case tending to prove that hybrid descendants from two clearly distinct species of animals are completely fertile. Some authors believe that prolonged domesticity reduces this great tendency towards sterility. The history of the dog , and that of some other domestic animals, renders this opinion very probable, if it be applied to the closely allied species; but it seems to me extremely reckless to generalize this hypothesis to the point of supposing that species that were originally so distinct, such as the Postal Service, the Cambalhotas, the Papudos and the Pavões, were able to produce perfectly fertile descendants “ inter se”.

These different reasons, which it is always good to recapitulate, that is, the improbability of man having once reduced seven or eight species of pigeons to a domestic state, and above all made them reproduce in this state freely; the fact that these supposed species are unknown everywhere in the wild state, and that domestic species do not become wild anywhere; the fact that these species present certain very abnormal characters, compared to all other species of columbidae, since they resemble the Woodcreeper in almost all aspects; the fact that the blue color and the different black stigmas reappear in all races, whether they remain pure or crossbreed; finally, the fact that crossbreeds are perfectly fertile - this complex of reasons leads us to conclude that all our domestic breeds derive from the Torcaz or Columba livia and its geographic subspecies.

I will add, in support of this opinion: first, that Columba livia or Torcaz is shown, in Europe and India, to be susceptible of easy domestication, and that there is a great analogy between its habits and the conformation of all domestic breeds; secondly, that, although the English Courier or the Short-faced Cambalhota differ considerably from the Torcaz in certain characters, it is possible, however, by comparing the various sub-varieties of these two breeds, and especially those originating from distant countries, to establish between the Torcaz and them an almost complete series connecting the two extremes (the same series can be established in some other cases, but not with all breeds); thirdly, that the principal characters of each breed are, in each of them, essentially variable, such as, for example, the caruncles and the length of the beak in the English Courier, the very short beak of the Cambalhota, and the number of tail feathers in the Peacock Pigeon (the obvious explanation of this fact will appear when we come to deal with selection); fourthly, that pigeons have been the object of the most extreme care on the part of a great number of fanciers, and that they have been reduced to a domestic state for thousands of years in different parts of the world. The oldest document in history concerning pigeons dates back to the fifth Egyptian dynasty, about three thousand years before our era; this document was pointed out to me by Professor Lepsius ; on the other hand, M. Birch teaches me that the pigeon is mentioned in a meal bulletin of the preceding dynasty. Pliny tells us that the Romans paid a considerable price for pigeons: “They went so far,” says the Latin naturalist, “as to take care of their genealogy and their breed.” In India, about the year 1600, Abker -Khan had such a great regard for pigeons that his dovecote had at least twenty thousand specimens. “The monarchs of Iran and Turan sent him very rare birds.” The royal chronicler then adds: "His Majesty, by crossing the breeds, which had never been done before, improved them extraordinarily." At the same time, the Dutch were also lovers of pigeons, as were the ancient Romans. When we come to consider selection, we will understand the great importance of these considerations in explaining the enormous number of variations that pigeons present. We will then also see how it is that different breeds often have monstrous characters. Finally, it is necessary to point out an extremely favourable circumstance for the production of distinct breeds, namely that male and female pigeons usually mate for life, and that many different breeds can be kept in the same cage.

I have just discussed at great length, and yet in an insufficient manner, the probable origin of our domestic pigeons; if I did so, it was because, when I began to care for pigeons and observe the different species, I was also unwilling to admit (knowing how faithfully the different races reproduce) that they all derived from a single parent species, and if had been formed from the moment they were reduced to the domestic state, as any naturalist would be in accepting the same conclusion with respect to numerous species of sparrows, or any other natural group of wild birds. One circumstance has struck me particularly, and that is that the greater part of the keepers of domestic animals, or the breeders with whom I have come into contact, or whose works I have read, are all firmly convinced that the different breeds with which each has been concerned in particular, they derive from many other primitively distinct species. Ask, as I did, a famous Hereford cattle-keeper , whether he might not derive his cattle from a long-horned breed, or whether the two breeds were descended from a common stock, and he will laugh at you. I have never met a keeper of pigeons, chickens, ducks, or rabbits, who was not entirely convinced that each principal breed was derived from a distinct species. Van Mons, in his treatise on pears and apples, categorically refuses to believe that different species, a pippin Ribsion and a Codlin apple , for example, may descend from seeds of the same tree. One could cite countless other examples. The explanation for this costume seems simple to me: they are deeply impressed, due to their long studies, by the differences that exist between the various races, and yet they know very well that each of them varies slightly, since they only win prizes in competitions by carefully choosing these slight differences, the keepers , however, ignore the general principles, and refuse to take into account the slight differences that have accumulated over a large number of successive generations . Naturalists, who know far less than pet owners about the laws of heredity, who know no more about the intermediate links which connect long genealogical series, and who nevertheless admit that the greater part of our domestic breeds are derived from a of the same kind, could they not become a little more prudent, and not scoff at the opinion that a species, in a state of nature, can be the direct posterity of other species?

The dove Columba livia illustrated by John Gould (between 1832 and 1837).

PRINCIPLES OF SELECTION OLD APPLIED AND THEIR EFFECTS

Let us now consider in a few lines the gradual formation of our domestic breeds, whether they are derived from a single species or from many neighbouring species. Some effects may be attributed to the direct and definite action of external conditions of existence, others to habits; but it would be necessary to be very cunning to explain by such causes the differences which exist between the draft horse and the racehorse, between the pointer and the greyhound, between the courrier and the tumbler. One of the most remarkable characteristics of our domestic breeds is that we see among them adaptations which contribute in no way to the well-being of the animal or the plant, but simply to the advantage and whim of man. Certain variations useful to man are probably produced gradually and successively by others; some naturalists, for example, believe that the combing thistle armed with hooks, which cannot replace any machine, is quite simply a variety of the wild Dipsacus ; and yet this transformation may be manifested in a single seed. It is equally probable that it was the same for the Tournebroche dog ; it is known, at least, that the Ancon sheep appeared suddenly. But it is necessary, if we compare the draft horse and the racehorse, the dromedary and the camel, the different breeds of sheep adapted both to the cultivated plains and to the pastures of the mountains, and whose wool, according to the breed, is suited to both; if we compare the different breeds of dogs, each of which is useful to man from different points of view; if we compare the fighting cock, so inclined to fight, with other breeds so peaceful, with the perpetual layers without ever brooding, and with the Bantam cock , so small and so elegant; If we consider, finally, this legion of agricultural and culinary plants, the trees that shade our orchards, the flowers that adorn our gardens, some of which are so useful to man in different seasons and for so many different uses, or simply so pleasing to the eye, it is necessary to look, I think, for something more than a simple effect of variability. We cannot suppose, in fact, that all these breeds have been successively produced with all the perfection and all the usefulness that they have today; we know, in fact, that in many cases this has not been the case. The power of selection, of accumulation, which man possesses, is the key to this problem; nature provides the successive variations, man accumulates them in certain directions that are useful to him. In this sense, it can be said that man has created useful breeds for his own benefit.

The great value of this selection principle is not hypothetical. It is true that many of our most eminent breeders have, during the mere age of a man, considerably modified their cattle and their flocks. To fully understand the results they have obtained, it is essential to read some of the numerous works they have devoted to this subject and to see the animals themselves. Breeders ordinarily consider an animal's organism as a plastic element, which they can modify at their leisure. If I were not short of space, I could cite, in this regard, numerous examples compiled from highly competent authorities. Youatt , who, more than any other, knew the labors of farmers, and who was himself an excellent judge in animal matters, admits that the principle of selection "enables the farmer not only to modify the character of his stock, but transform it entirely. It is the magic wand through which you can present the shapes and models that you like. Lord Somerville says, concerning what breeders have done to the sheep: "It seems as if they had drawn the outline of a perfect form, and then given it existence." In Saxony , the importance of the principle of selection in relation to Merino sheep is so well understood that it has become a profession; the sheep is placed on a table and a connoisseur studies it as he would a painting; This examination is repeated three times a year, and each time the rams are marked and classified in order to choose the most perfect ones for reproduction.

The considerable price attributed to animals whose genealogy is impeccable proves the results that English breeders have already achieved; their products are shipped to almost every part of the world. It would not be necessary to believe that these improvements were ordinarily due to the crossing of different races; The best breeders condemn this practice absolutely, and employ it only for closely related sub-breeds. When a cross of this kind is made, rigorous selection becomes even more indispensable than in ordinary cases. If selection consisted simply in isolating a few distinct varieties and making them reproduce, this principle would be so obvious that we would hardly have to deal with it; but the great importance of selection consists in the considerable effects produced by the accumulation in the same direction, during successive generations, of differences absolutely inappreciable to the inexperienced eye, differences which, so far as I am concerned, I have in vain attempted to appreciate. Not one man in a thousand has the sharpness of vision and the certainty of judgment necessary to become a skillful creator. A man endowed with these qualities, who devotes many years to the study of this subject, as long as he dedicates his entire life to it, applying all his energy and indomitable perseverance, will undoubtedly achieve good results and will be able to make immense progress; but the lack of just one of these qualities will necessarily determine a bad result. Few people realize how many natural skills are needed, and how many years of practice it takes to become a good pigeon breeder.

Horticulturists follow the same principles; but here the variations are often sudden. No one assumes that our most beautiful plants are the result of a single variation of the original source. We know that it has been otherwise in many cases about which we have exact knowledge. Thus, one can cite as an example the ever-increasing increase in the common gooseberry. If we compare today's flowers with designs made just twenty or thirty years ago, we can see improvements in most of the florist's products. When a breed of plants is sufficiently established, horticulturists no longer bother to select the best plants, they are content to visit the borderline plants to separate those that have reverted to the ordinary type. This kind of selection is also practiced with animals, because no one is negligent enough to allow the defective individuals of a herd to reproduce.

There is yet another way of observing the cumulative effects of selection in plants; it is enough, in fact, to compare, in a flowerbed, the diversity of flowers in the varieties of the same species; in a vegetable garden, the diversity of leaves, pods, tubers, or in general of the sought-after part of vegetable plants, in relation to the flowers of the same varieties; and, finally, in an orchard, the diversity of fruits of the same species, compared to the leaves and flowers of these same trees. Note how different the leaves of the cabbage are, and how similar the flower is; how different, on the contrary, are the flowers of the Pansy, and how uniform the leaves are; how the fruits of the various species of Gooseberry differ in size, color, shape and degree of villi, and how little difference there is in the flowers. It is only the varieties that differ greatly in one point, without differing in all the others, because I can affirm, after long and careful observations, that this never happens or almost never. The law of correlation of growth, the importance of which must not be forgotten, almost always involves some differences; but, as a general rule, it cannot be doubted that the continued selection of slight variations, whether in the leaves, flowers, or fruits, does not produce races different from each other, more particularly in one of the organs.

It might be objected that the principle of selection has been reduced to practice only for about three-quarters of a century. Undoubtedly, this subject has recently attracted more interest and numerous works have been published on the subject; the results have also been, as expected, quick and important; but it is not permissible to say that this principle is a modern discovery. I could cite many works from remote antiquity proving that, even then, the importance of this principle was recognized. We have proof that, even during the barbarous periods through which England has passed, purebred animals were often imported, and the laws prohibited their exportation; the destruction of horses that did not reach a certain height was ordered; which may be compared to the work which horticulturists do when they eliminate, among the products of their seed, all the plants which tend to deviate from the regular type. An ancient Chinese encyclopedia clearly formulates the principles of selection; certain classical Roman authors indicate some precise rules; It follows from certain passages in Genesis that, since that ancient period, some attention was already paid to the color of domestic animals. Even today, savages sometimes cross their dogs with wild canine species to improve the breed; Pliny confirms that it was done in the past. The savages of Southern Africa equip their yokes of oxen by color; the Eskimos use the same for packs of dogs. Livingstone notes that the blacks from the interior of Africa, who have no relation whatsoever to Europeans, value good domestic breeds at a high price. Undoubtedly, some of these costumes do not testify to direct selection; but they prove that, since ancient times, the culture of domestic animals was the object of very particular care, and that wild animals do the same today. It would be strange, moreover, that the heredity of good qualities and defects being so evident, choice should have constantly attracted man's attention.

UNCONSCIOUS SELECTION