Читать книгу Our Domestic Birds: Elementary Lessons in Aviculture - John H. Robinson - Страница 7

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The three general classes of domestic birds include few species but many varieties, and, outside of the distinct varieties, an indefinite number of individual types. Where varieties are as numerous as in the fowl, which has about three hundred, and the pigeon, which has a much greater number, the differences between them are often very slight. Sometimes the form of a single small character is the only distinguishing feature. But, if this is a fixed character, the variety is distinct. Where there are so many varieties it is hard to make short, appropriate descriptive names for all, if considered simply as varieties. For such diversity there must be a more extended classification. Such a classification, growing gradually with the increase in the number of varieties, will not be consistent throughout. Hence to understand clearly the relations of the artificial divisions of species in domestication we must know what a species is and how these divisions arise.

Definition of species. Species are the natural divisions of living things. Each plant and animal species retains its distinctive character through long ages because the individuals composing it can produce perfect offspring only (if asexual) of themselves, or (if bisexual) with others of their species.

The self-isolation of species is well illustrated when similar plants grow together, as grasses in the same field and practically on the same spot; yet year after year all the old kinds are found and no new ones such as might come from a mixture of two kinds, if they would mix. In the higher animals, where the parent forms are of different sexes, they choose mates of their own kind, and so each species remains distinct; but if in a species there are many different types, such as we find in domestic fowls, the members of the species, when free to do so, mate as readily with types quite different from their own as with individuals exactly like them, and produce offspring of intermediate types with all the essential characters of the species. In domestication individuals of distinct yet similar species are sometimes mated and produce offspring called hybrids, but these are sterile. The mule, which is a hybrid between the ass and the mare, is the most familiar animal of this kind. Hybrid, or mule, cage birds are produced by crossing the canary with several allied species. Among other domestic birds hybrids are almost unknown.

Origin of species. Until near the close of the last century it was commonly believed that each species had been created in perfect form and that species were unchangeable; but long before that time some keen students of the natural sciences and close observers of the changes that take place in plants and animals in domestication had discovered that species were not perfectly stable and were changing slowly. Geologists established the fact that the earth, instead of being only a few thousand years old, had existed for countless centuries. Among fossil remains of creatures unlike any now known they had found also other forms which appeared to be prototypes of existing species. The idea that the forms of life now on the earth had come from earlier and somewhat different forms had occurred to several scientists more than a hundred years ago, but it was not until about 1860 that a satisfactory explanation of progressive development of forms of life was given to the world. This mode of creation is called evolution.

The theory of evolution is that partly through their own inherent tendency to vary and partly through the influence of external things which affect them, all organisms change slowly; that things of the same kind, separated and living under different conditions, may in time so change that they become separate species; and that this process may be repeated indefinitely, the number of species constantly increasing and becoming more diversified and more highly developed.

Such a theory would not be entitled to serious consideration unless it was known that the earth was millions of years old, because we know that races of fowls separated for over three thousand years (and perhaps twice as long) and developed into quite different varieties will breed together as readily as those of the same variety. But when it is certain that the earth is so old that there has been ample time for changes in living forms that would require periods of time beyond our comprehension, some of the relations of varieties and species of birds have an important bearing on the theory of evolution.

As in the case of fowls just noted, we find that domestic ducks of the same species, after a separation of several thousand years, breed freely together. But our domestic ducks are not, like the fowls, all of the same species, and if individuals of different species are paired they produce only a few weak hybrids. Our domestic geese are probably descended from two wild varieties, but races that were not brought together for thousands of years after they were domesticated are perfectly fertile together, while when mated with the American Wild Goose, which is not domesticated but will breed in captivity, they produce only hybrids. The general resemblance between geese and ducks is very striking, yet they will not breed together at all.

A comparison of these facts indicates that while three thousand, or even five or six thousand, years of separation may not be enough to break down the natural affinity of varieties of the same species, separation and difference of development will eventually make of varieties distinct species, a union of which will produce only hybrids, while a longer separation and further increase of differences makes the break between the species absolute and they will not breed together at all.

Natural varieties. A species having developed as a variety of an earlier species will continue to develop as one variety or as several varieties, according to conditions. If a part of a species becomes so separated from the rest that intercourse ceases, each division of the species may become a well-defined variety.

Varieties in domestication. How a species when domesticated breaks up into varieties is well illustrated by the case of the fowl. The original wild species has long disappeared, but there is good reason to suppose that in size and color it was something between a Brown Pit Game and a Brown Leghorn. The birds were smaller than most fowls seen in this country to-day. The prevailing color was a dull brown, because that color best conceals a small land bird from its enemies. Fowls that were domesticated and given good care and an abundance of food would usually grow larger than the wild stock. Thus if any person, or the people generally in any community, systematically gave their fowls good care, a variety of unusual size would be developed.

Different colors would also appear in the flocks of fowls, because the birds of unusual colors would be protected and preserved, instead of being destroyed as they usually are in the wild state. Other peculiarities, too, such as large combs, crests, and feathered legs, would be developed in some lands and neglected in others. This is how it happened that after thousands of years in domestication the races of fowls in different parts of the world were quite different in size and form, but alike in being of many colors.

From a species in this condition modern poultry breeders have made hundreds of distinct varieties. The easiest method of making a variety in domestication is to select specimens for breeding as near the desired type as possible, and to breed only from a few individuals in each generation which come nearest to the ideal type. In this way a variety that breeds quite true to the type may be established in from three or four to eight or ten years, according to the number of characters to be established as distinctive of the variety. Varieties are also made by crossing unlike individuals. This process is longer than the other, and sometimes requires a series of crosses to produce specimens approximating the ideal sought. After such specimens have been obtained the method is the same as in the first case. A variety is commonly considered to be well established when the greater part of the specimens produced are easily identified as of that variety. But no domestic variety is ever established in the sense that a species is. All are artificial, produced by compulsory separation and preserved only as long as it is continued.

Classification of domestic varieties of birds. Domestic varieties of all kinds of live stock were at first mostly shape-varieties; that is, the individuals of a variety were alike in shape but of various colors. This is the case still with some varieties. These shape-varieties are mostly the common types of certain countries or districts. Thus the Leghorn fowl is the common fowl of Italy, and the Houdan is a type common in a small district in France. Such shape-varieties are called breeds. When other types were made by crossing such breeds they also were called breeds.

When people first began to be interested in the improvement of live stock, the popular idea of a breed was that it was a domestic species, and there are still many people who hold this view. This popular misconception of the nature of a breed is responsible for much of the inconsistency and confusion in the ordinary classifications of domestic varieties. To it also is due the use of the term "variety" to apply especially to color-varieties, which are the principal divisions of breeds.

In the classification of domestic birds a variety is properly a color-variety of a breed. Thus in the Plymouth Rock breed there are six color-varieties—barred, white, buff, partridge, silver-penciled, and ermine (called Columbian); and in Fantail Pigeons there are six color-varieties—white, blue, black, red, yellow, and silver. Birds of the same breed (shape) and the same variety (color) may differ in some other character, as the form of the comb or the presence or absence of feathers in certain places. In accordance with such differences varieties are divided into subvarieties. Thus, in Leghorn Fowls the brown, white, and buff varieties have single-combed and rose-combed subvarieties.

In any breed, variety, or subvariety certain families are sometimes distinguished for general or special excellence of form or color. Such a family is called a strain.

Systematic mixtures of breeds and varieties. Although so many distinct varieties have been developed from common domestic stocks, the improved races do not always displace the mongrels. When the old mongrels disappear their place is often taken by a new mongrel stock produced by mixtures of the distinct breeds with each other and with the old mongrel race. The greater part of such stock is so mixed that its relation to any established breed could not be determined or expressed, but systematic mixtures are sometimes made, and to describe these the following terms are used: Crossbred—having parents of different, distinct breeds, varieties, or subvarieties. A Leghorn male mated with a Cochin female produces offspring each of which is in blood one half Leghorn and one half Cochin. Grade—having more than half of the blood of a breed.

If the offspring of a cross such as is described in the preceding paragraph are mated with birds of one of the parent breeds, the offspring of this mating will have three fourths of the blood of that breed. If these in turn are mated to birds of the same pure breed, the offspring will have seven eighths of the blood of that breed. Animals bred in this way are called grades until the process has been carried so far that they are practically pure-bred. Mongrel stock is often graded up in this way. As a rule stock that is seven eighths pure is not distinguishable from average pure stock of the same breed.

Pure-bred, thoroughbred, and standard-bred. A pure-bred animal is, strictly speaking, one having the blood only of the variety to which it belongs. From what has been said of the making of breeds and varieties it is plain that absolute purity of blood is not a universal attribute of well-bred domestic birds. A thoroughbred animal is one that is thoroughly bred for some purpose or to some type. A standard-bred animal is one that is bred especially to conform to requirements agreed upon by breeders and exhibitors.

A great deal of misapprehension and confusion in the use of these terms has been caused by the attitude of those who maintain that the term "thoroughbred," having been used as a name for highly bred running horses, cannot properly apply to any other kind of live stock, and that "pure-bred" should apply to all thoroughly bred races. The noun "Thoroughbred" is the name of a breed of horses. The adjective "thoroughbred" is common property. Writers on aviculture who wish to be accurate prefer it in many instances to "pure-bred" because absolute purity of blood is rare and is not of the importance in breeding that novices usually suppose. Not only are many new varieties made by crossing, but in long-established breeds outcrosses are regularly made to restore or intensify characters.

To illustrate the use of the three terms in application to a single breed: A stock of Light Brahmas might be kept pure for half a century, yet at the end of that period might have changed its type entirely. It might be so deteriorated that it was worth less than common mongrels; yet it is pure-bred stock. Another stock of the same variety might be bred for table qualities, egg-production, and the same principal color-characteristics of the variety, but without attention to the fine points of fancy breeding. Such a stock is thoroughbred but not standard-bred.