The process is the same for all birds, except that those that take longest to develop in the shell take a longer rest after first breaking it. The young of aërial birds, which are naked when hatched, are ugly little things. Young poultry, too, are almost repulsive with their sprawling forms and the wet down plastered to the skin, but in a few hours they grow strong, the down dries and becomes fluffy, the bright little eyes seem to take in everything, and they are the most attractive of all baby animals.
CHAPTER III
SPECIES AND THEIR DIVISIONS IN DOMESTIC BIRDS
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.