If you take a hen's egg about the eighteenth or the nineteenth day of incubation and hold it closely in your hand, you may be able to feel the chick move. If your hand is a little bit cold, the chick is much more likely to squirm in the egg and may utter a peep. If, with the egg in a warm hand, you hold it to your ear, you will about this time hear an occasional tap, tap, caused by the chicken striking its beak against the shell. The tapping is kept up more or less steadily until the shell cracks where the point of the beak strikes it and a little piece is broken out. The chick usually rests awhile now,--perhaps for some hours,--then resumes the attack on the shell. It turns in the shell, breaking out little pieces as it turns, until there is a crack nearly all the way around, when, by pushing with its head and feet, it forces the shell apart and sprawls out of it. 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.