The mother of a young mammal knows that it came from herself, and she can see that it is like her and others of her kind. It at once seeks her care and responds to her attentions. The egg which a bird lays is as lifeless--to all appearances--as the stones which it often so closely resembles. Only after many days or weeks of tiresomely close attention does it produce a creature which can respond to the care lavished upon it. The birds incubating eggs not only give them the most unremitting attention, but those that fill their nests with eggs before beginning to incubate methodically turn the eggs and change their position in the nest, this being necessary because otherwise the eggs at the center of the nest would get too much heat and those at the outside would not get enough. A bird appears to know that if she begins to sit before she has finished laying, some of the eggs would be spoiled or would hatch before the others; and, as noted above, aërial birds seem to know better than to hatch more young than they can rear. But no bird seems to have any idea of the time required to hatch its eggs, or to notice the lapse of time, or to care whether the eggs upon which it sits are of its own kind or of some other kind, or to know whether the young when hatched are like or unlike itself. If eggs fail to hatch, domestic birds will, as a rule, remain on the nest until the eggs are taken away or until sheer exhaustion compels them to abandon the hopeless task. In domestication, however, those birds which continue laying most freely when their eggs are removed as laid, tend to lose the habit of incubation. Turkeys and geese will often begin to incubate after having laid about the number of eggs that they could cover. Many fowls will do the same, but most fowls lay for several months before attempting to incubate, and in many races not more than two or three per cent of the hens ever incubate. [Illustration: Fig. 4. Fresh egg[1]] [Illustration: Fig. 5. Infertile egg (after twenty-four hours' incubation)] [Illustration: Fig. 6. Fertile egg (after twenty-four hours' incubation)] [Illustration: Fig. 7. Embryo (after seventy-two hours' incubation)] [Illustration: Fig. 8. Embryo (after seven days' incubation)] [1] Photographs (Figs. 4-8) from Bureau of Animal Industry, United States Department of Agriculture. [Illustration: Fig. 9. Chick ready to break shell]