It is, and has been for ages, the common opinion that the wild birds and poultry, when first domesticated, were capable of laying only a small number of eggs each season, and that laying capacity has been enormously increased in domestication; but the oldest reports that we have of the amount of egg production indicate that the laying capacity of fowls was as great centuries ago as it is at the present time. Recent observations on wild birds in captivity show that even birds which pair and usually lay only a few eggs each season have a laying capacity at least equal to the ordinary production of hens. Quails in captivity have been known to lay about one hundred eggs in a season, and an English sparrow from which the eggs were taken as laid produced over sixty. The constitutional capacity to produce ovules is now known to be far greater than the power of any bird to supply the material for the nourishment of germs through the embryonic stage. The principal factors in large egg production are abundance of food and great capacity for digesting and assimilating it. =Incubation.= A bird before beginning to lay makes a nest. Some birds build very elaborate and curious nests; others merely put together a few sticks, or hollow out a little place on the ground. In birds that pair, the male and female work together to build the nest. Even in polygamous domestic birds like the fowl and the duck, a male will often make a nest for the females of his family and coax them to it as a cock pigeon does his mate. If the birds are left to themselves and the eggs are not molested, an aërial bird will usually lay a number of eggs equal to the number of young the parents can feed as long as they require this attention, while a terrestrial or aquatic bird will usually lay as many eggs as she can cover. The desired number of eggs having been laid, the process of incubation by the parents begins. The incubation of their eggs by birds is one of the most remarkable things in nature. We say that "instinct" leads birds to build their nests and to keep their eggs warm for a period varying from two weeks for small birds, to six weeks for the ostrich; but "instinct" is only a term to describe the apparently intelligent actions of the lower animals, which we say have not intelligence enough to know the reasons for the things that they do. [Illustration: Fig. 3. Sitting hen]