HATCHING, is exclusion of the young from the eggs of animals, either by the temperature of the circumam bient air, artificial heat, or the incubation of birds.
Nature has adopted some remarkable distinctions in the mode of propagating animals. Some are brought to per fection in the womb of the mother ; others, originally con centrated in eggs, are discharged either in that state, where the future concourse of the male is required to excite the vital spark, or where the latent embryo will be unfolded by the simple application of heat. To the first class belongs the offspring of all quadrupeds ; the second includes ma ny of the amphibious tribes, especially toads, frogs, and newts ; and the third, the whole class of birds, numerous fishes and reptiles, and most of the insect, molluscar, and vermicular tribes. Hatching and incubation have there fore different meanings, or the former, which applies to every kind of evolution of the nascent being, may be said to include the latter. Most animals that produce eggs, leave them simply to the cafe of nature, and certainly ne ver recognise their offspring after birth, at least with some exceptions, of which there is a prominent example in the crocodile. But in so far as we can yet ascertain, all birds impart their natural heat to their eggs by incubation, and watch the developement and subsequent growth of their young. Some distant analogy may therefore be conceived to subsist between gestation and incubation, in the respec tive period necessary for each, according to the genera and species of animals. The period of gestation is generally, perhaps universally, longer in the larger viviparous tribes ; and incubation is protracted, in proportion to the size of the bird. One irreconcileable difference, however, subsists, in there being no known method of accelerating the former, while exclusion of the young from the egg may be pro moted, by augmenting the intensity of the temperature. We are yet unacquainted with the process undergone by the egg of the largest of the feathered race, the ostrich : some assert that its exclusion is left entirely to the effects of the sun ; while others maintain that it is aided by the in cubation of the female. That of the swan requires incu bation during 42 lays ; that of the domestic hen 21 ; and that of the linnet 14. But we are told, that the period is somewhat abridged in the warmer climates ; that the egg of the common fowl has been brought out in 13 days, by the aid of artificial heat ; and that, by the diminution or in terruption of the temperature, it has been retarded for six weeks. It appears, that the heat of 104° of Fahrenheit's thermometer is required to hatch the eggs of all birds, the largest and smallest, and that the surface of the skin of the mother imparts it to that extent. Thus nothing more is required for the evolution of fecundated eggs, than the sim ple .application of any kind of heat.
These facts have been a long time established beyond controversy, whence they could not escape the notice of the ancients, as they now attract the consideration of the most unlearned observers. Instead of abiding by the ordina ry course which nature has herself committed to the parent, mankind, for the sake of deriving more profit from their own contrivances, have resorted to the means of hatching birds by artificial heat. The earliest information concern ing this process, is probably obtained from Aristotle in these words : " Although incubation be the common method em ployed by nature for bringing out eggs, it is not exclusive; for we see that in Egypt eggs will be hatched of them selves in the earth, if covered over and heated by litter." And he farther remarks," that when heated in certain vases wherein they are deposited, they hatch of themselves." Diodorus Siculus expresses his admiration of the contriv ance ; and Pliny, who lived a century after Diodorus, re marks, that "eggs are excluded of themselves quite natu rally, and without the aid of incubation by fowls, as in the dunghills of Egypt." Nevertheless, on comparing this with other passages of his works, it is obvious that Pliny could not have been ignorant that the application of simple heat was effectual. He and Suctonius relate, that Livia, the wife of Augustus, had before been married to Ti berius Nero, became pregnant, and desiring, with all the ardour of a youthful female, to discover of which sex her offspring would be, took an egg, that had undergone partial incubation, and kept it warm in her bosom. When obliged to desist, it was always committed to one of her women, that the same invariable heat might be preserved. Her ob ject was successful ; and the latter biographer informs us, that " from this egg a cock was hatched with a very re markable crest." Pliny therefore concludes, " It is pro bably on such principles, that a method was not long ago de vised, of placing eggs in straw heated by a fire, a man being occupied in turning them at different intervals un til their chickens were hatched." In another work of the ancients, the Geoponica, there is a whole chapter ascribed to Democritus, treating of " how it is possible to hatch eggs without the aid of fowls ;" and the principle is ana logous to the employment of dunghills. The eggs are to be placed in jars, observing to mark the date of deposition on the shell of each, on purpose to ascertain when the 20 days necessary for their exclusion shall have elapsed. Then the shell is to be broken, and the chickens supplied with food. It is unnecessary to insist farther on the practice of the ancients ; but as Herodotus passes over that of the Egyptians in silence, some authors have dated its com mencement between his era and the age of Aristotle, which were not remote from each other.