The eggs produced by the wheatear are uniform in color and similar in shape; but the eggs of birds in general vary much, and are occasionally very puzzling to identify when detached from their nests, as the colorings and markings differ greatly in the same species, and even nest. Those of one color, like this wheatear’s, retain it, with only shades of variation; but when there are blotchings or spots, these are at times very dissimilar, occasioned in great measure probably by the age of the bird; though this cannot account for the difference of those in an individual nest. None vary more than the eggs of the common sparrow. Those of marine birds, especially the guillemot (colymbus troile), are often so unlike each other, that it requires considerable practice to arrange them. The plumage of birds has probably never varied, but remains at this hour what it originally was: but whether these markings on the eggs have any connexion with the shadings on the feathers, it is difficult to determine; as we know that eggs entirely white will produce birds with a variety of plumage. The shell of the egg appears to be designed for the accomplishment of two purposes. One of the offices of this calcareous coating, which consists of carbonate and phosphate of lime, is to unite with the white of the egg, and form, during incubation, the feathers and bone of the future young ones; but as a large portion of this covering remains after the young are produced, its other object is to guard from injury the parts within. As far as I have observed, in eggs of one hue, the coloring matter resides in the calcareous part; but where there are markings, these are rather extraneous to it than mixed with it. The elegant blue that distinguishes the eggs of the firetail and the hedge-sparrow, though corroded away, is not destroyed by the muriatic acid. The blue calcareous coating of the thrush’s egg is consumed; but the dark spots, like the markings upon the eggs of the yellow-hammer, house-sparrow, magpie, &c., still preserve their stations on the film, though loosened and rendered mucilaginous by this rough process. Though this calcareous matter is partly taken up during incubation, the markings upon these eggs remain little injured, even to the last, and are almost as strongly defined as when the eggs are first laid. These circumstances seem to imply, that the coloring matter on the shells of eggs does not contribute to the various hues of the plumage; but, it is reasonable to conclude, are designed to answer some particular object, not obvious to us: for though the marks are so variable, yet the shadings and spottings of one species never wander so as to become exactly figured like those of another family, but preserve, year after year, a certain characteristic figuring. Few animal substances, in a recent state, contain more hepatic gas than an eggshell, as is manifest from the very offensive smell that proceeds from it when burned.