EGGS OF BIRDS. 



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 

 colouring matter resides in the calcareous part ; but 

 where there are markings, these are rather extra- 

 neous to it than mixed with it. The elegant blue 

 that distinguishes the eggs of the fire-tail and the 

 hedge-sparrow, though corroded away, is not de- 

 stroyed 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 

 yellowhammer, 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 colouring 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 



