EGGS OF BIRDS. 



birds in general vary much, and are occasionally 

 very puzzling to identify when detached from their 

 nests, as the colourings and markings differ greatly 

 in the same species, and even nest. Those of one 

 colour, like this wheatear's, retain it, with only 

 shades of variation ; but when there are blotchings 

 or spots, these are at times very dissimilar, occa- 

 sioned 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 re- 

 quires 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 con- 

 nexion with the shadings on the feathers, it is diffi- 

 cult to determine ; as we know that eggs entirely 

 white will produce birds with a variety of plum- 

 age. 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. 





