FAMILY LIKENESS OF EGGS. 177 



silica. Copper has also been detected in those 

 spots which variegate its surface. 



No person can examine a collection of eggs 

 without, in a very short time, becoming satisfied 

 that there is a certain family likeness among all 

 those which belong to a true genus. It is almost 

 as difficult to define in what this general resem- 

 blance of the eggs of a genus of birds consists, 

 as it is to define that subsisting between the mem- 

 bers of a human family, where a general re- 

 semblance is often to be traced among individuals 

 in many respects differing from each other. In a 

 communication to the Zoological Society by "W. 

 Yarrell, Esq., and kindly furnished by him to the 

 author, the following are the considerations to 

 which this distinguished ornithologist has arrived. 

 First, that the colour and markings which we find 

 deposited in the external surface of the shell afford 

 indications by which the classification of genera 

 of birds may be assisted ; and, secondly, that the 

 eggs of species belonging to the same genus will 

 resemble each other in colour and markings, what- 

 ever may be the geographical locality in which such 

 species are found. Exceptions and discrepancies 

 may be pointed out, but the application of this prin- 



