SHELLS OF EGGS. 37 



the film, thoiin-h loosened and rendered miioilag'inous 

 by this roni^h process. Though this calcareous mat- 

 ter is partly taken up during incubation, the mark- 

 iug's upon these egg's remain little injured even to 

 the last, and are almost as strongly defined ;is when 

 the eggs are first iaiil. These circumstances seem 

 to imply, tliat the colouring matter on the shells of 

 eggs does not contribute to the various hues of the 

 plumage, but, il is reasonable to conclude, are de- 

 signed 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 charac- 

 teristic figuring." 



Most of these remarks will apply to the colours of 

 the eggs of insects : but though we can in most in- 

 stances trace no connexion between the colours of 

 eggs and the perfect insect, there is a striking ex- 

 ception in the egg; of the brimstone-moth mentioned 

 above, which corresponds exactly in colour with the 

 wings of the moth, though the caterpillar is of a dull 

 brown. 



The eggs of insects, like those of birds, have a 

 shell enclosing the germ of the caterpillar, with a 

 peculiar matter for its nourishment, like the white 

 and yolk of a bird's eQ;g, provitled for the nourish- 

 ment of the contained chick. These several parts, 

 however, are very different in substance from the 

 eggs of birds. The shell of the bird's egg is brittle, 

 opaque, chiefly composed of cha!k (carbonate of 

 lime), and lined with a very thin tough membrane ; 

 while in the egg of an insect the shell is not brittle, 

 is transparent contains no lime ((or it is not per- 

 ceptibly acted upon by diluted sulphuric acid), and 

 no lining membrane can be detected. It appears, 

 indeed, very similar to the transparent portion of a 



