SHELLS OF EGGS. 37 



the^ film, though loosened and rendered mucilaginous 

 by this rough process. Though this calcareous mat- 

 ter is partly taken up during incubation, the mark- 

 ings 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 im- 

 ply, that the colouring matter on the shells of eggs 

 does not contribute to the various hues of the plum- 

 age; 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. 7 



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 egg, provided for the nourishr- 

 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, 

 opake, chiefly composed of chalk (carbonate of 

 /inie), 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 (for 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 



VOL. vi. 4 



