174 THE BIOLOGY OF THE SEASONS 



of the egg becomes the broader and the posterior the 

 narrower. 



But, while the primary reason why an egg has a 

 particular shape is doubtless to be found in the conditions 

 of pressure within the muscular oviduct, that is not incon- 

 sistent with finding that the shape is sometimes adaptive 

 to particular conditions in the life of the bird. Two 

 instances of this adaptiveness are well known. Eggs which 

 are markedly pear-shaped "are mostly those of birds which 

 invariably lay four in a nest ; and therein they lie with 

 their points almost meeting in the centre, and thus occupying 

 as little space as possible and more easily covered by the 

 brooding parent." But the guillemots and razor-bills 

 have also very markedly pear-shaped eggs, and the bird 

 usually lays only one or two. Darwin pointed out, however, 

 that this shape of egg was particularly well adapted to the 

 risks of the narrow rocky ledges on exposed cliffs, where the 

 eggs are laid without even an apology for a nest. If the 

 egg be jostled by a bird's foot or whirled by the wind, it 

 tends not to roll along, but to rotate on its short axis 

 without leaving the spot. 



Eggs differ not a little in the texture of the shell. In 

 different families there is, often at least, a characteristic 

 shell-structure. Thus a cuckoo's egg can be recognised in 

 a clutch when it has the same colour as the eggs of the 

 foster-parent. There are said to be differences between 

 the egg-shells of the carrion crow (Corvus corone) and those 

 of the hooded crow (Corvus comix), which are two very 

 closely allied species, or, as some would say, colour-varieties 

 of one species. In some cases, it is said, the shell registers 

 hybridism a very remarkable fact. It is another illustra- 

 tion of the great, though still vague, truth that the living 

 creature is a unity through and through, specific even in the 

 structure of the egg-shell within which it developed. For, 



