BIRDS' EGGS 275 



the oviduct during its formation, then, from the nature and 

 direction of motion of the peristaltic wave the pressure will 

 be greatest somewhere behind the middle of the egg ; in 

 other words, the tube is converted for the time being into 

 a more conical form, and the simple result follows that the 

 anterior end of the egg becomes the broader and the posterior 

 end the narrower. Extreme forms of this are seen in the 

 pointed egg of guillemot or razorbill. 



Texture. — In some families of birds the shell has definite 

 physical characters of grain or texture or gloss— an illustra- 

 tion of specificity. A kind of bird may sometimes be 

 identified from a single feather, and a fish from a square 

 inch of its skin, and a tree by a single leaf. A species has 

 usually individuality even in details — thus the blood of a 

 horse can be distinguished from that of a donkey by the 

 shape of the blood-crystals ; and sometimes the peculiarity 

 extends through all the members of the family. Though it 

 may not be of much vital importance, it is an index of blood- 

 relationship. The egg of the cuckoo is often almost in- 

 distinguishable in colour and markings from the eggs of the 

 foster-parents' clutch, but it is usually distinguishable in its 

 texture. 



The fine-grained white shell of the kingfisher's egg has a 

 beautiful gloss and opalescence : what a contrast to the rough 

 chalky surface of the grebe's ! The strangely coloured egg 

 of the Tinamou is like " highly burnished metal or glazed 

 porcelain : " what a contrast to the pitted surface of the eggs 

 of South African ostriches, or the rough surface in the Solan 

 Goose 1 In many water-birds, like ducks, the surface of the 

 shell is oleaginous, which may have some value in keeping 

 them from being over-wetted. Instances might be multi- 

 plied, but it would serve Uttle purpose, since the biological 

 significance of the differences in texture is obscure, and may 

 indeed be minimal. Professor Newton calls attention to 

 the fact that the egg-shells of hybrids between Grey and 

 Black Crows (probably varieties of one species) are dis- 

 tinguishable in texture from those produced when the two 

 forms pair with others like themselves. 



