REPRODUCTION— CONCERNING EGGS 197 



Blackbird, but the contrast in the sizes of their eggs is enor- 

 mous. 



Besides the variation as between different species, there are 

 to be found numerous instances of specific variations of a very 

 considerable range which have become normal to such species. 

 Of course variation within small limits occurs in eggs of all 

 species : to these we do not refer here, nor have we in mind 

 those abnormalities of size which so frequently occur. Re- 

 markably small eggs containing no yolk, for example, frequently 

 mark the first efforts of immature birds. But the variations to 

 which we refer are such as are afforded by the great White 

 Heron. This species occurs both in Europe and India, birds 

 from the two regions being indistinguishable, yet the eggs of 

 the European birds are always much larger than those taken in 

 India. European eggs in the collections of the British Museum 

 measure from 2*35 to 27 in. in length, while Indian specimens 

 range from r88 to 2"38. * 



Birds' eggs differ very considerably in the texture and 

 relative thickness of the shell. In the matter of texture some, 

 as in the case of the Tinamous, have their surface so highly 

 polished as to look like burnished metal or highly glazed por- 

 celain, while a very considerable gloss is commonly met with, 

 as in the case of the Kingfishers and Woodpeckers. Ducks' 

 eggs are remarkable for their peculiarly smooth, ' ' greasy " sur- 

 face, though the eggs of the Genus Erismatura must be taken as 

 exceptions to this rule, since they are covered with coarse granu- 

 lations ; and, curiously enough, the eggs of the allied Genus 

 Biziura, represented only by the Australian Musk-duck, are 

 frequently similarly granulated, and when this is not the case 

 they are of the normal Anserine type. The eggs of the Ibises 

 offer similar differences, since in some genera they have a 

 smooth surface, while in others it is rough. So far no ex- 

 planation for these differences appears possible. This roughness 

 of texture attains its maximum among the Grebes, Flamin- 

 goes and certain Cuckoos. The eggs of these birds have the 

 shell incrusted with a chalky layer often of considerable thick- 

 ness : often when this crust is scraped away a rich colouring 

 is revealed, as in the case of the Cuckoos of the Genus Croto- 

 phaga. In the allied Guira Cuckoo this chalky layer is deposited 

 in the form of a coarse and irregular meshwork forming a 



