582 F. C. R. Jourdain : 



up too much space, and it is sufficient to say that a 

 tendency towards albinism, or rather defect of colouring* 

 matter, is occasionally found in most coloured eggs. 



(2) In this division may be included those cases in which 

 birds, which, as a rule, lay eggs coloured solely by 

 Biliverdin, occasionally produce eggs showing distinct 

 markings of Oorhodein. Well known examples of this 

 are the red-spotted eggs of Sturnus vulgaris, Saxicola 

 oenanthe and Ruticilla phoenicurus, and similar instances 

 may be found among exotic species, such as the South 

 African Spreo bicolor. Certain birds which normally 

 lay white or nearly white eggs also occasionally show a 

 tendency to red or brown markings, e.g., Gyps fulvus, 

 the various species of the genus Circus, Fratercula ardita, 

 etc. (Besides these cases there are also the melanistic 

 eggs of Anas boschas and possibly also of Podiceps 

 cristatus and P. auritus, 1 but here further investigation 

 is recpiired. 



(3) This division forms the main subject of this paper and 

 is treated in greater detail. It will be seen that the range 

 of variation differs widely in different species. As an ex- 

 treme instance the case of Oisticola cisticola may be 

 mentioned, where the eggs are sometimes pure white, 

 at other times blue or with decided red markings on a 

 blue ground. 2 The more ordinary range is from a e blue ' 

 type, coloured chiefly by Biliverdin, to a f red ' or erythris- 

 tic type, coloured chiefly by Oorhodein, with numerous 

 intervening stages, and sometimes one or both of the 

 extreme types are wanting. 



The following table will give some idea of the limits of 

 variation in the more important cases of variable eggs : — 



1 See "«Journ. für Orn." 1S57, p. 302; " Naumannia," 1857, p. 53-4. 



- Similar instances are to be found in the African Cuticola textrix and C. 

 tinniens, in the Indian Cisticoïœ and Primes, and also among several Weaver 

 Birds of the genus II y pliant omis {S. velatus, H. nigrieeps, etc.). 



