THE BIEDS OF HELIGOLAND 467 



now no longer be any doubt as to the validity of this species, 

 which used once to be questioned. It used to be a saying of our 

 late revered master, Professor Blasius, that if during one night 

 all birds were to become black, there would be an end of 

 many species ; and I know of no case in which the realisation 

 of this statement would furnish clearer proof for the validity 

 of a species than in the present instance. We could then be no 

 longer led astray by the similarity of the markings of G.fulvus and 

 C. auratus, and only relations of form and shape would be admitted 

 as determining factors. These, however, differ to such a degree 

 that, if we regard them by themselves, the idea of the identity of 

 the two species could not be entertained for a moment. Badly pre- 

 pared skins may, perhaps, cause us for a moment to hesitate in our 

 judgment, but no one who has ever had the two birds in his hands, 

 in the fresh condition, could give admission to the least doubt in 

 the matter. Even the skeleton of the two species, the foundation 

 of all outward form and shape, displays the most striking differences. 

 Thus, the breast-bone of G. auratus is 2'52 ins. (64 iwrn.) in 

 length; while in C. fulvus it only measures 1-96 in. (50 mTJi.), 

 being shorter therefore by nearly a quarter of the length of the 

 former. Notwithstanding that the trunk of C. fulvus measures so 

 much less than that of C. OAiratus, certain separate portions of 

 its body not only equal corresponding parts in the other species, 

 but in some cases even exceed them. Thus, the beak of the finest 

 old C. auratus, which I Avas able to get for my collection, only 

 measured -78 in. (20 mm.), while that of an equally tine old G.fulvus, 

 shot here, measures a good 94 in. (24 m-m.); the tibia is of the 

 same length in both species ; but whereas, in C. auratus, that part 

 of it which is uncovered by feathers only measures -32 in. (8 mm.), 

 in G.fulvus, the naked portion measures 78 in. (20 mvi.). In C. 

 fulvus, the tip of the longest posterior flight-feather is equal to the 

 third of the primaries, and recedes -23 in. (6 mm.) from the point 

 of the wing ; in G. auratus the same feather is equal to the fifth 

 primary, and recedes '87 in. (23 inm.) from the apex of the wing. 

 We might enumerate several other absolute and relative divergen- 

 cies of this kind, but it is not so much these separate details as 

 the wide diff'erence of general shape determined by them, that so 

 markedly distinguishes the two species. 



Though in their colour and markings they bear a general 

 resemblance to each other, several differences are neveriheless 

 discernible. In G. fulvus the feathers on the under surface 

 of the wing are ashy grey at all stages of life; in G. auratus 

 they are pure white : further, old birds of G. fulvus in sum- 

 mer plumage display a diff'erence of colour and marking in 



