SPECIJ'IC CHARACTERS OF BIRDS. 525 
tlian the best systematist, or closest naturalist. The result 
of Mr Wilson's observation^ is, that, in general, in his 
opinion, no great reliance can be placed upon the colour of 
the iris, for it varies not a httle in a great proportion of the 
birds which have come through his hands. He adduces as 
examples the Falco uEsalon and F. Buteo, which, even after 
being full grown, exhibit in their iris all the intermediate 
tints between a deep brown and bright yellow. The iHs 
of the Sula alba, when young, is black, in the second year 
brownish or dusky, in the third whitish or light grey, and 
the white becomes purer as age advances. The iris of the 
Sea-Eagle, or young of the Albicilla, lightens from deep 
brown, or blackish, as the bird advances toward maturity, 
and continues to become paler and paler long after this 
period. And the same also he has observed of other birds. 
The colour of the beak, the legs, the claws, and other 
parts, may be somewhat more permanent ; but the variety 
exhibited here is of by far too little extent to afford specific 
characters : the almost universal colours are, dusky, horn- 
colour, and blackish, with a considerable proportion of yel- 
low, and some greenish, and a few other tints ; so that the 
colours of these parts can only at the utmost afford a very 
casual distinction. 
Finally, Has not an adherence to colour been productive 
of disjunctions which are every day becoming more appa- 
rent ? And have not individual species been split into two, 
three, even four, by this uncertain and misapplied distinc- 
tion ? The Colymbus glacialis and Immer, the Alca Torda 
and Pica, the Anas Clangula and Glaucion, the Tringa cin- 
clus and alpina, the Larus tridactylus and Rissa, and a 
numerous host of other binary schisms, support in so far 
the truth just stated ; and for ternary and quaternary ones, 
we have only to consult the Gmelinian edition of the " Sys- 
tema Naturae,'*' and even the more precise " Index Ornr^ 
VOL. IV. M m 
