SPECIFIC CHARACTERS OF BIRDS. 
comprehensive of all the varieties produced by circum- 
stances ; that is, the feathers are not absolutely alike in all 
the stages of life ; but surely no colourist can urge such 
objection as this, for it would instantly recoil upon himself. 
From what I have said, it will appear, that if the method 
which I propose were applied to the disputed species of 
birds, and should fail to determine whether they be really 
distinct or identical, this can form no objection to it. The 
form of the plumage, in fact, does vary in the same species 
with age ; although apparently not by any means so much 
as colour. In the breast of the old Goshawk, for example, 
the feathers are broad ; in that of the young bird they are 
narrow and elongated. In the first, the colour markings 
are transversal ; in the other, they are longitudinal. In 
this respect, the methods may be even admitted to be in 
equilibrio. And here it may be remarked, that, between 
the colour and form or texture of feathers, there is a cer- 
tain analogy, which, if strictly investigated, might lead to 
interesting results. A particular surface, or texture of fea- 
thers, seem, in general, to have a particular colour inva- 
riably attached to them ; insomuch, that when a feather 
is described as to form, &c., we may yet be able to pro- 
nounce with certainty, without seeing it, what might be its 
colour. 
But although form cannot identify the different varieties 
of a species, it would seem probable, that when two dispu- 
ted birds agree perfectly in plumage, both being of the 
same genus, they must necessarily be both of one and the 
same species. This, for example, is the case with the Alca 
Torda and Pica^ with the Tringa alpina and Cinclus, with 
the Anas Bernicla and Erythropus^ and many others. 
Whatever changes the same species undergoes as to co- 
lour, it often happens that the form of the feather remains 
unchanged ; as in the Falco buteo, where, while tlie colour 
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