THE RING-TAILED AND GOLDEN ExVGLES. 44S 
one or more pure, unmixed, and distinctly expressed 
colours, into those of an obscure, ill-defined, and mixed 
nature, is in opposition to all the known facts from which 
any thing like a general principle could be evolved. Novr, 
the change maintained to take place by Mr Selby is a 
change from a pure and unmingled black and white, to a 
mingled combination of black, brown, ferruginous, and ash 
colour. 
I beheve, that if any one general law regulates the 
changes in the plumage of birdsj it is this, That if any 
given species exhibits, in an important part of its plumage^ 
a combination of comparatively obscure shades of black, 
brown, dusky, ferruginous, and other colours, intermingled 
with each other, and, if other individuals of the same spe- 
cies exhibit the like important parts of their plumage, com- 
posed, not of these obscure and mingled colours, but of 
two simple and strongly contrasted colours, such as black 
and white, we may, with as much certainty as any induc- 
tive process warrants, infer, that the combined and less 
distinct colours belong to the immature,— -whilst the purer, 
unmixed, and more strongly contrasted colours, adorn the 
parent birds. This I look upon as a law of Nature : it is 
a general truth, derived from the consideration of common 
properties in individual facts, and m- therefore a legitimate 
induction. For the sake of those who may not have con- 
vinced themselves of this fact from observation, and who 
may naturally enough feel disinchned to rest their belief 
upon a mere assertion, I beg leave to mention a few ex- 
amples which at once occur to me out of the numerous 
instances which a special investigation of the subject would 
bring to light, in confirmation of the rule. In all the fol- 
lowing species, there are important parts of the plumage 
which change from various obscure shades of brov/n, dusky, 
VOL. IV. G g 
