OF THE GEXirs MERGUS. 
477 
other by no specific characters, but to differ in sex atone. 
When we observe that the prevailing colour of the one, 
said to be the male, is bkck and white, and of the other, 
said to be the female, ferruginous and lead colour, and 
when it is not asserted that they have ever been produced 
from the same brood, or otherwise proved to be one and 
the same, we are certainly authorised in withholding our 
assent, more especially if our prior belief, however vaguely 
-founded, should have been in opposition to such opinion. 
If, however, on examining all the other known species of 
the genus, we should find that the prevailing plumage of 
the males is invariably composed of black and white, and 
of the females of ferruginous and lead colour, there would 
certainly be nothing either rash or unphilosophical in be- 
lieving, that what was really applicable to those whose 
sexual characters had been ascertained, was probably also 
applicable to a solitary species in which they had not been 
ascertained ; and thus, that very difference in the plumage 
of the sexes, which had induced naturalists to class them 
as distinct species, would come to be adduced as the strong- 
est argument in favour of their being actually the same. 
Such a mode of determining the point in dispute, by re- 
ferring to the sexual relations in the plumage of other spe- 
cies of the same genus, I conceive to be particularly admis- 
sible in the present instance. There seems to be a uni- 
formity of distinction, both in the colours themselves, and 
in their distribution, as characteristic of the sexes, which 
strongly marks this genus, and distinguishes it from every 
other. This sexual distinction in plumage, though almost 
always perceptible in the species considered singly of other 
genera, is not, as far as I have observed, perceived to run 
so distinctly according to one model, through a whole group 
of species, as in the Mergus; and, therefore, any argu- 
VOL. IV. I i 
