OF THE GENUS MERGUS. 
479 
those which differ essentially in the plumage of the sexes), 
whether male or female, are fledged whilst immature in the 
plumage of the female only. In addition to this, the female 
of another species, the Mergus serrator^ or Red-breasted 
Merganser, so greatly resembles the Dundiver in plumage, 
that it is often confounded with that species in the young 
state ; and as I have ascertained that the young males of 
the Serrator also resemble the females whilst immature, 
we may see clearly in what manner it happens that indivi- 
duals in the female plumage are so much more numerous 
than those in the male ; in other words, how the Dundiver 
should be so much more common than the Goosander. 
Therefore no good argument can be drawn from this cir- 
cumstance against their forming one species. 
^dly. It has been said that the Dundiver cannot be the 
female of the Goosander, because, on dissection, indivi- 
duals of the former kind have been found to be males. It 
may be answered, that reasoning even on the general law 
already alluded to, which assigns to the young individuals 
of both sexes the plumage of the female, whenever there is 
a marked distinction in their adult plumage, we would be 
authorised in rejecting such circumstance as in any way 
conclusive ; but it fortunately happens that we are not 
left in doubt, because one of the specimens before you 
clearly marks the transition from the female plumage to 
that of the male. This is very perceptible in the black ring 
which is forming at the base of the ferruginous portion of 
the neck, and in the sooty hue which has begun to spread 
over the brown feathers of the head. There is also an evi- 
dent commencement, above the scapulary feathers, of that 
great portion of black which afterwards spreads down 
the back, and forms the most distinguishing sexual cha- 
racter of the adult male. Lastly, the mature plumage 
