Chap. XVI. THE YOUNG LIKE THE ADULT FEMALES. 189 
resemble (with the exception of the breast being spotted 
with bronze) the adult female in all respects including 
the length of her tail, so that the tail of the male 
actually becomes shorter as he reaches maturity, which 
is a most unusual circumstance.^ Again, the plumage 
of the male goosander (Mergus merganser) is more con- 
spicuously coloured, with the scapular and secondary 
wing-feathers much longer than in the female, but differ- 
ently from what occurs, as far as I know, in any other 
bird, the crest of the adult male, though broader than 
that of the female, is considerably shorter, being only a 
little above an inch in length ; the crest of the female 
being two and a half inches long. Now the young of 
both sexes resemble in all respects the adult female, 
so that their crests are actually of greater length though 
narrower than in the adult male.^ 
When the young and the females closely resemble 
each other and both differ from the male, the most ob- 
vious conclusion is that the male alone has been modi- 
fied. Even in the anomalous cases of the Heliothrix 
and Mergus, it is probable that originally both adult 
sexes were furnished, the one species with a much elon- 
gated tail, and the other with a much elongated crest, 
these characters having since been partially lost by the 
adult males from some unexplained cause, and trans- 
mitted in their diminished state to their male offspring 
alone, when arrived at the corresponding age of ma- 
turity. The belief that in the present class the male 
alone has been modified, as far as the differences be- 
tween the male and the female together with her 
young are concerned, is strongly supported by some 
^ I owe this information to Mr. Gould who shewed me the specimens ; 
see also his ‘ Introduction to the Trochilidse,’ 1861, p. 120, 
^ Macgillivray, ‘ Hist. Brit. Birds/ vol. v. p. 207-214. 
