'Chap. XVI. INHEEITANCE, LIMITED BY AGE. 
183 
CHAPTEE XVI. 
Birds — concluded. 
The immature plumage in relation to the character of the plumage 
in both sexes when adult — Six classes of cases — Sexual differ- 
ences between the males of closely-allied or representative species 
— The female assuming the characters of the male — Plumage of 
the youDg in relation to the summer and winter plumage of the 
adults — On the increase of beauty in the Birds of the World — 
Protective colouring — Conspicuously-^coloured birds — Novelty 
appreciated — Summary of the four chapters on Birds. 
We must now consider the transmission of characters 
;as limited by age in reference to sexual selection. 
The truth and importance of the principle of inheri- 
tance at corresponding ages need not here be discussed, 
as enough has already been said on the subject. Before 
giving the several rather complex rules or classes of 
cases, under which all the differences in plumage 
between the young and the old, as far as known to 
me, may be included, it will be well to make a few 
preliminary remarks*. 
With animals of all kmds when the young differ in 
colour from the adults, and the colours of the former are 
not, as far as we can see, of any special service, they 
may generally be attributed, like various embryological 
structures, to the retention by the young of the character 
of an early progenitor. But this view can be maintained 
with confidence, only when the young of several species 
closely resemble each other, and likewise resemble 
other adult species belonging to the same group; for 
the latter are the living proofs that such a state of 
things was formerly possible. Young lions and pumas 
