Chap. XVI. 
SUMMARY. 
237 
mission, then if the parents vary late in life — and we 
know that this constantly occurs with our poultry, and 
occasionally with other birds — the young will be left 
unaffected, whilst the adults of both sexes will be 
modified. If both these laws of inheritance prevail and 
either sex varies late in life, that sex alone will be 
modified, the other sex and the young being left un- 
affected. When variations in brightness or in other 
conspicuous characters occur early in life, as no doubt 
often happens, they will not be acted on through sexual 
selection until the period of reproduction arrives ; conse- 
quently they will be liable to be lost by the accidental 
deaths of the young, and if dangerous will be eliminated 
through natural selection. Thus we can understand 
how it is that variations arising late in life have chiefly 
been preserved for the ornamentation and arming of the 
males, the females and the young being left almost un- 
affected, and therefore like each other. With species 
having a distinct summer and winter plumage, the males 
of which either resemble or differ from the females 
during both seasons or during the summer alone, the 
degrees and kinds of resemblance between the young 
and the old are exceedingly complex ; and this com- 
plexity apparently depends on characters, first acquired 
by the males, being transmitted in various ways and 
degrees, as limited by age, sex, and season. 
As the young of so many species have been but little 
modified in colour and in other ornaments, we are 
enabled to form some judgment with respect to the 
plumage of their early progenitors ; and we may infer 
that the beauty of our existing species, if we look to the 
whole class, has been largely increased since that period 
of which the immature plumage gives us an indirect 
record. Many birds, especially those which live much 
on the ground, have undoubtedly been obscurely co- 
