222 
SEXUAL SELECTION: BIEDS. 
Paet II. 
will not pretend to say ; but the case is too remarkable 
to be passed over without notice. 
We have now seen in numerous instances under all 
six classes, that an intimate relation exists between the 
plumage of the young and that of the adults, either of 
one sex or both sexes. These relations are fairly well 
explained on the principle that one sex — this being in 
the great majority of cases the male — first acquired 
through variation and sexual selection bright colours 
or other ornaments, and transmitted them in various 
wavs, in accordance with the recognised laws of inhe- 
ritance. Why variations have occurred at different 
periods of life, even sometimes with the species of the 
same group, we do not know ; but with respect to 
the form of transmission, one important determining 
cause seems to have been the age at which the varia- 
tions first appeared. 
From the principle of inheritance at corresponding 
ages, and from any variations in colour which occurred 
in the males at an early age not being then selected, on 
the contrary being often eliminated as dangerous, whilst 
similar variations occurring at or near the period of 
reproduction have been preserved, it follows that the 
plumage of the young will often have been left unmo- 
dified, or but little modified. We thus get some insight 
into the colouring of the progenitors of our existing 
species. In a vast number of species in five out of our 
six classes of cases, the adults of one sex or both are 
brightly coloured, at least during the breeding-season, 
whilst the young are invariably less brightly coloured 
than the adults, or are quite dull-coloured ; for no in- 
stance is known, as far as I can discover, of the young 
of dull-coloured species displaying bright colours, or 
