Chap. XV. SEXUALLY LIMITED INHEEITAXCE. 
161 
young, such variations would be of no service until 
the age for reproduction had arrived, and there was 
competition between rival males. If we suppose that 
three-fourths of the young males of any species are 
on an average destroyed by various enemies ; then the 
chances would be as three to one against any one indi- 
vidual more brightly-coloured than usual surviving to 
propagate its kind. But in the case of birds which 
live on the ground and which commonly need the 
protection of dull colours, bright tints would be far 
more dangerous to the young and inexperienced than 
to the adult males. Consequently the males which 
varied in brightness whilst young would suffer much 
destruction and be eliminated through natural selec- 
tion ; on the other hand the males which varied in 
this manner when nearly mature, notwithstanding that 
they were exposed to some additional danger, might sur- 
vive, and from being favoured through sexual selection, 
would procreate their kind. The brightly-coloured young 
males being destroyed and the mature ones being suc- 
cessful in their courtship, may account, on the principle 
of a relation existing between the period of variation 
and the form of transmission, for the males alone of 
many birds, having acquired and transmitted brilliant 
colours to their male offspring alone. But I by no means 
wish to maintain that the influence of age on the form 
of transmission is indirectly the sole cause of the great 
difference in brilliancy between the sexes of many birds. 
As with all birds in which the sexes differ in colour, it 
is an interesting question whether the males alone have 
been modified through sexual selection, the females 
being left, as far as this agency is concerned, unchanged 
or only partially changed ; or whether the females have 
been specially modified through natural selection for the 
sake of protection, I will discuss this question at con- 
VOL. II. M 
