Chap. YIII. 
SEXUAL SELECTION. 
297 
transmitted to the same sex at the same age, the other 
sex and the young are necessarily left unmodified. 
When they occur late in life, but are transmitted to 
both sexes at the same age, the young alone are left un- 
modified. Variations, however, may occur at any period 
of life in one sex or in both, and be transmitted to both 
sexes at all ages, and then all the individuals of the 
species will be similarly modified. In the following 
chapters it will be seen that all these cases frequently 
occur under nature. 
Sexual selection can never act on any animal whilst 
young, before the age for reproduction has arrived. 
From the great eagerness of the male it has gene- 
rally acted on this sex and not on the females. The 
males have thus become provided with weapons for 
fighting with their rivals, or with organs for discovering 
and securely holding the female, or for exciting and 
charming her. When the sexes differ in these respects, 
it is also, as we have seen, an extremely general law 
that the adult male differs more or less from the young 
male ; and we may conclude from this fact that the 
successive variations, by which the adult male became 
modified, cannot have occurred much before the age 
for reproduction. How then are we to account for this 
general and remarkable coincidence between the period 
of variability and that of sexual selection, — principles 
which are quite independent of each other ? I think 
we can see the cause: it is not that the males have 
never varied at an early age, but that such variations 
have commonly been lost, whilst those occurring at a 
later age have been preserved. 
All animals produce more offspring than can survive 
to maturity ; and we have every reason to believe that 
death falls heavily on the weak and inexperienced 
young. If then a certain proportion of the offspring 
