Chap. VIII. SEXUAL SELECTION. 297 



When variations occur late in life in one sex, and are 

 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 be- 

 fore the age for reproduction has arrived. From 

 the great eagerness of the male it has generally 

 acted on this sex and not on the females. The males 

 have thus become provided with weapons for fight- 

 ing 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, have not generally occurred much before the 

 age for reproduction. Whenever some or many of the 

 variations have occurred early in life, the young males 

 will partake in a less or greater degree of the cha- 

 racters of the adult males. Differences of this kind 

 between the old and young males may be observed 

 with many animals, for instance with birds. 



It is probable that young male animals have often 

 tended to vary in a manner which would not only have 

 been of no use to them at an early age, but would have 

 been actually injurious, — as in the acquisition of bright 

 colours, which would have rendered them conspicuous 



