816 THE DESCENT OF MAN 



the individuals of either or both sexes have been affected 

 through sexual selection cannot fail to be complex in the 

 highest degree. 



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 left unmodified. When they occur late 

 in life, but are transmitted to both sexes at the same age, 

 the young alone are left unmodified. 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 are similarly modified. In 

 the following chapters it will be seen that all these cases 

 frequently occur in nature. 



Sexual selection can never act on any animal before the 

 age for reproduction arrives. 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 fighting with their rivals, with organs for dis- 

 covering and securely holding the female, and for exciting 

 or 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 sticcessive 

 variations by which the adult male became modified did 

 not generally occur much before the age for reproduction. 

 Whenever some or many of the variations occurred early in 

 life, the young males would partake more or less of the 

 characters of the adult males; and differences of this kind 

 between the old and young males may be observed in many 

 species of animals. 



, 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 actu- 

 ally injurious — as by acquiring bright colors, which would 

 render them conspicuous to their enemies, or by acquiring 

 structures, such as great horns, which would expend much 

 vital force in their development. Variations of this kind 



