24-0 The Descent of Man. Part II 



appeared in the parents. But these rules, owing to unknown 

 causes, are far from being fixed. Hence during the modification 

 of a species, the successive changes may readily be transmitted 

 in different ways ; some to one sex, and some to both ; some to 

 the offspring at one age, and some to the offspring at all ages. 

 Not only are the laws of inheritance extremely complex, but so 

 are the causes which induce and govern variability. The 

 variations thus induced are preserved and accumulated by 

 sexual selection, which is in itself an extremely complex affair, 

 depending, as it does, on the ardour in love, the courage, and 

 the rivalry of the males, as well as on the powers of perception, 

 the taste, and will of the female. Sexual selection will also 

 be largely dominated by natural selection tending towards 

 the general welfare of the species. Hence the manner in which 

 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 trans- 

 mitted 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 discovering 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 successive variations, by which the adult male became modi- 

 fied, 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 charac- 

 ters 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 actually injurious— 



