SEXUAL SELECTION. 235 



sexes liave 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 sim- 

 ilarly 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 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 par- 

 take more or less of the characters of the adult males; and dif- 

 ferences 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 — 

 as by acquiring bright colors, which would render them con- 

 spicuous to their enemies, or by acquiring structures, such as 

 great horns, which would expend much vital force in their devel- 

 opment. Variations of this kind occurring in the young males 

 would almost certainly be eliminated through natural selection. 

 With the adult and experienced males, on the other hand, the 

 advantages derived from the acquisition of such characters, would 

 more than counterbalance some exposure to danger, and some 

 loss of vital force. 



As variations which give to the male a better chance of con- 

 quering other males, or of finding, securing, or charming the oppo- 

 site sex, would, if they happen to arise in the female, be of no 

 service to her, they would not be preserved in her through sexual 

 selection. We have also good evidence with domesticated ani- 

 mals, that variations of all kinds are, if not carefully selected, 

 soon lost through intercrossing and accidental deaths. Conse- 

 quently in a state of nature, if variations of the above kind 

 chanced to arise in the female line, and to be transmitted exclu- 



