Chap. IV.] SEXUAL SELECTION. 107 



manner which protected them from their enemies. Yet 

 many of these eggs or seeds would perhaps, if not de- 

 stroyed, have yielded individuals better adapted to their 

 conditions of life than any of those which happened to 

 survive. So again a vast number of mature animals and 

 plants, whether or not they be the best adapted to their 

 conditions, must be annually destroyed by accidental 

 causes, which would not be in the least degree mitigated 

 by certain changes of structure or constitution which 

 would in other ways be beneficial to the species. But 

 let the destruction of the adults be ever so heavy, if the 

 number which can exist in any district be not wholly 

 kept down by such causes, — or again let the destruction 

 of eggs or seeds be so great that only a hundredth or a 

 thousandth part are developed, — yet of those which do 

 survive, the best adapted individuals, supposing that 

 there is any variability in a favourable direction, will 

 tend to propagate their kind in larger numbers than 

 the less well adapted. If the numbers be wholly kept 

 down by the causes just indicated, as will often have 

 been the case, natural selection will be powerless in 

 certain beneficial directions ; but this is no valid objec- 

 tion to its efficiency at other times and in other ways ; 

 for we are far from having any reason to suppose that 

 many species ever undergo modification and improve- 

 ment at the same time in the same area. 



Sexual Selection. 



Inasmuch as peculiarities often appear under domesti- 

 cation in one sex and become hereditarily attached to 

 that sex, so no doubt it will be under nature. Thus it is 

 rendered possible for the two sexes to be modified 



