T6 SEXUAL SELECTION. 



must be much fortuitous destruction, which can have little 

 or no influence on the course of natural selection. For 

 instance, a vast number of eggs or seeds are annually de- 

 voured, and these could be modified through natural selec- 

 tion only if they varied in some manner which protected 

 them from their enemies. Yet many of these eggs or seeds 

 would perhaps, if not destroyed, 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 favorable 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 improvement at the 

 same time in the same area. 



SEXUAL SELECTION. 



Inasmuch as peculiarities often appear under domestica- 

 tion 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 through natural 

 selection in relation to different habits of life, as is some- 

 times the case ; or for one sex to be modified in relation to 

 the other sex, as commonly occurs. This leads me to say a 

 few words on what I have called sexual selection. This 

 form of selection depends, not on a struggle for existence in 

 relation to other organic beings or to external conditions, 

 but on a struggle between the individuals of one sex, gener* 

 a)lv fcfte rnales, for $9 possession of ^e ; ot^er sex, Tk§ 



