230 READINGS IX EVOLUTION, GENETICS, AND EUGENICS 



destroyed by accidental causes, which would not be in the least degree 

 mitigated by certain changes of structure or constitution which w^ould 

 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 objection 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 domestication 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 sometimes 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 indi- 

 viduals of one sex, generally the males, for the possession of the other 

 sex. The result is not death to the unsuccessful competitor, but few 

 or no offspring. Sexual selection is, therefore, less rigorous than 

 natural selection. Generally, the most vigorous males, those which 

 are best fitted for their places in nature, will leave most progeny. 

 But in many cases, victory depends not so much on general vigor, as 

 on having special weapons, confined to the male sex. A hornless 

 stag or spurless cock would have a poor chance of leaving numerous 

 offspring. Sexual selection, by always allowing the victor to breed, 

 might surely give indomitable courage, length to the spur, and strength 

 to the wing to strike in the spurred leg, in nearly the same manner as 

 does the brutal cock-fighter by the careful selection of his best cocks. 

 How low in the scale of nature the law of battle descends, I know not; 

 male alligators have been described as fighting, bellowing, and whirl- 



