230 READINGS IN 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 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 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- 
