Chap. IV. 
SEXUAL SELECTION. 
87 
young in relation to the parent, and of the parent in 
relation to the young. In social animals it will adapt 
the structure of each individual for the benefit of the 
community; if each in consequence profits by the selected 
change. What natural selection cannot do, is to modify 
the structure of one species, without giving it any advan¬ 
tage, for the good of another species ; and though state¬ 
ments to this effect may be found in works of natural 
history, I cannot find one case which will bear investi¬ 
gation. A structure used only once in an animal’s whole 
life, if of high importance to it, might be modified to 
any extent by natural selection; for instance, the great 
jaws possessed by certain insects, used exclusively for 
opening the cocoon—or the hard tip to the beak of 
nestling birds, used for breaking the egg. It has been 
asserted, that of the best short-beaked tumbler-pigeons 
more perish in the egg than are able to get out of it; 
so that fanciers assist in the act of hatching. Now, if 
nature had to make the beak of a full-grown pigeon 
very short for the bird’s own advantage, the process of 
modification would be very slow, and there would be 
simultaneously the most rigorous selection of the young 
birds within the egg, which had the most powerful and 
liardest beaks, for all with weak beaks would inevitably 
perish: or, more delicate and more easily broken shells 
might be selected, the thickness of the shell being known 
to vary like every other structure. 
Sexual Selection ,—Inasmuch as peculiarities often 
appear under domestication in one sex and become 
hereditarily attached to that sex, the same fact pro¬ 
bably occurs under nature, and if so, natural selection 
will be able to modify one sex in its functional rela¬ 
tions to the other sex, or in relation to wholly different 
habits of life in the two sexes, as is sometimes the case 
