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 

 advantage, for the good of another species ; and though 

 statements to this effect may be found in works of 

 natural history, I cannot find one case which will bear 

 investigation. A structure used only once in an animal's 

 whole life, if of high importance to it, might be modi- 

 fied to any extent by natural selection ; for instance, 

 the great jaws possessed by certain insects, and 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 hatch- 

 ing. 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 pow- 

 erful and hardest 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 relations 

 to the other sex, or in relation to wholly different 

 habits of life in the two sexes, as is sometimes the ea.se 



