DO SEXUAL SELECTION. CriAr. IV. 



pods on his cotton-trces. Natural selection may modify and 

 adapt the larva of an insect to a score of continovncics, wholly 

 tlillcrent from tliose which concern the mature insect; and 

 these modilicalions may affect, throug'h correlation, the stnio 

 ture of the adult. So, conversely, modifications in the adult 

 may affect tlie structure of the larva; but in all cases natural 

 selection will insure that they shall not be injurious: for, if 

 they were so, the species would become extinct. 



Natural selection will modify the structure of the 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 

 indivitlual for the benefit of the Avhole community ; if this 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 spe- 

 cies ; 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 

 life, if of high importance to it, might be modified to any ex- 

 tent by natural selection ; for instance, the great jaws pos- 

 sessed by certain insects, used exclusively for opening the 

 cocoon — or the hard tip to the beak of nestling birds, used for 

 breaking th(^ eir'^. It has been asserted that, of the best short- 

 lieaked tumbler-pigeons, a greater number 2:)erish in the egg 

 than are al^le 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- 

 j^rown pigeon very short for the bird's own advantage, the pro- 

 cess of modification Avould be very slow, and there would be 

 simultaneously the most rigorous selection of all the young 

 birds within the egg, which had the most powerful and hardest 

 beaks, for all with weak beaks would inevitably perish ; or, 

 more delicate and more easily-l)rokcn shells might be se- 

 lected, the thickness of the shell being knoAvn 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 no doubt occurs under Nature, and if so, natural se- 

 lection will be able to modify one sex in its functional n'lations 

 to the otlier sex, or in relation to Avholly-ditrerent liabits of life 

 in the two sexes, as is sometimes the case with insects. And 



