106 NATURAL SELECTION. [Chap. IV 



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 

 whole community; if the community proiits by the se- 

 lected 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 ani- 

 mal's 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 

 unhatched birds, used for breaking the egg. It has 

 been asserted, that of the best short-beaked tumbler, 

 pigeons a greater number perish in the egg than arc 

 able to get out of it; so that fanciers assist in the act oi 

 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 

 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 

 broken shells might be selected, the thickness of the 

 shell being known to vary like every other structure. 



It may be well here to remark that with all beings 

 there must be much fortuitous destruction, which can 

 have little or no influence on the course of natural 

 selection. For instance a vast number of eggs or seeds 

 are annually devoured, and these could be modified 

 through natural selection only if they varied in some 



