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 profits 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 investiga- 

 tion. A structure used only once in an animal'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 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 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 



