NATURAL SELECTION 129 



natural selection will insure that they shall not be inju- 

 rious; 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 individual for the benefit of the whole 

 community; if the community 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 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 co- 

 coon — 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 

 perisli 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 



