NATURAL SELECTION. 105 



compared with another ; but they are now rapidly yield- 

 ing before the advancing legions of plants and animals 

 introduced from Europe. Natural selection will not pro- 

 duce absolute perfection, nor do we always meet, as far 

 as we can judge, with this high standard under nature. 

 The correction for the aberration of light is said by M til- 

 ler not to be perfect even in that most perfect organ, the 

 human eye. 



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 can not 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 can not 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 

 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 modifi- 

 cation would be very slow, and there would be simultane- 

 ously 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 ; 

 6 



