1490 THE STORY OF THE UNIVERSE 



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 giv- 

 ing 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 se- 

 lection; for instance, the great jaws possessed by cer- 

 tain 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 num- 

 ber 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 

 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 pow- 

 erful 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. 



Inasmuch as peculiarities often appear under do- 

 mestication in one sex and become hereditarily at- 

 tached to that sex, so no doubt it will be under nature. 

 Thus it is rendered possible for the two sexes to be 

 modified through natural selection in relation to dif- 

 ferent habits of life, as is sometimes the case; or for 



