68 THE ORIGIN OF SPECIES 



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 animal's life, if of high importance to it, might be modified to 

 any extent by natural selection; for instance, the great jaws pos- 

 sessed by certain insects, used exclusively for opening the cocoon 

 — or the hard tip to the beak of unhatched birds, used for break- 

 ing the eggs. 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 in- 

 evitably 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 manner which 

 protected them from their enemies. Yet many of these eggs or 

 seeds would perhaps, if not destroyed, have yielded individuals 

 better adapted to their conditions of life than any of those which 

 happened to survive. So again a vast number of mature animals 

 and plants, whether or not they be the best adapted to their condi- 

 tions, must be annually destroyed by accidental causes, which 

 would not be in the least degree mitigated by certain changes of 

 structure or constitution which would in other ways be beneficial 

 to the species. But let the destruction of the adults be ever so 

 heavy, if the number which can exist in any district be not wholly 

 kept down by such causes — or again let the destruction of eggs 

 or seeds be so great that only a hundredth or a thousandth part are 

 developed — yet of those which do survive, the best adapted indi- 

 viduals, supposing that there is any variability in a favorable direc- 

 tion, will tend to propagate their kind in larger numbers than the 

 less well adapted. If the numbers be wholly kept down by the 

 causes just indicated, as will often have been the case, natural 

 selection will be powerless in certain beneficial directions; but 

 this is no valid objection to its efficiency at other times and in 

 other ways ; for we are far from having any reason to suppose that 



