68 Natural Selection. Chap. iv. 



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 possessed by certain insects, 

 used exclusively for opening the cocoon— or the hard tip to the 

 beak of unbatched 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 advan- 

 tage, 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 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 con- 

 ditions, 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 individuals, sup- 

 posing that there is any variability in a favourable direction, 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 many species ever 



