CHAP. IV.] NATURAL SELECTION. 63 



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 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 conditions, 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, supposing 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 undergo 

 modification and improvement at the same time in the same 

 area. 



