80 NATURAL SELECTION. 



natural selection cannot 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 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 unhatched birds, used for breaking 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 simul- 

 taneously 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 vrould 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 dis- 

 trict 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 



