100 ORIGIN OF SPECIES 



works of natural history, I cannot find one case which will 

 bear investigation. A structure used only once in an ani- 

 mal'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 per- 

 ish ; 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 n'umber 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 ene- 

 mies. 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 sur- 

 vive. 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 bene- 

 ficial 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 propa- 

 gate their kind in larger numbers than the less well adapted. 



