■j^S EVOLUTION, GENETICS, AND EUGENICS 



selection may modify and adapt the larva of an insect to a score of 

 contingencies, wholly different from those which concern the mature 

 insect; and these modifications may affect, through correlation, the 

 structure of the adult. So, conversely, modifications in the adult may 

 affect the structure of the larva; but in all cases natural selection will 

 ensure that they shall not be injurious: for if they were so, the species 

 would become extinct. 



Natural selection will modify the structure of the young in relation 

 to the parent, and of the parent in relation to the young. In social 

 animals it will adapt the structure of each individual for the benefit of 

 the whole community, if the community profits by the selected 

 change. What 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 open- 

 ing 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 sheU 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 



