NATURAL SELECTION 79 



feed, a large part of their structure is merely the cor- 

 related result of successive changes in the structure of 

 their larvae. So, conversely, modifications in the adult 

 will probably often affect the structure of the larva ; but 

 in all cases natural selection will ensure that modifica- 

 tions consequent on other modifications at a different 

 period of life, shall not be in the least degree injurious : 

 for if they became so, they would cause the extinction 

 of the species. 



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 

 community; if each in consequence profits by the selected 

 change. What natural selection cannot do, is to modify 

 the structure of one species, without giving it any advan- 

 tage, for the good of another species ; and though state- 

 ments to this effect may be found in works of natural 

 history, I cannot find one case which will bear investi- 

 gation. A structure used only once in an animal's whole 

 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 

 nestling birds, used for breaking the egg. It has been 

 asserted, that of the best short-beaked tumbler-pigeons 

 more 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 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. 



Sexual Selection. — Inasmuch as peculiarities often 

 appear under domestication in one sex and become 



