Chap. XIV.] DEVELOPMENT AND EMBRYOLOGY. 249 



give value to his breeds, do not generally appear at a 

 very early period of life, and are inherited at a corre- 

 sponding not early period. But the case of the short- 

 faced tumbler, which when twelve hours old possessed 

 its proper characters, proves that this is not the 

 universal rule; for here the characteristic differences 

 must either have appeared at an earlier period than 

 usual, or, if not so, the differences must have been 

 inherited, not at a corresponding, but at an earlier age. 



Now let us apply these two principles to species in a 

 state of nature. Let us take a group of birds, descended 

 from some ancient form and modified through natural 

 selection for different habits. Then, from the many 

 slight successive variations having supervened in the 

 several species at a not early age, and having been 

 inherited at a corresponding age, the young will have 

 been but little modified, and they will still resemble 

 each other much more closely than do the adults, — just 

 as we have seen with the breeds of the pigeon. We 

 may extend this view to widely distinct structures and 

 to whole classes. The fore-limbs, for instance, which 

 once served as legs* to a remote progenitor, may have 

 become, through a long course of modification, adapted 

 in one descendant to act as hands, in another as paddles, 

 in another as wings ; but on the above two principles 

 the fore-limbs will not have been much modified in the 

 embryos of these several forms ; although in each form 

 the fore-limb will differ greatly in the adult state. 

 Whatever influence long-continued use or disuse may 

 have had in modifying the limbs or other parts of any 

 species, this will chiefly or solely have aftected it when 

 nearly mature, when it was compelled to use its full 

 powers to gain its own living; and the effects thus 



