Chap. XIV.] DEVELOPMENT AND EMBRYOLOGY. 249 



nis breeds, do not generally appear at a very early period 

 01 life, and are inherited at a corresponding 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 in- 

 lerited at a corresponding age, the yoling 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 ex- 

 tend 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 affected it when nearly ma- 

 ture, when it was compelled to use its full powers to 

 gain its own living; and the effects thus produced will 



