DEVELOPMENT AND EMBIiTOLOGT. 463 



the full-grown animal possesses them. And the cases just 

 given, more especially that of the pigeons, show that the 

 characteristic differences which have been accumulated 

 by man's selection, and which give value to his breeds, 

 do not generally appear at a very early period of 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 hkbits. 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 

 modiiied, 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 progen- 

 itor, may have become, through a long course of modifica- 

 tion, 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 

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

 gain its own living; and the effects thus produced will have 

 been transmitted to the offspring at a corresponding nearly 

 mature age. Thus the young will not be modified, or will 

 be modified only in a slight degree, through the effects of 

 the increased use or disuse of parts. 



With some animals the successive variations may have 

 supervened at a very early period of life, or the steps may 

 have been inherited at an earlier age than that at which 



