EMBRYOLOGY 259 



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 affected it when 

 nearly mature, when it was compelled to use its full 

 powers to gain its own living; and the effects thus pro- 

 duced 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 they first occurred. In either of these cases, the 

 young or embryo will closely resemble the mature parent- 

 form, as we have seen with the short-faced tumbler. 

 And this is the rule of development in certain whole 

 groups, or in certain sub-groups alone, as with cuttle-fish, 

 land-shells, fresh -water crustaceans, spiders, and some 



