CHAP. XIV.] DEVELOPMENT AND EMBRYOLOGY. 369 



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 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 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 super- 

 vened 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 members of the 

 great class of insects. With respect to the final cause of the 

 young in such groups not passing through any metamorphosis, we 

 can see that this would follow from the following contingences ; 

 namely, from the young having to provide at a very early age for 

 their own wants, and from their following the same habits of life 

 with their parents ; for in this case, it would be indispensable for 

 their existence that they should be modified in the same manner 

 as their parents. Again, with respect to the singular fact that 



