Chap. XIV. Development and Embryology. 393 



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 inhe- 

 rited 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 in- 

 herited, 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 modi- 

 fied, 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 cor- 

 responding nearly mature age. Thus the young will not be modi- 

 fied, 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 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 



