Chap. XIII. EMBKYOLOGY. 447 



age, the young of the new species of our supposed genus 

 will manifestly tend to resemble each other much more 

 closely than do the adults, just as we have seen in the 

 case of pigeons. We may extend this view to whole 

 families or even classes. The fore-limbs, for instance, 

 which served as legs in the parent-species, may be- 

 come, by a long course of modification, adapted in one 

 descendant to act as hands, in another as paddles, in 

 another as wings ; and on the above two principles — 

 namely of each successive modification supervening at 

 a rather late age, and being inherited at a corre- 

 sponding late age — the fore-limbs in the embryos of 

 the several descendants of the parent-species will still 

 resemble each other closely, for they will not have 

 been modified. But in each individual new species, 

 the embryonic fore-limbs will differ greatly from the 

 fore-limbs in the mature animal; the limbs in the 

 latter having undergone much modification at a 

 rather late period of life, and having thus been con- 

 verted into hands, or paddles, or wings. Whatever 

 influence long-continued exercise or use on the one 

 hand, and disuse on the other, may have in modi- 

 fying an organ, such influence will mainly affect the 

 mature animal, which has come to its full powers of 

 activity and has to gain its own living ; and the effects 

 thus produced will be inherited at a corresponding 

 mature age. Whereas the young will remain unmodified, 

 or be modified in a lesser degree, by the effects of use 

 and disuse. 



In certain cases the successive steps of variation 

 might supervene, from causes of which we are wholly 

 ignorant, at a very early period of life, or each step 

 might be inherited at an earlier period than that at 

 which it first appeared. In either case (as with the 

 short-faced tumbler) the young or embryo would closely 



