EMBRYOLOGY 401 



our domestic varieties. Fanciers select their horses, 

 dogs, and pigeons, for breeding, when they are nearly 

 grown up : they are indifferent whether the desired 

 qualities and structures have been acquired earlier or 

 later in life, if the full-grown animal possesses them. 

 And the cases just given, more especially that of 

 pigeons, seem to show that the characteristic differ- 

 ences which give value to each breed, and which have 

 been accumulated by man's selection, have not gener- 

 ally first appeared at an early period of life, and have 

 been inherited by the offspring at a corresponding not 

 early period. But the case of the short-faced tumbler, 

 which when twelve hours old had acquired its proper 

 proportions, 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 the 

 corresponding, but at an earlier age. 



Now let us apply these facts and the above two 

 principles — which latter, though not proved true, can 

 be shown to be in some degree probable — to species 

 in a state of nature. Let us take a genus of birds, 

 descended on my theory from some one parent-species, 

 and of which the several new species have become 

 modified through natural selection in accordance with 

 their diverse habits. Then, from the many slight suc- 

 cessive steps of variation having supervened at a rather 

 late age, and having been inherited at a corresponding 

 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 have become, 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 corresponding late age — the fore- limbs in the 



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