446 EMBRYOLOGY. Chap. XIII. 



young. But there was one remarkable exception to this 

 rule, for the young of the short-faced tumbler differed 

 from the young of the wild rock-pigeon and of the other 

 breeds, in all its proportions, almost exactly as much as 

 in the adult state. 



The two principles above given seem to me to explain 

 these facts in regard to the later embryonic stages of 

 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 differences 

 which give value to each breed, and which have been 

 accumulated by man's selection, have not generally first 

 appeared at an early period of life, and have been in- 

 herited 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 propor- 

 tions, 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 differ- 

 ences 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 



