i 



MUTUAL AFFINITIES OF ORGANIC BEINGS 401 



dragons, carriers, and tumblers. Now, some of these birds, when 

 mature, differ in so extraordinary a manner in the length and form 

 of beak, and in other characters, that they would certainly have 

 been ranked as distinct genera if found in a state of nature. But 

 when the nestling birds of these several breeds were placed in a 

 row, though most of them could just be distinguished, the propor- 

 tional differences in the above specified points were incomparably 

 less than in the full-grown birds. Some characteristic points of 

 difference — for instance, that of the width of mouth — could hardly 

 be detected in the 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 al- 

 most exactly the same proportions as in the adult stage. 



These facts are explained by the above two principles. Fanciers 

 lect their dogs, horses, pigeons, etc., for breeding, when nearly 

 own up. They are indifferent whether the desired qualities are 

 quired earlier or later in life, if the full-grown animal possesses 

 em. And the cases just given, more especially that of the pigeons, 

 show that the characteristic differences which have been accumu- 

 lated by man's selection, and which give value to his breeds, do 

 not generally appear at a very early period of life, and 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 super- 

 vened 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 



