258 THE ORIGIN OF SPECIES 



distinguished, the proportional difTercnccs 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 I'emarkable 

 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 almost exactly the same pro- 

 portions as in the adult state. 



These facts are explained by the above two principles. 

 Fanciers select their dogs, horses, pigeons, etc., for breed- 

 ing, when nearly grown up: they are indifferent whether 

 the desired qualities are acquired earlier or later in life, 

 if the full-grown animal possesses them. And the cases 

 just given, more especially that of the 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 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 character- 

 istic 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 

 3tate 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 



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