400 DEVELOPMENT Chap. XIII. 



of llic dciiiis and of threc-days-old colts of racc-horscs and 

 heavy cart-horses, I find tliat tliis is by no means tlic case. 



As we have conckisivc ev-idcnce that the breeds of the Pig- 

 con are descended from a single -wild species, I compared the 

 young within twelve hours after being hatched ; I carefully 

 measured in the wild jnu-ent-species, in pouters, fantails, runts, 

 barl)s, dragons, carriers, and tumblers, the i)roportlons (but 

 will not here give the details) of the beak, width of mouth, 

 length of nostril and of eyelid, size of feet and length of leg. 

 Now some of these birds, Avhen mature, differ in so extraor- 

 dinary a degree in the length and form of beak, and in other 

 characters, that they would certainly be 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, yet the projiortional 

 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 remark- 

 able excejition to this rule, for the young of the short-faced 

 tumbler difiered in all its proportions from the young of the 

 wald rock-pigeon and of the other breeds, almost exactly as 

 much as in the adult state. 



The above two principles explain these facts. Fanciers 

 select for breeding their dogs, horses, pigeons, etc., when 

 nearly grown \ip : they are indifferent whether the desired 

 qualities arc acquired earlier or later in life, if the full-grown 

 animal possesses them. And the cases just given, more espe- 

 cially that of pigeons, show that the characteristic differences 

 which give value to the breeds, and which have been accumu- 

 lated by man's selection, have not generally appeared at a very 

 early period of life, and have been inherited at a corresponding 

 not early period. But the case of the short-faced tumbler, 

 which when twelve hours old possessed its ]iroper characters, 

 proves that this is not the imiversal rule ; for here the charac- 

 teristic dillerences must either have appeared at an earlier 

 period than usual, or, if not so, the differences must have been 

 inlieritcd, not at a corresponding, but at an earlier age. 



Now let us ajiply these two principles to species in a state 

 of nature. Let us take a group of birds, desccmded from some 

 ancient form and modified through natinwl selection for differ- 

 ent habits. Then, from the many slight successive variations 

 having supervened in the several species at a not early age, 



