PIGEON'S BREAST-BONE. 67 



artificial selection, or to other modifications of 

 shape and proportion effected directly or indirectly 

 by the same cause. 1 The reduction is greatest in 

 the Pouter (i8 per cent.) and in the Pied -Scan- 

 deroon (\J\ per cent). In the former the body has 

 been greatly elongated by artificial selection and 

 three or four additional vertebrae have been acquired 

 in the hinder part of the body. 2 In the latter a 

 long neck increases the length of the bird, and so 

 causes, or helps to cause, the relative shortening of 

 the breast-bone. In the English Carrier which 

 experiences the effects of disuse, as it is too 

 valuable to be flown the relative reduction of 

 II per cent, is apparently more than accounted 



1 If a prominent breast is admired and selected by fanciers, the 

 sternum might shorten in assuming a more forward and vertical 

 position. If the shortening of the sternum is entirely due to disuse, 

 it seems strange that Darwin has not noticed any similar shortening 

 in the sternum of the duck. But selection has not tended to make 

 the duck elegant, or "pigeon-breasted " ; it has enlarged the abdo- 

 minal sack instead, besides allowing the addition of an extra rib in 

 various cases. 



2 Variation of Animals and Plants under Domestication, 144, 175. 



F 2 



