188 UlfUSUALLr DEVELOPED PARTS. [Chap. V. 



the points now mainly attended to by English fanciers. 

 Even in the same sub-breed, as in that of the short- 

 faced tumbler, it is notoriously difficult to breed nearly 

 perfect birds, many departing widely from the standard. 

 There may truly be said to be a constant struggle go- 

 ing on between, on the one hand, the tendency to re- 

 version to a less perfect state, as well as an innate 

 tendency to new variations, and, on the other hand, 

 the power of steady selection to keep the breed true. 

 In the long run selection gains the day, and we do not 

 expect to fail so completely as to breed a bird as coarse 

 as a common tumbler pigeon from a good short-faced 

 strain. But as long as selection is rapidly going on, 

 much variability in the parts undergoing modification 

 may always be expected. 



Now let us turn to nature. When a part has been 

 developed in an extraordinary manner in any one spe- 

 cies, compared with the other species of the same genus, 

 we may conclude that this part has undergone an 

 extraordinary amount of modification since the period 

 when the several species branched off from the com- 

 mon progenitor of the genus. This period will sel- 

 dom be remote in any extreme degree, as species rarely 

 endure for more than one geological period. An ex- 

 traordinary amount of modification implies an unusu- 

 ally large and long-continued amount of variability, 

 which has continually been accumulated by natural 

 selection for the benefit of the species. But as the 

 variability of the extraordinarily developed part or organ 

 has been so great and long-continued within a period 

 not excessively remote, we might, as a general rule, still 

 expect to find more variability in such parts than in 

 other parts of the organisation which have remained for 



