188 UNUSUALLY DEVELOPED PAETS [Chap. V, 



being 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 going on between, on the one hand, the 

 tendency to reversion 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 modifica- 

 tion may always be expected. 



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

 developed in an extraordinary manner in any one 

 species, 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 

 common progenitor of the genus. This period will 

 seldom be remote in any extreme degree, as species 

 rarely endure for more than one geological period. An 

 extraordinary amount of modification implies an un- 

 usually 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 



