STRUCTURES VARIABLE 163 



concerns tis is, that those points in our domestic animals, 

 which at the present time are undergoing rapid change by 

 continued selection, are also eminently liable to variation. 

 Look at the individuals of the same breed of the pigeon, and 

 see what a prodigious amount of difference there is in the 

 beaks of tumblers, in the beaks and wattle of carriers, in the 

 carriage and tail of fantails, &c., these 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 notori- 

 ously 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 modification may always be expected. 



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

 oped in an extraordinary manner in any one species, com- 

 pared with the other species of the same genus, we may con- 

 clude 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 extra- 

 ordinary amount of modification implies an unusually large 

 and long-continued amount of variability, which has con- 

 tinually 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 gen- 

 eral rule, still expect to find more variability in such parts 

 than in other parts of the organisation which have remained 

 for a much longer period nearly constant. And this, I am 

 convinced, is the case. That the struggle between natural 

 selection on the one hand, and the tendency to reversion and 

 variability on the other hand, will in the course of time 



