Chap. V. LAWS OF VARIATION. 153 



variability of all kinds, 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 far as to breed a bird as coarse as a 

 common tumbler from a good short-faced strain. But 

 as long as selection is rapidly going on, there may 

 always be expected to be much variability in the struc- 

 ture undergoing modification. It further deserves 

 notice that these variable characters, produced by man's 

 selection, sometimes become attached, from causes quite 

 unknown to us, more to one sex than to the other, gene- 

 rally to the male sex, as with the wattle of carriers and 

 the enlarged crop of pouters. 



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 species branched off from the common 

 progenitor of the genus. This period will seldom be 

 remote in any extreme degree, as species very 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 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 exces- 

 sively remote, we might, as a general rule, expect still 

 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 con- 

 vinced, is the case. That the struggle between natural 

 selection on the one hand, and the tendency to rever- 

 sion and variability on the other hand, will in the 



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