Chap. V. 
LAWS OF VARIATION. 
153 
variabiKty 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, generally 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 
