132 THE THEORY OF MUTATION 
An important point with regard to repeated parts 
is to be observed in the fact that in a pair of allied 
species, in which a series of repeated organs in the one 
is clearly comparable with a similar series in the other, 
all the parts in one form may differ from those in the 
second by the same kind of distinction, whether this 
be qualitative or numerical. The facts suggest strongly 
that such cases are to be accounted for by all the 
parts in question in one or both species having varied 
in a similar way at the same time rather than in suc- 
cession. The occurrence of such a similar and simul- 
taneous process of variation of repeated parts clearly 
simplifies in a marked degree the process of evolution, 
and greatly reduces the time which would be required 
for this process, if similar changes in repeated parts 
always took place successively. If we take an ex- 
treme case the latter supposition becomes absurd. In 
the albino or pure white types which occur as varia- 
tions in many species of birds and mammals it is 
obvious that every hair or feather has taken on the 
white colour at the same time and for the same reason, 
whatever that reason may have been. Hairs or 
feathers are very good examples of repeated parts of 
the kind of which we have been speaking. It appears, 
too, that colour patterns may originate and change in 
a similar manner. In the case of such a bird as the 
peacock we should expect on this view that the pattern 
varied in all the tail feathers simultaneously, nor is it 
necessary to suppose that even this process took place 
by a very long series of minute steps. If we find that 
the splendid coloration of the peacock’s tail arose 
