REPEATED PARTS 125 



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 



