Chap. VL] AFFINITIES AND GENEALOGY. 203 



Conclusion. — The best definition of advancement or 

 progress in the organic scale ever given, is that by Yon 

 Baer ; and this rests on the amount of differentiation and 

 specialization of the several parts of the same being, when 

 arrived, as I should be inclined to add, at maturity. Now 

 as organisms have become slowly adapted by means of 

 natural selection for diversified lines of life, their parts 

 will have become, from the advantage gained by the di- 

 vision of physiological labor, more and more differentiated 

 and specialized for various functions. The same part ap- 

 pears often to have been modified first for one purpose, 

 and then long afterward for some other and quite distinct 

 purpose ; and thus all the parts are rendered more and 

 more complex. But each organism will still retain the 

 general type of structure of the progenitor from which it 

 was aboriginally derived. In accordance with this view 

 it seems, if we turn to geological evidence, that organiza- 

 tion on the whole has advanced throughout the world by 

 slow and interrupted steps. In the great kingdom of the 

 Vertebrata it has culminated in man. It must not, how- 

 ever, be supposed that groups of organic beings are al- 

 ways supplanted and disappear as soon as they have 

 given birth to other and more perfect groups. The latter, 

 though victorious over their predecessors, may not have 

 become better adapted for all places in the economy of 

 Nature. Some old forms appear to have survived from in- 

 habiting protected sites, where they have not been ex- 

 posed to very severe competition ; and these often aid us 

 in constructing our genealogies, by giving us a fair idea 

 of former and lost populations. But we must not fall into 

 the error of looking at the existing members of any lowly- 

 organized group as perfect representatives of their ancient 

 predecessors. 



The most ancient progenitors in the kingdom of the 

 Vertebrata, at which we are able to obtain an obscure 



