THE DESCENT OB ORIGIN OF MAN 219 



pies of inheritance this state of inactivity would probably 

 be transmitted to the males at the corresponding age of 

 maturity. But at an earlier age these organs would be left 

 unaffected, so that they would be almost equally well devel- 

 oped in the young of both sexes. 



Conclusion. — Von Baer has defined advancement or prog- 

 ress in the organic scale better than any one else, as resting 

 on the amount of differentiation and specialization of the 

 several parts of a being — when arrived at maturity, as I 

 should be inclined to add. Now as organisms have become 

 slowly adapted to diversified lines of life by means of natu- 

 ral selection, their parts will have become more and more 

 differentiated and specialized for various functions, from the 

 advantage gained by the division of physiological labor. 

 The same part appears 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 ren- 

 dered more and more complex. But each organism still 

 retains 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 organi- 

 zation 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, however, 

 be supposed that groups of organic beings are always sup- 

 planted, and disappear as soon as they have given birth to 

 other and more perfect groups. The latter, though victori- 

 ous 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 inhabiting protected 

 sites, where they have not been exposed to very severe 

 competition ; and these often aid us in constructing our gene- 

 alogies, by giving us a fair idea of former and lost popula- 

 tions. 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. 



