186 THE DESCENT OF MAN. 



active. They are sympathetically affected under the influ- 

 ence of certain diseases, like the same organs in the female. 

 They often secrete a few drops of milk at birth and at 

 puberty; this latter fact occurred in the curious case, before 

 referred to, where a young man possessed two pairs of 

 mammae. In man and some other male mammals these 

 organs have been known occasionally to become so well 

 developed during maturity as to yield a fair supply of milk. 

 Now if we suppose that during a former prolonged period 

 male mammals aided the females in nursing their off spring,* 

 and that afterward from some cause (as from the production 

 of a smaller number of young) the males ceased to give this 

 aid, disuse of the organs during maturity would lead to their 

 becoming inactive; and from two well-known principles of 

 inheritance, this state of inactivity would probably be trans- 

 mitted 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 developed in 

 the young of both sexes. 



Conclusion. Von Baer has denned 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 nat- 

 ural 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 rendered 

 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 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 always 



* Maddle. C. Royer has suggested a similar view in lier " Origine 

 de 1'Homme," etc., 1870, 



