AFFINITIES AND GENEALOGY. 159 



mary glands and nipples, as they exist in male mammals, can 

 Indeed hardly be called rudimentary; they are merely not fully 

 developed, and not functionally active. They are sympathetically 

 affected under the Influence of certain diseases, like the same 

 organs in the female. They often secrete a few drops of milk at 

 hlrth 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 dur- 

 ing maturity as to yield a fair supply of milk. Now if we sup- 

 pose that during a former prolonged period male mammals aided 

 the females in nursing their offspring," and that afterwards 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 transmitted to the males at the 

 corresponding age of maturity. But at an earlier age these or- 

 gans would be left unaffected, so, that they would be almost 

 equally well developed in the young of both sexes. 



Conclusion.— Yon Baer has defined advancement or progress 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 diver- 

 sified lines of life by means of natural 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 afterwards for some 

 other and quite distinct purpose; and thus all the parts are 

 rendered more and more complex. But each organism still re- 

 tains 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 organization on 

 the whole has advanced throughout the world by slow and in- 

 terrupted 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 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 inhab- 



^1 Madlle. C. Royer has suggested a, similar view in her 'Origine de 

 I'Homme,' &c., 1870. 



