62 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



as already remarked, common to all the Mammalia, of 

 reduction in the number of teeth. Thus men with fewer 

 teeth are more advanced than those with more numer- 

 ous ones ; while people with tritubercular superior 

 molars have reverted to an ancient type ; and both re- 

 sults are probably attained by the same physiologic 

 process, i. e. defect of nutrition. It is to be remem- 

 bered also, in connection with our argument, that these 

 dental variations are modifications of generic charac- 

 ters, and that they are in definite directions, and are 

 not promiscuous. With regard to the question as to 

 whether dental variations in man are promiscuous or 

 not, we have better opportunities of investigation than 

 in the case of the lower animals generally. It may be 

 safely asserted that the dental variations above cited 

 are by far the most frequent in man, and that all others 

 put together are relatively insignificant. 



3. SUCCESSIONAL RELATION. 



As the biologic types are variations become perma- 

 nent, it is important to examine how the former stand 

 related to each other. These relations express the 

 direction which variation has taken, and throw a great 

 deal of light on the nature of the process. That exist- 

 ing types of all grades are the result of the isolation of 

 variations of species, is shown by the frequent exam- 

 ples of incomplete isolation, which follows inconstancy 

 of the definitive characters. Groups of individuals 

 which display this partial isolation are termed sub- 

 species. 



As an illustration of the mingling of isolated groups 

 of individuals (species) with imperfectly isolated groups 

 (subspecies), in a single genus, I refer to the American 



