INTERMINGLING OF EACES 249 



In general their taxonomists have had only to differen- 

 tiate strains which do not interbreed. The mission of the 

 ethnologist may be compared rather to that of the agri- 

 culturist who is called upon to produce a usable classifica- 

 tion of the numerous strains of a variable domesticated 

 species, such as cattle and swine, or even wheat and 

 maize. He must for the sake of convenience make a mor- 

 phological grouping that is non-existent in physical fact. 

 He does this by taking advantage of isolation; without 

 isolation it is impossible. 



An, appreciation of Mendelian inheritance shows the 

 fallacy involved in making such a system a basis for trac- 

 ing ethnic relationships. An immense number of heredi- 

 tary variations have occurred in man. Some can be 

 described by the main structural changes they effect, some 

 cannot. Some distinct changes, such as eye color, have a 

 very simple method of inheritance. They are the mark 

 of single-factor differences in the germ plasm. Other 

 changes, such as those expressed in stature and skull 

 form, appear to be controlled by numerous factors. There 

 are even numerous factor changes which seem to produce 

 no visible effect on the individual and whose existence can 

 be shown only by crossing. For example, it may be 

 assumed with considerable confidence that individuals can 

 have the same cephalic index and yet differ by several 

 hereditary factors whose chief functions are the control 

 of this character. At least such cases have been found in 

 other species and there is no reason for supposing they do 

 not occur in man. 



Now these various physical, physiological and psychic 

 characters are controlled by factors transmitted alter- 

 natively. They may be linked in various manners, it is 



