198 EVOLUTION AND GENETICS 



to another, that is sometimes necessary, it is essen- 

 tial that the blood corpuscles of the donor are not 

 agglutinated by the serum of the recipient. Thus it 

 is a matter of great importance to select a donor that 

 does not bring about such a catastrophe. The simple 

 rules are that individuals belonging to the same blood 

 group (I, II, III, or IV) do not agglutinate each 

 other's blood, but the blood corpuscles of an indi- 

 vidual represented hj AA or A a will be precipitated 

 if the donor contains the agglutinin represented by 

 aa, and similarly the blood corpuscles of an indi- 

 vidual represented by BB or Bh will be precipitated 

 if the donor contains the agglutinin represented by 

 bb. Inspection of the diagram will show that group 

 II (with serum bb) precipitates III and IV, and 

 group III (with serum aa) precipitates II and IV. 

 Further, the serum of group I (aa bb) precipitates 

 all of the other groups; while the serum of group IV 

 precipitates none of the others. 



Inheritance of Other Traits 



There are numerous other physical characters of 

 man that are evidently inherited but where the num- 

 her of factors involved is uncertain or entirely un- 

 known. Some of these characters are present in all 

 races. Others to some extent are racial characters. 

 Thus height in man is a very variable character. It is 

 obviously a complex of several or many elements 

 little understood. Differences in length of legs, or 



