It was first discovered by L. and H. Hirszfeld (1919) that the races of mankind differ in the relative frequencies of the four classical blood groups. Since that time an enormous amount of information has been collected from all parts of the world. To-day it can be said that more is known about the geographical variations of the human blood group genes than is known in the case of any other genes whatsoever, whether plant or animal (Dobzhansky, 1941 ). The four original blood groups depend upon the presence or absence of two agglutinable substances, or agglutinogens, in the red blood corpuscles, associated with the presence or absence of corresponding agglutinins in the serum. The agglutinogens are usually denoted by the letters A and B, the agglutinins by the letters a and b. Red cells containing A are agglutinated by serum containing a; similarly, red cells containing B are agglutinated by serum containing b.