7. The serum of certain humans may cause the destruction or agglutination of blood corpuscles of certain other humans. This fact of the existence of “isoagglutinins” seems to have been established for man, but Hektoen states that he has not been able to find any isoagglutinins in the serum of rabbits, guinea-pigs, dogs, horses, and cattle. Landsteiner found the remarkable fact that the sera of certain individuals of humans could hemolyze the corpuscles of certain other individuals, but not those of all individuals. A systematic investigation of this variability led him to the discovery of three distinct groups of individuals, the sera of each group acting in a definite way towards the corpuscles of the representatives of each other group. Later observers, for example Jansky and Moss, established four groups. These groups are, according to Moss,[56] as follows: Group 1. Sera agglutinate no corpuscles. Corpuscles agglutinated by sera of Groups 2, 3, 4. Group 2. Sera agglutinate corpuscles of Groups 1, 3. Corpuscles agglutinated by sera of Groups 3, 4. Group 3. Sera agglutinate corpuscles of Groups 1, 2. Corpuscles agglutinated by sera of Groups 2, 4. Group 4. Sera agglutinate corpuscles of Groups 1, 2, 3. Corpuscles agglutinated by no serum. [56] Moss, W. L., _Johns Hopkins Hospital Bulletin_, 1910, xxi., 62. The relative frequency of the four groups follows from the following figures. Of one hundred bloods tested by Moss in series of twenty there were found: 10 belonging to Group 1. 40 belonging to Group 2. 7 belonging to Group 3. 43 belonging to Group 4. Groups 2 and 4 are in the majority and in overwhelming numbers, which indicates that, as a rule, the sera agglutinate the blood corpuscles of individuals of the other groups, but not those of individuals belonging to the same group. The phenomenon that a serum agglutinates no corpuscles (Group 1), or that the corpuscles are agglutinated by no serum (Group 4), are the exceptions. It is obvious that, as far as our problem is concerned, only Groups 2 and 3 are to be considered. There is no Mendelian character which refers only to one half of the individuals except sex. Since nothing is said about a relation of Groups 2 and 3 to sex such a relation probably does not exist. 8. The facts thus far reported imply the suggestion that the heredity of the genus is determined by proteins of a definite constitution differing from the proteins of other genera. This constitution of the proteins would therefore be responsible for the genus heredity. The different species of a genus have all the same genus proteins, but the proteins of each species of the same genus are apparently different again in chemical constitution and hence may give rise to the specific biological or immunity reactions.