According to this theory, each quality in an organism which can be isolated and investigated independently of other qualities, is termed a “unit character,” and the appearance of each such unit character is determined by the presence of something called a “factor” in the sexual cells or “gametes,” male and female, by the union of which the individual is formed. Further, these unit characters are believed to occur in alternative pairs, and at first it was supposed that each alternative pair consisted of “dominant” and “recessive” characters, the second of which could only make its presence apparent in the individual if the dominant character were absent. Subsequently it was seen that the dominant and recessive characters need not necessarily consist of two positive, though opposite, qualities, but might better be regarded as consisting of the presence of a character and its absence. To use a classical illustration of this view, sweet peas may be classified into tall peas and dwarf peas. At first the unit characters were taken to be tallness (dominant) and dwarfness (recessive). Later this idea was modified, and it was said that potentially all peas are dwarf, but to some is added a factor producing tallness, this factor being absent in those that are dwarf. To represent this idea more simply a conventional notation has been used, according to which the large letters of the alphabet indicate the presence, and the small letters the absence, of each factor. In order to apply this theory to the case under consideration, it has been suggested that two pairs of factors are concerned: A the _presence_ of the character producing Group II. a the _absence_ of the character producing Group II. B the _presence_ of the character producing Group III. b the _absence_ of the character producing Group III. Each pair of factors is transmitted independently of the other. Both A and B may be absent, in which case the individual belongs to Group IV; or both may be present, and in this case the individual gives the reactions of Group I. It must be understood that the term “character producing Group II” is here used as a convenient way of expressing the obscure and probably complicated set of properties responsible for the reactions manifested by individuals of Group II. It includes not only the agglutinin or hæmolysin of the serum which reacts with corpuscles of Group III, but also the complementary iso-agglutinin or iso-hæmolysin by virtue of which the corpuscles react with serum of Group III.