NATURE OF THE INDIVIDUALITY DIFFERENTIALS 201 



its functions as host or donor. Furthermore, while the organismal differen- 

 tials exert primarily a direct effect, in determining the reactions of the host 

 against the transplant, secondarily they may function also as antigens and 

 call forth immune reactions, which may contribute to the intensity of the re- 

 actions, although to an unlike degree in different tissues. As to the number of 

 genes which are involved in these reactions, it is in all probability very large, 

 as especially the experiments with closely inbred strains suggest. There is no 

 indication that the genes determining the four primary blood-group differen- 

 tials are the genes which determine the nature of the individuality differen- 

 tials ; however, if we consider also the large number of additional differentials 

 already found in erythrocytes by means of agglutination or hemolysin reac- 

 tions, — a number which will probably increase still more in the future — , it 

 appears possible that the gene sets from which the blood cell antigens de- 

 velop and the individuality differentials will more and more tend to approxi- 

 mate each other. 



In this connection it may be stated that a distinction should be made between 

 the terms "individual differential" and "individuality differential." The for- 

 mer may be regarded as the more general term, including many characteristics, 

 which differentiate one individual from another, such as color of skin, hair, 

 eyes, size, shape of body and its parts, psychical attributes and which com- 

 prise thus the organ and tissue differentials. In contrast to these, "individuality 

 differential" is a more specific term, designating a definite characteristic 

 which is common to all or most parts of one individual and which differen- 

 tiates him from the common characteristic denominator in another individual. 



There has evolved, as the result of a long-continued series of experiments, 

 the concept of individuality and species differentials, and, in general, of 

 organismal differentials, in their interactions with tissue differentials and 

 various other factors characterizing the organism. This evolution was a 

 gradual one, taking place step by step, in close connection with concepts which 

 were prominent at certain periods in the development of biology and patholo- 

 gy. We may distinguish essentially four periods in this development: (1) In 

 the first period transplantations among more primitive invertebrates had 

 proved the importance of the polarity of tissues and of other structural pecu- 

 liarities affecting the harmony between grafted parts and the host, and these 

 observations were generalized and applied also to transplantations in higher 

 vertebrates; (2) In a second period the discoveries concerning active im- 

 munity and anaphylaxis, and especially those concerning agglutinins, hemoly- 

 sins and precipitins, very strongly influenced the interpretation of all subse- 

 quent experiments in transplantation ; (3) Later Mendelian concepts of hered- 

 ity, following the revival of the study of genetics at the beginning of this cen- 

 tury, suggested that the results of grafting were determined by the presence or 

 absence, in the host, of genes which the grafted tissue needed for survival ; (4) 

 The foregoing data, as far as they were found valid, and the addition of new 

 experimental data gave then origin to the concept of the various types of 

 organismal differentials as determiners of the mode and intensity of inter- 

 action between different organisms and their parts. 



