468 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



3. The term "specific adaptation" may be used to designate the difference 

 between the results of the interactions of two substances or tissues if they 

 take place on the one hand between individuals A and B, and on the other 

 hand between individuals A and C ; and likewise between species S and T and 

 species S and R. These differences in the results of interaction depend upon 

 the character of the organismal differentials of the different individuals or 

 species, and the degree of these differences should then be graded in corres- 

 pondence with the degree of genetic relationship between these organisms, 

 since the organs or substances involved find, in more nearly related organisms, 

 receptors to which they are better fitted than to those in less nearly related 

 organisms. It is the correspondence between the organismal differentials of 

 organs, tissues or substances in one organism and the receptors of organs, 

 tissues or substances in another organism, which characterizes the specific 

 adaptation in the reaction between them. 



Such a specific adaptation can be demonstrated most readily if we have to 

 deal with class and generic differentials. The finer the differentials are which 

 come into play, the more difficult it is to demonstrate a mutual adaptation. 

 Thus, a specific adaptation between substances carrying species differentials 

 can be demonstrated less readily than an adaptation between substances 

 carrying class or generic differentials ; and still greater is the difficulty when 

 individuality differentials interact. This increasing difficulty in recognizing the 

 presence of finer organismal differentials may be due to deficiencies in the case 

 of the very finely graded reactions by means of which finer organismal differen- 

 tials are tested. We would have, therefore, to face in this case the same 

 problem which arose when, in joining together more primitive tissues or 

 organisms, it was possible to demonstrate the presence of the coarser, but not 

 of the finer, organismal differentials. 



In all the instances considered so far, we have to deal with the interaction 

 of specifically adapted substances or tissues which are preformed. However, 

 a similar specific adaptation can also arise through active immunization, when 

 one substance serving as antigen, enters the system of an organism belonging 

 to another species ; immune substances may then develop, which react with 

 the antigens in a specific and graded manner, corresponding to the relation- 

 ship of the organisms or tissues and organs involved in these processes. We 

 have here, then, to deal (1) with a specific adaptation between an antigen 

 and an antibody, and in addition (2) with a gradation in specificity in the 

 interaction between antigen and antibody in the sense that other substances 

 may take their place the more readily, the more nearly related organs and 

 tissues, or the organisms are which substitute for the primary antigens or 

 antibodies. Conversely the degree of specific adaptation between these sub- 

 stituted antigens or antibodies may serve as the indicator of the degree of 

 relationship between the primary substance and the substitutes. 



In this discussion we have attributed the organismal reactions exhibiting 

 a specific adaptation to the presence of organismal differentials, with which 

 organ-specific substances may be combined in certain cases. However in 

 preceding chapters we have found instances in which specific reactions be- 



