232 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



tions is determined not only by the presence of the finely differentiated sub- 

 stances, which represent the organismal differentials in tissues and organs, 

 but also by the rate at which such substances are produced, given off by the 

 cells and allowed to diffuse into the strange organism ; finally, the results of 

 the reactions vary with the sensitiveness of the tissues involved. As far as the 

 injurious effects of incompatible bodyfluids are concerned, our ability to 

 discern these depends mainly on the sensitiveness of the tissues on which they 

 act; on the other hand, the strength of the cellular reactions against tissues 

 with strange organismal differentials is, to a certain extent, susceptible to 

 measurement, even if the tissues are more resistant. 



We see, thus, that we have to deal with a considerable number of factors 

 if we wish to grade the degree of compatibility of the organismal differentials 

 of different organisms. With regard to the higher organisms, only a beginning 

 has been made in the approximate quantitative grading of these incompati- 

 bilities; in the case of the more primitive organisms the observations which 

 bear on this question are more or less casual and are based largely on gross 

 aspects of the changes following transplantation. The introduction of micro- 

 scopical methods for the study of cellular reactions against strange differen- 

 tials, methods similar to those used in the case of mammalian tissues, has only 

 recently been undertaken in the study of transplantations in urodeles. The 

 results already indicate that reactions against strange tissues, endowed with 

 strange individuality differentials, are present in classes of animals where the 

 methods formerly used were not adequate for their demonstration. In addi- 

 tion, there are the immunological studies of the relationship of the bodyfluids 

 of various organisms ; these are based on the ability of bodyfluids to serve as 

 antigens and to call forth the production of antibodies, which latter, in their 

 interactions with antigens, indicate the relationship between various species 

 of animals. These investigations will be discussed in a subsequent chapter. 



We may then conclude that in the most primitive classes of animals the 

 substances which represent the organismal differentials or at least the reactions 

 against these differentials are as yet less finely differentiated than in higher 

 organisms, and that in general there is a correspondence between the lack of a 

 finer differentiation of organismal differentials and the lack in the finer differ- 

 entiation of organs and tissues, and an inverse correspondence between the 

 degree of development of organismal differentials and the degree of plasticity 

 of these organs. 



As stated above, the graded progression towards an increase in the speci- 

 ficity and fixity of organs and tissues in the course of evolution are clearly 

 discernible. With advancing phylogenetic development the various parts of 

 the body differentiate more and more into a variety of organs, tissues and 

 cells, interacting with each other according to patterns which are specific and 

 rigid. Hand in hand with this change from the relatively simple structures of 

 such primitive organisms as the hydrozoa, to the greatly differentiated types 

 of organs, tissues and cells of the anuran amphibia, there takes place a change 

 also in the kind of substances which regulate the interaction of the different 

 parts of the individual. In hydrozoa we must assume that substances repre- 



