156 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



examination, a reaction of the host may be demonstrated, which otherwise 

 would not have become manifest. In these respects, the primary blood group 

 antigens and individuality differentials differ from each other. (3) While in- 

 dividuality differentials have been found in all vertebrates so far studied, and 

 at least as far down in the phylogenetic series as the anuran amphibia, there 

 seem to be great variations among different species and classes in regard to 

 the presence, number and character of the blood groups. (4) While the 

 reactions against individuality differentials manifest very fine gradations in 

 response in accordance with the genetic relationship between host and trans- 

 plant, the blood group reactions are sharply defined into essentially two 

 classes, namely, those of compatible or incompatible individuals. 



In certain respects the blood group factors of primates have an intermediate 

 position between the organismal and tissue differentials, having certain fea- 

 tures in common with both. As far as the organismal differentials are con- 

 cerned, we shall discuss later their phylogenetic evolution ; it would be of in- 

 terest to trace in a similar manner also the evolution of the blood group anti- 

 gens. Such a study might help to clear up still further the relationship between 

 the blood group factors and the organismal or individuality differentials. 



