508 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



that goat serum may, in certain cases, kill rabbits by a mechanism comparable 

 to that which is effective in anaphylaxis. 



While these facts make understandable the lack of a strict quantitative 

 correlation between the systematic relationship of animals and the action 

 of heterogenous blood serum on cellular constituents, still, a specific adapta- 

 tion between sera and the cellular elements does exist. This is evident from 

 the fact that, as a rule, autogenous and homoiogenous blood sera are much 

 more favorable for the preservation of the cellular constituents of the blood 

 than are heterogenous sera. This is true also of man and of certain animal 

 species in which blood groups occur, if the group agglutinins are first removed 

 from the blood. There are indications that similar conditions hold good in the 

 case of the blood of invertebrates as well. Thus we found the blood serum 

 of Limulus on the whole more suited for the normal activities of experimental 

 amoebocyte tissue of Limulus, than the blood serum of decapode arthropods, 

 such as the lobster. However, through previous heating of lobster serum the 

 latter can be converted into a favorable medium. Furthermore, Limulus serum 

 on being mixed with the serum of other Limuli remains clear, but after mixing 

 it with the sera of various crustaceans, precipitates usually develop. 



While thus, in general, no very strict quantitative parallelism can be dem- 

 onstrated between the action of normal sera on heterogenous cells and the 

 relationship of the organismal differentials of the species used, such a specific 

 relationship is demonstrable in the case of immune sera, produced through 

 injection of sera or cells into an animal possessing different organismal dif- 

 ferentials. As a rule, the antigenic activity of the species differentials pre- 

 dominates under these conditions over that of other antigens contained in 

 the cells. 



We may then conclude that not only in the cells of an organism, but also in 

 its blood serum, there are present species differentials which can function as 

 antigens, and which, by means of experimentally produced immune sub- 

 stances, can readily be demonstrated ; also, that these differentials show a 

 gradation corresponding to the systematic relationship of the organisms. 

 Moreover, we have found that the cells of an organism and its normal blood 

 plasma contain species differentials which are mutually adapted to each 

 other; but a graded relationship between the organismal differentials of the 

 cells and bodyfluids belonging to different species and orders of animals, which 

 would correspond to the systematic relationship of the animals, cannot as a 

 general rule be demonstrated in the interaction between sera and cells. 



In these investigations, which concern the differentiation of different species 

 by serological methods in animals, substances and cells, which are constituents 

 of the blood, have been used in most experiments. While in plants substances 

 which are present in young tissues may also serve as species-specific antigens, 

 it seems to have been difficult, so far, to extract from organs of higher 

 animal species antigens which, after injection into other species, would give 

 rise to the production of precipitins or of complement-fixing substances. Such 

 antigens, if present at all, are found only in very small quantities and show 

 only a slight degree of specificity. But, it may be suspected that these 



