THE PRECIPITIN REACTION. IOI 



limits of error. Of the eight which failed to give correspondence 

 five involved horse serum and two involved dog serum. Of six 

 chicken reciprocal titrations three gave correspondence and three 

 failed to do so. Two of the latter involved horse serum. Clearly 

 the horse serum was aberrant in its reactions. Nuttall reports 

 similar results with horse serum. There may then be certain 

 animals whose blood sera are not suitable for studies of this kind. 

 If now only the beef, pig, sheep, and dog reciprocal titrations 

 are considered, out of fifteen pairs of values eleven gave agree- 

 ment, a rather high percentage. 



The entire series of reactions recorded in these titrations seems 

 to justify these conclusions: 



1. That there is a variation in the response of different animals 

 of the same species to the same protein. 



2. That in spite of this variation there is a general agreement 

 in the group reactions obtained with the majority of the antisera 

 tested. This correspondence is independent of the homologous 

 titers of the antisera compared (provided the antisera are suf- 

 ficiently strong to react with all the proteins tested) and may 

 be found even in antisera produced by such different animals as 

 rabbits and fowls. 



3. That in rabbit antisera there is an inhibition of response 

 to other rodent bloods which is absent in fowl antisera. 



4. That where there is a variation in the group reactions of 

 two antisera of the same kind it is possible to tell which is the 

 truer series of values by making reciprocal titrations. Only 

 those values which check within the limits of error of the re- 

 action may then be taken. 



DISCUSSION. 



The building of phylogenetic trees has practically ceased 

 today. This is not by any means because the problems of 

 animal relationships are believed to be finally settled. It seems 

 rather to be an expression of hopelessness as to the possibility 

 of solving these problems. It is true that there is general 

 agreement as to the relationships of animals within certain main 

 branches of the animal kingdom. But there is no such agreement 

 as to the affinities of many of the phyla of Invertebrates to each 

 other or to Vertebrates. But even within these main groups 



