300 GENERAL ZOOLOGY 



chimpanzee and human sera. If the sera are diluted still more, 

 a point is reached when the antihuman serum reacts only with 

 human serum. Thus on a basis of the susceptibility of the blood 

 proteins to precipitation by the antihuman bodies, the chim- 

 panzee ranks next to man, the gorilla next to the chimpanzee and 

 the monkeys farthest from man, all of which is in keeping with 

 the morphological findings. In general the precipitin tests 

 confirm the generally held view that apes are more closely 

 related to man than monkeys. 



Similar tests show that lizards and snakes are more closely 

 related to each other than to turtles, which in turn are more 

 closely related to crocodiles. When reptilian sera and bird sera 

 are tested, birds show a closer relationship to crocodiles. In 

 general when the serum reactions of two animals are tested with 

 the antiserum of a third, that animal whose serum in the higher 

 dilution produces a precipitate with the antiserum of the third is 

 more closely related to the third. From such experiments the 

 important fact emerges that the degree of relationship as ascer- 

 tained by ordinary methods of classification is borne out by 

 blood reactions. In some cases the precipitin tests have estab- 

 lished relationships between species when the usual morphological 

 criteria have failed or proved uncertain. 



Blood Groups. — Within a species, the blood of different 

 individuals may be classified into groups, depending upon the 

 conditions under which the serum of one individual will cause the 

 agglutination or clumping of the red blood corpuscles of another. 

 An understanding of the individual differences in properties of 

 human blood has become important in connection with blood 

 transfusions, because agglutination leads to the destruction of 

 the injected corpuscles and thus makes the transfusion ineffective. 

 When the blood of one individual is transferred into the blood 

 system of another, in some cases agglutination occurs and in other 

 cases not. Since agglutination is to be avoided, studies have 

 been made leading to the classification of human beings into 

 four blood groups known as 0, A, B, and A B. The designation 

 of the groups depends upon the presence or absence of one or 

 both of two substances A and B, located in the red corpuscles 

 and which are necessary for agglutination. Group contains 

 neither A nor B in its corpuscles. Group AB has both. The 

 principle underlying the agglutination reaction is simply this: 



