884 TEXTBOOK OF ZOOLOGY 



blood group contains neither, another contains one substance, a third 

 the second substance only, while a fourth group contains l)oth sub- 

 stances. Serum of the first group will agglutinate corpuscles when 

 mixed with any of the other three. The groups with the single sub- 

 stance will agglutinate corpuscles in blood having only the other 

 substance or blood with both substances. The serum from blood 

 with l)oth substances Avill not cause agglutination in any of the 

 others. 



In lower mammals some similar blood groups have been found, 

 but it is only in the apes that the groups correspond to those in the 

 human. This is an indication of the rather close relationship of 

 these animal groups. Whether in human or other animals this blood 

 characteristic is permanent in the individual and it is hereditary. 



Serum (blood minus cor^Duscles and fibrinogen) studies also estab- 

 lish certain relationships among animals. If small quantities of 

 human blood serum are introduced at intervals into the blood 

 stream of a rabbit, in time there will be developed in the blood of 

 the rabbit a substance (antibodies) which, when mixed with normal 

 human blood will cause precipitation of the proteins here. Serum 

 from such a rabbit is called antihuman serum. When this serum is 

 mixed with serum in a certain dilution taken from human, chimpanzee, 

 gorilla, or monkeys, it will cause precipitation. If the dilution is 

 increased there is no precipitation when mixed with monkey serum; 

 at still higher dilution there is no precipitation when mixed with 

 gorilla blood ; and higher none for chimpanzee blood, until finally none 

 for human blood. On the basis of these sera precipitation tests the 

 chimpanzee is closest in its relation to man, then gorilla, and then 

 the monkeys. 



In making similar tests on other vertebrate groups it is found that 

 crocodiles are more closely related to birds than are the other 

 reptiles; also among reptiles, that snakes and lizards are more closely 

 related to each other than to turtles. Too, crocodiles show a closer 

 relation to turtles than to the other groups of reptiles. Blood 

 studies of the various groups of vertebrates indicate that there is 

 more similarity in blood of closely related forms than of others. 

 At the same time, it is seen that a chemical relationship persists 

 in the blood throughout the chordate phylum. 



The hreeding of plants and animals through long series of genera- 

 tions of domestication and laljoratory experiments has yielded much 



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