BIOLOGICAL INDIVIDUALITY OF TISSUES 331 



very nearly identical with one another. Thus the casein of human 

 milk has been shown by Abderhalden to be chemically identical with 

 the casein of goat's milk, in so far as the relative yields of the various 

 amino-acids enable us to judge. Similarly the serum-albumins and 

 globulins of goose-blood are identical with those of horse-blood, and 

 the investigations of Abderhalden together with more recent analyses 

 by Gortner and Wuertz have shown that within the closest approxi- 

 mation attainable by present methods of analysis the amino-acid 

 yields from the fibrins of ox-blood, horse-blood, sheeps' blood and the 

 blood of swine, are all identical. Yet when the blood or blood- serum 

 of any species of animal is injected into the circulation of another 

 species it is treated as a foreign intrusion, and results in the appear- 

 ance of specific "antibodies" such as the Hemolysins or the Precipitins 

 which react with the blood of the species injected, but with no 

 other. Thus if a rabbit be injected repeatedly with human blood- 

 serum, the serum of this rabbit acquires the abnormal property of 

 causing a precipitate to form when it is mixed with human serum. 

 It makes no difference to the result what human being may furnish 

 the serum, but if we employ sera from other and unrelated mammals 

 we obtain little if any precipitate after mixing with the serum of the 

 immunized rabbit. With the sera of related species some precipitate 

 will be obtained, but it is not so abundant as that which is yielded by 

 human serum. The relationship of man to the primates was thus 

 established upon a quantative basis by Nuttal, to whom the following 

 measurements are due: 

 Anti-human serum mixed with : 



Blood of : Amount of precipit ate. Percentage. 



Man 0.031 100 



Chimpanzee 0.040 130 1 



Gorilla 0.021 64 



Ourang 0.013 42 



Dog 0.001 3 



Cat 0.001 3 



Tiger 0.0005 2 



Ox 0.003 10 



Sheep 0.003 10 



Guinea-pig . . 0.000 



Rabbit ..; 0.000 



Kangaroo (Macropus bennetti) . 000 



In a similar manner, if a rabbit be immunized against the serum of 

 some other vertebrate than man, the serum of the rabbit so treated 

 will develop a precipitin for that species and its near relatives, and not 

 for other vertebrates. The blood-serum of each species, and in fact the 

 tissues and tissue-fluids in general of each species are so many separate 

 Antigens, producing in immunized animals antibodies which may in 

 certain cases be related to one another but which are clearly not in 



1 The estimate of the quantity of precipitate yielded by chimpanzee-serum was 

 much too high, because, as occasionally happens, the precipitate did not settle properly 

 and its true value could not be estimated. 



