MECHANISM OF PROTEIN HYDROLYSIS AND IMMUNIZATION 51 



Curiously enough the blood corpuscles of new-born rabbits 

 are much more resistant to foreign blood than are those of the 

 adult rabbit. But a certain degree of resistance obtains in all 

 animals, and this may be accentuated by introducing a very small 

 quantity of foreign blood serum, and from time to time repeat- 

 ing and increasing the dose. In this way the system of the 

 animal becomes to some extent immune to the poisonous effect 

 of the foreign blood, through development of what for want of 

 a better term is called an anti-serum. 



The blood of a rabbit that has attained this condition may 

 then be used in testing for the presence of the particular type 

 of blood that was used in developing the anti-serum. For ex- 

 ample, if human blood was the kind injected into the system 

 of the rabbit, the rabbit's blood will now serve as a test for 

 human blood. 



MAN'S REMOTE RELATIVES 



It appears, however, that the anti-serum thus developed, while 

 its most pronounced reactions will be given with solutions of 

 human blood, will also react in a less marked degree with the 

 blood of other animals. 



If successive drops of the anti-serum are introduced into one 

 test-tube after another, as in Dr. Nuttall's experiments, it will 

 be observed that in some tubes there is an immediate reaction, 

 resulting in a white precipitate. In other tubes the reaction 

 will set in only after some minutes; in yet others after hours; 

 and the remaining test-tubes will remain permanently clear. It 

 is these graded results that enable the experimenter to test the 

 blood relationships of the different animals. 



It is found, for example, that when a test is made with human 

 anti-serum, an immediate reaction is observed only in test-tubes 

 containing human blood. Less prompt and less marked reaction 

 occurs in the tubes containing the blood of the man-apes; still 

 milder reaction in the case of baboons, monkeys, and marmosets 

 in succession ; and a long delayed or altogether negative result 

 in all other cases. It is obvious how similar tests with other 

 types of anti-sera enable the experimenter to follow out the 

 relationships of different tribes of animals. 



Professor Nuttall's experiments comprised sixteen thousand 

 individual tests, with a total of at least 586 species mammals, 

 birds, reptiles, batrachian, fishes, crustaceans coming from all 

 parts of the globe. These experiments are in themselves highly 

 interesting; in their implications they are nothing less than as- 

 tounding. 



Doubtless some hundreds of thousands of years have elapsed 



