A GENERAL RESUME OF IMMUNITY. 503 



tory experimentation. At the risk of being considered by some 

 readers as not possessing a sufficiently generalizing mind, I must 

 admit that I have been led to make my most important discoveries 

 by yielding tractably to the impulse of facts, by letting myself be 

 moved by my data without attempting to discipline them or sub- 

 ject them to systematic ideas of my own. This is quite evident to 

 one who follows the evolution of my researches. For example, I 

 have frequently been asked how I came to discover the law of bac- 

 teriolysis (that is to say the collaboration of two substances, alexin 

 and sensitizer), or how, following that deduction, I had the idea of 

 immunizing animals against the blood of an alien species or against 

 milk (hemolytic sera and precipitins for albuminous substances), 

 which enabled me to demonstrate the idea that the organism, by 

 employing the same mechanism and by the same proceedings, 

 immunizes itself against elements that differ markedly from bac- 

 teria; that is to say, the production of antibacterial antibodies 

 simply represents the application, in the struggle against infective 

 agents, of a faculty which the animal would have possessed even if 

 contagious disease had not existed. I may be permitted then to 

 note how these researches have been related to one another, and it 

 will be evident that no guiding theory has been necessary. 



Pfeiffer had just discovered lysis of the cholera vibrio in the peri- 

 toneum of guinea-pigs, and Metchnikoff had shown that, contrary 

 to Pfeiffer's opinion, this phenomenon occurs in vitro as well as in 

 the living animal; all that was necessary was to mix a little of the 

 peritoneal exudate from a normal guinea-pig with cholera vibrios 

 to which a trace of cholera serum had been added. I wondered 

 whether the exudate could be replaced by fresh defibrinated blood 

 from a normal animal. I found that it could ; vibrios mixed with 

 it in the presence of cholera serum showed granular transformation. 

 The question then was whether it was the cells or the serum in the 

 defibrinated blood which produce this effect, and experimentally I 

 found that the serum was the important substance and that neither 

 red nor white corpuscles were necessary. I could definitely dis- 

 card the idea of cellular participation. But why was fresh normal 

 serum necessary? The cholera serum, as we then employed it in 

 experiments, usually came from a stock that had been kept for some 

 time, or, as frequently happened, had been heated to 60 degrees. 



