178 THE KINGDOM OF MAN 



With regard to immunity, it must always be remem- 

 bered that we are liable to confuse two different condi- 

 tions under this one term. An animal may be said to be 

 immune to a blood-parasite because that parasite is 

 actually unable to live in its blood. On the other hand 

 an animal is often said to be immune to a parasite when 

 the parasite can and does flourish in its blood or tissues 

 but produces no poisonous effect. A more precise nomen- 

 clature would describe the attacked organism in the first 

 case as " repellent," for it repels the parasite altogether ; 

 in the second case as " tolerant," for it tolerates the 

 presence and multiplication of the parasite without suf- 

 fering by it. 



We have yet to learn a good deal more as to the 

 repulsion and the toleration of the trypanosome parasites 

 by mammals and man. Still more have we to learn 

 about the life-history of the trypanosome. At the 

 moment of writing, absolutely nothing has been ascer- 

 tained as to the life-history of the trypanosomes of mam- 

 malian blood, except that they multiply in the blood by 

 longitudinal fission. Our ignorance about them is all the 

 more serious since other trypanosomes, discovered by 

 Danilewsky in birds, have been studied and have been 

 shown to go through the most varied phases of multi- 

 plication and change of size and shape, including a 

 process of sexual fertilisation like that of the malaria 

 parasite, to which, indeed, it now seems certain the 

 trypanosomes are very closely allied. 



It is to Dr. Schaudinn, 1 that we owe a knowledge of 

 some most extraordinary and important facts with regard 

 to the trypanosomes parasitic in the blood of the little 

 stone-owl of southern Europe (Athenenoctua). These facts 

 are so remarkable that, were Dr. Schaudinn not known 



1 Dr. Schaudinn died in 1906. He was only 35 years of age. 



