IMMUNITY 227 



agents ; the brain extracted with ether loses much of its 

 neutralizing power, and the dye carmine neutralizes because it 

 contains lipoids derived from the cochineal insect (Metchnikoff). 



It is facts such as these which encourage investigators to 

 pursue the line opened up by Bordet's experiments, although 

 there is no absolute promise that they will find in this the key 

 to immunity. 



If we add now that phagocytosis is not incompatible either 

 with Bordet's theory or with Ehrlich's, it is true enough, but it 

 is not all the truth. It is not a question of reconciHng 

 theories. There are only two " theories," that of Ehrlich and 

 that of Bordet, which, with their conjectures, their uncertainty, 

 their attempts at explanation, and their continual state of 

 incompleteness, are striving to round off the positive doctrine, 

 the expression of undoubted facts, namely phagocytosis. 

 When a physiologist is studying digestion he founds his study 

 on facts which are essential and certain, such as the action of 

 trypsin and enterokinase ; this is no theory ; it is only when 

 he proceeds to interpret this action in terms of physics or 

 chemistry by fixations or combinations that he enters the 

 domain of theory. Similarly, it is no way of recognizing the 

 capital importance of phagocytosis to admit that antibodies 

 and other humoral properties are produced by the phagocytes. 

 The essential fact is the destruction of the microbes by 

 incorporation and digestion. Extraphagocytic destruction is 

 so much an exceptional case that it cannot even be brought in 

 as opposition. 



There would be no temptation to forget this fact if, instead 

 of limiting our attention to human pathology, we kept in view 

 the universality of intracellular digestion throughout the series 

 of living beings. Phagocytosis is quite different from a medical 

 theory. It is a doctrine as fundamental in general biology as 

 is the existence of the cell or the variation of species. 



Toxins and Phagocytosis. — The immunity of an animal 

 towards a toxin is not to be ascribed simply to the activity of 

 its body-fluid. We must take account of the body also in the 

 reaction. There are many facts indicating a similar state of 



Q 2 



