vi PREFACE TO THE GERMAN EDITION. 



what are at first sight very dissimilar biological processes is really 

 a conception of the simplest kind. 



The toxic metabolic products of bacteria, the artificially produced 

 bacteriolysins, haemolysins, and cytotoxins, and the majority of the 

 ferments, probably always produce their effects by the co-action of 

 two active groups in the molecule. One of these effects the union 

 with the substance to be acted upon, while the other really produces 

 the characteristic effect. 



It is not surprising, in view of the enormous multiplicity of the 

 vital phenomena, that this simple principle exhibits the greatest 

 variations in individual cases. Certainly this corresponds entirely 

 to what we constantly observe in the domain of biology. The cell, 

 for example, occurs as a type in every living form, from the lowest 

 plant to the highest animal. In principle it is ever the same; in the 

 details of its structure, however, it is of endless variety. 



But even from such complex phenomena as are exhibited, for 

 example, by the artificially produced hsemolysins, it is possible to 

 develop the fundamental principles of my theory, and thereby give 

 a harmonious uniform explanation of the manifold phenomena with 

 their peculiar specific relations. 



My theory has developed essentially on the basis of chemical 

 conceptions. I have been more and more forcibly impressed with 

 the idea that in a study of the fundamental biological phenomena, 

 the significance of morphological structure is far less than the sig- 

 nificance of the chemistry involved. It is obvious that in order to 

 effect a given chemical process certain mechanical conditions must 

 be fulfilled. In other words the production of any chemical action 

 necessitates the presence and the suitable arrangement of apparatus. 

 The essential feature, however, is neither apparatus nor form, but 

 the constituents involved; for without changing the apparatus 

 hundreds of different combinations can be effected according to the 

 components employed. Similarly in biology I believe that the morpho- 

 logical arrangement of the organs and cells is not the essential feature, 

 but that this is rather to be sought for in chemical differences of the 

 constituents. 



I am convinced that the influence exerted by my theory will 

 extend far beyond the limits of pure immunity studies, and that it 

 is of considerable significance for an appreciation of vital phenomena. 

 Furthermore, I believe that the theory is of great value in studying 

 certain phenomena which dominate all life, namely, intracellular 



