THE CAUSATION OF DISEASE. 343 



of the sum of the inter- act ions of the several cells composing 

 the organism and their individual environments. Where- 

 fore, in speaking of E, we must remember that we have 

 to deal, not only with an external-body-E, in the shape of 

 food, air, and so forth, but with an internal-cell-E also. This 

 latter is, in great measure, dependent upon the external- 

 body-E, but it is also very largely under the control of the body 

 itself ; so closely, indeed, are the several tissues of the body 

 bound up into one physiological whole that it is quite im- 

 possible to get a serious change in one part which shall not 

 lead to a more or less universal modification of cell environment. 

 And here let me remind the reader that a properly constituted 

 E not only includes certain positive conditions, such as the 

 presence of oxygen and proper food, but also certain negative 

 conditions, such as the absence of waste products. 



Now, disease being an improper inter-action of S and E, 

 it follows that the cause of disease must depend upon — (1) 

 peculiarity of S ; (2) peculiarity of E ; or (3) peculiarity 

 of both. Wherefore, in seeking for the causation of dis- 

 ease we must first inquire into the causation of S and E. 

 As regards S, at least -^ of its causation must be referred 

 to heredity, and there remains but one other factor capable of 

 affecting it — viz., the environment. The influence of E may be 

 summed up in the following proposition : whatever structural 

 peculiarity an individual exhibits which is not the pure out- 

 come of heredity is wrought through E. I have elsewhere shown 

 my reasons for what may seem a somewhat dogmatic asser- 

 tion, and we need not therefore discuss the matter again here. 

 Heredity and E are, then, the two great moulding powers. Let 

 S represent such of the structure of an individual as is the pure 

 outcome of heredity, and s that part of him which has been 

 shaped by E, then Ss expresses the causation of structure at 

 any particular time. If, now, the individual come into con- 

 tact with an agent producing disease in him, the formula 

 Ss -f E represents the causation of that disease. It represents, 

 in fact, a set of material conditions which necessarily issue in 

 unhealthy inter-action. Let us, for instance, suppose it proven 

 that cancer is due to parasitic action, then, if E stand for 

 the environment of the individual (including the parasite), 



A A 2 



