THE CAUSATION OF DISEASE. 1 79 



The external-body-E is not only complex ; it is also un- 

 stable. By a complex E is meant one that embraces many 

 different conditions ; by an unstable E, one that is not con- 

 stant from day to day, or even from moment to moment. 



The complexity of the external-body-E " in a civilized com- 

 munity is shown by the fact that every separate calling has 

 its own peculiar E ; while its instability can be readily realized 

 by remembering that, even in the case of unicellular organ- 

 isms, it is impossible for any two individuals to be " similarly 

 circumstanced in respect to incident forces." 



Structure. — In considering the part played by S in disease, 

 it is necessary to bear carefully in mind the fact that we can 

 never exclude it from the sphere of causation, seeing that 

 the causation of any particular disease is represented by the 

 formula S + E. 



When the S is normal (we may, for convenience, assume 

 an ideal normality) we are justified, from a practical point 

 of view, in excluding it from causation in certain cases, as 

 when an individual contracts scurvy through insufficiency of 

 vegetable diet. But, even viewing the matter from a strictly 

 practical standpoint, it is by no means easy to do this in all 

 cases where the S would certainly be accounted normal. If, 

 for instance, two individuals, enjoying perfect health, be 

 exposed for the first time to the scarlatina virus, and the one 

 contracts the disorder while the other escapes, it is obvious 

 that the S must be taken into practical account in stating 

 the cause of the disease. This one case, indeed, shows us that 

 the inclusion of S in the causation of all diseases is not a mere 

 logical quibble. 



Seeing, then, that the S takes a share in the causation of 

 all disease-processes, it behoves us to inquire most carefully 

 into its causation. 



The nature of S depends upon two great factors — (i) 

 Heredity ; (2) Environment. 



As regards the influence of E upon S, this is well shown 

 by the different ways in which different occupations mould 

 the individual. In estimating this influence, we must, be it 

 remembered, start at a period antecedent to the birth of 



