322 THE CAUSATION OF DISEASE. 



guarantee an equality of E, all brothers and sisters would, save 

 for sexual differences, be exactly alike. All other differences 

 are, as we have seen, due to differences of E, for E is capable 

 of modifying S. Let the letter S stand for the structural mean 

 to which the sons or the daughters of any given couple tend, 

 and let s 1? s^,, and s 3 represent the alterations which the 

 different E's work upon this S ; then Ss p Ss 2 , Ss 3 will represent 

 the structure of the several children. 



Although it is convenient to speak of an inherited structural 

 mean, such a mean is, be it remembered, entirely ideal, 

 since it assumes an ideal E, and one which it would be impos- 

 sible to define scientifically. In attempting the definition of 

 such an ideal E, all one can say is that it should be as negative 

 as possible — one, namely, tending to alter the S in as slight a 

 degree as possible. 



The formulae Ss,, Ss„, &c, represent the actual structure of 

 any individual at any particular period of life — S representing 

 that part of an individual which is the direct outcome of here- 

 dity, and s, such modification in it as has been brought about 

 by E. Now it is important to observe that the nature of s is 

 very largely dependent upon the nature of S ; I have already 

 remarked how particular Darwin was to emphasise the fact 

 that a variation depends more upon S than upon E ; and inas- 

 much as S, or the structural mean, is strictly the outcome of 

 heredity, we see what an enormous part heredity plays in 

 determining Ss, or the actual structure at any given time. 



Disease being the abnormal inter-action of the individual 

 and his E, it follows that Ss + E will express the causation 

 of any particular disease. Now the nature of Ss determines 

 whether or not it shall respond pathologically to a particular E, 

 and therefore heredity must play a large part in the causation of 

 every disease. Let me illustrate this by a very simple example. 

 The diversity of diseases among different species of animals 

 depends of course very largely upon differences of E — for, as 

 we have seen, every species has its own specific form of E ; 

 but the chief cause of the diversity is certainly difference of 

 structure. Thus, a child and a dog are exposed to a distemper- 

 giving E : the one contracts the malady, the other escapes. 

 This difference of response is obviously due to differences in 



