336 THE CAUSATION OF DISEASE. 



convenience, leave out of consideration those fundamental 

 particulars by which the different members are bonnd together 

 into a common class, and concentrate our attention on those 

 points only which are the criteria of the subdivisions, and it 

 is allowable to regard these differences as separate and inde- 

 pendent entities — call them species, varieties, or what we 

 choose. In like manner the pathologist takes a class of indi- 

 viduals — let us say, man — suffering from various diseases, and, 

 as every disease is probably characterized by certain structural 

 changes, these are natural variations. Here, also, we may 

 consider the structural peculiarities apart from those structural 

 characters which bind men together into one common class, and 

 thus we may speak of the different species of disease. 



My object is to draw an analogy, as far as possible, between 

 physiological variations and pathological variations, and to 

 show that, just as physiological peculiarities of structure are to 

 the biologist the criteria of his classification into species, 

 varieties, and so forth, so pathological peculiarities of structure 

 will be found to be the only rational basis for pathological 

 classification. 



How far it is possible to subdivide the several classes of 

 disease I will not attempt to decide. I do not wish to 

 push the analogy too far. All I want to emphasize is the fact 

 that when we use the term species in reference to disease, 

 we are simply referring to that structural state which constitutes 

 the disease, abstracted, as it were, from the rest of the body, 

 and in this sense, but in this sense only, it is allowable to 

 regard the disease as a separate entity, just as different classes 

 of animals or plants are separate entities. 



But if we now further compare the divisions of the biologist 

 with those of the pathologist, we shall be struck with a very 

 marked difference between them, one so great, indeed, as 

 to make us wonder how disease can be classified at all. 

 Let us instance the so-called species, the lowest accurate 

 subdivision of the biologist. In ordinary organic evolution 

 there are constant factors at work tending to fix the species. 

 Each is more or less perfectly adapted to its E, and if the E 

 remained exactly the same, would undergo no alteration, seeing 

 that perfect equilibrium (adaptation) between -S and E must 



