THE CAUSATION OF DISEASE. 335 



clencies are not wrought through E, then let me point out that the 

 above proposition is yet to a large extent true. Although the 

 forms of disease may undoubtedly be modified by the blending in 

 the offspring of disease-tendencies derived from the parents,* yet 

 it can very rarely happen that disease originates in this way. I 

 have suggested f that it may occasionally happen that a con- 

 dition of body rendering it liable to respond pathologically to 

 an average healthy E, or even leading to pathological states 

 directly, as in certain reversions — e.g., persistent branchial clefts 

 — may result from the crossing of two unlike individuals. In 

 the former instance, however, disease is set up by the E, which 

 must therefore be looked upon as the active cause, whether it 

 be markedly pathogenic to the community at large or not ; 

 and as regards the second class of disorders, they are very 

 rare, and even they, in almost all instances, result from pecu- 

 liarity in the ante-partem E. 



Seeing that the old notions concerning the nature of disease 

 cannot be entertained, but that it must be regarded as an 

 abnormal inter-action of S and E, we are prepared for the fact 

 that diseases do not adhere rigidly to definite types, but are, 

 on the contrary, ever variable. If no two S's and no two E's 

 are the same, how is it possible to get any two cases where 

 S + E = the same ? To assume that every disease is fixed 

 and unalterable is to assume not only that S never varies, 

 but that each form of specific mal-E is fixed and unalterable 

 also, both in respect of nature and of quantity — an assump- 

 tion which is obviously absurd. 



Our next point is how far it is possible to classify dis- 

 eases. Diseases, as we have seen, are natural variations, and, 

 consequently, the subject must be considered from the broad 

 point of view of biology. Let us ask how the biologist pro- 

 ceeds to classify ? A number of living organisms, agreeing in 

 certain fundamental particulars, by virtue of which they con- 

 stitute a particular class, % are taken. Certain differences are 

 observed among the various members, and by means of these 

 the class is split up into subdivisions. Now, we may, for 



* Vide Chapter X. Part I. f Pp. 78, 79. 



% I use this word in its simple logical sense. 



