340 THE CAUSATION OF DISEASE. 



maintain a certain degree of constancy, it is possible to recog- 

 nize diseases as belonging to a particular species, and to apply- 

 to them the term " specificity," with certain qualifications. No 

 one will doubt that rheumatism, tubercle, diphtheria, carcinoma, 

 and typhoid fever are distinct and definite diseases ; but even 

 these well-defined diseases deviate very widely from their classical 

 types, so much so, indeed, that they may be utterly unrecogniz- 

 able, in the truest sense of the term. But if this is true of such 

 well-recognized disorders, how much more is it true of others ? 

 Instance cutaneous and nervous diseases. Diseases of the skin are 

 so variable that they have completely baffled the most diligent 

 attempts at classification. Yet if a rare case come before the 

 specialist, he is not content unless he can refer it to one class in 

 his long category ; he is unwilling to regard it as a unique 

 disorder, as one the like of which has, perhaps, never occurred 

 before, and may never occur again. He might, I will venture 

 to say, learn a lesson from the horticulturist. Diseases are, 

 as we have seen, natural variations. Now, when a gardener 

 meets with a natural variation in one of his plants, he knows 

 very well that such variation may never have occurred before, 

 and may not be repeated, and in consequence, if the variation 

 be one favourable for his purposes, he is most careful to " fix" it, 

 as it were, to preserve the seeds, and to exert all his efforts to 

 accumulate and intensify the peculiarity during successive 

 generations of the plant. In like manner unique pathological 

 variations of the skin may occur. We have seen that variations 

 are more abundant among domesticated animals and plants, and 

 among civilized peoples than among organisms in their wild 

 u natural " state, and that this is largely due to instability of E. 

 Just as plants vary abundantly in an unstable E, and from the 

 crossing and recrossing of varieties, so may the human skin vary 

 abundantly in the unstable civilized E, and from the crossing 

 and recrossing of unlike individuals ; and, inasmuch as mal-E's 

 are numerous in civilized communities, there is no wonder that 

 pathological variations should be numerous, that they should 

 baffle the most careful attempts at classification, and that fresh 

 ones, quite unlike any which have before occurred, or which 

 may ever occur again, should from time to time appear. 



Then, as regards nervous diseases. Let us first instance the 



