74 ANALOGY OF DISEASE. [introd. 



Species arise through discontinuous transition from one state of 

 mechanical or chemical stability to another state of stability, there 

 nevertheless remain large classes of discontinuous variations, and 

 of Specific Differences still more, whose Discontinuity bears no 

 close analogy with these. To these phenomena inorganic Nature 

 offers no parallel. We may see that they are discontinuous and 

 that their course is in some way controlled, but as to the nature 

 of this control we can make no guess. 



Though the resemblance may be misleading, it is neverthe- 

 less true that in living Nature there are other phenomena, those 

 of disease, which present a Discontinuity closely comparable with 

 that of many variations. In problems of disease we meet again 

 the same problem which we meet in Variation, namely, changes 

 which may be complete or specific, though occurring so suddenly 

 as to exclude the hypothesis that Selection has been the limiting 

 cause. All this is familiar to everyone who has considered the 

 problem of Species. 



For though, like discontinuous variations, the mauifestations 

 of specific disease are not always identical, but differ in intensity 

 and degree, varying about a normal form, still these manifestations 

 may be specific in the sense in which the term is used with reference 

 to the characters of Species. If we exclude those diseases whose 

 specific characters are now known to be the result of the invasion 

 of specific organisms, there still remain very many which are known 

 and recognized by definite and specific symptoms produced in the 

 body, though there is as yet no evidence that they are due to 

 specific organisms. [Of course if it were shewn that these diseases 

 also result from the action of specific organisms, they then only 

 present to us again the original problem of Species ; for if the 

 definiteness, or Species, of a disease is due to the definiteness, or 

 Species, of the micro-organism which causes it, the cause of that 

 definiteness of the micro-organism remains to be sought, and we 

 are simply left with a particular case of the general problem of 

 Species.] But in the meantime we can see that the manifestations 

 are specific ; and while we do not know that they result from causes 

 themselves specific, the nature of the control in obedience to which 

 they are specific is unknown. 



The parallel between disease and Variation may be mis- 

 leading, but this much at least may fairly be learned from it : 

 that the system of an organized being is such that the result 

 of its disturbance may be specific. And in the end it may well 

 be that the problem of Species will be solved by the study of 

 pathology ; for the likeness between Variation and disease goes 

 far to support the view which Virchow has forcibly expressed, 

 that " every deviation from the type of the parent animal must 

 have its foundation on a pathological accident 1 ." 



1 R. Virchow, Journal of Pathology, i. 1892, p. 12. 



