CH. vii] VARIATIONS IN PATHOGENICITY 95 



distinguished by their pathogenicity. W. B. M. Martin (1911) 

 writes in this connection : " so far, in spite of the prevalence 

 of gonorrhoea and the periodical occurrence of great epidemics 

 of cerebrospinal fever, there is no satisfactory evidence that 

 the gonococcus ever causes a meningitis or the meningococcus 

 a urethritis. This is the more remarkable in that, on the one 

 hand, gonorrhoeal metastases are common enough elsewhere 

 and that, on the other, meningococci can frequently be isolated 

 from the urine of cases in which there is not the slightest 

 evidence of genito-urinary inflammation." He maintains that 

 the explanation must be in differences in "pathogenicity" on 

 the part of the two organisms and that such diflerences justify 

 our regarding them as distinct species. 



A study, however, of the pathogenicity of bacteria reveals 

 the fact that this, like every other character they possess, is 

 subject to variation — with respect to (i) the kind of animal 

 affected, (ii) the kind of symptoms caused and (iii) the kind 

 of lesions produced. 



I. As regards the first of these we have already shown 

 when speaking of virulence that the degree to which a 

 particular organism can cause disease in different species of 

 animals is subject to variation and can be artificially modified 

 {vide p. 81 et seq.). 



II. With regard to the second, variability in the symptoms 

 caused is displayed in several ways. 



1. In the first place; the same species of organism may 

 give rise in different cases to a totally different train of 

 symptoms. 



In manyinstances the explanation is obvious. Thesymptoms 

 naturally depend, to some extent, upon the particular organ, 

 ■either primarily or solely, affected in each case. The toxic 

 action of lead furnishes an analogy. Its absorption into the 

 body is followed by symptoms of arterio-sclerosis or of 

 peripheral neuritis or of renal disease according to whether 

 the blood-vessels or the nerves or the kidneys are primarily 

 affected. The pneumococcus, for example, may attack the 

 meninges, the lungs, the pericardium, the peritoneum or a 

 synovial membrane, and the difference in the symptoms in 



