EVOLUTION OP VIRULENCE 131 



the access of an anginal attack, or of measles, diphtheria, which 

 was recovering, gained fresh malignancy and became rapidly 

 fatal. 



There is another micro-organism frequently found in the 

 mouth and saliva of healthy persons, namely, the Talamon- 

 Erankel diplococcus. May it not be that in both these cases 

 a natural intensification of the virus is brought about by catarrhal 

 and other affections of the upper respiratory and pharyngeal 

 mucous membrane, and that to such intensification, rather than 

 to the entry afresh of virulent micro-organisms, is to be ascribed 

 the onset of these diseases ? 



I am well aware that in setting in order a series of facts and 

 observations upon deviations from the normal the tendency is 

 peculiarly strong to see in such deviations the normal, to see in 

 the normal the exceptional, and it may be that in the preceding 

 pages I have not dwelt with sufficient emphasis upon the fact 

 that employing well-recognized and standard methods the typical 

 forms of bacterial growth are easily and most usually to be 

 separated out from cases of disease. Still I shall feel that these 

 pages have not been written in vain if I succeed in drawing 

 increased attention to the fact that the bacteria are organisms 

 acutely susceptible to changes in environment, that as species 

 they are far from presenting constant characteristics, and that 

 to a variability which may impress itself upon a greater or less 

 number of generations is to be ascribed, in part, the differences 

 between successive epidemics, between the successive stages 

 of one epidemic, and between individual cases of disease. 



REFERENCES 

 The following papers may also be consulted : 



Hansen (E. C). Annales de Micrographie, ii., 1889, p. 214. (" On the Produc- 

 tion of Varieties among the Saccharomycetes.") 



Metchnikoef. Annales de l'lnst. Pasteur, iii., 1889, pp. 61 and 265. (" On a 

 Remarkable ' Spiro Bacillus ' (Polymorphic) Parasitic in Daphnia.") 



Winogradsky. Annales de l'lnst. Pasteur, iii., 1889, p. 244. (" On Poly- 

 morphism.") 



Htjeppe. Berliner klin. Wochenschrift, 1884. (" On the Relationship of 

 Saprophytic to Pathogenic Bacteria.") 



Delepine (Sh.). Transactions, Pathological Society, London, 1891. (" Muta- 

 bility of Pathogenic Aspergillus, as also of the Characters of the B. Leprae.") 



Unna. Deutsch. Naturforscherversammlung, Halle, September 1891, and 

 Centralblatt f. Bakt. xi., 1892, p. 638. ("Races of the Favus Fungus 

 (Achorion Schonleinii).") 



