CH. XXI] PATHOGENIC ORGANISMS 535 



by direct experiment, but will only limit myself to reference to 

 the researches of Schroter, Cohn, and Wernich on that group 

 of micro-organisms known as pigment bacteria, i.e. bacteria 

 which only under certain conditions, notably temperature and 

 soil, produce definite pigments' (Cohn's Beitrdge zur Biologie 

 d. Pflanzeri) ; to those of Hansen (Carlsberg Laboratory) on 

 yeast ; to those of Neelsen on the bacilli producing the blue 

 colour of milk, the bacillus syncyanus {Beitr. zur. Biol. d. 

 Pflanzen, iii. 2, p. 187); to the works of Toussaint, Pasteur, 

 Chauveau, Koch, and others on the bacillus anthracis ; 

 Arloing, Thomas, and Cornevin on the bacillus of symptomatic 

 charbon ; of Koch on the bacillus of tuberculosis ; of Israel 

 on actinomyces, and many others ; and particularly would I 

 refer to the many valuable suggestions and considerations 

 expressed by v. Nageli in these respects in his book, Die 

 niederen Pilze, Miinchen, 1877 and 1882. 



While from these observations it would appear that both 

 saprophytic and parasitic micro-organisms are capable of 

 suffering modifications in their morphological and physio- 

 logical behaviour, sometimes small, sometimes great and 

 pronounced, it is nevertheless still an open question whether 

 an organism which under ordinary conditions is only 

 associated with septic changes in dead organic material, 

 and which cannot under these ordinary conditions grow and 

 multiply within the living body, can, under certain extra- 

 ordinary circumstances, acquire the nature of a parasite, 

 become endowed with the power of growing and multiplying 

 within the body of a living animal, creating there a patho- 

 logical condition, inducing there an infectious disease. 



It is a common laboratory experience that many specific 

 microbes, owing to medium, temperature, &c., or to successive 

 subcultures, while retaining their general morphological 

 characters nevertheless gradually change their physiological 

 action, becoming more and more attenuated, and ultimately 



