It has already been mentioned that we are not quite certain whether all the species, genera, or even orders of bacteria are natural divisions, or whether the same organism under various conditions of nutrition and development may not present such different appearances as to be included in different orders and under different names. Yet this is a matter of very great importance in regard to the causation of disease, for if it be true that organisms which are usually innocuous may undergo an opposite process to that which occurs in anthrax bacilli by cultivation, and may in certain conditions of soil be changed from innocuous into pathogenous forms, we can understand how diseases may appear to originate de novo.

1 Cash: Proceedings of the Physiological Society, Dec. 12, 1885. Journal of Physiology, vol. vii.

It has been stated by Naegeli that bacteria may be so modified by cultivation as to form entirely different fermentative products. Thus he says that the bacterium which produces lactic acid fermentation in milk may be changed by cultivating it in extract of meat and sugar, so that it will no longer produce a lactic but an ammoniacal decomposition in milk. He considers also that innocuous may be transformed into virulent bacteria, and back again into an innocuous form, and Buchner thinks that he has succeeded in transforming the ordinary hay-bacillus (bacillus sub-tilis) into anthrax bacillus by cultivating it for a number of generations in Liebig's meat extract, peptone, and sugar. This observation is denied by Klein1 and others, but observations which partly support Buchner and partly Klein have been made by F. Kohler,2 who finds that while the ordinary hay-bacillus (bacillus subtilis) is not altered in its appearance by repeated cultivations, it acquires a progressive virulence which renders it so fatal to animals as to resemble the anthrax bacillus in its deadly properties.

H. C. Wood and Formad3 have also come to the conclusion that the micrococci found in diphtheria resemble those on furred tongues in all respects excepting in their greater tendency to grow. When cultivated successively, they lose their contagious power and grow less readily. These authors, therefore, consider that circumstances outside the body are capable of converting the slower growing or common micrococcus into the rapidly growing micrococcus of diphtheria, which, when cultivated again, reverts to the common type.