PLEOGENY 29 



same species (Coccobacieria septica), can be no longer held. Like Zopf's 

 far-reaching speculations on the cycle of forms among bacteria, they are 

 matters of history, and they are nothing more. The similar ideas of 

 Nageli have been unable to hold out in the face of more recent dis- 

 coveries, and there can be no doubt that the justification of Cohn's original 

 conception of the possible systematic classification of the bacteria is now 

 generally recognized. 



Less simple to decide than the question of pleomorphism is the question 

 of physiological change or pleogeny*. All bacteria possess to a certain 

 extent the power of living on different kinds of substrata, and the com- 

 position of these determines in a great measure the nature of the chemical 

 changes which the bacteria set up. According to the degree to which this 

 power is developed, we may conveniently divide the bacteria into two 

 classes, the monotrophi c and the polytrophic. Among the members of the 

 first group the requirements of nutrition are specific and clearly defined . 

 The composition of the substrata may vary only within very narrow limits, 

 and, as a result, the metabolic products also of these bacteria and the func- 

 tions they are able to perform are specific and well defined. As typical 

 examples of the monotrophic group may be mentioned the sulphur bacteria, 

 the nitrifying organisms, all the tnilv parasitic bacteria, a nd those found 

 in the root-nodules of the leguminosae. Among the great host of ferments 

 and putrefactive organisms, too, there are monotrophic forms with specific 

 properties. Such are the bacteria of the acetic, lactic, and some butyric 

 fermentations, and the bacteria which decompose urine (urea ferment). 

 Some of the butyric bacteria, on the other hand, are polytrophic, and, 

 besides possessing specific fermentative powers, are able to break up 

 albuminous compounds ; they are saprogenic as well as zymogenic. Others 

 of the same class are pathogenic also, and can live in the tissues of the 

 animal body (malignant oedema, quarter evil). We find, too, forms like 

 B. vulgaris with pronounced saprogenic (putrefactive) powers, which are yet 

 able to grow on non-putrefactive substances and cause them to ferment. 



Again, many saprophytic bacteria are unable to exist in the animal 

 body, while others (typhoid, cholera) are decidedly polytrophic in this 

 respect, and multiply rapidly in the tissues. It is not necessary to give 

 parallel examples of differences among other organisms. 



As might be expected, it is in monotrophic forms that the 'specific 

 characters,' both morphological and functional, are most sharply marked. 

 But they undergo no lasting change even in those bacteria which are physio- 

 logically the most versatile ; and the attainment in our cultures of new 

 varieties, varieties with acquired characters that are permanent and heredi- 

 tary, is not to be expected. We are able, it is true, to obtain cultures of 



* Detailed instances will be found in Chaps. V, XI, XII, XIII, and XV. 



