VARIABILITY AMONG BACTERIA. 31 



idea long held by agriculturists that the growth of a crop of beans or 

 peas fertilises the soil and improves subsequent crops. 



The Occurrence of Variability among Bacteria. The 



question of the division of the group of bacteria into definite 

 species has given rise to much discussion among vegetable 

 and animal morphologists. 



In 1872 Colin stated the opinion that bacteria showed as distinct 

 species as the other lower plants and animals. He recognised the 

 great divisions into which bacteria naturally fall when the forms under 

 which they appear are considered, but he carefully guarded himself 

 against the error that mere considerations of form are sufficient for a 

 proper natural classification of the group. In such a classification 

 the history of the whole life cycle of an organism, and especially the 

 course of its development, must be taken into account. Variations in 

 form occurring in particular cases have, however, been made the basis 

 for criticism of the statement that the large numbers of bacteria which 

 have been identified are really to be looked upon as distinct species. 

 The extreme case for the existence of variability was put by Nageli, 

 who held that "the same species in the course of generations might 

 present different morphological and physiological forms which might 

 give rise at one time to the souring of milk, at another to butyric acid 

 fermentation, at another to the putrefaction of albuminous matter, at 

 another to diphtheria, at another to typhoid, at another to cholera." 

 Such an extreme view was advanced before the elaboration by Koch 

 of methods by which growths of a single kind of bacteria without 

 admixture of any other variety can be obtained. Undoubtedly many 

 of the earlier observations were made on mixtures of different organisms. 

 Thus the upholders of the occurrence of great variability founded their 

 view very largely on observations of a group of organisms closely 

 allied to the bacteria which, from a peculiar pigment called " purpurin " 

 in their protoplasm, have been called the purpurin bacteria (German, 

 Purpurbakterien). From a more recent study of the group by 

 Winogradski, however, these seem to be a mixture of many different 

 species, each of which maintains during multiplication its characteristic 

 form. Practically no one at the present day holds that an organism 

 can appear now as a bacillus, now as a coccus, now as a spirillum. 

 Nor is such a view in any way supported by the occurrence in isolated 

 cases among the higher bacteria of coccus-like and bacillus-like seg- 

 mentation, such as we have seen to take place in the streptothrix 

 group. 



With regard to the bacteria as a whole we may say that 

 each variety tends to conform to the definite type of structure 



