INTRODUCTION 13 



alone have been described, and, in all probability, with greater 

 knowledge of the capacity of bacteria to accomplish morpho- 

 logical changes it should be possible to reduce this number 

 considerably. Organisms like Bacillus subiilis, for example, 

 must have been described under numerous names, for the 

 aggregate of its characters is never exactly the same from 

 one generation to another. 



The difference between these slight changes in the form 

 of bacterial organisms and pleomorphism is one of degree 

 not of kind. The change of form in pleomorphism is more 

 pronounced and probably more sudden in its appearance. 

 The sulphur bacteria are at a stage in which, compared with 

 the lower bacteria, a slight advance in evolutionary progress 

 has taken place. Developments that are mere tendencies in 

 the lower bacteria have become more or less crystallized in 

 them. In some the filamentous condition is normal for the 

 species, slime formation is a characteristic feature of develop- 

 ment, and the zoogloea condition with its enclosed mass of 

 bacteria makes its appearance. Such developments have not, 

 however, resulted in the stabilization of any one of the forms 

 of growth, and so in the life-history of some of the species it 

 is possible to find forms of growth of different morphological 

 features. The changes in form may take place in successive 

 generations or the differences may all appear in the same 

 generation. Or again, the culture may be unimorphic for 

 several generations and then, under the influence of certain 

 (at present unknown) changes, become pleomorphic. It is 

 evident that the proof of the existence of pleomorphism can 

 be obtained only by chance, or by the long-continued in- 

 vestigation of an organism, when at length some proof may 

 be expected. It cannot be demonstrated with regularity, 

 because the conditions w^hich bring about its manifestation 

 are at present unknown. 



The evidence for the existence of pleomorphism in the 

 sulphur bacteria may now be set forth. 



