VI PREFACE. 



the civilized world. For the present we probably can- 

 not do better than to adhere to the artificial classifica- 

 tion, based upon morphological characters alone, which 

 Cohn has given us. 



It must be remembered, however, that SPHERO- 

 BACTERIA micrococci are not always round; that 

 there is no well-defined line of demarcation between 

 the MICRO-BACTERIA and the DESMO-BACTERIA, between 

 the genus BACTERIUM and the genus BACILLUS, or be- 

 tween the last-named genus and the LEPTOTHRIX. 



The systematic naturalist, in his attempt to establish 

 genera and species among these lowly organisms, meets 

 with difficulties even greater than those encountered in 

 the classification of the higher cryptogams and flower- 

 ing plants. These difficulties arise from the multitude 

 of species and minute size of the unicellular organisms 

 under consideration ; from the various phases which 

 the same species may present at different epochs in 

 the life-history of the plant ; from the morphological 

 identity of species having different physiological char- 

 acters ; and, finally, from the influence of the environ- 

 ment in modifying both morphological and physiological 

 characters. 



Most writers continue to speak of the Bacteria as 

 fungi. The observations of the writer are, however, 

 in favor of the view of Cohn, that they are more nearly 

 related to the algae. It would be idle, however, to 

 discuss this question, as the border-line of these two 

 great classes of the VEGETABLE KINGDOM is not well 

 defined ; and here, as in the attempt to establish gen- 

 era and species, the systematic naturalist must ever 

 encounter the stubborn fact that NATURE is contin- 

 uous, and, consequently, that ell attempts at classifi- 

 cation are artificial. 



The writer ventures to hope that the rtsumt given in 

 the present volume will be found to fairly represent the 



