99 



be possible to find a series of characters which will be adhered 

 to in the strictest sense. It may even be frequently necessary 

 as a schematic aid to orientation, to divide organisms into 

 groups although it is very uncertain how stable the characters 

 are, and even though Nature provides even gradations and free 

 combinations so that none of the three requirements of a 

 rational grouping are present; and in the more favourable 

 cases one must usually be content with a good approximation 

 to constancy, discontinuity, and correlation, the characters being 

 proved to be stable only within certain limits, and the stepping- 

 stones between the different gradations of the individual quality, 

 not being absent, but only occurring rarely. 



These considerations are the most important influence in 

 all the. following investigations into the classification of the 

 haemoglobinophilic bacteria. 



The reason Pfeiffer's bacillus has been exclusively dealt 

 with so far is because it is the most important of the true 

 haemoglobinophilic bacteria, and we will therefore begin with 

 an analysis of this microbial group with the twofold object 

 in view: to define the boundaries of the group and to investi- 

 gate its inner „structure". 



There is frequently mention of other nutritive media as 

 well as those already discussed. As the whole question of 

 nutritive media will be dealt with in a special chapter, the 

 reader is referred to this for various details in connection 

 with the preparation of media, and only the most essential 

 information about the different media will be given, step by 

 step, as they come under notice. 



To begin with it will be advantageous to deduce a pro- 

 visional separation * in the form of a definition, of the bacteria 

 we are going to class as Pfeiffer's bacilli. In the course of the 

 investigation it will appear whether any of the points in this 

 definition should perhaps be modified, and what factors can 

 further be included in a general description of this species of 

 bacterium. 



1. As the presence of Pfeiffer's bacillus is investigated by 

 cultivation on various media containing blood or blood derivati- 



* The description on pp. 68—70 contains too many facts to be 

 used as a definition. 



