176 CLASSIFICATION OF THE MICRO-ORGANISMS. 



sents so many transitional appearances that they cannot 

 be employed as decisive distinctive characters. Here it 

 is evidently of importance to make use of the manifold 

 and valuable distinctions described above, those, namely, 

 which the bacteria show when growing on certain sub- 

 strata, especially on nutrient jelly. When sub-classes 

 have been successfully formed by the help of these 

 differences, the further subdivision of the relatively 

 small number of bacteria in any small group may be 

 made by the help of morphological characters, of be- 

 haviour with staining re-agents, of relation to oxygen, or 

 of the power of exciting fermentation or disease, and 

 thus we are ultimately able to recognise a given species by 

 the study of its various characters as compared with 

 other species. Of course the species thus formed can- 

 not in the slightest degree be regarded as natural 

 species or varieties; they have an entirely provisional 

 character, and are only meant to serve as a means 

 of rendering the further recognition of the bacteria 

 easy, and of paving the way for a natural classification ; 

 they have only arisen from, and are only suitable for the 

 Practical practical necessities. Hence the classification given 

 tiiaSioing the ^ ere res ^ s on that practical method which almost every 

 bacteria. one now employs who wishes to ascertain what bacteria 

 are present in any unknown mixture of fungi. The 

 first step is always an attempt at isolation by means 

 of gelatine plates ; the colonies which have developed on 

 the plates can then be easily investigated by making a 

 .__ microscopical preparation with a small quantity, and ascer- 



taining whether the colony consists of micrococci, bacilli, 

 or spirilla. It is almost always easy to decide this point, 

 because when we have a large number of similar indivi- 

 duals all at the same stage of development we have none of 

 those uncertainties mentioned above which might render 

 the diagnosis of an individual cell a matter of great diffi- 

 culty. When the chief division to which the species in 

 question belongs is thus ascertained, the appearance of 

 the colonies, both macroscopically and under low powers 

 of the microscope is investigated^ and then with the help 

 of a simple table (see p. 178) the relation of the species 



