SPECIES AND " STRAINS " 149 



form growing in rectangular masses), Nitrobacter (oxi- 

 dising nitrates in soil), etc. 



The species of bacteria are often called after the name 

 of the discoverer, or from their form, colour, activity 

 or habitat : in the case of the pathogenic (disease 

 producing) forms, the species is often called after the 

 name of the disease, e.g. Bacillus tuberculosis, the form 

 causing phthisis and other forms of tuberculosis. Some 

 species are so closely alike in form that they can only 

 be distinguished by their effect upon the medium in 

 which they are cultivated, or upon the animal in which 

 they live. Thus Bacillus typhosus of typhoid fever 

 resembles so closely B. coli communis, a regular inhabi- 

 tant of the human intestine, that they can only be 

 distinguished by the wide difference in their actions 

 (for instance whether they can or cannot ferment 

 various kinds of sugars), the former producing a dan- 

 gerous disease through the substances it excretes. 



So far as we know the species of bacteria are as 

 constant as those of other organisms, like always pro- 

 ducing like, but different " strains " belonging to the 

 same species may differ rather widely in their activities ; 

 for instance one may be specially virulent as a disease 

 producer, while another may give rise to quite mild 

 symptoms, so that, as in the higher plants, we have 

 what are called " aggregate species " and " elementary 

 species" (pure strains). How far strains of widely 

 different activities are produced solely by the effect 

 of changed conditions, we do not certainly know. 

 Changed conditions undoubtedly do produce great 

 differences ; thus by growing Bacillus coli in a culture 

 medium containing a trace of carbolic acid it is easy 

 to obtain a race which is much more resistant to the 

 disinfectant action of the phenol than is the normal 



