ABSTRACT OF PROCEEDINGS. lxxxvii. 



the number of feeders ; where initially many species are 

 present there often seems to occur a sort of selection which 

 results in the survival of those best suited by the food 

 material present. 



Bacteria are by no means indifferent as to the quality of 

 their nutriment, and from this point of view they may be 

 divided into three groups. The first group comprises species 

 such as the nitrifying bacteria, which feed almost entirely 

 upon inorganic material. They are capable of surviving in 

 water and play a certain part, but as they are non-parasitic 

 and do not call for removal, we need not consider them 

 further. The second group comprises the strict parasites 

 which feed only on material within the living bodies of 

 their hosts. They are incapable of prolonged existence in 

 water, and may also be set aside for the present. The 

 third group comprises species which can feed upon dead 

 organic matter and which are in many instances indifferently 

 parasites or not. This is the dangerous group from the 

 point of view of the water hygienist. Yet they are not all 

 capable of living in water because not only must they have 

 organic matter to feed upon, but individual species must 

 have it in a particular form. It is sufficient to appreciate 

 the fact that certain bacteria will not grow unless the food 

 material present is such as they are able to utilize. If 

 proteid were not present in water, the proteid bacteria 

 would not be able to live ; if proteid were originally present 

 but became by purification converted into other material 

 the proteid bacteria would perish of starvation, and similarly 

 as regards the other forms. It might happen that whilst 

 proteid was being reduced through the stages of amido- 

 compounds to ammonia, the water would successively be 

 dominated by different species, first the proteid bacteria, 

 then the amido-bacteria, then the ammonia bacteria would 

 in turn flourish and succumb. Pinal extinction would be 



