Report to the Water Research Committee. 



423 



fact that pathogenicity, like the other characters, is variable, as is 

 abundantly shown by Dr. Kanthack's experiments. 

 In conclusion, the results show that — 



1. Very many forms occur in the Thames, some of which are 

 pathogenic under certain conditions. 



2. The " species " of the descriptive hand-books — principally 

 medical — are frequently not species at all, in the botanical sense, but 

 varieties, or growth-forms, the distinctive characters of which are 

 not constant. These so-called species need revision and grouping 

 around types, which may turn out to be the true species. 



3. The characters derived from the behaviour of colonies are not 

 sufficient for the determination of species, and how far they may be 

 employed in conjunction with other characters will only be eluci- 

 dated by advances in our knowledge of the way the colonies are built 

 up by the growing bacteria on the given media. 



4. The effects of definite changes in the environment on the media 

 are of great importance, but have hardly been noticed as yet. Plate- 

 colonies on gelatine, for instance, develop quite differently, according 

 to the condition of- the gelatine ; so that a feeble and slow-growing 

 bacterium produces colonies quite unlike those developed by the same 

 species when vigorous and quickly growing, not only owing to its 

 peculiarities of growth as a feeble form, but also because the gelatine 

 has altered during the intervening period. 



5. The effect of changes of the environment on the growing 

 organism itself is recognised as important. 



6. With especial reference to the Thames bacteria, the past history 

 of the organism isolated from the river implies causes of variation. 

 The river water is a poor nutritive medium, and the organism is 

 exposed to great changes of temperature, light, movement, &c, during 

 its sojourn therein. Consequently the time it has been in the river 

 affects the behaviour of the organism when isolated, just as we know 

 that a bacterium is affected by the previous conditions of its culture 

 in other media. Hence two colonies on a plate may look very differ- 

 ent, and yet belong to the same species, one being developed from a 

 cell that had been many days or weeks in the water, the other from 

 one that had only been there a few hours. It may need weeks or 

 months of cultivation under constant conditions to establish the 

 identity of the two. 



VOL. LXT. 



