416 



Prof. II. Marshall Ward. 



Others have been referred with less certainty to less well known 

 iforms, such as Bacterium urece (Jaksch), B. fulvus (Zimm.), B. 

 •aureus (Adam.), Ascococcus (Cohn), Micrococcus cameus (Zimm.), and 

 M. candicans (Fl.). 



The remainder are either new, or only doubtfully identified as 

 already described forms, or evidently varieties of some of the fore- 

 going. 



During the progress of the cultures, large numbers of coloured 

 drawings were made, with the intention of affording means of identi- 

 fication, but it was found that variations are so numerous and so 

 large, that some study of these variations had to be undertaken. 



This led to an investigation of the growth of the colonies in 

 gelatine and other media, and an attempt to explain why, and how 

 far, the colonies vary in culture. This necessitated a careful exami- 

 nation of the factors concerned in the development of the shapes, 

 markings, movements, and so forth of the colonies themselves, and 

 to a classification of the characters furnished by these colonies. 



One outcome of the above studies was the conviction that two sets 

 of factors are at work in causing the variations found in the colonies. 

 First, the slight variations in the food-materials, temperature, 

 moisture, &c, which cannot be avoided, however carefully the work 

 is done ; and, second, variations in the bacterium cell itself as it 

 -comes from the river, owing to the exigencies it has been subjected 

 ifco during its sojourn there. The water of the river is, in fact, a very 

 dilute and indefinite food- solution, and just as changes occur when 

 we remove a bacterium from broth to milk or to gelatine, so do such 

 result when we transfer from the river to these media, and the 

 changes induced in all cases depend on how long the bacterium has 

 been in the one medium or the other, as well as upon other factors. 



The river water is a very poor food medium, and so we cannot be 

 surprised that in many cases the recently isolated bacteria behave as 

 weakened forms ; the recognition of these enfeebled varieties suggests 

 •explanations of many of the bad " species " in the literature of water 

 bacteria. My work goes to show, not that species cannot be made 

 out, but that the limits of the species are, in most cases, far wider than 

 is assumed in descriptions — in other words, that many so-called species 

 in the books are merely varietal forms, whose characters, as given, 

 are not constant, but depend on treatment. How far this is true for 

 any given case will have to be tested on the particular form in 

 question. 



Very slight variations in rapidity of growth of the individual 

 bacterium, its power of liquefaction, pigment production, capacity of 

 fermenting, and so on, lead to comparatively very great differences in 

 the appearance of the colonies formed in a given time on, or in, a 

 medium like gelatine, the composition, aeration, hygroscopicity, 



