THE PHYLOGENY OF THE SULPHUR BACTERIA 227 



in its turn could serve as a starting-point for a further 

 advance. 



From the ease with which it is possible to effect slight 

 morphological and physiological changes in bacterial species it 

 may be argued that the rate of advance of bacterial organisms, 

 and probably of all lowly organisms, is greater than in more 

 complicated forms. Indeed, it is not beyond possibility that 

 some of the organisms described in the early days of Bacteri- 

 ology may have completely disappeared, and their places taken 

 by species that were not in existence at that time. 



The following factors appear to be those most concerned 

 in the evolution of the group : — ■ 



A. Slime Formation. — In varying degree all the sulphur 

 bacteria form slime from the outermost layers of the cell. In 

 Beggiatoa alba, even in motile individuals, a thin layer of slime 

 is always present. Under unfavourable conditions of growth 

 its amount may be considerable. 



In Thiothrix, on the other hand, slime development is 

 normal, and begins at an early stage in its growth. As the 

 slime subsequently hardens, and as the cells enclosed by it 

 continue their growth before the slime has completely har- 

 dened, the structure peculiar to this organism is developed. 

 The filament continues to grow inside a hollow sheath, and 

 its fission is limited to the apical parts that have emerged 

 from the sheath. The fissured fragments when short may be 

 regarded as the precursors of the conidia of more highly 

 developed plants. It is of interest to note that a similar line 

 of development appears to have been followed in the related 

 Iron Bacteria in such organisms as Cladothrix dichotonia and 

 Crenothrix polyspora. 



The zoogloea condition is also in some phases a characteristic 

 of Lankesteron roseo-persicina. If this condition is one that 

 is peculiarly favourable to a new environment, a condition 

 temporary under normal conditions may well have become 

 permanent. This probably occurred in the evolution of such 

 organisms as Thiocystis violacea in which the cocci are em- 

 bedded in a mass of slime of a permanent character. The 

 cocci escape in this organism at a definite point in the slime. 



15* 



