THE PHYLOGENY OF THE SULPHUR BACTERIA 231 



The bud formation of Thioporphyra volutans appears to 

 be not an advance in methods of reproduction, but rather the 

 recurrence of what is probably a more primitive method of 

 reproduction, for the products of multipHcation are of a lower 

 type. Support is given to this view by the fact that it occurs 

 under unfavourable circumstances. The formation of zoo- 

 spores in Achroniatimn oxaliferutn marks a decided advance, 

 but much stress cannot be laid on the fact because it is found 

 only in one organism, and this, one which in its general 

 structure is widely different from the other sulphur bacteria. 



These suggested lines of development are schematically 

 represented (Fig. 64). 



Summary. 



The coloured sulphur bacteria have probably all arisen 

 from the development along several lines of one (or a few) 

 highly pleomorphic primitive organism of a type which is 

 represented among modern forms by such organisms as 

 Lankesteron roseo-persicina. It is conjectured that with changes 

 of environment certain pleomorphic forms became stabilized 

 on account of their greater fitness for the changed environ- 

 ment. The uncoloured sulphur bacteria have probably 

 sprung from similar types that had no colouring matter. 

 The sulphur bacteria as a whole are polyphyletic in origin. 



The following lines are suggested as having resulted in the 

 modern forms : — 



1. Development of hardened slime at an early stage in 

 development, resulting in a hardened sheath surrounding the 

 organism. 



2. Fixation of the zoogloea condition with the consequent 

 retention of the units within the slime. 



3. The enlargement of the coccus with freedom from 

 excessive slime formation, and greater freedom of action. 



4. The contraction of the filament to a length suitable for 

 free and active movement. 



