THE PHYLOGENY OF THE SULPHUR BACTERIA 229 



E. Colour. — The following facts are important in esti- 

 mating the phylogenetic importance of colour : — 



(i) There is little, if any, to choose in point of morphological 

 complexity between the coloured and the uncoloured sulphur 

 bacteria. 



(2) The occurrence of colour is not correlated with the 

 possession of any other feature either in structure or in life- 

 history. 



(3) There is no distinction in the quality of the pigment 

 between the most primitive and the more highly developed 

 members of the group. 



It may be concluded from the third statement that the 

 coloured sulphur bacteria were just as highly coloured on their 

 emergence from still more primitive conditions. Hence we 

 must conclude either that the uncoloured forms were once 

 coloured, and have lost their pigments, or that the sulphur 

 bacteria are of polyphyletic origin. As there are very strong 

 grounds for the belief that photosynthesis occurs in the col- 

 oured sulphur bacteria a polyphyletic origin is very probable. 

 There is nothing in common between the two groups apart 

 from what is common to all the microorganisms classed under 

 the Schizophytes, except a common facility for oxidizing 

 hydrogen sulphide to elementary sulphur. In view of the 

 advantage which would be derived from photosynthesis, it 

 is not probable that organisms that flourish in the full light 

 of day, as do the uncoloured sulphur bacteria, would abandon 

 such an obvious advantage. It is much more probable that 

 their ways have lain apart from very early beginnings, 

 and that the uncoloured sulphur bacteria have been derived 

 from still more primitive uncoloured forms. Issatchenko (4), 

 who accepts the photosynthetic view of the coloured sulphur 

 bacteria, in consequence regards the origin of the group as 

 polyphyletic. He states that coloured bacteria like Lampro- 

 cystis roseo-persicina thrive in a purely mineral solution, 

 drawing their carbon from the carbon dioxide of the atmo- 

 sphere, and considers that this is possible only to organisms 

 that are capable of photosynthesis. 



When the mode of life and the general structure of 



