Chemical Basis of Genus and Species 41 



higher animals in general, was not so readily accepted 

 in the case of the micro-organisms, which on account 

 of their minuteness and simplicity of structure are not 

 so easy to differentiate. There existed for a long time 

 serious doubt whether or not the simplest organisms, 

 the bacteria, possessed a definite *' specificity" like 

 the higher organisms, or whether they were not en- 

 dowed, as Warming put it, with an *' unlimited plasti- 

 city, " which forbade classifying them according to 

 their form into definite species as Cohn had done. An 

 interesting episode in this discussion, which was settled 

 about twenty-five years ago arose concerning the sulphur 

 bacteria, which often develop in large masses on parts 

 of decaying plants or animals along the shore. Sir E. 

 Ray Lankester found collections of red bacteria cover- 

 ing putrefying animal matter in a vessel and forming 

 a continuous membrane along its wall. These red 

 bacteria were of very different shape, size, and group- 

 ing, but they seemed to be connected by transition forms. 

 They had a common character, however, namely, their 

 peach-coloured appearance. This common character, 

 together with their association in the same habitat, 

 led Lankester to the then justifiable belief that they 

 all belonged to one species which was protean in char- 

 acter and that the different forms were only to be 

 considered as phases of growth of this one species. 

 The presence of the same red pigment *'Bacterio-pur- 

 purin" seemed justly to indicate the existence of 



