The idea of definiteness and constancy of species, a matter of daily observation in the case of man and 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 endowed, as Warming put it, with an “unlimited plasticity,” 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 covering putrefying animal matter in a vessel and forming a continuous membrane along its wall. These red bacteria were of very different shape, size, and grouping, 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 character 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-purpurin_” seemed justly to indicate the existence of common chemical processes. Cohn, on the contrary, considered the different forms among these red bacteria (they are today called sulphur bacteria since they oxidize the hydrogen sulphide produced by bacteria of putrefaction to sulphur and sulphates) as definite and distinct species, in spite of their common colour and their association. Later observations showed that Cohn was right. Winogradsky[30] succeeded in proving by pure culture experiments that each of these different forms of sulphur bacteria was specific and did not give rise to any of the other forms of the same colour found in the same conditions. [30] Winogradsky, S., _Beiträge zur Morphologie und Physiologie der Bacterien_. Leipzig, 1888. The method of pure line breeding inaugurated by Johannsen[31] has shown that the degree of definiteness goes so far that apparently identical forms with only slight differences in size may breed true to this size; but for reasons which will become clear later on we may doubt whether they are to be considered as definite species. [31] Johannsen, W., _Elemente der exacten Erblichkeitslehre_. 2d ed., 1913.