The two classes of bacteria and chromacea seem, in the present condition of our knowledge, on account of their simple organization, to be the simplest of all living things, real monera, or organisms without organs. Hence we have to put them at the lowest stage of the protist kingdom, and must regard the difference between them and the most highly differentiated unicellular beings (such as the radiolaria, ciliated infusoria, diatomes, or siphonea) as no smaller than the difference (in the realm of the histona) between a lower polyp (_hydra_) and a vertebrate, or between a simple alga (_ulva_) and a palm. But if the kingdom of the protists is badly divided, on the older rule, into a plant kingdom and an animal kingdom, the only discriminating mark we have left is the difference in metabolism; in that case we have to include the plasmophagous bacteria in the animal kingdom (as Ehrenberg did in 1838) and the plasmodomous chromacea in the plant kingdom. The remarkable class of the flagellata, which includes ciliated unicellulars of both groups, contains several forms which are only distinguished from the typical bacterium by the possession of a nucleus. If it is true that in some of the protists which were counted as bacteria a real nucleus has been detected, these must be separated from the others (unnucleated) and included in the nucleated flagellata. The monera which I described in 1866, and on which I based the theory of the monera in my monograph, belong to a different division of the protists from the classes of bacteria and chromacea. These are the forms which I described as _protamœba_, _protogenes_, _protomyxa_, etc. Their naked mobile plasma-bodies thrust out pseudopodia, or variable "false feet," from their surface, like the (nucleated) real rhizopods (=sarcodinæ); but they differ essentially from the latter in the absence of a nucleus. Afterwards (in my _Systematic Phytogeny_) I proposed to separate these unnucleated rhizopods from the others, giving the name of _lobomonera_ (_protamœba_) to the amœba-like monera with flap-shaped feet, and the name of _rhizomonera_ (_protomyxa_, _pontomyxa_, _biomyxa_, _arachnula_, etc.) to the gromia-like, root-feet forming monera. However, of late years, real nuclei have been detected in each of these large monera, and so they have been proved to be true cells. This discovery was made possible by the improved modern methods of coloring the nucleus which I had not the use of thirty years ago in my first observations. On the strength of these recent discoveries many scientists claim that all the monera I described are true cells, and must have nuclei. This baseless assertion is much employed by the opponents of the theory of evolution in order to deny the existence of the monera altogether.