6. THE SYSTEMATIC POSITION OF THE BACTERIA
The foregoing pages have, we feel, justified the claim that a phylogenetic approach to the taxonomic problems of the bacteria is capable of yielding fruitful results. It is true that there are a small number of organisms of whose relationships we are still ignorant, but if it be remembered that these are mostly microbes not as yet studied under laboratory conditions, it may be expected that further work will result in an elucidation of their taxonomic positions. All the other bacteria can be readily subdivided into three large groups whose sharp and easy separation on the basis of one or two fundamental morphological characters is possible. It is more difficult to give a precise yet adequate definition covering all the organisms in these three groups. The morphological criteria that can be used are exclusively negative-viz., the earlier mentioned absence of true nuclei, of sexual reproduction, and of plastids. But these alone are not enough to characterize the bacteria, since they apply equally well to the Myxophyta. Thus, we are forced to realize that on a morphological basis alone the separation of the bacteria and the blue-green algae is impossible. Whereas the higher groups of the Myxophyta (e.g., the Beggiatoaceae) can be adequately distinguished from the bacteria proper on a strictly morphological basis, we must face the fact that so far as the Chroococcales are concerned this is not so.
Hence, if the Myxophyta are to be retained in their present form as a division, and if a purely morphological definition of the bacteria fails to keep out the members of the blue-green algae, the obvious conclusion would seem to be the creation of a kingdom in which the Myxophyta and the bacteria constitute the two at present recognizable divisions. Other taxonomists (e.g., Smith 1933, Copeland 1938) have been previously forced to the same conclusion. Copeland has pointed out that the name Monera, proposed by Haeckel, should be used for this kingdom in preference to Schizophyta-the name given by Cohn to the combined groups of blue-green algae and bacteria. Since Copeland's arguments seem sound, the name Monera will be adopted here. A subdivision of the kingdom Monera can be achieved along at least two rather different lines, each one recognizing certain definite relationships.
The first method would be a splitting into three divisions on the basis of the method of locomotion. These three divisions would be: firstly, the Myxophyta in the usual sense, with the addition of the Myxobacteriales and the Beggiatoaceaei.e., all organisms which, if motile, show creeping motility; secondly, the Eubacteriales, the Actinomycetales and the Rhodobacteriales, which, if motile, possess flagella; and thirdly, the spirochaetes, which are motile by means of an axial filament or a fibrillar membrane. Whereas this scheme accords most closely with our concept of the phylogeny of the Monera (see fig. 2), it has at present one grave disadvantage. This is the allocation of non-motile organisms.
Since permanent immotility is so common in the first two groups, we feel that this scheme would be at present unworkable. It has been mentioned nevertheless because future work may lead to the discovery of one or more correlating morphological characters which would obviate the arbitrary assignment of a non-motile species to either one of the first two units.
The second possibility is suggested by the desirability of maintaining the Myxophyta as an independent unit. This implies that the subdivision of the Monera must be based on physiological characters, a subdivision all the more difficult because the designation "photosynthetic" for the Myxophyta has to be qualified. The basic reason for this unfortunate predicament is the fact, mentioned previously, that the only possible consistent basis for distinguishing between the Eubacteriales and the Chroococcale&- both of them key groups which it is obviously desirable to keep apart, since they form the starting points for two major evolutionary lines-is physiological.
To make this point perfectly clear the consequences of the two alternative methods of subdivision will be elaborated. In the former, an immotile organism, either belonging to the Myxophyta or to the Eubacteriales, but which is not clearly related to motile organisms in one of these two groups, is in a doubtful position. Yet a taxonomist would not have the least hesitation in placing a typical (immotile!) blue-green Chroococcus in the first group, an action which would imply that he had made use of the photosynthetic mechanism in determining its position. This shows that in doubtful cases (i.e., all immotile forms!) for the ultimate determination of systematic position physiological characters would have to be used.
In the latter case, it is the position of all colorless, non-photosynthetic members of the Myxophyta which is uncertain. By recognizing that in some cases morphological criteria alone can satisfactorily place an organism in the Myxophyta, we can make possible the inclusion of such forms as Beggiatoa here. But we must resign ourselves to the fact that the colorless representatives of the larger part of the Chroococcales cannot be so treated and will probably be placed among the bacteria. In such cases the question whether a colorless organism shall be considered to belong to the bacteria or to the Myxophyta will, at least for the present, remain a matter of scientific tact. In order to forestall possible criticism on this matter, we should like to point out that exactly the same difficulties, although perhaps not so clearly apparent, exist in all other systems of classification, including that of Bergey et al.
For practical reasons, then, we propose the adoption of a subdivision of the kingdom Monera into a division of blue-green algae (Myxophyta) and a division of bacteria (Schizomycetae) in accordance with the second scheme outlined above. The definitions of the kingdom and its two divisions are as follows:
KINGDOM MONERA Haeckel.
Microorganisms which do not possess true nuclei or plastids, and which do not exhibit sexual reproduction.
DIVISION I. MYXOPHYTA. Unicellular or multicellular organisms, which, if motile, show creeping motility. The predominant type of metabolism is photosynthetic with oxygen production. The photosynthetic pigments are chlorophylls a and b, accompanied by phycocyanin and sometimes phycoerythrin. Non-photosynthetic colorless organisms which are clearly recognizable as counterparts of photosynthetic genera are also included.
DIVISION II. SCHIZOMYCETAE. Unicellular or mycelial organisms, which, if motile, may creep or may move by means of flagella or an elastic axial filament or fibrillar membrane. Metabolism is predominantly non-photosynthetic, but if photosynthetic is without oxygen production. The photosynthetic members of the division never contain chlorophylls a or b, phycocyanin or phycoerythrin, and, if motile, are always flagellated.