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<strong>2. PHYLOGENY AND EMPIRICISM IN BACTERIAL SYSTEMATICS</strong> <p>In most biological fields it is considered a truism to state that the only satisfactory basis for the construction of a rational system of classification is the phylogenetic one. Nevertheless, "realistic" bacteriologists show a curious aversion to the attempted use of phylogeny in bacterial systematics. This is well illustrated, for example, by the statement of Breed (1939): Realistic workers have on their side been impatient with idealists who have introduced many ... unjustified speculations regarding relationships between the various groups of bacteria.</p> <p>To what may we ascribe this distrust of phylogeny? In part it is undoubtedly due to the unsatisfactory nature of certain systems, purportedly based on phylogeny, which have been proposed in the past. However, the mere fact that a particular phylogenetic scheme has been shown to be unsound by later work is not a valid reason for total rejection of the phylogenetic approach.</p> <p>Another important reason for the "realistic" attitude is the widespread belief that bacteria present too few characters on which schemes of relationships (and hence a natural system) can be based. It is our belief that such pessimism is not entirely justified, and that at present some relationships can be recognized which can well be incorporated in a system of classification. Even granting that the true course of evolution can never be known and that any phylogenetic system has to be based to some extent on hypothesis, there is good reason to prefer an admittedly imperfect natural system to a purely empirical one. A phylogenetic system has at least a rational basis, and can be altered and improved as new facts come to light; its very weaknesses will suggest the type of experimental work necessary for improvement. On the other hand, an empirical system is largely unmodifiable because the differential characters employed are arbitrarily chosen and usually cannot be altered to any great extent without disrupting the whole system. Its sole ostensible advantage is its greater immediate practical utility; but if the differential characters used are not mutually exclusive (and such mutual exclusiveness may be difficult to attain when the criteria employed are purely arbitrary) even this advantage disappears. The wide separation of closely related groups caused by the use of arbitrary differential characters naturally enough shocks "idealists," but when these characters make it impossible to tell with certainty in what order a given organism belongs, an empirical system loses its value even for "realists."</p> <p>It seems unnecessary to give here an exhaustive review of bacterial systematics. The reader is referred to Buchanan's scholarly treatise on general systematic bacteriology for an excellent survey of this field up to 1925. More recent literature has been briefly reviewed by Breed in the latest edition of Bergey's Manual.</p> <p>We shall, therefore, restrict ourselves to a critique of Bergey's system which illustrates well the weaknesses of the empirical approach.</p>
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