VAN NIEL: SYSJEMATICS OF THE RACTERIA AND BLUECREEN ALGAE 107 



... It cannot be denied that the studies in comparative morphology made by botan- 

 ists and zoologists have made phylogeny a reality. Under these circumstances it seems 

 appropriate to accept the phylogenetic principle also in bacterial classification. 



The question then arises in what characters phylogeny expresses itself. There is no 

 doubt that in this respect morphology remains the first and most reliable guide. 



But is this inference concerning the superior value of morphological prop- 

 erties actually applicable to the bacteria and bluegreen algae ? It has been used 

 to justify the establishment of taxa above the rank of species for organisms 

 with similar outward shape, and the tacit implication has been that such taxa 

 reflect truly "natural relationships." This, however, is open to serious doubt, 

 as illustrated by the genus Sarcina, comprising bacteria of spherical shape, di- 

 viding in two or three perpendicular directions, thus producing squares, flat 

 sheets, or cubical packages. It would not be surprising to find that bacteriolo- 

 gists familiar with these organisms balk at the notion that the aerobic ;S^. lutea, 

 the anaerobic S. ventriculi, S. maxima, and S. methanica, exhiljiting an alco- 

 liolic, butyric acid, and methane fermentation, respectively, the lialophilic ;S'. 

 f/igantea, and the motile, sporeforming S. ureae represent a group of phylo- 

 genetically closely related types. 



It seems to me that the most important reason for much confused thinking 

 about bacterial classification is that Cohn's careful appraisal of the meaning 

 of his "form genera" has not been given the attention it deserves. Proponents 

 of the view that morphological characters are of primary importance for the 

 establishment of natural relations appear often to have failed to realize that 

 only those associated with the developmental history or embryology of a higher 

 plant or animal have served to trace its phylogeny. Even though a sufficiently 

 advanced knowledge of the various types of organisms may sometimes permit 

 the use of a special shape as the only character needed for the determination 

 of relationships, this approach can be very precarious, as shown, for example 

 by Ginkgo hiJoha and the whales. Now, most bacteria and bluegreen algae do 

 not exhibit the kind of developmental history that can be useful in reconstruct- 

 ing phylogeny. Once this is recognized, genera such as Sarcina stand revealed 

 as signifying no more than the "form genera" of Cohn. 



It should thus be evident that many of the morphological features used in 

 the past as differential characters in the classification of bacteria and blue- 

 green algae cannot be depended upon as guides to phylogeny. Is there any 

 reason to believe that physiological and biochemical properties are more sig- 

 nificant in this respect? A priori this possibility cannot be dismissed; there 

 does not seem to be any valid basis for Prevot's insistence that these can be 

 used only for the differentiation of species but not of higher taxa. In fact, the 

 group of photosynthetic bacteria (green and purple sulfur bacteria, and non- 

 sulfur purple and brown bacteria), as also that of the lactic acid bacteria in the 

 sense of Orla- Jensen can easily be regarded as phylogenetically much more 

 homogeneous than the Sarcina group, in spite of a considerably diversified 

 morphology among the organisms comprising the first two assemblages. In 

 the photosynthetic bacteria the cell shapes range from small spheres and short 

 rods to large vibrios, rods, and spirals, and the lactic acid bacteria include strep- 

 tococci, tetraeocci, short rods, and long rods, even to the point of becoming 

 filamentous. 



