VAN NIEL: SYSTEMATICS OF THE BACTERIA AND RLUEGREEN ALGAE 95 



tures, experimental evidence tended more and more to favor the view that even 

 bacteria display a remarkable constancy in both morphological and physiological 

 respects. This further implied the existence of numerous intrinsically different 

 types of bacteria. 



At this stage the needs for methods of differentiation and recognition became 

 apparent, and it was Cohn who early made some notable contributions towards 

 filling this need. As one of the leaders in the fight against pleomorphistic dogma, 

 Cohn (1872, p. 133) had raised the question: 



. . . ob es denn bei den Bacterieu iiberhaupt Arten in dem namlichen Sinne giebt 

 wie bei den hoheren Organismen. Selbst wer von der Metamorphosenlehre jener Myko- 

 logen nichts wissen will, die Alles aus Allem entstehen und zu Alles sich entwickeln 

 lassen, wird docli beim Anblick eines Bacterienhaufens oft verzweifeln, unter diesen 

 zahlreichen Korperchen von alien moglichen Formen eine Sondeiung natiirlicher Arten 

 vorzunehmeu. 



Cohn's conclusion was in the affirmative, as follows from the statement {ibid.) : 



Gleichwohl bin ich zu der Ueberzeugung gekommen, dass die Bacterien sich in eben so gute 

 und distincte Arten gliedern, wie andere niedere Pflanzen und Thiere, und dass nur 

 ihre ausserordentliche Kleinheit, das meist gesellige Zusammenwohnen verschiedener 

 Species so wie die Variabilitat der Arten die Unterscheidung in vielen Fallen fiir unsere 

 heutigen Mittel unmoglich macht. 



In the same paper a beginning was made with the systematic differentiation and 

 naming of bacterial "species." Differentiation was based on morphological char- 

 acteristics exclusively. This does not mean, however, that Cohn was not aware 

 of the existence of physiological dift'erences as well. He clearly recognized that 

 two morphologically indistinguishable organisms might yet be found to exhibit 

 clear-cut and constant physiological differences. But he found it difficult to deter- 

 mine how far such differences should be accepted as grounds for species differen- 

 tiation. The pertinent passage in Cohn's paper is, it appears to me, so significant 

 that it is worth quoting in full; a free translation follows. After pointing out 

 that perhaps physiological differences may later be correlated with morphological 

 ones, he stated {ibid., pp. 135-136) : 



But, on the other hand, I suspect that in the class of bacteria similar conditions ob- 

 tain as found in higher animals, and particularly among cultivated plants. Of two almond 

 trees which cannot be distinguished by their growth, their leaves, blossoms, and fruits, 

 not even by the external and microscopic aspects of their seeds, one produces only bitter 

 seeds that contain amygdalin and emulsin and produce toxic hydrocyanic acid, whereas 

 the other always yields sweet almonds. We assume that these two trees belong to the 

 same species and originated from a common ancestor from which the two, physiologi- 

 cally so different, came about through variation. . . . Perhaps there exist also among 

 the bacteria which are morphologically indistinguishable, yet exhibit differences in 

 chemical and physiological activity, similar varieties or races which, initially derived 

 from a common germ, always produce the corresponding products through continued, 

 natural or artificial, cultivation under identical conditions and on the same medium. 

 With various yeast types Rees has demonstrated the formation of special races through 

 artificial cultivation. Just as summer rye is unsuitable for winter seed, though initially 

 both races have the same origin and can be interconverted by prolonged cultivation, 

 so Is a top yeast unsuitable for the production of a Bavarian type beer, and nearly every 

 kind of wine or beer is made with its own special yeast. Nonetheless, it is most prob- 

 able that many alcohol-producing yeasts belong to only one species, comprising nu- 

 merous "cultured races." I suspect that also among the bacteria, which act as ferments 

 in totally different chemical and pathological processes, there occur, besides a small 



