THREE DECADES PROGRESS IN MICROBIOLOGY 



many of these species can rightly be claimed as such, and if so cannot 

 they nearly always be connected with other well-known forms, from 

 which they only differ in secondary characters? 



It will be clear that the conclusion of the exploration period in micro- 

 biology did not put an end to the growth of this science. It only meant 

 that new trends of investigation manifested themselves, all tending to 

 consolidate the occupied territory by gaining a deeper insight into the 

 properties of the newly discovered world. Evidently a first necessity 

 hereto was to replace the chaotical conglomerate of known living 

 forms by a more or less systematic arrangement. In first instance this 

 asked for a recognition and evaluation of natural relationships be- 

 tween the various forms, and the demarcation of natural groups as 

 building stones for a satisfactory classification. 



If for a moment we restrict ourselves to the bacteria, we have to 

 acknowledge that the older workers too had already made several 

 efforts to arrive at some system of classification. But these systems all 

 had the limitation that they were based solely on the scanty morphol- 

 ogical data derived from the microscopical examination of the organ- 

 isms in question. 



It is now the great merit of the nestor of Danish bacteriologists, 

 Orla-Jensen, to have inrroduced a new era in the study of bacteriol- 

 ogical classification. In several papers Jensen has made an eloquent 

 and broadly documented plea that physiological characters should 

 determine the main lines of a bacterial system. In their entirety Jen- 

 sen's views have not been accepted, and yet they mark a milestone in 

 the development of bacterial taxonomy. For they have opened the 

 eyes of generations of bacteriologists to the truth that, given the enor- 

 mous diversity of metabolic properties amongst bacteria which con- 

 trasts so sharply with the relatively great uniformity characteristic of 

 the metabolism of higher plants and animals, it is not only permissible, 

 but even imperative also to take the physiological properties into ac- 

 count in the ordering of the bacterial kingdom. For those bacteriol- 

 ogists who have not yet been reconciled with this idea it may be re- 

 marked that nowadays there is increasing evidence that differences in 

 catabolic properties of bacterial cells can be considered as expressions 

 of variations in submicroscopical morphology. Anyhow, there is no 

 doubt that Orla-Jensen's views have greatly influenced the numerous 



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