94 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



The difficulty will be to devise adequate criteria for such a taxon; this remains a 

 task for the future. 



The Species Concept in Bacteriology 



These two criteria — practical expedience in the interpretation of biological phe- 

 nomena, and the application of an effective system of nomenclature — are the elements 

 from which the systematist must fashion his concept of species. — Camp and Gilly, 1943, 

 p. 381. 



The peculiar difficulties encountered in attempts to give formal expression 

 to the general relationships of the bacteria and bluegreen algae to other living 

 organisms can evidently be referred to the paucity of salient characteristics 

 among the former. This same feature is responsible for the fact that also at the 

 other extreme end of the classification problem, concerned with the species con- 

 cept, no clear-cut solution within the framework of accepted taxonomic procedure 

 has been possible. 



Until 1872, advances in this field had been greatly handicapped by the pre- 

 vailing notion, purportedly based on unambiguous experimental results, that 

 bacteria exhibit an enormous range of variability. It stands to reason that one 

 can hardly expect to "classify" organisms that behave in the manner claimed 

 for them by the early protagonists of the doctrine of pleomorphism, according 

 to whom practically any bacterium could assume the shape of any other, depend- 

 ing largely on the conditions under which it had developed. 



There had been some responsible claims and observations to the contrary. 

 Going back to the pioneering studies of Louis Pasteur, one can find considerable 

 evidence in favor of the view that the transformations claimed by the pleo- 

 morphists were, to say the least, not always observed. The experienced eye of 

 the great French chemist-turned-microbiologist, together with his uncanny ability 

 to devise experimental methods apt to give clearly interpretable results, soon 

 convinced him, as they should have convinced others, that there is often a close 

 and consistent correlation between the chemical changes brought about in a par- 

 ticular environment by the organisms growing therein and the microscopic 

 aspects of the cultures. Pasteur had unhesitatingly taken this to mean that there 

 are different and recognizable types among these microorganisms and had pro- 

 ceeded to describe and name them. But some later workers insisted on the occur- 

 rence of drastic transformations in the appearance of the organisms themselves 

 with changes in environmental conditions. It was, however, not always appre- 

 ciated that their observations might equally well be interpreted as resulting from 

 the use of impure cultures, by the mechanism of preferential development of 

 different organisms elicited by modifications of the external milieu. As long as 

 this fundamental ambiguity had not been resolved, the picture remained too 

 confused to permit serious attempts at classification. 



It must have been with much relief that bacteriologists who had learned from 

 Lister and Koch how pure cultures could be procured and who had started experi- 

 menting with such material became increasingly convinced that the concept of 

 pleomorphism was untenable. Their results clearly indicated that, provided 

 pure cultures, sterile media, and aseptic techniques were employed, transforma- 

 tions of the sort claimed by the pleomorphists simply did not occur. With the 

 gradual development of rigorous techniques and criteria for work with pure cul- 



