98 ^ CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



be regarded as distinct clones of the same species. But the problem is an alto- 

 gether different one when the question is transferred to a number of isolates from 

 natural sources showing similar differences. Here the practice has been to indi- 

 cate such differences by the use of different specific designations. Hence the 

 bacteriological literature is replete with descriptions of "species" that differ no 

 more from one another (as far as the actual characterization has proceeded) than 

 by properties that might well be the result of "single gene" differences. And it 

 should be remarked that it is not only biochemical characters, but also morpho- 

 logical ones that may be aft'ected in a similar manner. The now widely recognized 

 "smooth-rough" variation, determining the appearance of bacterial colonies, may 

 well be a case in point. 



In consequence of this situation some students of microbial genetics have 

 expressed the view that the separation of species among the bacteria cannot be 

 taken seriously. And, admittedly, the evidence for the occurrence of variation, 

 even in pure cultures, is so overwhelming that its implications have to be con- 

 sidered. Naively, one might formulate the problem in some such form as : How 

 many differences, equivalent to single-gene differences, shall one accept as justi- 

 fication for the establishment of a species 1 It will be clear that even this formu- 

 lation is hardly conducive to a solution of the problem. The geneticist will counter 

 that, by the use of an appropriate methodology, it is easy to produce from a pure 

 culture offspring that differ from it by one-, two-, three-, four-, etc., gene char- 

 acters. Where, then, shall one draAV the line? 



The developments sketched in the above paragraphs seem to lead to the con- 

 clusion that the problem of speciation in bacteria — and, by a similar reasoning, 

 this would apply equally to the bluegreen algae — has not been solved, and that 

 the recent work on variability and induced mutations has led us back to the stage 

 before Cohn's contributions, when an almost unlimited variability was accepted. 

 Obviously, this new emphasis on variation is not the result of "inadequate tech- 

 niques"; it is well established, and it is also in much closer agreement with the 

 Darwinian approach to biology. In a sense, one would call Cohn's ideas on clas- 

 sification of bacteria the outcome of the Linnean philosophy; this now has to be 

 abandoned. 



It is an interesting problem to consider how far the "evolutionary" approach 

 can ever render service in reaching a more satisfactory basis for establishing some 

 rationale in clarifying the meaning of a bacterial species. Is it really true tliat 

 we have now to admit that Cohn's predecessors and antagonists have "won," and 

 that an unlimited variability or mutability has to be reckoned with, thus invali- 

 dating any and all attempts to arrive at an acceptable concept of a bacterial 

 species! This I do not believe; it will be necessary to recognize, not merely that 

 Cohn's ideas on the constancy of characters was based on inadequate informa- 

 tion, but also that his insistence on "constancy" had an equally sound basis in fact. 

 As happens so often in scientific and other controversies, the ultimate answer is 

 not to be found by application of the "either-or" approach, but by synthesis. It 

 is in this respect that the recent contributions of the botanists and zoologists 

 have done so much in bringing about a considerable clarification in problems of 

 "biosystematy," as Camp (1951) calls this branch of science, and the question 

 arises how far similar approaches are possible as a means of reaching the same 

 level with respect to the classification of bacteria and bluegreen algae. 



