96 ^ CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



number of independent species, a far greater number of natural and "cultivated" races, 

 the latter tenaciously retaining their individual physiological particularities because 

 they multiply exclusively by asexual means. 



So keen an appreciation of the value of physiological and biochemical char- 

 acteristics for systematic purposes inevitably led Cohn to refrain from using 

 them. Nor did this practice cause, at the time, serious inconveniences. In 1872 

 knowledge of the bacteria was still so rudimentary that the twenty-one species 

 which Cohn proposed satisfactorily consolidated the existing information. 



But not for long did this state of sophomoric bliss persist. With the rapidly 

 growing interest in Pasteur's "infiniment petits" as biological agents of economic 

 and particularly sanitary importance, it was only a matter of years before the 

 accumulated information led to the realization that an enormously larger num- 

 ber of "different" bacteria existed, and it thus became necessary to devise more 

 adequate methods for systematizing this Imowledge. The approach generally 

 adopted was the creation of a new "species" for every organism that in some 

 respects differed from the previously proposed ones, generally without the least 

 attempt at formulating what was to be understood by a "species" of bacteria. Not 

 until 1912 was this matter clearly discussed by Benecke, and his answer to the 

 question "What is a bacterial species?" was far from reassuring to those who 

 might have felt that it should be possible to establish definite criteria for such 

 entities. With considerable candor Benecke (1912, p. 212) stated: "Die Antwort 

 lautet: Das, was der Forscher, welcher die Art aufstellt, nach seinem 'wissen- 

 schaftlichen Takt' darunter zusammenfasst." This statement bears a striking 

 resemblance to Dobzhansky's remembrance of a definition by "an affable sys- 

 tematist": "A species is what a competent systematist considers to be a species." 

 Dobzhansky, however, continued (1941, p. 372) : 



The cause of this truly amazing situation — a failure to define species which is sup- 

 posedly one of the basic biological units — is not too difficult to fathom. All of the at- 

 tempts, mentioned above have striven to accomplish a patently impossible task, namely 

 to produce a definition that would make it possible to decide in any given case whether 

 two given complexes of forms are already separate species or are still only races of a 

 single species. Such a task might be practicable if species were separate acts of crea- 

 tion or arose through single systematic mutations. If species evolve rather than sud- 

 denly appear, there will necessarily be a residue of situations intermediate between 

 species and races. This need not, however, deter biologists from attempting to elucidate 

 the nature of species, provided it is clearly realized that no rigid standard of species 

 distinction can be secured. 



Even at the time Dobzhansky wrote this passage new concepts had been devel- 

 oped which render the systematic treatment of special groups of higher plants 

 and animals much less arbitrary than the quotations above would seem to imply. 

 Elsewhere in this volume a discussion of such developments may be found; suffice 

 it here to refer to the important contributions by Babcock and Stebbins (1938), 

 Dobzhansky (1941), Petrunkevitch (1952), and Camp (1951). Unfortunately, 

 in the realm of bacteria and bluegreen algae no comparable advances have been 

 made. In large part this is connected with the lack of conclusive evidence for 

 the occurrence of sexual reproduction in these organisms, and Dobzhansky has 

 concisely treated this aspect in the last chapter of his book (1941, p. 379), con- 

 cluding that "the species as a category which is more fixed and therefore less 

 arbitrary than the rest is lacking in asexual and obligatorily self-fertilizing 



