VAN NIEL: SYSTEMATICS OF THE BACTERIA AND BLUEGREEN ALGAE 97 



organisms. All the criteria of species distinction utterly break down in such 

 forms." A similar verdict was rendered earlier by Babcock and Stebbins (1938, 

 p. 64) : "The species, in the case of a sexual group, is an actuality as well as a 

 human concept; in an agamic complex it ceases to be an actuality." Even if 

 future investigations were to reveal a more or less common and frequent sex- 

 uality in bacteria and bluegreen algae, a phenomenon which at present is sus- 

 pected to characterize some actinomycetes (Lieske, 1921; Stanier, 1942; Bisset 

 et al., 1951), and perhaps some few strains among the eubacterial groups (Leder- 

 berg et al., 1951), the situation hardly warrants the hope that the modern tax- 

 onomic concepts of the botanists and zoologists will soon be successfully applied 

 to these microorganisms so as to render the bacterial and myxophycean species 

 "actualities" rather than merely "human concepts." 



The arbitrariness of such "species" is now generally conceded. Also, it is 

 well-nigh impossible to escape the conclusion that "scientific tact" in delineating 

 these taxa must carry different connotations for different investigators. This is 

 quite understandable if we realize that it is often imperative, even for no other 

 than strictly practical purposes, to distinguish between individual strains (pure 

 cultures), differing from one another with respect to only one type of property, 

 such as pathogenicity, serological reactions, growth factor requirements, or utili- 

 zation of special carbohydrates. As has been pointed out in more detail elsewhere 

 (van Niel, 1946) the relative weight given to various possible differential charac- 

 teristics thus depends to a large extent on the nature of the investigation in which 

 the organisms in question play a role. 



In this respect there has been a shift in emphasis in the direction of physio- 

 logical and biochemical studies. Consequently there has also developed a tendency 

 to use physiological and biochemical criteria for the delineation of species among 

 the bacteria ; studies on the physiology of the bluegreen algae have not progressed 

 far enough to include them in the present argument. But this departure from 

 Cohn's approach has rarely been justified, except perhaps on the basis of the 

 consideration that the paucity of morphological characteristics makes it inevitable 

 to resort to the use of differential properties other than morphological ones, and 

 that physiological differences can be regarded as the detectable expressions of 

 differences in submicroscopic morphology (Winslow, 1914; Kluyver and van Niel, 

 1936). The implications of this procedure have, however, become very clear and 

 very disturbing during the past decade as a result of the important investiga- 

 tions with naturally occurring or artificially induced "mutants" of bacterial 

 cultures. Apart from demonstrating that the properties of a pure culture are not 

 firmly and irrevocably fixed, many of these studies have also indicated that 

 especially the biochemical characteristics of the "mutant strains" show the same 

 sort of relationship to those of the "wild type" as those that have been recognized 

 as the result of single-gene differences in organisms in which the occurrence of 

 sexualitj^ has permitted a genetic analysis. This very fact has sharply raised the 

 question as to how far strains exhibiting such differences should be regarded as 

 distinct species. AYhat Cohn, without benefit of genetic knowledge, had intuitively 

 grasped and clearly expressed, has now once more become a point that has to be 

 seriously analyzed; and it is not an easy problem. 



Few taxonomists will challenge the opinion that a series of mutants, produced 

 by the action of mutagenic agents from a pure culture of bacteria, should still 



