2 RELATIONSHIPS OF THE COCCACE^! 



If, however, one passes to the study of more closely 

 related types under these main groups and attempts to 

 determine just what forms will breed true, the problem 

 of classification is found to be a hard one. Morphological 

 differences among the bacteria are so slight that physio- 

 logical properties must necessarily be relied upon in 

 most cases. Yet such properties are frequently ex- 

 tremely variable, as in the case of pathogenic power. 

 Among asexual forms there is no swamping effect of 

 amphimixis to bring exceptional variations back to the 

 specific mean. With unicellular organisms again there 

 is no bar to the persistence of acquired characters. 

 Bacteria respond in many ways to the direct influence 

 of environmental conditions; and each strain transmits 

 to a degree the impress of its recent history. Selection, 

 too, has exceptional opportunities to modify these 

 quickly reproducing forms. The immense number 

 of generations which may succeed each other in a 

 short space of time makes boundary lines as shifting 

 as they would become among the higher plants if a 

 dozen geological epochs were considered at once. Such 

 variability makes detailed bacterial classification diffi- 

 cult; and in some of those groups which have been most 

 carefully studied confusion is greatest. A certain 

 amount of " physiological pleomorphism " is strongly 

 suspected, for example, among the allies of B. coli 

 and B. diphtheria; and judgment has been suspended 

 by good bacteriologists as to the existence of one, or a 



