ROGER G. PERKINS 125 



tion of the genot>'pe. If this is properly done, the personal equation involved in the 

 selection of various group characters for inclusion is checked and crystallized by the 

 selected type example. This has long been theoretically necessary, but has been much 

 neglected. Buchanan {loc. cit.) and Bergey {loc. cit.) have been the most consistent, 

 not only in their demands for this, but in actually supplying the types. 



There are five main headings which influence classification. Beginning with the 

 earliest, "Morphology," we have added "Chemistry and Physiology," "Evolution," 

 "Habitat," and "Immunology." The most successful work has been accomplished 

 with the first two. 



MORPHOLOGY 



In the beginnings of biological classification, this was the only basis, the alpha and 

 omega, and until discoveries multiplied was quite satisfactory. But as more and more 

 individuals were discovered, groups became very large and needed further or differ- 

 ent subdivisons. Under the head "Morphology" we generally admit not only form 

 and arrangement, including organelles, capsules, and spores, but also tinctorial dis- 

 tinctions such as gram stain, acid fastness, and granule formation, though of course 

 these might also be considered chemical. But even with these admissions the number 

 of distinctive characters is limited. Moreover, as noted earlier, even form is unstable, 

 and subject to temporary or permanent modification which may readily become in- 

 herited. 



But whatever may be the changes in morphology under extraordinary conditions, 

 we must still admit that under ordinary conditions, or under readily obtainable stand- 

 ard laboratory conditions, a rod form remains a rod form, a sphere continues as a sphere 

 a spiral form clings to its spirals. Moreover, a coccus with the habit of dividing in 

 certain planes tends to retain this habit with some obstinacy, and a rod form which 

 after division tends to retain two or more individuals in a chain may be reasonably 

 expected to continue this activity. What constants may we add to form and planes of 

 fission? Obviously, the most conspicuous are motility and spore formation. 



Migula (1897-1900) went so far as to make a major division on the basis of mo- 

 tility, speaking of the motile form as "bacillus," the non-motile as "bacterium." Al- 

 though this was rather widely accepted for a time, it finds little support at present. 

 Leaving aside the question as to whether motility is a higher or lower divisional point, 

 there is marked variation in this character, to such a degree that insistence upon it 

 will make wide separations of closely allied strains. In this group, moreover, not only 

 is the actual motility a differentiating feature, but inasmuch as in the Eubaderiales 

 (Buchanan) motility is dependent on flagella, the arrangements of these may also be 

 important. Orla- Jensen's two orders are based on such arrangement, separating those 

 with flagella at the end from those with flagella all around. Breed and others (191S) 

 state: 



In passing, it is of interest to notice that there is a close analogy between the generally 

 recognized groups of bacteria and those of protozoa. Thus the cephalotrichic and peritrichic 

 bacteria find their analogies respectively in the flagellates and ciliates. This analogy goes 

 further than a mere resemblance in the arrangements of organs of locomotion ; for the ciliates 

 and peritrichic bacteria are both highly specialized grc ups, while both flagellates and cephalo- 

 trichic bacteria contain all gradations between primitive forms and highly specialized human 

 parasites. 



