EVOLUTION AND CLASSIFICATION OF BACTERIA 447 



between the arrangement of their orders and the arrangement 

 of their families. 



Buchanan, in his recent classification (1917), recognizes six 

 orders instead of four. He obtains this number by giving ordinal 

 rank to the Spirochaetes and Actinomycetes. There is con- 

 siderable reason to question whether the Spirochaetes belong 

 with the Schizomycetes or with the Protozoa, so perhaps the 

 committee's treatment of them is as satisfactory as Buchanan's. 

 The Actinomycetes, however, differ so decidedly from true bac- 

 teria and embrace such a diversity of species^ that there is good 

 reason for accepting Buchanan's suggestion that there be a 

 separate order Actinomycetales. 



In passing, it is of interest to notice that there is a close an- 

 alogy between the generally recognized groups of bacteria and 

 those of Protozoa. Thus the cephalotrichic and peritrichic 

 bacteria find their analogues respectively in the Flagellates and 

 Ciliates (Infusoria). This analogy goes further than a mere 

 resemblance in the arrangement of organs of locomotion; for the 

 Ciliates and the peritrichic bacteria are both highly speciahzed 

 groups, while both Flagellates and cephalotrichic bacteria con- 

 tain all gradations between primitive forms and highly special- 

 ized human parasites. There is also a less striking analogy 

 between the Rhizopods and the cocci, both groups with equal 

 diameters. More striking is the resemblance between the two 

 highly specialized, but apparently unrelated groups, the Myxo- 

 bacteria and the Mycetozoa. Probably this analogy has no 

 greater significance than the similar one so often mentioned 

 between marsupials and placental mammals, in both of which 

 groups the lines of development show much superficial similarity. 

 Under similar conditions, different groups of living things fre- 

 quently appear to undergo evolutionary development along 

 parallel lines. This similarity between the groups of bacteria 

 and of Protozoa does not indicate interrelationship; but it does 

 increase the probability that the orders of bacteria just men- 



2 The diversity of species of Actinomycetes has been discussed by Lachner- 

 Sandoval (1898), Neukirch (1903) and Conn (1917). 



