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THE AMERICAN NATURALIST [Vol. LII 



as the Lactobacillus of sour milk and the diphtheria types. 

 It is possible that these organisms together with a few 

 genera from the Eubacteriales represent an entirely dis- 

 tinct series. The Spirochcetales apparently constitute a 

 group showing combinations of characters which relate 

 them to the Eubacteriales and the Thiobacteriales on the 

 one hand, and the Protozoa on the other. The group 

 Myxobacteriales is apparently related to the true bac- 

 teria, but not to higher groups of plants or animals, unless 

 there may be some as yet undiscovered relationship be- 

 tween these forms and the slime molds as suggested by a 

 superficial study of their fruiting forms. 



The interrelationships just discussed among the vari- 

 ous great groups of bacteria may be illustrated by the 

 following diagram in which the connecting lines are in- 

 tended to show relationship, but not necessarily deriva- 

 tion. 



\^ BLUE - GREEM ALGAE ^ / 



5L/nE MOLD5 X 



From the standpoint of the student of evolution the 

 order Eubacteriales (possibly also Thiobacteriales) is of 

 special interest, for within it is probably to be found 

 greater variation in physiological activity than in any 

 other group of plants or animals. A comparison of the 

 modern forms belonging to this group may well give some 

 hint as to their evolution. Too much can not be expected, 

 however, without getting far into the realms of specula- 

 tion. 



After rather careful consideration a committee of the 

 Society of American Bacteriologists has proposed a list 



