THE EVOLUTIONARY RELATIONSHIPS OF BACTERIA I55 



accident of anacrobiosis (perhaps associated with the habit of parasitism in the 

 animal gut) has lost this opportunity, the spore itself is much larger. This 

 is readily explicable as a retrogressive step in evolution, the loss of a character 

 which has ceased to be of service, for which innumerable parallels could be 

 cited from other fields. An analogous difference exists between Streptomyccs 

 and Micromonospora. The latter has adopted a thermophilic life in manure 

 heaps and a semi-aquatic life ni lake muds, and its spore is much less freely 

 airborne than in the, probably less degenerate, Streptomyccs. True ActinoniYCcs 

 and their sporogenous relatives may well be still more degenerate, parasitic 

 descendants of freely-sporing ancestors. 



The Gram-positive cocci resemble the sporing bacilli not onlv m their 

 septate structure, but also in a high degree of adaptation to a terrestrial environ- 

 ment. The individual coccus is much reduced in size, and is not only capable 

 of drifting in the dust like a spore, but, possibly because of a similar if less 

 extreme concentration of the proteins, is among the most resistant of vegetative 

 bacteria. 



A parallel, but apparently independent line of evolution is found in the 

 myxobacteria, of which the most highly evolved types are completely adapted 

 to a terrestrial life. As already described, they are able to crawl upon moist 

 surfaces, and their distribution is achieved by the enclosure of entire swarms 

 in fruiting-bodies, often borne upon stalks to catch the air currents, which 

 dry up and blow away in the wind. 



F: RELATIONSHIPS OF AUTOTROPHIC BACTERIA 



(3,4) 



Although inappropriate to detailed discussion in this book, it is an interesting 

 confirmation of the validity of an evolutionary scheme for the classification 

 of bacteria, such as has been outhned in this chapter, that the conclusions 

 derivable from the parallel concept of progressive loss of synthetic power in 

 the course of evolution are in excellent accordance with those based upon 

 purely morphological reasoning. 



This is well seen if the systematic relationships of the autotrophic bacteria 



