THE EVOLUTIONARY RELATIONSHIPS OF BACTERIA I5I 



able from typical, moiiofibrillar, bacterial flagella, beat as a single organ, and 

 take their origin, not individually, but in tufts from single blepharoplasts. 



In other respects the spirilla are admirably suited to the role of ancestral 

 bacteria, since they may be Gram-positive or Gram-negative, septate or 

 unicellular, and are capable of forming resting cells of various types, as well 

 as of tiny, motile gonidia. But the most striking piece of evidence suggestive 

 of a spirillar origin for bacteria is the observation of Pijper (1946) that many 

 short rod-like bacilh are slightly spiral in morphology. The conclusions 

 which Pijper drew from this observation, especially in respect of bacterial 

 motility, have not been accepted by other workers in this field, but this 

 tendency to a spiral form has proved to be general in almost all bacteria except 

 cocci, and is ot the utmost consequence in the present argument. 



Thus, between the spirilla and the more specialised bacteria of every type 

 there exists a complete series of morphological types, through which a line 

 of descent may be traced. The main line appears to lead to an adaptation to a 

 terrestrial environment, but specialised aquatic genera, such as caulobacteria 

 and chlamydobacteria may also be brought into the scheme of reference 

 and accorded their evolutionary position. 



D; CHANGES IN FLAGELLAR PATTERN 



{2, 3, 5, 6, 7) 



The series of forms which connects Spirillum through Vibrio and Pseudomonas 

 with Bacterium, by a gradual simplification of the elongate spiral into a short 

 and only slightly spiral rod, and the change from polar, through lophotrichous 

 to peritrichous flagellation, is reasonably obvious. But further progress 

 among the more specialised Bactcriaceae leads on the one hand to Proteus 

 with an enormous number of peritrichous flagella, and on the other to Aero- 

 hacter and related types which may have discarded their flagella altogether and 

 adopted, in some cases, an almost coccal morphology. It is apparent that the 

 latter have become completely adapted to a terrestrial existence, but the 

 significance of peritrichous flagellation is obscure. 



