150 THE CYTOLOGY AND LIFE-HISTORY OF BACTERIA 



C: THE ANCESTRAL BACTERIUM 



(2, 3, 4, 5, 7) 



The preceding chapters have described the cytological characters of 

 bacteria, but it is expedient, if it is wished to determine the nature of the 

 ancestral bacterial type, to consider which of these characters are diagnostic 

 of bacteria as a group. 



The form of the vegetative nucleus, as a haploid, reductionally-dividing 

 chromosome, lying at right-angles to the long axis of the cell, is such a 

 character ; so also is the possession of monofibrillar flagella. In septate rods 

 and filaments, the cells are separated by cross-walls which contain an element 

 of true cell wall material. Leaving aside the question of the form of the 

 nucleus, the two, latter characters serve quite conclusively to demarcate 

 bacteria from blue-green algae, which have no flagella, and the filamentous 

 forms of which are subdivided, not by true cross-walls but by relatively 

 delicate septa. 



The suggestion that the simpler bacteria may be derived by fragmentation 

 from streptomyces-like forms of fungal origin is again unacceptable, since it 

 suggests that flagella, lost for countless generations, might be regained. At 

 the same time, while not entirely impossible, it is a priori improbable that an 

 entire biological group, such as the bacteria, should have evolved trom a 

 complex terrestrial, to a simple aquatic form, whereas plants, animals and 

 fungi alike seem to have done precisely the reverse. 



If, on the other hand, it is considered more probable that the terrestrial 

 bacteria are derived from the aquatic, it is obviously among the latter that the 

 ancestral type must be sought. The most completely aquatic of all bacteria 

 are the spirilla, and among these are to be found examples which appear to 

 have several characters intermediate between typical bacteria and those small, 

 saprophytic flagellates from which, in common with the fungi, it is reasonable 

 to believe that they may have been derived. 



The cell wall of these spirilla is less rigid, in some cases, than is that ol 

 other bacteria, but more so than that of the flagellates. And their polar 

 flagella, although consisting of numerous separate fibrils, almost indistinguish- 



