THE PRINCIPLES OF CLASSIFICATION 77 



(3) Size of Cells. — Of limited application, on account of the 

 variations both in length and in thickness. Certain ranges 

 of dimensions may be given, if they are not regarded as having 

 an absolute value. 



(4) Ciliation. — 1 he mode of distribution of cilia is constant. 

 Bacteria may be divided into three classes on this basis : 

 (i) With peritrich ciliation ; (ii) with polar ciliation ; (iii) with 

 no cilia. The number of cilia is not constant, except where an 

 organism has not more than one cilium. 



(5) Pleomorphism constitutes a factor which may be used in 

 the grouping of the sulphur bacteria. Our knowledge of this 

 phenomenon in the sulphur bacteria is at present too scanty 

 to make much use of it. 



(6) Mode of Germination. — Not applicable. 



(7) Method of Cell Division. — Two methods are known : (i) 

 with formation of transverse wall in the plasma ; (ii) without 

 such a formation. The value of this difference is considerably 

 lessened because practically the whole of the sulphur bacteria 

 belong to the second class. 



(8) Intimate Structure of the Cell. — The differences are not 

 sufficiently marked to give them value for grouping purposes. 



(9) Colour. — The presence or absence of colour is the most 

 important distinction that is found in the sulphur bacteria. 



(10) Diversity of Habit. — As many, perhaps all, bacteria do 

 not follow only one mode of life, a grouping based on the mode 

 of life of the sulphur bacteria can have very little value for 

 the major partitions, although there is a possibility of its use 

 in the minor divisions. 



There are thus many difficulties to encounter in the 

 classification of the sulphur bacteria. The confused state of 

 the classifications of the present day arises from the bestow- 

 ing of names on what appear to be new species, after only 

 an inadequate investigation. Genera have been founded on 

 characters whose constancy is open to question ; and very 

 little pains have been taken to ascertain whether the life- 

 history of the organism under investigation is limited to the 

 one phase that is presented to the observer. 



Probably more classifications of bacteria have been issued 



