THE PRINCIPLES OF CLASSIFICATION 71 



It is therefore highly probable that the capacity of 

 movement is a distinctive feature of all bacteria under certain, 

 at present incompletely known, conditions. It is well to bear 

 in mind that motility is one of the fundamental properties of 

 protoplasm, and is more readily manifested in unicellular 

 aqueous organisms because of the obvious advantage which 

 they obtain by its occurrence. At any rate it is certain that 

 the fact of motility in an organism can only have a restricted 

 use in a natural system of classification. 



(3) The Size of the Cells. — This is an unreliable criterion for 

 classification, as may be demonstrated by the cultivation of 

 any one species of bacteria for an extended period, and under 

 difi^erent conditions of growth. The production of long or 

 short rods is often dependent upon variable external conditions, 

 whose effect upon the organism cannot be predicted before- 

 hand. The tendency of a normal bacillus to form long filaments 

 under certain conditions is well known. Even the thickness 

 of any particular species of bacteria varies. At the same time 

 a large organism, e.g. Bacillus megatherium, is always large 

 when cultivated under normal circumstances ; and others, as 

 for example some of the nitrate bacteria, are always very small. 

 It is therefore possible to assert that an organism is large or 

 that it is small, but unless the given range of its measurements 

 is a wide one, any statement of the size of an organism cannot 

 be relied upon as a help in its identification. 



Some of the sulphur bacteria show an extraordinary 

 variation in size. This applies not only to differences in 

 different generations, but frequently to differences in the same 

 generation. Bacterium sulfuratum may show hundreds of 

 different sizes in the same field. It is a sound rule, and one 

 followed in this book, to ascribe to the same species all those 

 organisms that are similar except as to size, and in which the 

 sizes are so graded that the individuals form a series separated 

 only by minute dimensional differences. This attitude is 

 justified by the frequent occurrence of pleomorphism (see 

 pp. 14 et seq.) in the sulphur bacteria. In the cultivation of 

 some of the sulphur bacteria a very small change in the environ- 

 mental conditions sometimes results in the production of indivi- 



