24 GENERAL MORPHOLOGY AND BIOLOGY 



of the occurrence of pleomorphism. This is especially the case with 

 certain bacilli, and it may lead to such forms being classed among the 

 higher bacteria. Pleomorphism is, however, a rare condition, and with 

 regard to the bacteria as a whole we may say that each variety tends to 

 conform to a definite type of structure and function which is peculiar to 

 it and to it alone. On the other hand, slight variations from such type 

 can occur in each. The size may vary a little with the medium in 

 which the organism is growing, and under certain similar conditions the 

 adhesion of bacteria to each other may also vary. Thus cocci, which are 

 ordinarily seen in short chains, may grow in long chains. The capacity 

 to form spores may be altered, and such properties as the elaboration 

 of certain ferments or of certain pigments may be impaired. Also the 

 characters of the growths on various media may undergo variations. 

 As has been remarked, variation as observed consists largely in a tendency 

 in a bacterium to lose properties ordinarily possessed, and all attempts 

 to transform one bacterium into an apparently closely allied variety 

 (such as the b. coli into the b. typhosus) have failed. This of course 

 does not preclude the possibility of one species having been originally 

 derived from another or of both having descended from a common 

 ancestor, but we can say that only variations of an unimportant order 

 have been observed to take place, and here it must be remembered that 

 in many cases we can have forty-eight or more generations under obser- 

 vation within twenty- four hours. 



