26 GENERAL MORPHOLOGY AND BIOLOGY. 



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 anothner 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 observation within twenty-four hours. 



