32 GENERAL MORPHOLOGY AND BIOLOGY. 



and function which is peculiar to it. 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 

 often have forty-eight or more generations under observa- 

 tion within twenty-four hours. If we accept De Bary's 

 definition of a species, we can have little difficulty in saying 

 that species exist among the bacteria. The definition is : 

 " By the term species we mean the sum total of the separate 

 individuals and generations which, during the term afforded 

 for observations, exhibit the same periodically repeated 

 course of development within certain empirically determined 

 limits of variation." 



The Death of Bacteria. The death of bacteria is 

 usually judged of by the fact that, when they are trans- 

 ferred to a fresh quantity of an artificial medium in which 

 they previously grew, no growth takes place. Under the 

 microscope the counterpart of this of course would be the 

 cessation of division when surrounded by such a medium. 

 All bacteria can be killed by heat, drying, starvation, and 

 chemical agents, as we have seen. Great attention has been 

 paid to the latter, which are usually called antiseptics, though 



