24 PATHOGENIC BACTERIA 



theory of evolution, viz., the impossibility of defining species, 

 holds equally through microscopic and macroscopic forms of life. 

 Nevertheless, of the different methods of classifying bacteria, 

 those founded upon form are the only ones at present practica- 

 ble. Of such classifications there are almost as many as there 

 are authors. For all practical purposes, however, the patho- 

 genic bacteria are reducible to spheres and rods; the latter being 

 of the most various lengths, at times produced into long fila- 

 ments, which may be straight or spirally twisted, or, in a few 

 instances, branched. 



As to the terms bacterium and bacillus, the former is now 

 obsolete; every straight, rod-shaped organism is a bacillus. If 

 the first is still employed, it is only in deference to pre-estab- 

 lished usage; the early distinction between the two, founded as 

 it was upon difference in length, is rendered valueless by the var- 

 iations in this respect presented by one and the same organism; 

 in the more highly pleomorphic examples, the shortest forms of 

 bacilli may be indistinguishable from spheres or cocci. The pro- 

 posal to name as "bacilli" straight, rod-like organisms which 

 form spores, and "bacteria" those that do not, has not been 

 followed, since there is nothing whatever in the terms indicative 

 of such a difference. Besides the straight rod or bacillus, the 

 other chief forms of pathogenic bacteria are the spherical, or 

 cocci; and the twisted rod or spirillum. 



It may be observed, in passing, that micrococci are not in 

 all cases geometrically spherical. The faces of subdivision are 

 very frequently flattened so long as the resulting elements re- 

 main united, whatever disposition the grouping takes whether 

 in twos (diplococci), fours (tetracocci), eights (sarcinacocci), or 

 linear series (streptococci); it is not rare again to find the ele- 



