XIII. 

 BACTERIOLOGICAL DIAGNOSIS. 



THE researches made by bacteriologists during the past ten years 

 show that there is an extensive bacterial flora, especially in water 

 and in the soil, and that many of the species known are widely dis- 

 tributed and may be recognized by their morphological and biologi- 

 cal characters wherever they may be found ; but they also show that 

 we cannot depend upon morphology alone for the differentiation of 

 species, and that in many cases a careful study of the mode of 

 growth in various culture media, and of pathogenic power by inocu- 

 lations in the lower animals, shows slight differences in bacteria 

 which resemble each other so closely in form and in certain biologi- 

 cal characters that a less careful study would lead to the belief that 

 they were identical. That there are true species among the bacte- 

 ria, in the same sense as among the higher plants, is well established; 

 but, as among the higher plants, it. is often difficult to determine 

 whether the differences observed should be considered sufficient to 

 justify the description of allied forms as distinct species, or whether 

 they should simply be considered as varieties of a single species. 

 For example, the well-known streptococcus of pus has morphologi- 

 cal characters which enable us to distinguish it from many other 

 bacteria, but streptococci have been obtained from various sources 

 which present slight differences as to their growth in certain media 

 and in their pathogenic power. The question arises as to whether 

 these differences are to be considered "specific" or otherwise. If 

 the differences noted are permanent, and enable a bacteriologist to 

 distinguish one streptococcus from the other wherever it may be 

 found, we are justified in considering them as two distinct micro- 

 organisms. And the decision as to whether they are to be consid- 

 ered as different species, or as varieties of a single species, need not 

 detain us. As a matter of fact, nature is continuous, and specific 

 lines are not sharply drawn except in systematic text books of bot- 

 any, etc. The more complete our knowledge of any class of animal 

 or vegetable organisms, the less sharply differentiated are the so- 

 called species on account of the introduction of intermediate forms 

 in a series having common characters. 



