14 CLASSIFICATION. 
alone. Thus, for example, there are among the spherical bacteria, or 
micrococci, numerous well-established species which the most expert 
microscopist could not differentiate by the use of the microscope 
alone ; the same is true of the rod-shaped bacteria. The assump- 
tion often made by investigators who are not sufficiently impressed 
with this fact, that two" microdrganisms from different sources, or 
even from the same source, are the same because stained prepara- 
tions examined under the microscope look alike, has led to serious 
errors and to much confusion. As an example of what is meant we 
may refer to the pus organisms. Before the introduction of Koch’s 
‘‘plate method” micrococci had been observed in the pus of acute 
abscesses. Some of these were grouped in chains—streptococci— 
and some were single, or in pairs, or in groups of four ; but whether 
these were simply different modes of grouping in a single species, or 
whether the chain micrococci represented a distinct species, was not 
determined with certainty. That there were in fact four or more 
distinct species to be found in the pus of acute abscesses was not 
suspected until Rosenbach and Passet demonstrated that this is the 
case, and showed that not only is the streptococcus a distinct species, 
but that among the cocci not associated in chains there are three 
species which are to be distinguished from each other by their color 
when grown on the surface of a solid culture medium. One of these 
has a milk-white color, one is of a lemon-yellow color, while the thira 
is a golden-yellow. 
Those microérganisms which form pigment are called chromo- 
genes, or chromogenic; those which produce fermentations are 
spoken of as zymogenes, or zymogenic ; those which give rise to dis- 
ease processes in man or the lower animals are denominated patho- 
genes, or pathogenic. We cannot, however, classify bacteria under 
the three headings chromogenes, zymogenes, and pathogenes, for 
some of the chromogenic species are also pathogenic, as are some 
of the zymogenes. These characters must therefore be considered 
separately as regards each species, and in studying its life history and 
distinguishing characters we determine whether it is chromogenic 
or non-chromoggnic ; whether it produces special fermentations ; 
and whether it is or is not pathogenic when inoculated into the 
lower animals. In making the distinction between pathogenic 
and non-pathogenic microdrganisms we must remember that .a 
certain species may be pathogenic for one animal and not for an- 
other. Thus the anthrax bacillus, which is fatal to cattle, sheep, 
rabbits, guinea-pigs, and mice, does not kill white rats ; the bacillus 
of mouse septicemia kills house mice, but field mice are fully im- 
mune from its pathogenic effects ; on the other hand, the bacillus of 
glanders is fatal to field mice but not to house mice. 
