INFLUENCE ONE SPECIES UPON ANOTHER. 137 
to direct sunlight) hydrogen peroxide (H,O,) is formed. 
This is demonstrated by exposing an agar plate half- 
covered with black paper, upon which a weak solution 
of iodide of starch is poured, and over this again a 
dilute solution of sulphate of iron; the side exposed 
to the light turns blue-black. In gases containing no 
oxygen, hydrogen peroxide is not produced, and the 
light has no injurious effect. Access of oxygen also 
explains the effect which light produces on culture 
media which have been exposed to the action of sun- 
light, as standing in the sun for a time, when after- 
ward used for inoculation. The bacteria sulsequently 
introduced into such media grow badly—far worse than 
in fresh culture media which are kept in the shade. - 
Influence of One Species upon the Growth of Another. 
While it is the custom of bacteriologists to have pure 
cultures to work with, we should never forget that in 
nature bacteria often occur in mixed cultures. If we 
examine water, milk, or the contents of the intestines 
of either sick or healthy persons we shall always find 
several species of bacteria occurring together. This 
admixture may, perhaps, seem to us at first merely 
accidental, but on further investigation it will appear 
that also in the department of bacteriology there exist 
synergists and antagonists, or at least bacteria which 
assist or oppose one another mutually or one-sidedly. 
Nencki speaks of symbiosis and enantobiosis. 
Gassé has demonstrated experimentally the existence 
of antagonisms by inoculating gelatin streak cultures of 
various bacteria; it is found that many species will not 
grow at all or only sparingly when in close proximity 
to some other species. This antagonism, however, is 
often only one-sided in character; for instance, the 
