INFLUENCE ONE SPECIES UPON ANOTHER. 137 



to direct sunlight) hydrogen peroxide (H 2 O 2 ) 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 subsequently 

 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 enantobiosix. 



Gasse" 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 



