_Antagonism of Bacteria._ Study of the life-history of many of the water bacteria will reveal the fact that they can live and multiply under conditions which would at once prove fatal to other species. Some of these water organisms can indeed increase and multiply in distilled water, whereas it is known that other species cannot even live in distilled water, owing to the lack of pabulum. Thus we see that what is favourable for one species may be the reverse for another. Further, we shall have opportunity of observing, when considering the bacteriology of water and sewage, that there is in these media in nature a keen struggle for the survival of the fittest bacteria for each special medium. In a carcass it is the same. If saprophytic bacteria are present with pathogenic, there is a struggle for the survival of the latter. Now whilst this is in part due to a competition owing to a limited food supply and an unlimited population, as occurs in other spheres, it is also due in part to the inimical influence of the chemical products of the one species upon the life of the bacteria of the other species. Moreover, in one culture medium, as Cast has pointed out, two species will often not grow. When Pasteur found that exposure to air attenuated his cultures, he pointed out that it was not the air _per se_ that hindered his growth, but it was the introduction of other species which competed with the original. The growth of the spirillum of cholera is opposed by _Bacillus pyogenes fœtidus_. _B. anthracis_ is, in the body, opposed by either _B. pyocyaneus_ or _Streptococcus erysipelatis_, and yet it is aided in its growth by _B. prodigiosus_. _B. aceti_ is, under certain circumstances, antagonistic to _B. coli communis_.