In the world at large bacteria perform many offices of the utmost usefulness to other living things. They fix nitrogen in the soil, sweeten polluted rivers, reduce animal and vegetable matter to a condition in which it is available as plant-food. Their presence within the alimentary canal is inevitable; but it is somewhat doubtful whether, with the exception of the fermentation of cellulose, they do the economy any service with which it could not dispense. As parasites of the alimentary canal, some kinds are less desirable than others. Recently a method of limiting their variety has been introduced and advocated with much enthusiasm, as favourable to the hygiene of the digestive tract. In countries in which the cows are driven, in summer, to mountain pastures, the peasants of the plains live during their absence largely upon milk brought down at intervals, and allowed to turn sour. Sour milk, in Bulgaria, develops a bacterium of extraordinary vigour. It can live in a medium containing as much as 10 per cent. of lactic acid, a concentration fatal to other forms of _Bacterium acidi lactici_. It is easily cultivated, and when ingested continues to multiply in the alimentary canal. So peculiarly lusty is this bacterium that it makes life impossible for other germs. As it dies out after two or three months, it seems unlikely that a man who swallows the Bulgarian milk-germ runs a risk of inviting a repetition of the tragedy which followed the acclimatization of the mongoose in Jamaica. Its supremacy has been attributed to its capacity of developing a concentration of lactic acid too high for the well-being of other bacteria; but it is improbable that it has the opportunity of doing this in the alimentary canal of a person living on a mixed diet. The extinction of other bacteria (if they are extinguished) is more likely to be due to an antagonism of a more subtle kind, at present inexplicable, but not without parallel. The purifying influence of the water of the Ganges has for ages been an article of faith. Pilgrims from fever-stricken districts bathe in it, foul it, drink it, with the corpses of their fellows floating down the stream. Recently it has been shown that this belief is not without foundation. The water of the Ganges at Benares contains bacteria which are as tigers among lesser vermin. The germs of cholera and typhoid fever disappear from cultures into which these overbearing microbes are introduced.