[Illustration: Fig. 24.--Spirillum Volutans. (Kutscher.) Stained to show Flagellae.] [Illustration: Fig. 25.--Spirillum Volutans. (Kutscher.) Unstained Preparation.] I have pointed out previously the importance of the nutrient medium, or substratum, in which the bacteria grow, on the species surviving. In it one can see on a small scale the Darwinian process of natural selection. There is a great struggle for existence between the various species, and the circumstances determining the survival of this or that organism are extremely complicated, and we are yet very much in the dark as to the action of the various chemical compounds contained in the puer, so that it is unsafe to neglect even those which are present in only small amounts. Very minute quantities of certain bodies, almost too small for detection by chemical means, are sufficient to cause large differences in the growth of certain organisms. For instance, Raulin found that the addition of a trace of zinc to his nutrient liquids increased the crop of the mould Aspergillus niger more than four times the weight of a crop grown in the same liquid free from zinc. If we inoculate a nutrient material with a pure culture of bacteria, and the medium is not exactly adjusted to the needs of the particular organism, it will not thrive, and will speedily be overgrown by some other species obtaining access from the air. This fact very much discounts the use of pure cultures of bacteria which have been proposed for bating, although in the case of erodin, where the medium has been adjusted to suit the organism, considerable success has been attained. The whole of the enzymes and chemical compounds essential for a perfect bate, are not present in the dung when it leaves the animal’s body, but these compounds are produced by the continued action of the intestinal bacteria and other organisms which obtain access from the air. The production of the enzymes depends, too, upon the composition of the nutrient medium, since this exerts a selective influence on the species of bacteria obtaining access to it. Just as in the spontaneous souring of milk numerous bacteria have free access to it, yet the lactic ferment is generally so pure that it may be, and is, used as a pure culture on a large scale in the manufacture of lactic acid. Coming to the action of the bacteria on the skin fibres, from the work of Abt and Stiasny,[88] we may conclude that the substance of the conjunctive fibres is less profoundly decomposed by bacterial fermentation than by the action of lime. The latter dissolves about 2 per cent. of skin substance from a fresh skin, whereas a puer acting normally dissolves about 1 per cent. [88] Sensibilité de la peau verte, et de la peau après l’échauffe, les pelains, et les confits, à l’égard de la chaux du sel, et de l’acide acétique. Georges Abt et Edmund Stiasny, Collegium 1910, p. 1 9.