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 Ehrlich. At the same time, a careful analysis of the processes effected by the ferments gives us the chance of determining, with more certainty than has hitherto been the case, what is the nature of the injuries set up by the presence of cells that are out of harmony with the body. Sometimes the parasite takes an active part, sometimes only a passive one, while at other times its influence is extremely varied.

The proof, that ferments play an important part in the means of defence of animal cells against disharmonious substances, opens up new paths for experimental research. It may be long before the true nature of ferments is elucidated; nevertheless, we shall always be getting nearer to the possibility of excluding the second unknown—the substrate. The more we extend our knowledge of the composition and structure of the food material, and of the components of the cells, the more do we find ourselves in a position to make use of substrates of a known structure, which enable us to investigate the ferments in a much surer manner, and to determine exactly how they decompose a particular product. We shall be able to get hold of individual decomposites, and study their properties, so as gradually to penetrate into the mysteries of the effects of infectious diseases, as well as into the principles of immunity.

There is scarcely, in the whole domain of biology, a more stimulating task than that of finding