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 between hypotheses and facts. For these reasons I have limited myself to making mention of those works which either are closely connected with my own researches, or else will be of special service to the reader, in that the full lists of references they contain will be a guide to further study in this field of research. This limitation alone has enabled me to present a picture—which, I hope, is quite clear—of the development of my own investigations, and to show how I arrived at the doctrine of the active part played by ferments in connection with disharmonious substances.

The comprehensive survey, which I now present, has resulted from the fact that many problems have been so far advanced, recently, by means of experimental work, that it seemed advisable to take stock of the observations that lie stored in numerous publications. And, on the other hand, I find that the further study of particular problems can be carried on only in institutions supplying means and apparatus which I cannot command. One man by himself is able in certain problems to reach only a certain point. He takes over, as it were, an edifice which has been built up to a certain height from all possible sides. He tests the scaffolding—the existing working theories—to see whether it will last any longer, or whether it must be replaced; and, more important than that, decides