The question has been raised whether vibrios, especially those which we have shown to be the ferment of butyric and many other fermentations, are, in their nature, animal or vegetable. M. Ch. Robin attaches great importance to the solution of this question, of which he speaks as follows[150]:—“The determination of the nature, whether animal or vegetable, of organisms, either as a whole or in respect to their anatomical parts, assimilative or reproductive, is a problem which has been capable of solution for a quarter of a century. The method has been brought to a state of remarkable precision, experimentally, as well as in its theoretical aspects, since those who devote their attention to the organic sciences consider it indispensable in every observation and experiment to determine accurately, before anything else, whether the object of their study is animal or vegetable in its nature, whether adult or otherwise. To neglect this is as serious an omission for such students, as for chemists would be the neglecting to determine whether it is nitrogen or hydrogen, urea or stearine that has been extracted from a tissue, or which it is whose combinations they are studying in this or that chemical operation. Now, scarcely any one of those who study fermentations, properly so called, and putrefactions, ever pay attention to the preceding data.... Among the observers to whom I allude even M. Pasteur is to be found, who, even in his most recent communications, omits to state definitely what is the nature of many of the ferments which he has studied, with the exception, however, of those which belong to the cryptogamic group called _torulaceæ_. Various passages in his works seem to show that he considers the cryptogamic organisms called _bacteria_, as well as those known as _vibrios_, as belonging to the animal kingdom (see _Bulletin de l’Académie de Médecine_, Paris, 1875, pp. 249, 251, especially 256, 266, 267, 289, and 290). These would be very different, at least physiologically, the former being _aërobian_, whilst the vibrios are _anaërobian_, that is to say, requiring no air to enable them to live, and being killed by oxygen, should it be dissolved in the liquid to any considerable extent.”