We are unable to see the matter in the same light as our learned colleague does; to our thinking, we should be labouring under a great delusion were we to suppose “that it is quite as serious an omission not to determine the animal or vegetable nature of a ferment as it would be to confound nitrogen with hydrogen, or urea with stearine.” The importance of the solutions of disputed questions often depends upon the point of view from which these are regarded. As far as the result of our labours is concerned, we devoted our attention to these two questions exclusively:—1. Is the ferment, in every fermentation properly so called, an organized being? 2. Can this organized being live without air? Now, what bearing can the question of the animal or vegetable nature of the ferment, of the organized being, have upon the investigation of these two problems? In studying butyric fermentation, for example, we endeavoured to establish these two fundamental points:—1. _The butyric ferment is a vibrio._ 2. _This vibrio may dispense with air in its life, and, as a matter of fact, does dispense with it in the act of producing butyric fermentation._ We did not consider it at all necessary to pronounce any opinion as to the animal or vegetable nature of this organism, and, even up to the present moment, the idea that vibrio is an animal and not a plant is, in our minds, a matter of sentiment rather than of conviction. M. Robin, however, would have no difficulty in determining the limits of the two kingdoms. According to him, “every variety of cellulose is, we may say, insoluble in ammonia, as also are the reproductive elements of plants, whether male or female. Whatever phase of evolution the elements which reproduce a new individual may have reached, treatment with this reagent, either cold or raised to boiling, leaves them absolutely intact under the eyes of the observer, except that their contents, from being partially dissolved, become more transparent. Every vegetable, whether microscopic or not, every mycelium, and every spore thus preserves in its entirety its special characteristics of form, volume, and structural arrangements; whilst in the case of microscopic animals, or the ova and microscopic embryos of different members of the animal kingdom, the very opposite is the case.”