We may add that the general result of our researches has convinced us that “high” yeast cannot transform itself, any more than “low” yeast can, into the ferment of which we are speaking, and that whenever a beer produced by means of “high” or “low” yeast develops a foreign ferment, this ferment must have existed in the original yeast in the form of germs, which, from their extreme scarcity, often fail to be detected by means of the microscope. The best proof that we can give of this is the fact that a beer produced by means of “high” or “low” yeast, if left to itself for months or years, will never contain in its deposit anything besides the yeast that was used in its manufacture, provided that that was pure to begin with. Now this can never be the case in dealing with actual commercial beers, no matter what they may be or in what brewery they may be produced. In all beers, in the course of time, in addition to diseased ferments, ferments essentially different from those used in their manufacture will appear, and notably _saccharomyces pastorianus_; this result must be attributed to the general impurity of commercial yeasts. In certain cases the intermixture of ferments is to be feared almost as much as the presence of diseased ferments, when these latter have not developed to any great extent. We have often seen our fermentations invaded by ferments differing absolutely from those which we originally employed. The repetition of growths, and more particularly changes in the composition of our fermentable media, purposely made with the view of attaining certain results, often produce complications of this kind. For a long time we were unable to realize the true significance of the results of some of our experiments, in consequence of the facts which we have just explained, as well as those detailed in the preceding paragraph, having escaped our notice; indeed, our ignorance of those facts added greatly to the difficulty and length of our researches. Our labours from the commencement of this work to the date of its publication have extended over not less than five years, and no one can know better than ourselves with what advantage we might devote a still longer time to it; but, as Lavoisier says, one would never give anything to the world if he delayed doing so until he fully attained unto his ideal aims, which always seem more distant the more one increases one’s efforts in the attempt. Our preceding observations show how extremely important it is to employ pure yeasts to obtain, on the one hand, well flavoured beers, whilst adhering to the processes at present existing in breweries, and on the other, beers of good keeping qualities, less liable to injury, less dependent on actual commercial requirements, capable, that is, of withstanding conditions favourable to the development of ferments prejudicial to the soundness of the produce, what we have named ferments of disease.