During the weeks and months over which our observations extended, there was not the least formation of yeast in these flasks; moreover, we have frequently repeated this and other experiments of a similar kind, without ever detecting the appearance of either ordinary yeast or any other true alcoholic ferment. The experiments may be made with saccharine juices that are highly favourable to the development of _bacteria_ and lactic ferment. These latter appear equally incapable of transformation into yeast, which has never been seen to develop in experiments where they were used, if proper precautions have been taken to secure a pure growth. Should we neglect any of the precautions that are necessary to secure the purity of our spores, we may of course obtain different results. If, for instance, we sow spores of _penicillium_ grown in free contact with the atmosphere, and consequently exposed to the particles of dust floating therein, we shall frequently observe, mixed with the fruiting hyphæ of the fungoid growth, yeast and _mycoderma vini_ and _torulæ_, or even _bacteria_ and lactic ferment. Thus we shall be led to believe, in all good faith, that we have under our eyes examples of the changes of spores of fungoid growth into cells of ferment, or proofs of the conversion of _bacteria_ or lactic ferment into the same cells. Causes of error of this nature have induced some German naturalists to believe that they have succeeded in proving, beyond the possibility of doubt, that a number of fungoid growths may produce alcoholic ferment, and that they have clearly demonstrated that spores of these fungi may become transformed into yeast. In 1856 M. Bail, and, about the same time, Berkeley, and, later on, H. Hoffmann and Hallier, have successively entertained these views, which were introduced into science by M. Turpin. We have combated them since the year 1861.[56] Since that period they have lost rather than gained ground abroad, in spite of the growing favour bestowed on the Darwinian system. One of the mycologists who enjoy the most legitimate authority beyond the Rhine, M. de Bary, has arrived, as we have, at absolutely negative results. A simple perusal of what has been written in favour of the transformations which we are discussing causes us to entertain the gravest doubts as to the correctness of results which are quoted as decisive. We need only give one example, which we extract from a paper by M. H. Hoffmann.