Pasteur studied the scientific point for a long time, for he never abandoned a subject, but was ever ready to begin again after a failure. He modified the disposition of his first tests, and by the use of special vessels and slightly complicated apparatus succeeded in eliminating the only imaginable cause of error--the possible fall, during the manipulations, of exterior germs, that is, the fortuitous sowing of yeast cells. After that he saw no more yeast and no more active alcoholic fermentation; he had therefore formerly been the dupe of a delusion. In his _Studies on Beer_ Pasteur tells of his error and its rectification: “At a time when ideas on the transformations of species are so readily adopted, perhaps because they dispense with rigorous experimentation, it is somewhat interesting to consider that in the course of my researches on microscopic plants in a state of purity I once had occasion to believe in the transformation of one organism into another, the transformation of the _mycoderma vini_ or _cerevisiae_ into yeast, and that this time I was in error; I had not avoided the cause of illusion which my confirmed confidence in the theory of germs had so often led me to discover in the observations of others.” “The notion of species,” writes M. Duclaux, who was narrowly associated with those experiments, “was saved for the present from the attacks directed against it, and it has not been seriously contested since, at least not on that ground.” Some failures are blessings in disguise. When discovering his mistake, Pasteur directed his attention to a strange phenomenon. We find in his book on beer--a sort of laboratory diary--the following details on his observation of the growth of some mycoderma seed which he had just scattered over some sweetened wine or beer-wort in small china saucers. “When the cells or articles of the mycoderma vini are in full germinating and propagating activity in contact with air on a sweetened substratum, they live at the expense of that sugar and other subjacent materials absolutely like the animals who also utilize the oxygen in the air while freeing carbonic acid gas, consuming this and that, and correlatively increasing, regenerating themselves and creating new materials.