remain sterile, or multiply very slowly. If we place in a young wine some _mycoderma aceti_, we shall obtain _mycoderma vini_; by placing some _mycoderma vini_ in an old wine, especially if it is a little acid, we shall obtain _mycoderma aceti_.[50] It is from facts of this kind, wrongly interpreted, that many errors have crept into our knowledge of the lower organisms, and that we are constantly seeing old discussions crop up, both on the subject of so-called _spontaneous_ generation and on that of the theory of fermentation. At every step in the course of this work we shall see the trace of these complications, as well as the influence they have had on the progress of our knowledge. In opposition to these results, we will study the case of wort sown directly with germs distinctly of one kind, unmixed with any other. § I.—Growth of Penicillium Glaucum and Aspergillus Glaucus in a State of Purity—Proofs that these Fungoid Growths do not become Transformed into the Alcoholic Ferments of Beer or Wine.—Preliminary Enquiry into the Cause of Fermentation. Let us again take one of the flasks furnished with two necks such as we have already described, and let it be supposed that this flask contains a quantity of saccharine wort, brewed some considerable time ago, which has undergone no change whatever, except in colour, the slow process of oxidation having gradually darkened the original colour of the liquid. What we have to do is to drop into this unchanged and fertile liquid some grains or spores of _penicillium_ which are free from the slightest trace of the spores or germs of other microscopic organisms.