This I have proved by experiment; and 1 have recently obtained additional proof of the correctness of this view from the examination of spontaneous yeast of the tan-pits kindly forwarded to me from Kingston, Canada, by Professor Lawson. This in no way differs from brewer's yeast which has been long kept. The favorite fruit of the yeast-cell is sugar, upon which it acts in such a manner as to disturb the feeble combination of its chemical elements. This process, which is termed catalysis by chemists, causes decomposition of the sugar and a new arrangement of its particles, giving rise to carbonic acid and alcohol. Sugar is essential to the maintenance of yeast in its integrity. As soon as its requirements in this respect fail to he supplied, the plant turns for its prey upon the new element it has evoked, the alcohol, which is at once converted, by a similar process, into vinegar. Here the ceil becomes changed in form. It is now oval, and this condition, which has received the name of Torula, it may be made to retain indefinitely; but under ordinary circumstances, it proceeds to convert the acid into other compounds, and its development goes on rapidly until it has assumed the form of a filamentous mycelium.

In this stage, again, it can be retained at will, as the vinegar plant, or, as it is popularly termed, the "mother" of vinegar, which possesses the power of at once converting saccharine matter into add, apparently without the intermediate alcoholic fermentation. If now exposed to the air, it completes its growth by producing spores, which in their turn go through the same cycle.

With this slight sketch of the natural history of one of these minute beings, we will now notice some of the more important effects which they are reputed to produce. And, first, let us glance at their influence on the higher forms of vegetables.

It appears certain that before any great damage can be done by these parasites, there must pre-exist in the objects of their attack an unhealthy condition of structure, resulting partly from being deprived of some chemical element essential to healthy growth, and partly to atmospheric changes which tend to foster a too rapid formation of cellular tissue, at the same time that they favor the rapid development of the parasite. The result of these changes in the plant is a lowered vitality, rendering it prone to the attacks of the fungus, which, once having found a habitat, spreads with prodigious rapidity, and by setting in motion chemical changes similar to those already spoken of, soon involves the whole plant in decay. Examples of this will be familiar to you, as in the case of the potato disease, which not many years ago brought England to the verge of famine, and in Ireland, which depends almost solely on this crop, was the cause of untold misery and destitution.

The failure of the vine crops in Spain and Portugal was owing to the ravages of another species, Oidium Tuckeri; and in some seasons the wheat crops in this country are to a great extent damaged or destroyed by another of these minute pests, which, under the name of mildew, often in the course of a single night, converts whole fields of waving corn into black, useless rubbish. Dry-rot in timber is another example of the destructive power of this group. Nor are these the only commercial interests which thus suffer. The production of silk is often a complete failure, owing to the silk-worm being infested by a minute fungus, the Botrytis Bas-siana, which, entering, probably by the spiracles or breathing apertures, insinuates itself into the blood-vessels and destroys the insect. Damp and want of cleanliness are found to be the cause of the attack. Other species again have been found in flies, beetles, eggs, in the air sacs of birds, on fish, reptiles, and animals, the mention of which would encroach too much upon your time.

A great part of those which have received distinct names, as well as nearly the whole of those from the human subject, I have proved to be mere initial or imperfect forms of one or two common species of mould which occur every where upon decaying organic matter, as cheese, apples, oranges, etc. The number of plants thus degraded from the rank of species is about thirty-four, and I doubt not that many others might be placed in the same category.

The first discovery of a vegetable parasite on man was, as I have said, made by M. Schoenlein, of Berlin, while examining the crusts from the head of a person affected with favus, (Porrigo lupinosa, or scald head.) The plant has been since known under the name of Oidium Schoenleinii. Another parasite was subsequently discovered in the hairs of persons affected with the disease termed plica polonica; also a similar one in ulcer was found by Mr. Robin.

Others have been found in Tinea, Porrigo, Pityriasis, Lichen, and Sycosis, etc, etc. Others again in the lungs and on the mucous surfaces of the body. Now the whole of these are referable to a common origin; the characters which have caused them to be raised to the rank of species being due to the plant having been retained in a state of immaturity. So singular is this power of being so retained, that we might almost reduce it to a formula. Giving a certain quantity of sustenance, we might predicate the form which the parasite would exhibit, and thus we find no difficulty in accounting for the great variety which is met with on the human subject alone; difference in density and chemical constitution of textures, in degrees of warmth and moisture, in greater or less facility of access to external air, will readily account for these differences in form, and will render it no matter for surprise that microscopists should have given distinct specific names to upwards of thirty plants which are in truth referable to one or two.

There remains one very peculiar variety to be mentioned. This consists of minute, square-shaped cells arranged in fours. It was discovered by Professor Goodsir, in a disease of the stomach, and was named by him Sarcina ventriculi. A similar one has been observed by Dr. Gardner and others, from the kidney. There is now good reason to believe that both these are merely varieties of the common fungi of which we have been speaking, Penicillium and Aspergillus, for identical growths have been found by Mr. Stephens on bones from South America; by Dr. Fox on the human subject, in a case of skin disease; and by myself in a vial containing crystals of cholesterine.