The intimate structure and fructification of the higher fungi are for the most part microscopic but an innumerable mass of the lower fungi are themselves invisible to the naked eye, living upon all kinds of vegetable and animal substances, dead, alive, fresh or putrid. They vegetate upon decayed linen, flannel, leather, and even on metallic and poisonous solutions. They yield myriads of minute spores wafted by every breeze. They float in the air we breathe, seeking a nidus in anything that will supply them with suitable food. There is scarcely a spot on the earth where these minute spores may not exist, and being insoluble, they wait where they fall for the growth or decay of the plant or animal which suits them. As parasites they are most destructive, producing disintegration, disease, and even death, both in vegetables and animals. It is the mycelium or spawn which does the mischief, by supplying the fungus with food at the expense of the victim. The spores of the Botrytis Bassiana find a nidus in the breathing pores which open into the trachea of the silkworm; they develop their mycelium in the air-tubes, which are soon filled up; it then extends into the fatty matter under the skin, which nourishes the worm during its dormant state, and, as soon as that matter is exhausted, the victim dies. In autumn, the common fly, though quite dead, may be seen adhering to many parts of a room, especially to the window glass, as if it were alive. In this state it is always surrounded by a halo about an inch in diameter of whitish dust, consisting of the spores of the Empusa Muscæ, or fly fungus. The body of the fly is much distended, the rings of its abdomen are separated by the growth of the mycelium from within, and all the contents of the body having been consumed by the parasite, nothing remains but a hollow shell with a thin felt-like layer of the interlaced mycelia of innumerable fungi, for the fly fungus increases with wonderful rapidity within the insect. Mr. Berkeley believes the fly fungus to be merely a condition or phase of one of those anomalous moulds which grow on dead fish, making them conspicuous as they float on the surface of the water, by the foggy halo which surrounds them. Different kinds of parasitic fungi may exist at the same time. Dr. Leidy found a variety in the stomach of the Passalus cornutus, a beetle that lives upon decayed wood. Fungi do not attack the carnivorous beetles.