The unit of the olfactory sense-organ is the olfactory cell. This, which does not vary in structure from one end of the animal kingdom to the other, is microscopically seen to consist of an elongated body like a tiny rod, bearing on its free end a small enlargement or prominence, on the surface of which is a cluster of extremely fine protoplasmic filaments, the olfactory hairs. These hairs project into and are immersed in a thin layer of mucus, at all events in air-breathing animals, an environment which is necessary for their functional activity, because, if the nose becomes desiccated, as it does in some diseases, the sense of smell is lost (anosmia). The hairs are, without doubt, the true receptive elements of the olfactory cells. It is these which come into contact with and are stimulated by odours—whatever the nature of Odour may be. The deep (proximal) end of the rod-like olfactory cell tapers into a nerve-fibre, which passes by way of the olfactory nerve to a special lobe of the brain—the olfactory lobe—in the vertebrates, or to a nerve-ganglion in the invertebrates. Olfactory cells in man are only found in the upper—the olfactory—region of the nose, spread over a surface of about one square inch, the olfactory area—part lying on the outer (lateral) wall of each nasal passage and part on the septum, or partition between the nasal passages. In macrosmatic animals the olfactory area is relatively greater than in man, but there is apparently no other difference between them. Olfactory cells are held in place by ordinary epithelial cells—the sustentacular cells—which contain pigment. Olfactory cells are found in animals as low in the scale as the sea-anemone. They occur in the integument of the animal, and their structure is the same as in man, the only difference evolution has brought about being that in the higher animals they are protected by lodgment in a _cul-de-sac_. Their function in the sea-anemone is probably limited to the sensing of food, but we do not yet know much about this particular organism. It is otherwise with the olfaction of insects. Here the work of painstaking observers like Lubbock, Fabre, and Forel, has supplied us with a mass of information of the utmost interest, which we shall now proceed to discuss in some detail, commencing with the work of that remarkable French naturalist, Fabre, whose interest in the subject was aroused by an accident—the accident of which the genius of observation knows so well how to take advantage. Having by chance a living female Great Peacock moth captive in his house, Fabre was surprised one night by the advent of some forty others of the same species—males in search of a mate. At once the question arose in his mind: How was it that they had been attracted?