Of animal substances only four are used, namely: _musk_, _castor_ or _castoreum_, _civet_, and _ambergris_; the separation of their characteristic odoriferous substances has, however, not yet been accomplished. The odor of plants is generally due to volatile substances called _volatile_ or _essential oils_. Their occurrence is not limited to special parts, they being found in the flower, seed, wood, bast, bark, leaves, and root. However, in every plant the oil occurs chiefly in certain organs, and it even happens that the oil differs with the part of the plant whence it is derived. The odors exist already formed in the living plant, or else are generated, as in the instance of bitter almonds, by some reaction between the elements which takes place during fermentation or distillation. From the strength of the odor of a plant no conclusion can be drawn as to the quantity of volatile oil present. If this were the case, the hyacinth, for instance, would contain more oil than the coniferae, whilst in fact it contains so little that it can be separated only with the greatest difficulty. The odor does not depend on the quantity, but on the quality of the oil; a plant may diffuse but little odor and still contain much volatile oil. Of the various families of plants, the _labiatae_, _umbelliferae_, and _coniferae_ are richest in volatile oils. In every climate plants diffuse odor, those growing in tropical latitudes being more prolific in this respect than the plants of colder regions, which, however, yield the most delicate perfume. Although the East Indies, Ceylon, Peru, and Mexico afford some of the choicest perfumes, Central Europe is the actual flower garden of the perfumer, Grasse, Cannes, and Nice being the principal places for the production of perfume-materials. Thanks to the geographical position of these places, the cultivator, within a comparatively narrow space, has at his disposal various climates suitable for the most perfect development of the plants. The _Acacia Farnesiana_ grows on the seashore, without having to fear frost, which in one night might destroy the entire crop, while at the foot of the Alps, on Mount Esteral, the violet diffuses a much sweeter odor than in the hotter regions, where the olive and the tuberose reach perfect bloom. England asserts its superiority in oils of lavender and peppermint. The volatile oils obtained from plants cultivated in Mitcham and Hitchin command a considerably higher price than those from other localities, this preference being justified only by the delicacy of their perfume. Cannes is best suited for roses, acacias, jasmine, and neroli, while in Nimes, thyme, rosemary, and lavender are chiefly cultivated. Nice is celebrated for its violets, while Sicily furnishes the lemon and orange, and Italy the iris and bergamotte.