ascertaining the relation between the odor and the chemical constitution of substances in general. Hydrocarbons as a class possess considerable similarity in odor, so also do the organic sulphides and, to a much smaller extent, the ketones. The subject waits for some one to correlate its various physiological, psychological and physical aspects in the same way that Helmholtz did for sound. It seems, as yet, impossible to assign any probable reason to the fact that many substances have a pleasant odor. It may, however, be worth suggesting that certain compounds, such as the volatile sulphides and the indoles, have very unpleasant odors because they are normal constituents of mammalian excreta and of putrefied animal products; the repulsive odors may be simply necessary results of evolutionary processes." (_Loc. cit._, _Nature_, December 27, 1900.) Many of the perfumes in use are really combinations of a great many different odors in varying proportions, such as oil of rose, lavender oil, ylang-ylang, etc. The most highly appreciated perfumes are often made up of elements which in stronger proportion would be regarded as highly unpleasant. In the study and manufacture of perfumes Germany and France have taken the lead in recent times. The industry is one of great importance. In France alone the trade in perfumes amounts to £4,000,000. It is doubtless largely owing to the essential and fundamental identity of odors--to the chemical resemblances even of odors from the most widely remote sources--that we find that perfumes in many cases have the same sexual effects as are primitively possessed by the body odors. In northern countries, where the use of perfumes is chiefly cultivated by women, it is by women that this sexual influence is most liable to be felt. In the South and in the East it appears to be at least equally often experienced by men. Thus, in Italy Mantegazza remarks that "many men of strong sexual temperament cannot visit with impunity a laboratory of essences and perfumes."[56] In the East we find it stated in the Islamic book entitled _The Perfumed Garden of Sheik Nefzaoui_ that the use of perfumes by women, as well as by men, excites to the generative act. It is largely in reliance on this fact that in many parts of the world, especially among Eastern peoples and occasionally among ourselves in Europe, women have been accustomed to perfume the body and especially the vulva.[57]