It is worthy of remark that girls who live in cities develop about a year earlier than girls living in the country, and that the larger the town the earlier, _ceteris paribus_, the development takes place. Heredity, however, has no small influence on libido and sexual power. Thus there are families in which, with great physical strength and longevity, great libido and virility are preserved until a great age, while in other families the vita sexualis develops late and is early extinguished. In women the time of the activity of the reproductive glands is shorter than in men, in whom the sexual function may last until a great age. Ovulation ceases about thirty years after puberty. This period of cessation of activity of the ovaries is called the change of life (climacterium). This biological phase does not represent merely a cessation of function and final atrophy of the reproductive organs, but also a transformation of the whole organism. In Middle Europe the sexual maturity of men begins about the eighteenth year, and their virility reaches its acme at forty. After that age it slowly declines. The potentia generandi ceases usually at the age of sixty-two, but potentia cœundi may be present even in old age. The existence of the sexual instinct is continuous during the time of sexual life, but it varies in intensity. Under physiological conditions it is never intermittent (periodical), as in animals. In men it manifests an organic variation of intensity in consonance with the collection and expenditure of semen; in women the increase of sexual desire coincides with the process of ovulation, and in such a way that libido sexualis is greater after the menstrual period. Sexual instinct—as emotion, idea, and impulse—is a function of the cerebral cortex. Thus far no definite region of the cortex has been proved to be exclusively the seat of sexual sensations and impulses.[22] Owing to the close relations which exist between the sexual instinct and the olfactory sense, it is to be presumed that the sexual and olfactory centres lie close together in the cerebral cortex. The development of the sexual life has its beginning in the organic sensations which arise from the developing reproductive glands. These excite the attention of the individual. Readings and the experiences of every-day life (which, unfortunately, to-day are too early and too frequently suggestive) convert these notions into clear ideas. These become accentuated by organic sensations which are pleasurable. With this accentuation of erotic ideas by lustful feelings, an impulse to induce these (sexual desire) is developed. Thus there is established a mutual dependence between the cerebral cortex (as the place of origin of sensations and ideas) and the reproductive organs. The latter, by reason of physiological processes (hyperæmia, secretion of semen, ovulation), give rise to sexual ideas, images, and impulses.