"Each of the two sexes is then at the same time under the influence of a corresponding excitement. The unusual development of the genital organs reacts upon the entire system, and produces a state of peculiar activity and excitability, known as the condition of 'erethism.'" "It is a remarkable fact, in this connection, that the female of these animals will allow the approaches of the male only during and immediately after the oestral period; that is, just when the egg is recently discharged, and ready for impregnation. At other times, when sexual intercourse would be necessarily fruitless, the instinct of the animal leads her to avoid it; and the concourse of the sexes is accordingly made to correspond in time with the maturity of the egg and its aptitude for fecundation." "The egg, immediately upon its discharge from the ovary, is ready for impregnation. If sexual intercourse happens to take place about that time, the egg and the spermatic fluid meet in some part of the female generative passages, and fecundation is accomplished.... If, on the other hand, coitus do not take place, the egg passes down to the uterus unimpregnated, loses its vitality after a short time, and is finally carried away with the uterine secretions." "It is easily understood, therefore, why sexual intercourse should be more liable to be followed by pregnancy when it occurs about the menstrual epoch than at other times.... Before its discharge, the egg is immature, and unprepared for impregnation; and after the menstrual period has passed, it gradually loses its freshness and vitality." [Footnote 11: Dalton.] The law of periodicity, as it affects the sexual activity of males of the human species, is indicated in the following remarks by the same author:-- "The same correspondence between the periods of sexual excitement in the male and female, is visible in many of the animals [higher mammals], as well as in fish and reptiles. This is the case in most species which produce young but once a year, and at a fixed period, as the deer and the wild hog. In other species, on the contrary, such as the dog, the rabbit, the guinea-pig, etc., where several broods of young are produced during the year, or where, as in the human subject, the generative epochs of the female recur at short intervals, so that the particular period of impregnation is comparatively indefinite, the generative apparatus of the male is almost always in a state of full development; and is excited to action at particular periods, apparently by some influence derived from the condition of the female." The facts presented in the foregoing quotations from Dr. Dalton may be summarized as follows:-- 1. The sexual function is for the purpose of producing new individuals to take the place of those who die, and thus preserve the species from becoming extinct.