[14] The best modern account of these complex subjects will be found in the advanced text-book, "The Physiology of Reproduction," pp. xvii., 706, by F. H. A. Marshall. Reference may be made to original papers by J. Beard in the _Anat. Anzeiger_ for 1897; and by Heape in the _Philosophical Trans._ Royal Society, 1894, 97.
The most that modern science appears to have attained is briefly summarised in the following quotation from Marshall ("The Physiology of Reproduction," p. 69): "According to Martin and certain other writers, the human, female often experiences a distinct post-menstrual oestrus [Modern research has recognised a period when the female animal is ready for impregnation, which is called the oestrus, and a preparatory series of physiological changes called the pro-estrous phase.--M.C.S.], at which sexual desire is greater than at other times; so that, although conception can occur throughout the intermenstrual periods, it would seem probable that originally coition was restricted to definite periods of oestrus following menstrual or pro-estrous periods in women, as in females of other mammalia. On this point Heape writes as follows: 'This special time for oestrus in the human female has very frequently been denied, and, no doubt, modern civilisation and modern social life do much to check the natural sexual instinct where there is undue strain on the constitution, or to stimulate it at other times where extreme vigour is the result. For these reasons a definite period of oestrus may readily be interfered with, but the instinct is, I am convinced, still marked.'"
In nearly all wild animals there is a definite period for sexual excitement, very commonly just at that time of the year which fits into the span of gestation, so that the young are born at the season which gives them the best chance to grow up. In animals the period of desire, the ovulation (or setting free of the female germ or unfertilised egg-cell) and the time of the birth of the young, are all co-related harmoniously. The male animal is only allowed to approach the female when the natural longing for union is upon her. Among human beings, the only race which seems to have long periods of sexual quiescence at all comparable with those natural to the animals are the Esquimaux, who appear to pass many months without any unions of the men and women.