For a summary of the chief researches into this question, see Ploss and Bartels, _Das Weib_; also, Rosenstadt, "Zur Frage nach den Ursachen welche die Zahl der Conceptionen, etc," _Mittheilungen aus den embryologischen Institute Universität Wien_, second series, fasc. 4, 1890. Rosenstadt concludes that man has inherited from animal ancestors a "physiological custom" which has probably been further favored by climatic and social conditions. "Primitive man," he proceeds, "had inherited from his ancestors the faculty of only reproducing himself at determined epochs. On the arrival of this period of rut, fecundation took place on a large scale, this being very easy, thanks to the promiscuity in which primitive man lived. With the development of civilization, men give themselves up to sexual relations all the year around, but the 'physiological custom' of procreating at a certain epoch has not completely disappeared; it remains as a survival of the animal condition, and manifests itself in the recrudescence of the number of conceptions during certain months of the year." O. Rosenbach ("Bemerkungen über das Problem einer Brunstzeit beim Menschen," _Archiv für Rassen und Gesellschafts-Biologie_, Bd. III, Heft 5) has also argued in favor of a chief sexual period in the year in man, with secondary and even tertiary climaxes, in March, August, and December. He finds that in some families, for several generations, birthdays tend to fall in the same months, but his paper is, on the whole, inconclusive. Some years ago, Prof. J.B. Haycraft argued, on the basis of data furnished by Scotland, that the conception-rate corresponds to the temperature-curve (Haycraft, "Physiological Results of Temperature Variation," _Transactions of the Royal Society of Edinburgh_, vol. xxix, 1880). "Temperature," he concluded, "is the main factor regulating the variations in the number of conceptions which occur during the year. It increases their number with its elevation, and this on an average of 0.5 per cent, for an elevation of 1° F." Whether or not this theory may fit the facts as regards Scotland, it is certainly altogether untenable when we take a broader view of the phenomena.