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.
Recently Dr. Paul Gaedeken of Copenhagen has argued in a detailed statistical study ("La Réaction de l'Organisme sous l'Influence Physico-Chimiques des Agents Météorologiques," Archives d'Anthropologie Criminelle, Feb., 1909) that the conception-rate, as well as the periodicity of suicide and allied phenomena, is due to the action of the chemical rays on the unpigmented skin in early spring, this action being physiologically similar to that of alcohol. He seeks thus to account for the marked and early occurrence of such periodic phenomena in Greenland and other northern countries where there is much chemical action (owing to the clear air) in early spring, but little heat. This explanation would not cover an autumnal climax, the existence of which Gaedeken denies.
In order to obtain a fairly typical conception-curve for Europe, and to allow the variations of local habit and custom to some extent to annihilate each other, I have summated the figures given by Mayr for about a quarter of a million births in Germany, France, and Italy,[156] obtaining a curve (Chart II) of the conception-rate which may be said roughly to be that of Europe generally. If we begin at September as the lowest point, we find an autumn rise culminating in the lesser maximum of Christmas, followed by a minor depression in January and February. Then comes the great spring rise, culminating in May, and followed after June by a rapid descent to the minimum.
In Canada (see e.g., Report of the Registrar General of the Province of Ontario for 1904), the maximum and minimum of conceptions alike fall later than in Europe; the months of maximum conception are June, July, and August; of minimum conception, January, February, and March. June is the favorite month for marriage.
It would be of some interest to know the conception-curve for the well-to-do classes, who are largely free from the industrial and social influences which evidently, to a great extent, control the conception-rate. It seems probable that the seasonal influence would here be specially well shown. The only attempt I have made in this direction is to examine a well-filled birthday-book. The entries show a very high and equally maintained maximum of conceptions throughout April, May and June, followed by a marked minimum during the next three months, and an autumn rise very strongly marked, in November. There is no December rise. As will be seen, there is here a fairly exact resemblance to the yearly ecbolic curve of people of the same class. The inquiry needs, however, to be extended to a very much larger number of cases.
Mr. John Douglass Brown, of Philadelphia, has kindly prepared and sent me, since the above was written, a series of curves showing the, annual periodicity of births among the educated classes in the State of Pennsylvania, using the statistics as to 4,066 births contained in the Biographical Catalogue of Matriculates of the College of the University of Pennsylvania. Mr. Brown prepared four curves: the first, covering the earliest period, 1757–1859; the second, the period 1860–1876; the third, 1877–1893; while the fourth presented the summated results for the whole period. (The dates named are those of the entry to classes, and not of actual occurrence of birth.) A very definite and well-marked curve is shown, and the average number of births (not conceptions) per day, for the whole period, is as follows:—
| Jan. | Feb. | Mar. | Apr. | May | June | July | Aug. | Sept. | Oct. | Nov. | Dec. |
| 10.5 | 11.4 | 11 | 8.3 | 10.2 | 10.5 | 11.5 | 12.6 | 12.3 | 11.6 | 12 | 11.7 |
There is thus a well-marked minimum of conceptions (a depression appearing here in each of the three periods, separately) about the month of July. (In the second period, however, which contains the smallest number of births, the minimum occurs in September.) From that low minimum there is steady and unbroken rise up to the chief maximum in November. (In the first period, however, the maximum is delayed till January, and in the second period it is somewhat diffused.) There is a tendency to a minor maximum in February, specially well marked in the third and most important period, and in the first period delayed until March.
A very curious and perhaps not accidental coincidence might be briefly pointed out before we leave this part of the subject. It is found[157] by taking 3000 cases of children dying under one year that, among the general population, children born in February and September (and therefore conceived in May and December) appear to possess the greatest vitality, and those born in June, and, therefore, conceived in September, the least vitality.[158] As we have seen, May and December are precisely the periods when conceptions in Europe generally are at a maximum, and September is precisely the period when they are at a minimum, so that, if this coincidence is not accidental, the strongest children are conceived when there is the strongest tendency to procreate, and the feeblest children when that tendency is feeblest.
Nelson, in his study of dreams and their relation to seasonal ecbolic manifestations, does not present any yearly ecbolic curve, as the two years and a half over which his observations extend scarcely supply a sufficient basis. On examining his figures, however, I find there is a certain amount of evidence of a yearly rhythm. There are spring and autumn climaxes throughout (in February and in November); there is no December rise. During one year there is a marked minimum from May to September, though it is but slightly traceable in the succeeding year. These figures are too uncertain to prove anything, but, as far as they go, they are in fair agreement with the much more extensive record, that of W. K. (ante p. 113), which