The considerations so far brought forward clearly indicate that among primitive races there are frequently one or two seasons in the year—especially spring and autumn—during which sexual intercourse is chiefly or even exclusively carried on, and they further indicate that these primitive customs persist to some extent even in Europe to-day. It would still remain, to determine whether any such influence affects the whole mass of the civilized population and determines the times at which intercourse, or fecundation, most frequently takes place.
This question can be most conveniently answered by studying the seasonal variation in the birthrate, calculating back to the time of conception. Wargentin, in Sweden, first called attention to the periodicity of the birthrate in 1767.[[151]] The matter seems to have attracted little further attention until Quetelet, who instinctively scented unreclaimed fields of statistical investigation, showed that in Belgium and Holland there is a maximum of births in February, and, consequently, of conceptions in May, and a minimum of births about July, with consequent minimum of conceptions in October. Quetelet considered that the spring maximum of conceptions corresponded to an increase of vitality after the winter cold. He pointed out that this sexual climax was better marked in the country than in towns, and accounted for this by the consideration that in the country the winter cold is more keenly felt. Later, Wappäus investigated the matter in various parts of northern and southern Europe as well as in Chile, and found that there was a maximum of conceptions in May and June attributable to season, and in Catholic countries strengthened by customs connected with ecclesiastical seasons. This maximum was, he found, followed by a minimum in September, October, and November, due to gradually increasing exhaustion, and the influence of epidemic diseases, as well as the strain of harvest-work. The minimum is reached in the south earlier than in the north. About November conceptions again become more frequent, and reach the second maximum at about Christmas and New Year. This second maximum is very slightly marked in southern countries, but strongly marked in northern countries (in Sweden the absolute maximum of conceptions is reached in December), and is due, in the opinion of Wappäus, solely to social causes. Villermé reached somewhat similar results. Founding his study on 17,000,000 births, he showed that in France it was in April, May, and June, or from the spring equinox to the summer solstice, and nearer to the solstice than the equinox, that the maximum of fecundations takes place; while the minimum of births is normally in July, but is retarded by a wet and cold summer in such a manner that in August there are scarcely more births than in July, and, on the other hand, a very hot summer, accelerating the minimum of births, causes it to fall in June instead of in July.[[152]] He also showed that in Buenos Ayres, where the seasons are reversed, the conception-rate follows the reversed seasons, and is also raised by epochs of repose, of plentiful food, and of increased social life. Sormani studied the periodicity of conception in Italy, and found that the spring maximum in the southern provinces occurs in May, and gradually falls later as one proceeds northward, until, in the extreme north of the peninsula, it occurs in July. In southern Italy there is only one maximum and one minimum; in the north there are two. The minimum which follows the spring or summer maximum increases as we approach the south, while the minimum associated with the winter cold increases as we approach the north.[[153]] Beukemann, who studied the matter in various parts of Germany, found that seasonal influence was specially marked in the case of illegitimate births. The maximum of conceptions of illegitimate children takes place in the spring and summer of Europe generally; in Russia it takes place in the autumn and winter, when the harvest-working months for the population are over, and the period of rest, and also of minimum deathrate (September, October, and November), comes round. In Russia the general conception-rate has been studied by various investigators. Here the maximum number of conceptions is in winter, the minimum varying among different elements of the population. Looked at more closely, there are maxima of conceptions in Russia in January and in April. (In Russian towns, however, the maximum number of conceptions occurs in the autumn.) The special characteristics of the Russian conception-rate are held to be due to the prevalence of marriages in autumn and winter,[[154]] to the severely observed fasts of spring, and to the exhausting harvest-work of summer.
It is instructive to compare the conception-rate of Europe with that of a non-European country. Such a comparison has been made by S. A. Hill for the Northwest Provinces of India. Here the Holi and other erotic festivals take place in spring; but spring is not the period when conceptions chiefly take place; indeed, the prevalence of erotic festivals in spring appears to Hill an argument in favor of those festivals having originated in a colder climate. The conceptions show a rise through October and November to a maximum in December and January, followed by a steady and prolonged fall to a minimum in September. This curve can be accounted for by climatic and economic conditions. September is near the end of the long and depressing hot season, when malarial influences are rapidly increasing to a maximum, the food-supply is nearly exhausted, and there is the greatest tendency to suicide. With October it forms the period of greatest mortality. December, on the other hand, is the month when food is most abundant, and it is also a very healthy month.[[155]]
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.
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:—