That in the human species also, fertility is influenced to a notable degree by nutritive conditions, is shown by statistical investigation. After years distinguished by an exceptionally good harvest the number of children born is considerably greater than in normal conditions; whereas after a famine the opposite is observed. Malthus’s law of population states, inter alia, that the population increases when the amount of available nutriment increases, that is, that favourable nutritive conditions cause an increase, that unfavourable nutritive conditions cause a decrease, of population. Hardships and exhausting occupations diminish the fertility of women. The remarkable fertility of the Kaffirs is referred to the fact that this people, possessing large herds of cattle, lead a life comparatively free from care; it is no less true that the Boer women, who lead a life of well-fed leisure, have very large families; whereas the Hottentot women, poor, ill-nourished, and hard working, seldom bear more than three children.
Generally speaking, it may be said that fertility of the soil, in connection with an easily gained livelihood, favours also human fertility, notwithstanding the fact that certain statistical data seem to conflict with this proposition. Sadler, for instance, concludes that an increase in the price of the necessaries of life does not per se check fertility, but, indeed, rather increases it; he considers that the apparent decline in fertility is due to the fact that the number of marriages diminishes, owing to the rise in prices. We must, however, point out, that an increase in price of the necessaries of life is often associated with a rise in wages, and is therefore not necessarily identified with deficient nutrition; when, however, such a rise in prices leads to actual want, a limitation of fertility will certainly result; this has been proved by Legoyt and Villermé with regard to failure of the crops. Famine and disease lower the number of births; a less severe deficiency of nutriment often lowers only the quality of those born. Malthus was of opinion that the population of a country at any time was related to the quantity of nutriment produced or imported therein, on the one hand, and, on the other, to the liberality with which this nutriment was distributed to the individual. In countries where corn forms the principal crop, we find a thicker population than in pasture lands; and where rice is the principal crop, the population is even more abundant than it is in corn growing countries.
Passing to the consideration of the individual nutritive elements, we find that these also influence fertility. Above all, it has been proved that alcohol notably diminishes the fertility of women. Lippich states that of 100 women in Kärnten and Krain suffering from chronic alcoholism, 28.3 were barren. In England, where the abuse of alcoholic beverages is also very frequently observed in women, the same phenomenon has been noted. Matthews Duncan held that alcohol exercised a specific deleterious influence on fertility. Moreover, in addition to the constitutional disturbances produced by the abuse of alcohol, this beverage also exercises a well-known pathogenetic influence upon the female reproductive organs; with especial frequency, chronic oöphoritis may be shown to depend on this exciting cause.
A diet consisting mainly of fish is known to increase the sexual impulse, and is said also to increase fertility. Further, a diet consisting mainly of potatoes or rice is said to favour reproduction; compare, for instance, the fertility of the Hindoos, who abstain entirely from animal food, and of the Chinese, who live chiefly on rice. Davy maintained that the women of races living chiefly on fish were handsomer and more fertile than others: and Montesquieu suggested that there was an association between the abundant population of sea-ports and also of Japan and China, and the large quantity of fish consumed in those places. On the other hand, a diet consisting chiefly of meat is said to have an unfavourable influence in this direction; in support of this view it is pointed out that races living by the chase, and living therefore almost entirely on meat, have very small families. This generalization is invalidated by the fact that Englishwomen, who eat far more meat than the women of the Latin races, are nevertheless distinguished by their great fertility.
In his “History of Civilisation in England” Buckle writes: “The population of a country, although influenced by many other conditions, unquestionably rises and falls in proportion as the supply of nutriment is abundant or the reverse.” Herbert Spencer also states that “every increment in the supply of nutriment is followed by an increment in fertility.”
It must not be forgotten that, in addition to the more or less abundant supply of nutriment, there are always other influences affecting fertility; the general mode of life, race, climatic conditions, etc., may, in various ways, co-operate with or countervail the influence of nutritive conditions. If, with the best possible supply of nutriment, there is associated a luxurious and enervating mode of life, the abuse of alcohol, severe intellectual exertion, or sexual excesses, the general result will be a diminution in fertility. And it is easy to understand why Cros, although perhaps with little justification, goes so far as to regard easy circumstances as an active cause of depopulation. “It is the poor,” he writes, “and the less wealthy departments of France, in which we find the most children.” In estimating fertility, however, we must never fail to take into consideration the more extensive employment of means for the prevention of pregnancy among the upper classes of society.
To a certain extent we can trace the influence of climate and of season upon fertility. Heat appears to favour fertility; Haycraft’s figures for the eight largest towns of Scotland show clearly how the number of conceptions rises and falls pari passu with the temperature. Lower animals also, when brought from a colder to a warmer neighbourhood, exhibit an earlier and more frequently recurring “heat.” In Europe, however, the Northern races appear more fertile than those of the south.
Of the seasons, spring is the one especially favourable to fertility. Quetelet, who proves by numerous statistical data that the maximum of conceptions occurs in May, attributes this fact to a general increase in the vital forces occurring in spring, after the cold of winter. Villermé, however, goes back to the older explanation, that the increase in the number of conceptions in May and June is due to social and economic conditions. The return of spring, especially the end of spring and the beginning of summer, a time of year in which the means of subsistence are provided in exceptional quantity, and of especially good quality, the season also of festivals and social reunion, when the two sexes are brought into more intimate contact and when the majority of marriages occur—these are the conditions associated with the season of greatest fertility. The figures of Wappaeus also confirm the influence of spring in favouring fertility. He found, however, that there were two seasons of maximal fertility. The first at the end of spring and the beginning of summer; the second in winter, especially in December. Mid-winter is for most people a period of domestic amusement and relaxation, one of exceptionally good nutrition, and of social reunion; the spring increase in fertility is a part of the awakening and increase of the reproductive forces of nature at large, which recurs every spring-time.
Every marked and sudden change in the mode of life has an unfavourable influence on fertility. Darwin reports that mares who have for some time been stall-fed with dry fodder and are then put out to grass are at first infertile after the change. Europeans going to reside in the tropics experience a notable decline in fertility as a result of the change of climate. According to Virchow, the fertility of European women who become acclimatized in the tropics declines very gradually, but in the course of a few generations is almost completely annulled.
The marriage of near kin is believed also to diminish fertility. As regards inbreeding in the lower animals, it is well known that when nearly related animals copulate, the number of the offspring is below the average. Nathusius paired a sow with its own uncle, the boar having proved productive in intercourse with other sows; the litter numbered five to six only. This sow, which belonged to the great Yorkshire race, was then paired with a small black boar, which in intercourse with sows of its own variety had procreated litters numbering six or seven; as a result of her first pairing with the black boar, the sow cast a litter numbering twenty-one whilst the second attempt produced a litter of eighteen. Similar results were obtained by Crampe, in his experiments in the inbreeding of rats.