We learn from these figures that the maximum fertility is exhibited by a woman 18 years of age, when married to a man 25 years of age; less fertile is a woman 25 to 30 years of age married to a man 28 years of age; still less fertile is a woman 35 years of age married to a man 29 years of age. Neither the age of the mother alone, nor that of the father alone, is determinative of the fertility of the marriage, for the fertility of young wives married to elderly husbands is quite different from that of young wives married to young husbands. Very various age-combinations are possible, and each exhibits an average fertility peculiar to itself.
We can also regard the question from the standpoint of the difference between the ages of husband and wife respectively. In this connection, Körösi is led by his tables to the conclusion that wives between the ages of 18 and 20 years attain their maximum fertility when married to men 7 years older than themselves; women of 25 years when married to men 3 years older than themselves; women of 29 years when married to men of the same age; women of 30 years and upward attain their maximum fertility only when married to men younger than themselves. Men, on the contrary, always attain their maximum fertility when married to women younger than themselves. The age of maximum fertility differs in the two sexes, and those marriages will be most fruitful in which husband and wife are each of the age most favorable to fertility. This will be the case when the age of the wife is 18 to 20 years, and that of the husband 24 to 26 or perhaps 29 years.
In connection with the question of fertility, we have also to take into consideration the vitality of the children born, that is, what proportion of those born survive. According to Körösi’s interesting papers regarding the fertility of the inhabitants of Buda-Pesth, we learn that for every 100 marriages which have persisted for thirty years and upward, there were born, on the average, 539 children, of whom during this period 241 died, so that the percentage of survivals was 55.28. Parents who have lost one only of several children must, therefore, regard themselves as exceptionally favoured by fortune.
Social position, occupation, and religion, have, according to the last-quoted author, a notable influence on fertility. His investigations showed that the Roman Catholics and the Jews exhibited the greatest fertility; among the Catholics there were 541 children, and among the Jews 557 children, per 100 marriages. Amongst 100 Protestant families, on the other hand, only 479 children had been born. It will be seen that the theory of the comparatively enormous fertility of the Jewish race is not supported by these statistics. The Jews do, however, exhibit a greater power of rearing children, for among them the marriages of more than 30 years’ duration had 61⅔ % of the children still living; among the Protestants 57¾% survived; and among the Catholics only 52–⅗%. It thus appears that the surviving offspring per 100 marriages of 30 years’ duration were, among the Catholics 278, among the Protestants 252, and among the Jews 349.
The question whether, and to what extent, the age of the parents has an influence on the vitality of the children, is answered by Körösi’s mortality statistics in the sense that mothers below 20 years of age give birth to a larger proportion of children deficient in vital power. Where the mothers had married at the age of 16, the mortality of their offspring was, among Catholics 43%, among Jews 33%; married at 17, Catholic mortality 44%, Jewish 30%; married at 18, Catholic mortality 42%, Jewish 32%; married at 19, Catholic mortality 41%, Jewish 29%; married at 20, Catholic mortality 40%, Jewish 26%. Of the children whose fathers had married at the age of 24, 32% had died; of those whose fathers had married at 23, 37% had died; of those whose fathers had married at 20, 42% had died; and of those whose fathers had married before 20, actually 44% had died. It thus appears that the children alike of very young mothers and of very young fathers have a lessened chance of survival.
Inasmuch as the fertility of the wife is a product of two factors, her own peculiar fertility, and that of the procreating male, the question of the fertility of women cannot be accurately treated independently of this second consideration; hereby, however, is introduced a multiplicity of obscure combinations, by which the value of all the statistical data of fertility in women is seriously impaired.
These data give as the measure of fertility, the number of children per marriage actually brought up, embracing fruitful marriages, sterile marriages, and those not yet fruitful. In Berlin, in Copenhagen, and in Buda-Pesth, the average thus attained was slightly less than three births to each family, whilst the number of children actually living averaged two per family. A more accurate representation of fertility is obtained by ascertaining the number of children born, and the number of children living in relation to the duration of marriages reckoned in years, that is beginning with marriages of one year’s duration, and proceeding year by year to the highest recorded duration of marriage. In this way interesting statistics have been obtained; for example, one who has completed thirty years of married life may count on the average that five or six children will have been born to him, but may also reckon on having buried two or three at least of these. (Körösi.)
Fertility is, as many facts indicate, also dependent on nutrition. A distinct proof, says Spencer, writing on the “Coincidence between high Nutrition and Genesis,” that abundant nutriment increases the number of births, and vice versa, is found among the mammalia; compare, for instance, the litter of the dog with that of the wolf and the fox. Whilst the dog’s litter numbers 6 to 14, that of the wolf numbers 5 to 7, that of the fox 4 to 6. The wild cat gives birth to 4 or 5 kittens once a year, the domesticated cat to 5 or 6, twice or thrice annually. The most remarkable contrast, in this respect, exists between the wild and the domesticated breeds of swine. The wild sow gives birth once a year to a litter of 4, 8, or 10 pigs (the number increasing in successive litters); the domesticated sow has often as many as 17 in a single litter, whilst in two years five litters, each numbering 10 pigs, are commonly born.
Darwin also draws attention to the fact that animals under domestication, being fed more abundantly and regularly than their wild allies, procreate at shorter intervals and are markedly more fertile than the latter. He states that the wild rabbit has four litters annually, each numbering 4 to 8 young; whereas the tame rabbit reproduces its kind six to seven times annually, and gives birth to litters numbering 4 to 11. Among birds, analogous phenomena are observed. The wild duck, for instance, lays 5 to 10 eggs in the course of the year, whereas the tame duck lays from 80 to 100; the wild grey goose lays 5 to 8 eggs, the domesticated goose 13 to 18.
It must be added that this exceptional fertility is manifested in animals that are quite inactive in comparison with their wild allies; not only are they richly fed, but they get their food without working for it. Moreover, it is easy to observe that among the domesticated mammals the well-fed are more fertile than the ill-fed.