It is undeniable that degeneracy does in some cases follow from the marriage of near kin, and probably with greater frequency than from non-related marriages. But it is likewise true that many of the world's greatest men have been the products of close inbreeding, sometimes continued through several generations. Frederick the Great of Prussia was the product of three successive cousin marriages between descendants of William the Silent,[[42]] and among his seven brothers and sisters at least three others ranked among the ablest men and women of the generation. Cousin marriage has always been frequent in the "first families of Virginia" which have produced a phenomenal percentage of able men. In fact, few persons who have traced their pedigrees back through a number of generations, do not find some names duplicated, as a result of cousin marriage.
The ills which have at one time or another been attributed to consanguineous marriage include nearly all those which cannot otherwise be satisfactorily accounted for. But with the progress of pathology the list has greatly been reduced: for instance, cretinism is now known to be a product of local conditions. The remaining counts in the indictment against consanguineous marriage may roughly be classified as: 1. The production of infertility, some forms of physical degeneracy, and deformity. 2. The production or aggravation of mental and nervous disorders. 3. The production of certain defects in the organs of special sense. These three divisions will be discussed separately.
1. INFERTILITY AND DEGENERACY
Although there has never been any considerable evidence for the first of these charges, it has frequently been repeated. Professor Montegazza of the University of Pavia collected data in regard to 512 cases of consanguineous marriage of which between 8 and 9 per cent were sterile, and with this basis he asserts that sterility is the only fact which can safely be deduced from his cases, since it cannot be hereditary.[[43]] But if in the nature of things absolute sterility is not inheritable, comparative infertility may be. And even then 8 or 9 per cent does not seem to be an excessively high proportion of sterility, especially if late marriages be counted. Boudin bases his assertion on this point on even less tenable grounds.[[44]] On the other hand some writers assure us that cousin marriages are even more prolific and less liable to sterility than the average.
The most important statistical investigation was made by G.H. Darwin.[[45]] From his genealogical data he compiled the following table:
| TABLE XVI. | ||||
|---|---|---|---|---|
| Number of marriages. | Average number sons to marriages. | Per cent sterile marriages. | Ave. no. sons to fertile marriage. | |
| Not consanguineous | 217 | 1.91 | 15.9 | 2.26 |
| Parents 1st cousins[A] | 97 to 105 | 2.07 to 1.92 | 14.7 to 20.9 | 2.43 |
| One parent offspring of 1st cousin marriages. | 93 | 1.93 | 17.2 | 2.34 |
| [A] Eight cases of doubtful fertility. | ||||
It will readily be seen that the conclusion is negative, since the variation is slight, but the higher fertility of the cousin marriages is interesting.
On the other hand de Lapouge quotes a case of a community founded two centuries ago by four families and populated almost entirely by their descendants, in which from 1862 to 1886 there were 273 marriages of which 63 were consanguineous and 26 were between first cousins. Among the non-consanguineous 3 per cent were uniparous, as against 7.95 per cent among the consanguineous. 7.5 per cent of the non-consanguineous were sterile as against 16 per cent of the consanguineous.[[46]] The importance of these percentages is impaired by the fact that they involve only five uniparous families and ten sterile ones, and that of these latter only five were sprung from first cousins.
It is almost impossible to get any accurate statistics of sterility from genealogies, for when no children are given in the record, there is always a strong possibility that there were children of whom the genealogist has no record. However, of 16 first-cousin marriages of which the record expressly stated "no issue," or where it was practically certain that no issue was possible, the average age of the brides was 34.3 years and that of the grooms was 39 years, showing that consanguinity could not have been the only cause of their sterility.
In regard to relative fertility the figures are reliable, but they fail to indicate any effect of consanguinity upon fertility, as will be noted in Table XVII.