Where then related persons differ greatly in mental and physical traits, and generally speaking, belong to different types, it is very improbable that there would be any ill effects resulting from the mere fact of consanguinity. A case in point is furnished me by a correspondent. A first cousin marriage which turned out exceedingly well was between strongly contrasted individuals; the husband was "short, stocky and dark complexioned" while the wife was "tall, slight of figure, and of exceedingly light complexion." In other cases in which the results were not so good the husband and wife bore a close resemblance to one another, physically and mentally.
This, however, does not agree with the results obtained by Professor Karl Pearson. Basing his conclusions on the correlation of stature between husband and wife, he believes that homogamy is a factor of fertility. Taking 205 marriages from Mr. Francis Galton's Family Records, Professor Pearson found the correlation between husband and wife to be .0931 ± .0467, while weighted by their fertility the correlation was .1783 ± .0210, practically doubling the intensity of assortative mating.[[100]] The value of these correlations, however, is impaired, as he says, by the insufficient number of observations, and by the fact that absolutely taller mothers are the more fertile.
In a subsequent investigation of from 1000 to 1050 pairs of parents of adult children, Professor Pearson found the correlation in stature to be .2804 ± .0189; of span .1989 ± .0204; and of forearm .1977 ± .0205; with cross coefficients varying from .1403 to .2023. If, as he believes, "The parents of adult children are on the average more alike than first cousins, then it follows that any evils which may flow from first cousin marriage depend not on likeness of characters, but on sameness of stock."[[101]]
But even if it were true, as is very improbable, that parents of adult children are more alike than first cousins, it would still be likely to follow that first cousins who married would be more alike than first cousins in general. A certain degree of resemblance is undoubtedly necessary to complete fertility: husband and wife must be physically compatible, and must both enjoy a certain degree of health and physical strength. These facts are admitted by all, but it does not follow that resemblance beyond a certain point is not in itself detrimental.
Professor Pearson's own experiments in this line, however, do not give consistent results, for in correlating eyecolor with fertility, heterogamy seems to increase fertility. The highest average fertility (4.57) is in those cases where the father is dark-eyed and the mother light-eyed, while the lowest is where both parents have blue-green or gray eyes.[[102]]
In a recent study an attempt has been made to measure the coefficient of correlation between cousins.[[103]] In the characteristics of health, success, temper and intelligence the coefficients ranged between .25 and .30. These values differ but little from those found to obtain for the resemblance between avuncular relatives for eye color (.265), or between grandparent and grandchild for the same characteristic (.3164).[[104]] Positive results were also found, with one doubtful exception, for the occurrence of insanity and tuberculosis in cousins. The writer concludes: "The grandparent, the uncle and aunt, and the cousin are on practically the same footing with regard to relationship or intensity of kinship as measured by degree of likeness of character; and it seems probable that any scientific marriage enactments would equally allow or equally forbid marriage between grandparent and grandchild, uncle and niece, aunt and nephew, and between first cousins."[[105]]
As we should expect the resemblance between near relatives has been found to be much greater. From a measurement of from 4000 to 4886 pairs, the average correlation of the characteristics of stature, span, forearm length and eyecolor between parent and child was .4695. By similar computations and measuring the same characteristics, the fraternal correlation was found to be .508.[[106]] From measurements of a greater variety of characteristics in school children the mean fraternal correlation was .539.[[107]] In athletic power the coefficient was still higher, .72 between brothers, .75 between sisters and .49 between brothers and sisters. Measurements of mental characteristics—vivacity, assertiveness, introspection, popularity, conscientiousness, temper, ability and handwriting proved to be as easily correlated, the mean coefficients being; brothers, .52, sisters. .51, brothers and sisters .52.[[108]]
The relative amount of degeneracy and disease among the offspring of consanguineous marriages has been enormously exaggerated, and the danger is by no means as great as is popularly supposed. Nevertheless, since it is undoubtedly true that on the average such marriages do not produce quite as healthy offspring as do non-consanguineous unions, and since public sentiment is already opposed to the marriage of cousins, it is perhaps just as well that existing laws on the subject should remain in force. From the standpoint of eugenics however, it is much more important that the marriage of persons affected with hereditary disease should be prevented. Dr. Bell has pointed out the danger of producing a deaf-mute race by the intermarriage of congenitally deaf persons,[[109]] and this warning should be made to apply to other congenital defects as well. Some states already prohibit the marriage of the mentally defective, and persons under the influence of intoxicants. Such provisions are wise, and are the most practical means of achieving eugenic ideals—by preventing the propagation of the unfit. The interests of society demand that the mentally and physically defective should not propagate their kind.
From the broader viewpoint of social evolution the problems of inbreeding or crossing of stocks merge into the discussion of the endogamous and exogamous types of society. Whatever may have been the origin of exogamy, the survival of the exogamous type in progressive societies may easily be explained on the ground of superior adaptability, variability and plasticity, which enables such societies to survive a change of environment while the more rigid structure of the endogamous clan brings about its extermination.
Inbreeding leads to caste formation and a rigid and stratified social structure, which is in the end self-destructive, and cannot survive a change of environment. The governing caste may, as Reibmayr says, favor the growth of culture, but it is usually the culture of that caste, and not of the people at large. The ruling caste is usually the result of selection of the strongest and ablest, but after it becomes a caste, the individuals are selected on account of hereditary social position and not primarily on account of ability. Now biological experiments show that although artificial selection may be carried to a point where animals will breed true to a characteristic to within 90 per cent, yet if selection is stopped, and the descendants of the selected individuals are allowed to breed freely among themselves, they will in a very few generations revert to the original type. This is what happens in a social caste, unless, as in the case of the English aristocracy, it is continually renewed by selection of the ablest of the other classes.