In passing judgment on a proposed marriage, therefore, the vital question is not, "Are they related by blood?" but "Are they carriers of desirable traits?"
The nature of the traits can be told only by a study of the ancestry. Of course, characters may be latent or recessive, but this is also the case in the population at large, and the chance of unpleasant results is so small, when no instance can be found in the ancestry, that it can be disregarded. If the same congenital defect or undesirable trait does not appear in the three previous generations of two cousins, including collaterals, the individuals need not be discouraged from marrying if they want to.
Laws which forbid cousins to marry are, then, on an unsound biological basis. As Dr. Davenport remarks, "The marriage of Charles Darwin and Emma Wedgewood would have been illegal and void, and their children pronounced illegitimate in Illinois, Indiana, Iowa, Kansas, Missouri, Nebraska, New Hampshire, Oklahoma, Oregon, Pennsylvania, South Dakota, Utah, Washington, Wyoming, and other states." The vitality and great capacity of their seven children are well known. A law which would have prevented such a marriage is certainly not eugenic.
We conclude, then, that laws forbidding cousin marriages are not desirable. Since it would be well to make an effort to increase the opportunities for further play of sexual selection, the lack of which is sometimes responsible for cousin marriages, consanguineous marriage is by no means to be indiscriminately indorsed. Still, if there are cases where it is eugenically injurious, there are also cases where its results are eugenically highly beneficial, as in families with no serious defects and with outstanding ability.