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Ptolemy X. m. Cleopatra IV, his full sister.
|
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| Berenice II m. Ptolemy XI (brother of X), her uncle.
| |
| |
| Ptolemy XII, d. without issue, succeeded by his uncle.
| |
| |
---Ptolemy XIII.
|
|
Cleopatra VII.

shows an amount of continued inbreeding that has never been surpassed in
recorded history, and yet did not produce any striking evil results. The
ruler's consort is named, only when the two were related. The
consanguineous marriages shown in this line of descent are by no means
the only ones of the kind that took place in the family, many like them
being found in collateral lines.

It is certain that consanguineous marriage, being the mating of like
with like, intensifies the inheritance of the offspring, which gets a
"double dose" of any trait which both parents have in common. If the
traits are good, it will be an advantage to the offspring to have a
double dose of them; if the traits are bad, it will be a disadvantage.
The marriage of superior kin should produce children better than the
parents; the marriage of inferior kin should produce children even worse
than their parents.

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 ma