The crime of criminal and unnatural commerce with a person within the degrees forbidden by the law. By the rules of the church, incest was formerly very absurdly extended even to the seventh degree; but it is now restricted to the third or fourth. Most nations look on incest with horror, Persia and Egypt excepted. In the history of the ancient kings of those countries we meet with instances of brothers marrying their own sisters, because they thought it too mean to join in alliance with their own subjects, and still more so to marry into any foreign family. Vortigern, king of South Britain, equalled, or rather excelled them in wickedness, by marrying his own daughter. The queen of Portugal was married to her uncle; and the prince of Brazil, the son of that incestuous marriage, is wedded to his aunt. But they had dispensations for these unnatural marriages from his holiness. "In order, " says one, "to preserve chastity in families, and between persons of different sexes brought up and living together in a state of unreserved intimacy, it is necessary, by every method possible, to inculcate an abhorrence of incestuous conjunctions; which abhorrence can only be upheld by the absolute reprobation of all commerce of the sexes between near relations.
Upon this principle the marriage, as well as other cohabitation of brothers and sisters of lineal kindred, and of all who usually live in the same family, may be said to be forbidden by the law of nature. Restrictions which extend to remoter degrees of kindred than what this reason makes it necessary to prohibit from intermarriage, are founded in the authority of the positive law which ordains them, and can only be justified by their tendency to diffuse wealth, to connect families, or to promote some political advantage. "The Levitical law, which is received in this country, and from which the rule of the Roman law differs very little, prohibits marriage between relations within three degrees of kindred; computing the generations not from but through the common ancestor, and accounting affinity the same as consanguinity. the issue, however, of such marriages are not bastardized, unless the parents be divorced during their lifetime." Paley's Mor. Phil. p. 316, vol. 1.