Nature itself seemed to abhor the marriage of close kin, since such unions are often childless and their offspring seem subject to grave physical and mental weakness (epilepsy, deaf-muteness, weak eyes, nervous diseases), and incur easily and transmit the defects, physical or moral, of their parents, especially when the interbreeding of blood-relations is repeated (Santi-Leitner, op. cit.    From Wordnik.com.  [The Catholic Encyclopedia, Volume 4: Clandestinity-Diocesan Chancery] Reference