Woods, Heredity in Royalty, pp. 74-75. The Great Elector, a great-grandson of William the Silent, married his 1-1/2 cousin, a granddaughter of William and also a great-granddaughter of Admiral Coligny. Frederick I married his second cousin, daughter of the Duchess Sophia of Brunswick, and a descendant of William. Frederick William I married his first cousin, Dorothea, granddaughter of Sophia, and also a descendant of William the Silent. Unfortunately the Hohenzollern line was continued by a mediocre brother of Frederick II, but through his sister, Queen Ulrica, the line of genius lasted still another generation to Gustavus III of Sweden.
See Darwin, "Marriages between First Cousins in England and Their Effects," Journal of Statistical Society, June, 1875, p. 178.
Boudin, "Croisement des familles, de races et des espèces." In Memoires de la Société d' Anthropologie, vol. i, p. 518.
Op. cit., p. 181.