He is also a leading broodmare sire and an emerging sire of sires. At times, a horse of mediocre ability sires foals of outstanding quality. He is remembered as one of the better broodmare sires. Lightning is one of the only three silver foals Thowra sires. She also chose the sires of litters that were bred in her kennel. Today, two foundation sires of the breed are recognized, both Thoroughbred crosses. Consider an experiment with a group of sires and their progeny from random dams. Bulls have been used as sires for crossbreeding. In rodents, the highest-ranking male frequently sires the most offspring. Litters from multiple sires tend to be more genetically diverse than litters of single sires. Northern Dancer was one of the 20th century's most successful Thoroughbred sires. He sires his last foal after Sarah's arrival in England. In this marriage he sires the son Fridleif and two daughters. Wajima was a son of one of America's greatest sires, Bold Ruler. This group formed an organization called the Sons of Revolutionary War Sires (SRWS).