Eclipse died of colic at the age of 24, his skeleton is on display at the Royal Veterinary College in Hartfordshire. He sired 344 race winners in his day. A necropsy at his death revealed an abnormally large heart weighing 14 pounds, this has been referred to as the X-Factor over the years and has been seen in others such as Phar Lap with a 13.6 pound heart and Secretariat, his heart was an amazing 22 pounds. It is called the X-Factor because it is passed through the x chromosome, so it is carried on through the generations from their daughters. Eclipse has been honored through the years by the American Thoroughbred industry by the Eclipse Awards. So it is no wonder that his sire line has become so dominant in racehorses all over the world. Which offspring of his that first made it into the United States I am not sure of.
Two horses of the Eclipse are very dominant contributors in USA pedigrees. Pharos who is sired by Phalaris and St. Simon who is sired by Galopin.
St. Simon won all 9 of his races before siring over 400 foals.
Pharos won 14 of his races before retiring to stud and becoming a leading sire in Great Britain, Ireland and France.
Today's technology has allowed some DNA testing on a tooth of Eclipse to determine what made him superior to other racehorses. Scientists also want to research the remains of the Godolphin Arabian, a founder of the breed. While modern methods can hack away and try to determine the greatness of these horses and how it trickles down through the generations and spans over centuries, there is no denying the greatness of Eclipse and his progeny.