Thoroughbred
Later development in Britain

By the end of the 18th century, the English Classic races had been established. These are the St. Leger Stakes, founded in 1776, The Oaks, founded in 1779, and The Derby in 1780. Later, the 2,000 Guineas Stakes and the 1,000 Guineas Stakes were founded in 1809 and 1814. The 1,000 Guineas and the Oaks are restricted to fillies, but the others are open to racehorses of either sex aged three years. The distances of these races, ranging from one mile (1.6 km) to 1.75 miles (2.82 km), led to a change in breeding practices, as breeders concentrated on producing horses that could race at a younger age than in the past and that had more speed. In the early 18th century, the emphasis had been on longer races, up to 4 miles (6.4 km), that were run in multiple heats. The older style of race favored older horses, but with the change in distances, younger horses became preferred.

Selective breeding for speed and racing ability led to improvements in the size of horses and winning times by the middle of the 19th century. Bay Middleton, a winner of the Epsom Derby, stood over 16 hands high, a full hand higher than the Darley Arabian. Winning times had improved to such a degree that many felt further improvement by adding additional Arabian bloodlines was impossible. This was borne out in 1885, when a race was held between a Thoroughbred, Iambic, considered a mid-grade runner, and the best Arabian of the time, Asil. The race was over 3 miles (4,800 m), and although Iambic was handicapped by carrying 4.5 stone (29 kg; 63 lb) more than Asil, he still managed to beat Asil by 20 lengths. The improvement of the breed for racing in this way was said by noted 19th century racing writer, Nimrod, to have created "the noblest animal in the creation".

An aspect of the modern British breeding establishment is that they breed not only for flat racing, but also for steeplechasing. Up until the end of the 19th century, Thoroughbreds were bred not only for racing but also as saddle horses.

Soon after the start of the 20th century, fears that the English races would be overrun with American-bred Thoroughbreds because of the closing of US racetracks in the early 1910s, led to the Jersey Act of 1913. It prohibited the registration of any horse in the General Stud Book (GSB) if they could not show that every ancestor traced to the GSB. This excluded most American-bred horses, because the 100-year gap between the founding of the GSB and the American Stud Book meant that most American-bred horses possessed at least one or two crosses to horses not registered in the GSB. The act was not repealed until 1949, after which a horse was only required to show that all its ancestors to the ninth generation were registered in a recognized Stud Book. Many felt that the Jersey Act hampered the development of the British Thoroughbred by preventing breeders in the United Kingdom from using new bloodlines developed outside the British Isles.
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