[Riot]

Originally Posted by cmorioles

Since Man O' War is usually included, along with Sir Barton, I'm not sure that is going to fly.

Yes, they are usually included in "modern era". I just wanted to point out that there was indeed a pretty significant change in the American Thoroughbred world, in the genetic composition of the bloodstock itself, that resulted in what we describe as the "modern Thoroughbred".

Good example: Blenheim

From TB Pedigree:
Owner: A Syndicate in U.S.
Breeder: Lord Carnarvon
Winnings: 10 Starts: 5 - 3 - 0, £ 14,533 ($73,067)

1929: 1st New S. (ENG), Hopeful S. (ENG), Manton Plate (ENG), Speedy Plate (ENG)
2nd Middle Park S. (ENG), Champagne S. (ENG)
AT 3: Won Derby S. (ENG) - only start at 3

Won First Epsom Derby for HH Aga Khan III, as an 18-to-1 outsider, beating the Aga Khan's Rustom Pasha who was the second favourite at 9-to-2. Entered stud in 1931. Imported to USA (1937). Died 1958, buried at Claiborne Farm. Properly US-registered as "Blenheim II".

And from Wiki, basic TB history, one of the reasons why imports took off in the US (so we could sell them back). But that changed our bloodlines,and the look of the horse:

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.[36] It prohibited the registration of any horse in the [ English ] 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 The act was not repealed until 1949, after which a horse was only required to show that all its ancestors to the ninth generation were registered in a recognized Stud Book.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.

So we imported those horses here, so we could interbred them, and sell them back internationally. The reinfusion of the Euro lines with the extant American lines changed the racehorse into the modern physical and genetic specimen we are most familiar with.