Complex sentence: Prior to this there had been a single race for both colts and fillies, the Poule d'Essai, which was first run in 1840 at the Champ de Mars, moving to Longchamp in 1857.
Split into simpler sentences: Prior to this there had been a single event for both colts and fillies, the Poule d'Essai. This was first run in 1840 at the Champ de Mars, and it moved to Longchamp in 1857.