Complex sentence: Its success was such that it reached a circulation of 6 million copies in 1900, by 1914 it sold 7 million copies, it was still used in schools until the 1950s and still in print to this day.
Split into simpler sentences: Its success was such that it reached a circulation of 6 million copies in 1900, and by 1914 it had sold 7 million copies. It was still used in schools until the 1950s and is in print to this day.