There is no one specific source for this information; it is quoted by many sources that animation developed in the early 1900s. The first animated feature film was made in 1918, but there are animations discovered before 1910. Everything changed when celluloid was introduced in 1913; this is when Walt Disney was the first one to add sound to animation, which premiered with steamboat Willie in 1928. Ten years later, he followed it up with a full-length feature - Snow White and the Seven Dwarfs.