During the beginning of the Baroque period, as a companion for operatic and vocal music, the orchestra evolved. By the mid-1600s the orchestra was growing into its own entity and the concerto was one of the most popular forms of music performed. The concerto featured a solo instrumentalist, or small ensemble of soloists, playing in opposition to the orchestra, thus creating an interesting contrast of texture and volume. In addition, during the Baroque period composers began exploring music's ability to express the human spirirt and to depict natural phenomena. Vivaldi's The Four Seasons was the most famous set of concertos that typified this.