sentences:  While still in his teens, he toured as a violinist with the Adolf Busch Chamber Orchestra, but the youthful performer's mastery of the viola came to the fore when, at a few days' notice, he was asked to take over from the ailing second violist in a performance of a Mendelssohn Quintet with the Busch Quartet.His achievement was so remarkable that he was invited to join the Quartet in presenting a series of Mozart quintets at London's Wigmore Hall.From then on, Paul Doktor stuck to the instrument fate had chosen for him, and became the first violist ever to have been awarded unanimously the First Prize at the International Music Competition in Geneva.