2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/210509Invited contribution to the collection of articles: `The Birth of String Theory', edited by Andrea Cappelli, Elena Castellani, Filippo Colomo and Paolo Di Vecchia.16 pages. A few typos corrected and changed abstractHigh Energy Physics - TheoryHistory and Philosophy of Physicstext