Complex sentence: His graduation thesis was on the progress of the working classes, the ambitious subject of "The historical development of epidemic and contagious diseases all over the world, with the laws of their diffusion", which showed the influence of Schönlein.
Split into simpler sentences: His graduation thesis was on the progress of the working classes. The ambitious title of "The historical development of epidemic and contagious diseases all over the world, with the laws of their diffusion" showed the influence of Schönlein.