Complex sentence: Per Boas, these plays, distinguished by their themes and treatment, require classification beyond comedy or tragedy so, adopting the popular classification of his time, he called them Shakespeare's problem-plays.
Split into simpler sentences: Per Boas, these plays, which are distinguished by their themes and the manner in which they handle these themes, require a discrete classification beyond comedy or tragedy. To name this genre, Boas adopts the popular classification of his time and defines these plays as Shakespeare's problem-plays.