Obviously, some screenplays are adapted from other works, whereas others are written from scratch.
Intuitively, it seems like there would be fewer original scripts now, given the prevalence of superhero films (based on comic books) or adaptations of YA novels, and the increasing amount of older material to draw upon. But perhaps that's simply perception.
How, if at all, has the ratio of original screenplays to adaptations changed over time in Hollywood, from its inception to the present day?