When Rubber Footballs Were All The Rage

Early folk football games occurred post-harvest using the inflated bladders of recently-slaughtered pigs. Eventually, they covered the pig bladders with cowhide, and the bladders transitioned to rubber, yet the combination continued to be called pigskins. — www.footballarchaeology.com
The era and inception of rubber footballs and how they were perceived int he game. Football Archaeology does a nice bit of research on the subject.