Yesterday was the 100th anniversary of the forward pass. I've just finished reading a book that traced the evolution of baseball in the U.S. with soccer everywhere else, and one of the interesting things that characterizes the sport the rest of the world calls "football"* is that it was developed by committee. Several committees, in fact; the early Football Association basically consisted of a number of gentlemen representing various towns and "public schools", each of which played their own variation of the game, and in the mid-nineteenth century, the two most popular versions included one developed at the Rugby School, which allowed players to carry the ball. The committee ultimately split, with the majority favoring no hands, and that sport became popularly known as "association football" ("soccer" is a bastardization of the first word), and the sport known as rugby favored by the seceding minority.
In the fullness of time, soccer became less of a "gentleman's game," while rugby continued to have strong ties to English public schools, from which the early American version of the game spread to Ivy League students in the late-19th Century. Since the forward pass is inarguably the most distinctive difference in the rules between American football and rugby, it is interesting (at least to me) how the two sports differed prior to 1906. Both sports feature running with the ball towards the other team's goal, permit the other side to physically stop the ball carrier via tackling, and allow for more points if you cross the goal line with the ball as opposed to kicking it through the goal posts.
The impression I have is that prior to 1906, the American version had a great deal more violence, with several college players dying every season. President Theodore Roosevelt, whose son had played the sport at Stanford, threatened to outlaw the game, and a number of schools (including my alma mater, Cal) switched to rugby at around that time. If I had to fathom a guess, I would say the big rules difference before 1906 was that the "scrum" was more open-ended in American football, and was used as the primary means of moving the ball forward, as opposed to rugby, where it seems to be used to resolve possession disputes. Today, of course, no American football fan would even notice a "scrum"; it's the routine battle that takes place during every play at the line of scrimmage. In any event, the sports seem to have evolved quite dramatically in different directions even before the forward pass was legalized in the U.S.
*Except the U.S. and Canada, which favor "soccer", and Italy, which inexplicably calls the game "calcio." Since they're the champs, who's to argue?