I collect old cartoon books. This one's called "Cartoon Cavalcade". It's edited by Thomas Craven, 1943. It traces American cartoons back to its roots and features a wide variety of styles, from comic strips to magazine cartoons and even animation. There was a much greater variety of cartoon styles in the first half of the 20th century than there is today or even that there was in animation in its whole history.
These are just a handful of cartoons from the 20s that influenced the "UPA Style".
Even though cartoons had a huge variety of individual styles, most people agreed they had one thing in common - the thing that made them cartoons, as opposed to illustrations:
More to come...