My guess, is that this is from the 1930s and was probably a give-away premium for the Professor Bunko radio show. In the corner box on the envelope is a contest sponsored by The Magic Sponge, a Great Western Products Corporation. I'm not familiar with a Professor Bunko radio show or a Magic Sponge. Internet searches came up with zilch.
It's interesting that some of the artwork is very similar to the work of Theodor Geisel, better known as Dr. Seuss. He did a lot of work for advertisers before World War II and it makes me wonder if he worked on this set. Even the names on the back of the cards sound like Seuss type of names.
Unfortunately this set is missing a few cards (the pieces to make the creature on the envelope), but I've put together a nice animal show below.