Long before Sam went to extraordinary lengths to peddle discolored breakfast foods to obstinate citizens, Theodor Seuss Geisel (Dr. Seuss, if you please) made his living as an advertising illustrator–and in retrospect, his work is unmistakable.

Seuss became the father of the modern day children’s stories not solely through his inventive lexicon molded into clever syntax and anapestic meter, but also through vivid imaginary worlds and the charming characters within them. Take one look at his early creations for brands including GE, Ford, and NBC, and there’s no denying the framework of his style that would later turn into the denizens of Whoville, Cat in the Hat and Fox in Socks. And, according to the keepers of the Seuss collection at the UC San Diego Library, the enduring brilliance that is Seuss’ legacy can be traced back to a very unlikely source: bug spray.

As is also recounted in Judith and Neil Morgan’s book Dr. Seuss & Mr. Geisel, newly married and having left his Ph.D. studies at Oxford University (his “Dr.” title was later legitimatized with an honorary from his alma mater Dartmouth), Seuss found work as an illustrator at the satirical magazine Judge in New York City where a cartoon he drew featuring a knight with a pesky dragon problem caught the attention of an ad executive’s wife. The ad exec just so happened to be in charge of the agency’s Standard Oil account (the company that owned Flit), and Seuss’ illustration evolved into a 17-year campaign that got his family through the Great Depression and aided in funding the early stages of his writing.

Seuss is one of many cultural icons who cut his teeth in advertising; he’s in company with the likes F. Scott Fitzgerald, Bob Newhart, Jim Henson, and Sir Ridley Scott.

See his work in the slide show above.