"I can only assume that your editorial writer tripped over the First Amendment and thought it was the office cat"

This quote by E. B. White is a funny method of expressing his shock at the actions of an editorial writer. He is indicating that the writer was so careless and uninformed of the First Amendment that they mistook it for the workplace feline. This suggests that the writer was not knowledgeable about the First Amendment, or was not taking it seriously. White is suggesting that the writer was not taking their task seriously, and was refraining from doing their due diligence in looking into and comprehending the First Amendment. This quote is a humorous way of revealing White's disappointment in the writer's lack of understanding and understanding of the First Amendment.