In George Orwell's Animal Farm, written during World War II and published in 1945, the animals of Manor Farm overthrow their human owners and take over the farm. In the novella's first chapter, a pig named Old Major sows the first seeds of this revolt and this saying when he tells the animals that
Whatever goes upon two legs is an enemy. Whatever goes upon four legs, or has wings, is a friend. Whatever goes upon two legs is an enemy. Whatever goes upon four legs, or has wings, is a friend.
Eventually, after the revolt against the humans, the last part of Major's mandate becomes the second of the seven commandments that the animals adopt in Chapter 2: "Whatever goes upon four legs, or has wings, is a friend."
By Chapter 3, this slogan has undergone further revision and becomes the summary for entire set of commandments since the pigs discover that many of the other animals are too ignorant to learn all of the seven commandments. Thus, Snowball declares that all even commandments can be reduced to a single maxim, namely: "Four legs good, two legs bad." The other animals accept this explanation and revision, especially the sheep, who delighted in repeating the phrase over and over again. As with the other commandments, the revised version was written on a prominent wall of the farm.
The subtle changes in this saying from their original conception by Major to Snowball's revision in Chapter 3 are paralleled in the rise to complete domination of the pigs in the course of the book. By Chapter 9, the sheep use their chants of this saying to drown out the voices of anyone who might complain about the Spontaneous Demonstration.
By the novella's final chapter, though, pigs begin walking on two legs, thus violating the core of their previously expressed beliefs. This gives way to a further revision of the commandment by the pigs: "Four legs good, two legs better!"
Thus, by the conclusion of Animal Farm, the pigs have become the very thing that Old Major had declared as evil at the novella's beginning. Orwell's repetition of this commandment and his subtle changes to it throughout his work thus demonstrate its importance.