How Good Is a Mimic on the Tail of a Snake?

There are different levels of evolutionary difficulty. A poor mimic might be good enough to fool a bird at 10 meters. And so we can see more or less the path that an eye would travel, where it is valuable at each of the intermediate steps. But he also goes through an explanation of a crude eye and its value. He discusses there the battle, the arms race, between a caterpillar that mimics maybe it mimics bird shit.