by Richard Dawkins.
I'll try to do justice to it here. There are lots of caterpillars in a particular species, and they all look quite similar to each other, but each slightly little different. Some of them look a little tiny bit like snakes, if you see them in the dim light of dawn or dusk, or deep forest, or just catch a glimpse of them as you pass. That small difference in look makes predators pause, or pass them by, or back away at a just slightly higher rate than their brethren. They tend to live just a little bit longer, and mate and have more offspring. And so the population of caterpillars that look just a little bit like snakes grows larger. Their children all look just a little bit like snakes, but some of them are a little more green, or just slightly more bulgy, or happen to have a slight dark patch on their haunch. This, for some of them, makes the illusion just a little bit more convincing, giving them just a tiny edge against predators in low light, in marginal situations - but it is just enough or the effect to continue, over millions of generations, to create and select for mimicry that eventually is good enough to be very convincing indeed, in full light.
Evolution is so beautiful, and powerful, and amazing.
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