The four-eyed fish sees all

Anableps, the four-eyed fish, frequents the rivers and estuaries from southern Mexico to northern South America. Ac-tually, this curious fish has only two eyes, but each is divided in half horizontally; that is, each eye had two separate optical systems, each with its own focal length. The top half is for seeing in the air; the bottom half is for underwater. Thus equipped, Anableps can see prey and predators above and below the surface at the same time and increase its opportunities to get meals as well as escape from its enemies.

(Zahl, Paul A.; National Geographic Magazine, 153:390, 1978.)

Comment. Nature is full of such "marvelous" adaptations, but it is hard to see how fish bifocals could develop gradually, given the intolerance of optical systems to minute changes in dimensions, position, and refractive index.