His most direct contribution to medicine was the invention for his own use of bifocal eyeglasses.
Any other fish or mammal would have been just as likely as Anableps to begin the development of a bifocal eye.
bifocal bi·fo·cal (bī-fō'kəl, bī'fō'-)
Having two focal lengths.
Having one section that corrects for distant vision and another that corrects for near vision.