English: Blinking has evolved repeatedly. (A) A phylogeny showing the distribution of blinking in osteichthyans (bony fishes). A black square indicates that blinking is present within a clade. A white square indicates that blinking is absent in the group. A pink hash mark indicates where blinking is hypothesized to have originated. In some clades labeled with black squares, where blinking generally characterizes the group, blinking has been secondarily lost (e.g., snakes and some fully aquatic frogs). Among actinopterygians, select clades are shown to convey both the general condition among the more than 30,000 actinopterygians, the absence of blinking, as well as the phylogenetic position of mudskippers. (B) In humans, blinking involves lowering of the upper eyelid. Other tetrapods, however, can blink with other membranes. (C) A yellow mongoose (Cynictis penicillata) closing its eyes with a nictitating membrane, which moves in a rostrocaudal direction. (D) A mallard duck (Anas platyrhynchos) closing its eye by raising the lower eyelid. (E) A pug-headed mudskipper (Periophthalmodon freycineti) blinking and showing movement of the eye and dermal cup. All images by John Morris.