Intermediates

Thanks Nuggin for breaking down so many examples of "half-an-eye" (which as you pointed out, is really just "an eye").Maggots are photophobic and seek to avoid light. In the presence of a light source they will sway their heads back and forth giving them the ability to compare light on one side to light on the other, in the hope of heading for darker places. This works fantastically for horror movies where a decaying corpse is illuminated and the maggots all burst into action.In addition to the salient point that individuals do not evolve, populations do, I think it is also important to consider the vast numbers of unique species along the tree of life that have not survived, and of these only a very tiny proportion has left behind any fossil evidence for us to examine.Before you ask "what good is half an eye" you need to ask yourself how you define "an eye". As other posters have mentioned, there really is no such thing as "half an eye". You either have the ability to detect light, or you don't. The difference between a minor ability (such as that of the maggot's eye) and a dramatic ability (such as that of a human's eye) is represented by a continuum of varying degrees of sophistication in between. For any particular location in that continuum, chances are you will find an extant creature with that sort of eye.Edited by Briterican, : Typo in topic subtitle - meh.