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This is a difficult case. Is a peacock's tail display an unintended consequence of the fact that peahens are able to see? Certainly (if one believes the evolutionary story), peacocks grew tail displays to impress peahens, which would not have been possible were they not able to see. Of course, the fact that they are impressed involves more than just the ability to see. It's also not clear how the fact of being impressed and the fact of growing impressive tail displays mutually reinforce each other. That's another long story. But at some level, the ability to see is a fundamental prerequisite to it all. None of it would have happened were peahens blind. So one can truly say that peacocks used the mechanism that allows peahens to see to achieve an end that probably wasn't initially "intended" as the end to be achieved by vision. That makes vision much more teleological than it really is. But the point is that vision is a mechanism that appeared in the world. Its existence (along with other things having to do with the way that peahens react to tail displays) was then exploited by peacocks to achieve an end.