Convergent evolution besides being a major mechanism of evolution itself, is one of the major methologies of speculative evolution. When we see that eyes or hair have evolved several times on Earth, it becomes very plausible that they would continue being independently discovered further on in Earth history or during evolution on an alien world. Better than that even, we can examine the similarities and differences amongst these semi-independent occurrences, and get a better idea of what the general space of possibilities looks like.