Don't rely on your eyes - they'll lie to you far too often. Two animals that may look similar may be totally different species.
This isn't the eyes. This is actually the result of breeding experimentation for tens of centuries. There _are_ observable species boundaries.
Also, the lines between species isn't always clear. I think there are cases where breeding can still occur between genera. Classification isn't all its cracked up to be.
A progressive fossil record is the evidence, showing change through time. This is not equivalent with proof for evolution. Creationism allows for both (a) progressive creation, and (b) certain kinds of speciation. Both of which equate to change over time. However, thousands of years of breeder experimentation has shown that there are, indeed, limits to change.
Darwin's view on this was simply that nature both causes and allows more change than breeding. However, this is pretty much just conjecture.