Here's a summary of a proof stated by Ian Johnston at http://www.mala.bc.ca/~johnstoi/essays/courtenay1.htm

1) All living things come from living parents. (Evidence: Spontaneous generation was disproven centuries ago. Apart from that, lack of opposing evidence or alternate theories.)
2) There are many species alive today that are very different from each other. (Self-evident, I would hope.)
3) Very long ago, fewer and simpler organisms existed than exist now. (Evidence: Fossil record.)

Conclusion: Unless God is constantly creating new species out of nothing, "macroevolution" has to have occurred. Darwin's theory is the best naturalistic explanation of how that has happened.

Addendum in Ian Johnson's words: "To make the claim for the scientific truth of evolution in this way is to assert nothing about how it might occur. Darwin provides one answer (through natural selection), but others have been suggested, too (including some which see a divine agency at work in the transforming process). The above argument is intended, however, to demonstrate that the general principle of evolution is, given the scientific evidence, logically unassailable and that, thus, the concept is a law of nature as truly established as is, say, gravitation."