The full title of Darwin's most famous book is "On the Origin of Species by means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life." (Emphasis mine). Whereas natural selection and Evolution are commonly believed to be the same thing, this title is proof that not even Darwin believed this. Rather, natural selection is the process by which Darwin believed Evolution occurred.
Many modern Evolutionists still believe this to be the case, however there is a significant weakness to this theory. Specifically, natural selection has always been a culling process. The weakest members of a species die and cease to reproduce, whereas the strongest tend to reproduce. But of course, the strongest must exist before they can reproduce. Therefore, natural selection cannot account for the origin of species.
In the above diagram, I have demonstrated this by presenting a hypothetical sample of alleles, the dominant producing rough scales, and the recessive producing smooth. You can put these into as many punnet squares as you like, feathers will never be produced. Kill off all the carriers of the smooth scales, all you have is a rough scaled population. Kill off all the carriers of the rough scales, all you have is a smooth scaled population. This process neither explains the origins of the scales, nor does it demonstrate that feathers could ever have evolved from scaly creatures.
You see, then, that by demonstrating natural selection, which Creationists actually discovered first, and have always believed, you have not demonstrated Evolution. If anything, you have demonstrated the opposite. If God created a genetically diverse group of animals to reproduce according to their own kinds, as Genesis says, natural selection could have occurred since the first day, as genetic information is lost and reshuffled regularly, eventually leading to less diversity as time goes on. But Evolution requires the opposite to happen! It requires that new genetic information be produced, and only once it has been produced can it possibly be selected for. Yet this has never been demonstrated. We've never seen the development of a brand new feature in a creature that did not previously have it. For this reason, natural selection can be said to have been scientifically proven, but Evolution, its polar opposite, cannot.