Allow me to compare evolution to geology. We can see mountains rise a little tiny bit as time passes for us. Logically, we have no reason to believe there was a dramatic change in the mechanics at work in the growth of a mountain: plate tectonics. So we can assume that the mountain has always grown like that, rather than suddenly being there and growing slowly after that. The same goes for evolution. Though it takes long periods of time to see these huge changes, longer than many human lifespans, we can still see the small changes and accept that that has consistently been how it works as the most logical explanation.