Camels love the desert, right? There are deserts all over the world – Asia, Australia, Africa, North America, and South America. Yet camels are only native to Asia. Sure, you can find camels in other places now, but it's just because people moved them there. If a creator made a camel for living in the desert, why restrict them just to Asia? (Hint: it's not for fear of camels taking over the world.)
Darwin was a big fan of biogeography, and spent quite a bit of time thinking and writing about it as he traveled the world. Just as we saw with the camel, Darwin noted that the physical environment was clearly not the only factor determining where a given species lived; the same exact habitat on two different continents might have completely different kinds of organisms. He considered this observation evidence for evolution.
What was his reasoning? As we said before, if a creator made all the living things on earth, you would expect to see them distributed anywhere they could survive. Yet there are no camels in Arizona, no giraffes in Hawaii, and no chimpanzees in Florida—or at least, they're not native to those places. However, if we accept that species evolve, then each species must have had a region of origin, or a place where they evolved from an ancestor. From there, that species and/or any closely related species could spread out. Thus, we would only expect to find them in the region they originated or in areas they could feasibly reach.
Darwin also noticed that different kinds of organisms on one continent were usually more similar to each other than to organisms on other continents. For example, South America has some crazy kinds of rodents—take the capybara, which, at over 100 pounds, could probably squish a lot of dogs. Anyway, the capybara hangs out in the water a lot, so we might expect it to be pretty similar to something like a beaver. Even though the beaver and the capybara live in similar habitats, each one is more similar to other rodents that live in its own geographic area. Again, evolution can explain this pattern. If a group of related species all descended from a common ancestor, you'd expect them to share many similarities and to cluster together geographically, even if they evolved different lifestyles.
Image from here.