It is sometimes argued that the world in itself must bear a close resemblance to the world of appearance, because otherwise humans would not have been able to survive and evolve; we must be fitted to our real environment. It is said that Kant was therefore wrong to argue that things-in-themselves are unknowable, and that he would probably have changed his mind if he had lived to read Darwin's theory.
However, the fact that there is such a thing as evolution at all is something we learn through sense experience (reading and listening), just like any other fact such as that there are mountains, or that ice is cold, or that Julius Caesar ruled Rome. The truth of evolution is therefore no more certain than any of these other 'obvious' truths, including the existence of the entire world. It thus seems to beg the question if we assume the reality of evolution as evidence of the reality of the world. If our direct sense experience is insufficient to convince us of the external reality of everyday objects (i.e. if sense experience is considered a 'weak link' in the chain of inference), why should our apparent knowledge of evolution (which we learn about via the same 'weak link') provide a firmer basis?
1. It is possible that things in the world of appearance do not correspond to any things-in-themselves (because the former are mediated by the structure of our mind).
2. Evolution is a thing in the world of appearance.
3. Therefore, it is possible that evolution does not correspond to any thing-in-itself.
4. Therefore, things-in-themselves need not be such as to enable humans to survive over an extended period.
5. Therefore, things-in-themselves may be completely different from things in the world of appearance. Perhaps, for example, I am a brain in a vat, or (per Dave Oblad) part of a mathematical formula (one of an infinite number of such formulae, covering all possible states of affairs).