Passage Summary: Some people used to think evolution makes animals perfect for their surroundings, but that's wrong because the world changes too much for any one creature to be ready for everything.

Conclusion: The idea that natural selection eventually produces perfectly adapted organisms is incorrect.

Reasoning: Environments are highly variable, and it is impossible for an organism to have a set of traits that allows it to cope with every possible condition it might encounter.

Analysis: The first sentence contains the main claim, signaled by the phrase 'It is a mistake to think.' The rest of the passage provides the 'why' (the premise), specifically using the indicator 'After all.' When you see 'It is a mistake to think X,' the conclusion is usually 'Not X.' Focus on the structural relationship where the second sentence supports the rejection of the old ecological belief.