A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first line because it introduces the main topic. It talks about a mystery that Darwin thought about. This is a broad overview of the subject. It does not refer to any previous information. Why other lines are not chosen? Paragraph 2 starts with 'The zoologist Marion Petrie...' which uses 'the' and refers to a specific person, so it is not independent. Paragraph 3 starts with 'Darwin argued...' which uses 'Darwin' and assumes prior knowledge. Paragraph 4 starts with 'That is...' which is a transitional phrase, so it is not independent.

Concluding sentence: Why this is the last line? This paragraph is the last line because it wraps up the ideas. It summarizes Darwin's theory of natural selection. It gives a clear conclusion about survival in nature.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the discussion about Darwin. The phrase 'a century after Darwin's death' links back to the mystery mentioned in Paragraph 1. This shows a logical order of ideas.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it explains what Darwin argued. It gives evidence for the work that Marion Petrie is doing. This shows a logical order from the current research to Darwin's ideas.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it explains Darwin's idea of 'natural selection'. It gives more detail about how the 'fittest' survive. This shows a broader to specific strategy.