A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first line because it introduces the main topic about Australia's separation from Southeast Asia. It gives a broad overview of the geographical history of Australia. This follows the rule of Topic Sentence. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with 'In that time,' which is a conjunction. Paragraph 3 starts with 'The best-known examples,' which refers back to a topic. Paragraph 4 starts with 'But there are countless smaller creatures,' which is also a conjunction. Paragraph 5 starts with 'Many other species,' which uses the definite article 'the' and assumes prior knowledge.

Concluding sentence: Why this is the last line? This paragraph is the last line because it wraps up the discussion about species in Australia. It provides a summary of how many species came to be in Australia, which follows the rule of Summary Style.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it talks about what can happen in evolution over time. This shows a logical order from the idea of separation to the effects of that separation.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it gives specific examples of species that evolved in Australia. This follows the Broader to Specific Strategy, where the general idea of evolution leads to specific examples.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it continues the idea of unique species in Australia. It uses the Connector 'But' to show that there are even more examples beyond the well-known ones.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 connects to Paragraph 4 because it talks about other species that are now in Australia. This shows a logical order from smaller creatures to a broader idea of species that came from elsewhere.