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I already learned how the mammal circulatory system (double circulatory system) evolved from the reptilian circulatory system. How did natural selection evolve the reptilian circulatory system and the amphibian circulatory from the fish circulatory system (single circulatory system)? Did the reptilian circulatory system evolve from the amphibian circulatory system? If so, how? http://www.shmoop.com/animal-movement/animal-circulation.html shows what the fish, reptilian, and mammal circulatory system look like and https://universe-review.ca/I10-82-circulatory.jpg shows what the amphibian circulatory system looks like.

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