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Men who might be considered early ecologists such as Alexander Von Humboldt , Thomas Belt , Henry Walter Bates , and even Charles Darwin sailed to tropical locations and wrote extensively about the exotic flora and fauna they encountered .
 Alexander Von Humboldt, Thomas Belt, Henry Walter Bates, Charles Darwin
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 Alexander Von Humboldt, Thomas Belt, Henry Walter Bates, Charles Darwin