The Brain Teaser Challenge

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Across
  1. 2. The theory that species evolve over time by natural selection (9 letters) (The reason we are solving this puzzle today.. Haha!)
  2. 4. The study of knowledge, reality, and existence (11 letters)
  3. 5. The basic unit of a chemical element (4 letters)
  4. 6. Unit A unit of astronomical distance roughly equal to the distance from the Earth to the Sun (8 letters)
  5. 9. The longest river in the world! eeszy peesy!! (4 letters)
  6. 12. A subatomic particle with a positive electric charge (7 letters)
  7. 13. The branch of science concerned with the classification of organisms (10 letters)
  8. 14. A branch of mathematics dealing with shapes and spaces (8 letters)
  9. 15. The smallest bone in the human body (7 letters)
Down
  1. 1. The ancient Greek physician known as the "Father of Medicine"!! (12 letters) Thats easy too!
  2. 3. A poetic term for twilight (7 letters)
  3. 5. A rare, naturally occurring radioactive element (8 letters)
  4. 7. A 19th-century artistic and literary movement that emphasized emotion and individualism (9 letters)
  5. 8. The term for a word that has the same spelling but different meanings (8 letters)
  6. 10. A philosophical doctrine that emphasizes the role of experience and evidence (10 letters)
  7. 11. A colorless, odorless gas essential for respiration (6 letters) (fits in perfectly! ;) )
  8. 16. The mathematical constant approximately equal to 3.14159 (2 letters) (Is also a 3rd word of a great movie!)