Answer: The correct answer is : The central idea is the need to make an observation carefully, to look again and again and to be able to study the reality in front of us.
Explanation: Agassiz was the founder of the Harvard Museum of Zoology and a professor there as well. The detailed observation that he ordered his student to make initially from a fish and then to make the comparison between many fish, cost the student a lot of work but finally he recognizes that it was a great legacy that his teacher left him.