Simplify the following text for a Grade 15 native English-speaking person.
Original: Incorporating the full range of animal diversity into the Ark story was becoming increasingly difficult , and by the middle of the 18th century few natural historians could justify a literal interpretation of the Noah 's Ark narrative .
Simplified: Incorporating the full range of animal diversity into the Ark story was becoming increasingly difficult , and by 1700 few natural historians could see any reasons to believe in a literal interpretation of the Noah 's Ark narrative .