Simplify the following text for a Grade 8 native English-speaking person.
Original: He is best known for coining the concept `` survival of the fittest '' , which he did in Principles of Biology ( 1864 ) , after reading Charles Darwin 's On the Origin of Species .
Simplified: He is best known for coining the phrase , `` survival of the fittest '' . He did this in Principles of Biology ( 1864 ) , after reading Charles Darwin 's On the Origin of Species .