Simplify the following text for a Grade 8 native English-speaking person.
Original: Herbert Spencer first used the phrase â '' after reading Charles Darwin 's On the Origin of Species â '' in his Principles of Biology of 1864 in which he drew parallels between his economic theories and Darwin 's biological , evolutionary ones , writing , â This survival of the fittest , which I have here sought to express in mechanical terms , is that which Mr. Darwin has called ` natural selection ' , or the preservation of favored races in the struggle for life . '' In the first four editions of On the Origin of Species , Darwin used the phrase `` natural selection '' .
Simplified: Darwin agreed that the phrase survival of the fittest was better than natural selection .