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 ( 1864 ) , in which he drew parallels between his own economic theories and Darwin 's biological ones , writing , `` 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 . ''
Simplified: He first used the term in his book Principles of Biology , published in 1864 . In that book ( vol 1 p444 ) he wrote : `` This survival of the fittest , which I have here sought to express in mechanical terms , is that which Mr. Darwin has called â natural selectionâ , or the preservation of favored races in the struggle for life '' .