Why do we say "Survival of the fittest"?

Meaning:

Species adapt and change by natural selection with the best suited mutations becoming dominant

Background:

When setting this Wordplay we had in mind "Survival of the Fittest", but on seeing the answers we realized that "Sins of the Father" or "Sins of the Flesh" were also valid answers so included these when drawing the winner.

Although most people would likely attribute this expression to Charles Darwin, it was actually first coined by British polymath Herbert Spencer who used the phrase, after reading Charles Darwin's On the Origin of Species (1859), in his Principles of Biology (1864).  In the book Spencer compared his own economic theories and Darwin's biological ones, using the expression twice:

"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 favoured races in the struggle for life"

"This survival of the fittest, implies multiplication of the fittest."

Darwin first used Spencer's phrase as a synonym for "natural selection" in the fifth edition of On the Origin of Species (1869).  By 'fittest', of course, Spencer and Darwin didn't have in mind the commonly used meaning of the word today (the most highly trained and physically energetic) but rather those animals most suited to their environment and thus best fitted to survive.

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