<P> "Survival of the fittest" is a phrase that originated from Darwinian evolutionary theory as a way of describing the mechanism of natural selection . The biological concept of fitness is defined as reproductive success . In Darwinian terms the phrase is best understood as "Survival of the form that will leave the most copies of itself in successive generations ." </P> <P> Herbert Spencer first used the phrase, after reading Charles Darwin's On the Origin of Species, in his Principles of Biology (1864), in which he drew parallels between his own economic theories and Darwin's biological ones: "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 ." </P>

What is meant by the phrase survival of the fittest