The idea that species adapt and change by natural selection with the best suited mutations becoming dominant.
This expression is often attributed to Charles Darwin and, although it appears in the fifth edition of his Origin of Species, 1869, it is there attributed to Herbert Spencer:
"The expression often used by Mr. Herbert Spencer of the survival of the fittest is more accurate..."
Spencer had published The principles of biology in 1864. In that he referred to 'survival of the fittest' twice:
"This survival of the fittest, implies multiplication of the fittest."
"This survival of the fittest... is that which Mr. Darwin has called 'natural selection, or the preservation of favoured races in the struggle for life'."
By 'fittest', of course, Spencer and Darwin didn't have in mind the commonly used meaning of the word now, i.e. the most highly trained and physically energetic. The 'fittest' referred to here are those animals which are the most suited to their environment, i.e. those which are best fitted to survive.