The phrase 'the survival of the fittest' is a classic example of something that cannot be properly understood without thinking about its context
The phrase was coined by Herbert Spencer, the Victorian social theorist. Spencer had read Darwin, but his view of evolution was not Darwinian, because he believed (like the French biologist Lamarck) that an organism's adaptations to its environment could be passed on. This is completely opposite to Darwin, who thought that all variation happened by chance and the only mechanism was inheritance of variations by offspring - those organisms that survived to pass on their variations were the 'successful' ones.
Spencer's phrase, then, described his own understanding of Darwin's idea, in which the idea of adaptation and purpose were important. It's essential to say that for Darwin there is no such thing as adaptation, and there is no such thing as purpose. Evolution has no purpose beyond its own operation. There is no goal towards which it is moving and no end to the process. To speak about our 'evolutionary purpose', therefore, is completely wrong in Darwin's terms (though not in Spencer's or Lamarck's).
Unfortunately, Spencer's phrase was immediately, and has been ever since, taken out of context and misunderstood. In particular it has become the one of the slogans of social Darwinism (the application of the theory of evolution by natural selection to human society) and thus to eugenics.
This, if I may say so, is a really good example of why context is so necessary. Unless 'survival of the fittest' is understood in the context of the Victorian debates over evolution it will be misunderstood.