Survival of the fittest is a result of selection and proliferation of only those organisms which were most suitably adapted to the environment and get selected by nature. To survive in a particular environment, anorganism need to adapt in that environment. If the organism be comes success in adapting them that organism is selected by nature.
Related Theory
Darwinism can explain the survival of fittest but is unable to account for arrival of the fittest. It is difficult to suppose that small variations could have formed organs like wings and mimicry colouration which are useful only in their fully developed state but what about their presence in early stages.