One thing Darwin didn't do on the voyage was propound the theory (or even a theory) of evolution. For a start, evolution as a concept was already decades old by the 1830s. Darwin's own grandfather, Erasmus, had paid tribute to evolutionary principles in a poem of inspired mediocrity called 'The Temple of Nature' years before Charles was even born. It wasn't until the younger Darwin was back in England and read Thomas Malthus's "Essay on the Principle of Population" (which proposed that increases in food supply could never keep up with population growth for mathematical reasons) that the idea began to percolate through his mind that life is a perpetual struggle and that natural selection was the means by which some species prospered while others failed. Specifically, what Darwin saw was that all organisms compete for resources, and those that had some innate advantage would prosper and pass on that advantage to their offspring. By such means would species continuously improve.

It seems an awfully simple idea - it is an awfully simple idea - but it explained a great deal, and Darwin was prepared to devote his life to it.
'How stupid of me not to have thought of it!' T. H. Huxley cried upon reading "On the Origin of Species". It is a view that has been echoed ever since.

Interestingly, Darwin didn't use the phrase 'survival of the fittest' in any of his work (though he did express his admiration for it). The expression was coined in 1864, five years after the publication of "On the Origin of Species", by Herbert Spencer in "Principles of Biology". Nor did he employ the word 'evolution' in print until the sixth edition of "Origin"
(by which time its use had become too widespread to resist), preferring instead 'descent with modification'.