Darwin, Wallace, Huxley, Mendel, Spencer...
Wallace sent his letter outlining a theory of natural selection to Darwin in 1858 because the latter was a well-known scientist at the centre of an extensive web of biological and geological correspondence. If he had not had Darwin to send it to, there were several alternatives - Huxley, Hooker, Lyell to name three. It is likely that his work would have been published, but quite possible it would not have made the "splash" Origin of Species did: Wallace was an impoverished and obscure collector (and, horror of horrors, a socialist), while Darwin was rich and respectable. Moreover, Darwin had taken the central idea much further than Wallace, and had amassed far more evidence for the fact (as well as the mechanism) of evolution. It should be remembered that the theory of natural selection, unlike evolution itself, was not widely accepted until the early 20th century; and indeed, Darwin's ignorance of the mechanism of heredity made this quite reasonable - if the hereditary mechanism blended the parents' characteristics, as he thought, rather than recombining them, as it does and as Mendel's experiments suggested when rediscovered in 1900, natural selection could only act to eliminate deleterious mutations. However, Darwin's ideas were then available to be synthesised with Mendel's, while Wallace's might have been forgotten. So all in all, I'd say that if Darwin had died young, or fallen overboard from the Beagle and drowned, there would probably have been a considerable delay both in establishing the fact of evolution, and in appreciating the central role of natural selection.
On a related topic, and the reason I came here - I am sure I have read an alternate history short story in which the Soviet Union never comes into existence, but the USA undergoes a "Spencerist" revolution, inspired by Herbert Spencer, who probably coined the term "survival of the fittest" (although Darwin also used it), and whose ideas underlie much of what is often called "social Darwinism". Does anyone know of this? Thanks.