The Origin acted as a seed in a supersaturated cultural solution. The solution was already set to crystalise, and he came along and made evolution, phylogeny, heredity, dispersal, and other topics respectable. So far from being a “class traitor” as Desmond and Moore make him out to be, Darwin’s bourgeois respectability is what made him effective as a motivator of evolutionary biology, where evolution had previously been seen as politics in naturalists’ dress. He was in many ways more prescient than those who followed him. But had he not lived, had he drowned on the Beagle voyage, as Peter Bowler is presently writing a what-if history, would we have had evolution anyway? Almost certainly. If Owen hadn’t shaken off the shackles of the clergy who ran Cambridge, very likely others would have. But the tenor of the field would have been somewhat different. Here’s how I think it would have played out, based on a discussion with Bowler about his work. Anything original is his.
Common descent/phylogeny might not have developed as early. Heinrich Bronn had given an “evolutionary tree” in 1858, but his mechanism was no different to Buffon’s – the degeneration of types from an original stock; basically evolution was localised and played out on existing potentialities.
Natural selection might not have been seen to be a mechanism of evolution for another fifty years, and when it was, it would not have been in terms of an analogy with artificial selection. Wallace would have been “rediscovered” the way Mendel was, without being all that influential directly. Sexual selection might not have popped up until the mid-20th century or so.
There would have been a much stronger emphasis on developmental biology, coming out of von Baer’s work and later experimental developmental biology in Germany. Evo-devo would have been the first state of the field.