The Theory of Natural Selection proposed independently by these two men was the cohering conclusion to several strands of wildly different data:
- The graded similarities among adults of related species: we look more like our dogs than either looks like frogs - it's the nipples, stupid
- The divergence of embryonic development: a 6 day whale fetus is almost indistinguishable from a 6 day bat fetus
- Trends in the fossil record
- Biogeographical distribution: that are no marsupials in Europe outside of zoos; the flora of Ireland is really similar to the flora of England and wildly different from Brazil's - the explosive difference shook European naturalist [Darwin, Hooker, Wallace, Bates, Huxley, Humboldt, Bonpland etc.]
Wallace was also remarkable for his longevity, dying at age 90 in 1913: a year an a half after Alfred Wegener published his iconoclast suggestion that the continents have been romping about the face of the globe crashing into each other and groaning apart. That's why you find marsupials in Australia and South America but not in Ukraine or Ireland. But everyone ignored Wegener until 50 years after Wallace was dead.