That seems a wierd way to look at it Ned. Considering the nature of many of the best known examples Darwin studied, differing species on islands in a chain, it seems pretty obvious why he would consider geographic isolation to be a significant factor. To see how non-surprising it is you only have to notice the Alfred Russel Wallace reached the same conclusions after his studies in the Malay archipelago.
It is also worth noting that geographic isolation is
not neccessarily required for evolution to occur. There is a PNAS review of an article on sympatric evolution in
Drosophila which discusses the the differing possible mechanisms of speciation.