Isolation---by distance as well as barriers?

Hi Folks. Help me understand Darwin when he wrote that two or more varieties could be formed in a strictly continuous area. He also wrote he believed "many perfectly defined species have been formed on strictly continuous areas".Isn't isolation neccesary for evolution? So if there are no geographical barriers to separate different groups of a species, can mere distance serve to isolate them?Hypothetically, I'm thinking of a species of hare that has a range...let's say 300 miles wide. Wouldn't the genes of all the would-be varieties keep criss-crossing across the range, and keep the species pure? Or if the distance is great enough, can varieties form?Thanks for any help.

Thanks for the replies and examples. You've answered my question. Basically, natural selection can act faster than crossbreeding over a wide range can counter it.This is important to me when contemplating the theory of evolution, because it effects the speed at which evolution can proceed. It would take a lot longer if evolution had to wait for a species to migrate through a narrow pass in a barrier and fill the other side, or for geologic upheaval. Edited by InGodITrust, : No reason given.Edited by InGodITrust, : No reason given.Edited by InGodITrust, : No reason given.Edited by InGodITrust, : No reason given.