In Darwin’s day species were thought to have originated where we find them, so disjunct distributions, that is, similar species found in widely separated areas, were though to have been created separately in both areas.
Darwin maintained that this sort of distribution was more sensibly explained by a single origin flowed by migration, followed by extinction over part of the range, creating the current separated occurrences. He wrote:
“…it seems to me, as it has to many other naturalists, that the view of each species having been produced in one area alone, and having subsequently migrated from that area as far as its powers of migration and subsistence under past and present conditions permitted, is the most probable... the several species of the same genus, though inhabiting the most distant quarters of the world, must originally have proceeded from the same source, as they have descended from the same progenitor.”
Figure: disjunct distribution of the jellyfish Aurelia
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