Note 21 — Chapter VI · Geographical Distribution.

The only exception is in the case of the fish on each side of the Isthmus of Panama, where about 30 per cent, of the species are identical. But it is possible enough that at some previous time this narrow Isthmus may have been even narrower than at present, if not actually open. At all events, the fact that this partial exception occurs just where the land-barrier is so narrow, is more suggestive of migration than of independent creation.