Despite the impassable barriers of land which now sever the tropical oceans, we can yet speak of a =circumtropical= zone possessing many species common to its most widely separated parts. Such circumtropical species, occurring on both the Atlantic and Pacific coasts of tropical America, on the West African coast, and in the Indo-Pacific, are various Grapsidae, _Calappa granulata_ and its allies, and certain _Albunea_. The most striking instance of all is that of the Land-crabs. Of _Ocypoda_, the greater number of species occur in the Indo-Pacific, but representatives are also found on the tropical Eastern and Western American coasts and on the West African coast, and the same is true of _Gelasimus_. The genus _Cardisoma_, belonging to a different group of Land-crabs, is also typically circumtropical. For this community of the circumtropical species we may certainly advance in explanation the comparatively recent formation of the Isthmus of Panama. Besides the resemblance of the Crustacea on the east and west coasts of the isthmus, we have an actual identity of species in several cases, e.g. _Pachycheles panamensis_ and _Hippa emerita_, and the same thing has been observed for the marine fish. Another connexion, at any rate during early tertiary times, which probably existed between now isolated tropical coasts, was across the Atlantic from the West Indies to the Mediterranean and West African coasts. Numerous facts speak for this connexion. Species of _Palinurus_ and _Dromia_ occur in the West Indies and the Mediterranean, which only differ from one another in detail, and a connexion between these two regions has been urged from the minute resemblances of the late Cretaceous Corals of the West Indies with those of the Gosau beds of S. Europe, and also of the Miocene land-molluscs of S. Europe with those at the present time found in the West Indies. To account, then, for the present distribution of littoral Crustacea we must imagine that great changes have taken place during comparatively recent times in the coast-lines of the ocean, but the guiding principle in both the past and present has been temperature, and this factor enables us, despite the immense changes in the configuration of the globe that must have taken place, to divide the coasts latitudinally into Arctic, Antarctic, and Circumtropical zones.