THE MARINE EAST-INDIES - CENTER OF ORIGIN

Much of the argument about the existence or nonexistence of centres of origin has been focused on the marine East Indies. During the past 10 years, considerable new information has become available. Patterns suggesting historic dispersals from the East Indies have been described. It is now apparent that there is, in several animal groups, an increasing trend in average generic age from the East Indies outward. Paleontological studies have revealed significant onshore to offshore evolutionary trends during much of the Phanerozoic. Antitropical and other disjunct patterns in the Indo-West Pacific suggest that the extinction process often begins in the East Indies and spreads outward. Widespread Indo-West Pacific species, that possibly originated in the East Indies, occasionally surmount biogeographic barriers to colonize adjacent regions. The biogeographic patterns described are compatible with, and provide support for, the centre of origin hypothesis. Successful evolution, which results in the formation of new phyletic lines, takes place in the nearshore waters of centres of origin. From such centres, new species, which may eventually give rise to new higher taxa, spread out both vertically and horizontally. As they spread vertically, they replace older forms from the mid- and outer shelf and eventually from the slope. As they spread horizontally, they replace their more generalized, peripheral relatives, leaving as evidence the historic dispersal and age patterns which have been noted.