_Gygis a. microryhncha_ was for a long time treated as a species distinct from _G. alba_ but has recently been considered as a subspecies _G. alba_ by Peters and others. On the islands of Hatutu and Motane in the Marquesas, the Whitney South Sea Expedition obtained some birds which appear to be intergrades between the two subspecies of White Terns in the area. The measurements of nine birds which show intergradation between _G. a. microrhyncha_ and _G. a. pacifica_ are listed in table 24. Probably the Marquesas population is tending toward complete reproductive isolation. [Illustration: FIG. 12. Geographic distribution of _Gygis alba_ in the Pacific area. (1) _G. a. candida_; (2) _G. a. pacifica;_ (3) _G. a. microrhyncha;_ (4) _G. a. royana_.] Peters (1934:348, 349) recognizes three other subspecies from the Pacific area: _G. a. rothschildi_ Hartert from Laysan, Lisiansky, and Krusenstern islands; _G. a. candida_ (Gmelin) from "the Carolines east to Christmas Island and south to the Tonga and Society Islands"; and _G. a. royana_ Mathews from Norfolk and the Kermadec Islands. Birds from Revilla Gigedo, Cocos and Clipperton islands, although geographically isolated, are placed in _G. a. candida_. On the basis of a critical study of specimens at hand, the populations in the Pacific fit into three groups. Small birds, _G. a. candida_, are found in the North Pacific from the Bonins and Marianas east to Wake and the Hawaiian Chain and south to the Marshall, Phoenix, Christmas and the Fanning islands (see figure 12). Larger birds, _G. a. pacifica_, are found in the Central Pacific and South Pacific from the Carolines in the west southeastward through Melanesia and eastward through Samoa, to the Tuamotus and Easter to Cocos, Clipperton, and Revilla Gigedo islands. In the Southwest Pacific, at Norfolk and the Kermadec Islands, a longer-winged population occurs; it is separable as _G. a. royana_. The measurements of these birds are given in table 24. TABLE 24. MEASUREMENTS OF _Gygis alba_ FROM THE PACIFIC AREA Column headings: A: No. B: Wing C: Longest tail feather D: Shortest tail feather E: Exposed culmen F: Depth Culmen G: Tarsus