_Remarks._--Superficially, the White-browned Rail of Micronesia is distinct from its near relative, _P. c. collingwoodi_, but the differences are not so well marked as they are between insular populations of other species of rails. It is probably a comparatively recent addition to the Micronesian avifauna, and its pattern of distribution may represent an early stage in the development of endemism in contrast to the pattern of later stages in the development of insular forms shown by the isolated rails, _Rallus owstoni_ and _Aphanolimnas monasa_. The fact that _Poliolimnas cinereus_ is found only on widely separated islands in Micronesia does not necessarily mean that it has become "extinct" on the intervening islands, but that it may be partial to the larger, "high" islands, or that it is actually present but remains to be discovered on these intervening islands when more intensive field investigations are made. Hachisuka (1939a:151), in naming the Micronesian form, comments that it has a shorter bill than _P. c. collingwoodi_ of the Philippines and Celebes, and that it is intermediate between this subspecies and _P. c. brevipes_ of the Volcano Island to the north. Within these three subspecies there are trends toward a shorter culmen and shorter tarsus and, less markedly, toward a shorter wing. From the evidence at hand, it can be concluded that _Poliolimnas_ first colonized Micronesia probably from the Philippine area (or Papuan area) through the Palaus and Carolines, to the Marianas and north to the Volcano Islands. Further, this has probably been a relatively recent invasion, although the subspecies in the Volcano Islands shows marked change in length of tarsus and culmen. This extension of range to the islands north of the Marianas is unusual and resembles somewhat the distribution of _Nycticorax caledonicus_ in the same general area.