Another possibility is that the birds, which are less differentiated from their ancestral stocks, may be less differentiated because of suppression of newly evolved characters by dilutions, which result from interbreeding with new birds, which may be arriving at irregular intervals from the ancestral home. Interbreeding of the resident population with newcomers may overshadow any modifications which might have appeared as a result of insular isolation, especially modifications which have little adaptive significance. One would suspect, from their modifications, that _Rallus owstoni_, _Metabolus rugensis_, _Corvus kubaryi_, and other endemic forms have experienced less of this "dilution," than such birds as _Rallus philippensis pelewensis_, _Artamus leucorhynchus pelewensis_, _Myzomela cardinalis_, and others. Murphy (1938) mentions this "dilution" effect in his discussion of "strong" and "weak" subspecies among warblers of the Marquesas. He writes that "strong" subspecies may develop if the birds are present on islands which are upwind from islands containing related subspecies. The wind acts to block interisland migration in these weak-flyers. On the other hand, "weak" subspecies may show the effect of "dilution," being situated on islands downwind from islands containing related subspecies. The direction of the wind acts to aid the weak flyers to move to the downwind islands and continually "dilute" the resident subspecies. Similar examples can be cited for Micronesian birds. Hesse, Allee, and Schmidt (1937:87) write, "Endemism on islands is most frequent in forms for which the difficulty of reaching the island is most extreme, so that new increments of the parent form are unlikely to follow." Employing the criteria mentioned above, the birds of Micronesia can be tentatively divided into four groups as regards the relative time when they arrived at the islands: 1. Birds of ancient colonizations which reached certain individual islands, became modified, and dispersed no farther. Examples are _Aphanolimnas_, _Rallus owstoni_, _Aplonis corvinus_, _Metabolus rugensis_, and _Corvus kubaryi_. 2. Birds of ancient colonizations which reached or dispersed through a number of islands but are now restricted to relatively few islands. Examples are _Ducula oceanica_, _Ptilinopus porphyraceus_, _Megapodius lapérouse_, _Asio flammeus_, and _Acrocephalus luscinia_. 3. Birds of ancient, or possibly more recent, colonizations which initially reached or subsequently dispersed to many of the islands of Micronesia possessing habitat suitable for them. Examples are _Myzomela cardinalis_, the two species of _Halcyon_, _Aplonis opacus_, and _Zosterops conspicillata_. 4. Birds of rather recent colonizations, which may have reached only a few islands and are relatively unmodified from their parental stocks. Examples are _Artamus leucorhynchus_, _Caprimulgus indicus_, _Poliolimnas cinereus_, and _Nycticorax caledonicus_. FACTORS CAUSING DISPERSAL