Based on the facts and principles concerning the evolution and distribution of marine animals, mostly fishes, in the Japan Sea and its neighboring waters in the North Pacific, which were discussed in the previous part of the present study, a consideration was made on the history of development of this marginal sea and its fauna during the Quaternary period. 1. During the early phase of the Quaternary period, possibly Pre-Glacial to First Glacial stage, there must have been a shallow and wide freshwater lake in the place of the present-day Japan Sea. In this lake, freshwater fishes of the Amur River system (Acipenseridae, Leuciscinae, Lefua, etc.) as well as of the survival forms from the ancient or paleo-Japan Sea (mostly salmonids), with a minor component of the tropical sea origin (Epinephelinae : the ancestral forms of Stereolepis and Coreoperca), must have been flourishing. In early half of this phase, the paleo-Amur may have poured directly into the lake and a drain river of the lake may have started somewhere on its southern border and flowed southward across the "East China Sea Plain" and, after uniting with the lower part of the paleo-Yantse Kiang, discharged into a lagoon situated in the north of the presentday Miyako Islands, southern Ryukyu. In the latter half of this phase, the area bordering the southern sides of the lake must have been raised, forming a mountain ridge, and the flow pattern of the river system in the surrounding region markedly changed; the lower reaches of the paleo-Amur are now considered to have worked as a drain river of the lake. 2. It is presumed to have been in First Interglacial stage that the lake first received an inundation of sea water and was transformed through a brackish environment eventually into a marine basin. The invasion of sea water must have been made from north, possibly via the lower part of the paleo-Amur or the drain river of the lake. In the course of thalassification, some of the freshwater animals might have been exterminated or taken refuge in less saline or quite freshwater environment in coastal lagoons and upper reaches of rivers, but others succeeded to adapt or re-adapt themselves to the marine environment. The first marine fish invaders and colonizers of this newly-formed seawater environment are supposed to have been the ancestral forms of the families Stichaeidae and Agonidae, both of which, finding the greater part of the habitat unoccupied in the new environment, have adapted themselves to every (bottom) niches, and carried out a prodigious differentiation somewhat resembling the adaptive radiation, producing many important subfamilies such as Opisthocentrinae, Xiphisterinae, Neozoarcinae, etc. and Tilesininae, Brachyopsinae and Agoninae, respectively. 3. The Japan Sea thus originated must have remained in the connected condition with the Pacific possibly throughout First Interglacial to Second Interglacial stages. In Second Interglacial stage, another sea channel is supposed to have been opened somewhere near the present-day Tsugaru Straits. Through these channels, active interchange of marine fauna may have been done between this marginal basin and the Pacific ; of particular significance may be the emigration of a part of the diversely differentiated stichaeid and agonki fishes to the northern North Pacific and further to the west coast of North America, and also the invasion of the ancestral forms of some primitive groups of the Pleuronectinae (Cleisthenes, Hippoglossoides and Acanthopsetta) and of the Cottidae, particularly the Pseudoblenninae and its related groups (Atopocottus, Alcichthys, Ricuzenius, etc.), from the tropical waters in southeastern Asia and the west coast of North America, respectively. 4. In Third Glacial stage, the Japan Sea was possibly isolated , and within this isolated basin most of the animals may have been differentiated into the species or genera endemic to the

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