of the Early Vertebrates" gives a philosophical discussion of the question which is extremely interesting and suggestive, though not backed up by much data. He calls attention to the fact that eurypterids and fishes are found associated in the Ludlow (Upper Siluric) of England, in the Island of Oesel in the Russian Baltic, in Podolia, Russia, in Galicia, and in the Waterlime group of North America. "The physical conditions in all these cases seem to have been peculiar," he continues, "and in the case of the Waterlime group they were singularly so, for they permitted a host of these large Eurypterids and other Crustaceans to flourish in seeming luxuriance, while only a meagre and pauperate marine fauna found an occasional entrance into the series. The conditions seem to have been congenial to the fish and Eurypterids, but not to a typical marine fauna" (32, 401, 402). The association of eurypterids and fishes in the Old Red sandstone where marine life was only occasional and meagre does not, as Chamberlin points out, imply prevalent marine conditions, for the Old Red and its homologues are the deposits of fresh water, and yet both the fishes and eurypterids found congenial conditions of life there. Chamberlin, recalling that fishes and eurypterids are found both earlier and later than the Devonic in marine deposits, puts the following question: "Were the fishes and eurypterids primarily marine and later became adapted to fresh water, or were they primarily fresh water forms which were occasionally carried out to sea and which later became adapted to salt water?" He reminds us that we are always in the habit of considering all life at first marine, then terrestrial, but, though this is true in general, the idea should not be held to with too great tenacity in every case. That the eurypterids may well furnish an example of an exceptional case is shown by various lines of evidence which Chamberlin cites. First, of the dozen genera of eurypterids known in 1900 only two or three of the least well known are without associations with formations regarded as fresh water; secondly, he says: "The relics found in marine sediments may be attributed to transportation from the land just as is one in the case of terrestrial plants and land insects not infrequently found in marine beds; but transportation in the opposite direction cannot be assigned" (32). One may, however, take exception to this last statement, for many marine forms migrate up rivers in the spawning season, as for instance, the crabs which go up the Hudson as far as Albany; and there are many marine molluscs which become adapted to conditions in rivers and may even in time migrate
