404 T. C. CHAMBERLIN 



presumptions, rather than specific ones. There is a strong general 

 presumption, based on theory and observation, that the earliest 

 life was marine and hence that in the gross the course of 

 migration has been from the sea to the land and to the air. 

 But this should weigh nothing in particular cases not in conflict 

 with it, for the descent of reptiles and mammals from the land to 

 the sea is well established, and this in no way contravenes their 

 remote ascension from a marine ancestry. It may be equally 

 true that the fish and the Eurypterids descended from the rivers 

 to the sea in the mid-Paleozoic, though their remote ancestors 

 may have ascended from it. 



In dealing with the specific presumptions of the case it is to 

 be noted that the relics of river faunas are imminently liable to 

 be borne down to the sea, while transportation in the opposite 

 direction is unassignable. The presumption is that a land or 

 fresh-water fauna will be somewhat represented in contempora- 

 neous marine sediments if it be readily fossilizable. The frag- 

 ments of fish and Eurypterids in the marine beds previous 

 to the transition stage at the close of the Silurian are not 

 more than could be expected if fish and Eurypterids were 

 living in the streams of those times, but entirely absent from 

 the seas. Indeed the record is rather scant even on this 

 assumption. 



A more or less widely accepted presumption regarding the 

 early states of the land has possibly also weighted against the 

 hypothesis that the fishes had their early development in the 

 land waters, viz., the presumption that the land was without vegetal 

 clothing, and that hence its waters were sterile and unsuited to 

 life. Against this presumption there are several important con- 

 siderations. If the land were naked, not only would the 

 streams be sterile and silty from the unrestrained wash of the 

 surface, but the waters of the sea border would also be similarly 

 affected in some notable degree. Sea life should have avoided 

 rather than sought the sterile, silty, shore waters. But the 

 abundance of littoral life in the early Paleozoic fails to support 

 this view. 



