PHYSICAL CONDITIONS— POSTGLACIAL. 503 



coast;" Callista convexa occurs sparingly in shallow sheltered 

 localities in Casco Bay; "but the oysters (Ostrea virginiana) 

 and ' scollops ' (Pecten irradians) had apparently become extinct 

 in the vicinity of Portland Harbour before the period of the 

 Indian shell-heaps, for neither of these species occurs in the 

 heaps on the adjacent islands, while the quahogs lingered on 

 until that time, but have subsequently died out everywhere in 

 this region, except at Quahog Bay." The position of the beds 

 of oyster-shells, pectens, etc., shows that " no important change 

 in the relative level of the land and water can have occurred in 

 that region since they were formed." Professor Verrill says that 

 he can explain the presence of the southern species in no other 

 way than by supposing " that they are survivors from a time 

 when the marine climate of the whole coast, from Cape Cod to 

 Nova Scotia and the Bay of Fundy, was warmer than at present, 

 and these species had a continuous range from Southern New 

 England to the Gulf of Saint Lawrence." 



All these facts plainly show that the temperature of our 

 northern seas has been exceptionally high at some recent period. 

 In no other way can we account for the northern immigration 

 of the southern species. These species tell of a time when the 

 Gulf Stream carried into the North Atlantic a much greater body 

 of heated water than now reaches such high latitudes. At one 

 time I was inclined to assign that latest immigration of southern 

 forms to the last interglacial epoch, and therefore looked upon 

 the isolated colonies and individual species in our postglacial 

 deposits and present seas as the few survivors who were able to 

 outlive the rigour of the latest glacial epoch. But when we 

 come to consider the nature of the conditions which obtained 

 during that latest phase of the Ice Age, it seems hardly possible 

 that any southern species whatsoever could have survived them. 

 Few geologists, save those who have specially worked at the 

 subject, have realised the extent of the glaciation that took place 

 toward the close of the Glacial Period. So far as Scotland and 

 Scandinavia are concerned, the ice-sheet which then covered 

 them seems to have been hardly, if at all, less thick than that 



