PHYSICAL CONDITIONS— POSTGLACIAL. 505 



presents. The evidence for this we find in the "submarine 

 forests and peat" of our own shores and the opposite coasts of 

 France and the Low Countries. It is now generally admitted 

 by geologists that these old forests indicate a time when the 

 British Islands were united to themselves and the Continent. 

 The purely geological evidence points to this conclusion and no 

 other. No one doubts that the flora and fauna of our islands 

 could only have immigrated by a land-passage, and as neither 

 our animals nor our plants could have existed here during the 

 last glacial epoch, it follows that they must be of postglacial age. 

 I have shown how we read in the postglacial shell-beds a 

 history of the gradual change from arctic to temperate conditions. 

 The same history is repeated by our peat-bogs, and it is clearly 

 evinced by the present distribution of plants in North-western 

 Europe. It will be remembered that underneath peat-bogs in 

 various parts of Central Europe the traces of an arctic or 

 northern flora have been discovered, principally by Mr. Nathorst. 

 He has recorded them from Switzerland, Bavaria, Mecklenburg, 

 Denmark, and Southern England. It is highly probable that 

 some of these finds belong to the last glacial epoch itself, that is 

 to say, that they represent the flora which characterised Central 

 Europe at a time when the great mer de glace still occupied the 

 basins of the Baltic and the North Sea. As the snow and ice 

 disappeared from Northern Germany, Denmark, and England, 

 the dwarf birches and arctic willows gradually crept north and 

 overspread the barren grounds. At what particular point, and 

 by what passage, the northern flora entered Sweden we do not at 

 present know. No trace of that flora is found under the peat of 

 the low-lying coast-lands of Scania and Southern Norway. It 

 may be, as Mr. Axel Blytt has suggested, that those regions were 

 still under water when the arctic willow and its congeners 

 became established in Sweden. But if that were so, one might 

 ask by what route that flora immigrated. Was it conveyed by 

 chance ice -rafts from the German and Danish shores? This, 

 although possible, is hardly likely. How, then, and at what 

 time, did it enter Sweden ? If it crossed to Sweden by a land- 



