101(5.] 137 



That there were land couuections available between Scotland and 

 Scandinavia when our flora and fauna and those of Scandinavia 

 were in the making, can be readily demonstrated. Take, for 

 example, the two mosses Anclreaea alpina Sm., and Grimmia 

 marifima Turn. The former, so abundant in suitable localities with 

 us, is entirely absent from Continental ones, except for a few in 

 Scandinavia, from which we can only conclude that it advanced from 

 Scotland via some old and long since submerged land. In the case of 

 Grimmia maritima, the reverse is apparently true, i.e., that it passed 

 to our islands from Norway, because if one judges from its present 

 North Atlantic distribution, the form has originated on American 

 shores. Clearly, it seems possible and probable that Bimorpha 

 versicolnra reached its northern stations with other (and more truly 

 northern) migrants from Scandinavia. That this occurred at a later 

 period than that which marked the passage of most of the Northern 

 forms can be shown by its absence from Ireland in areas affected 

 by Northern species which passed to Ireland via the Mull, Islay, 

 Donegal, and Londonderry route, which lasted long after the Wales- 

 Ireland causeway broke down. This would throw the origin of the 

 Southern colony of Biynorj^ha versicolora in our islands far anterior 

 to the Northern one, because the former had time to reach Ireland by 

 a connection which broke early, whilst the latter failed to reach the 

 same goal by a route which lasted an infinitely longer period. 



There seems, thus, to have been some disturbances in the Scandi- 

 navian stations after these had been settled, which caused the species 

 once more to resume its wanderings ; this disturbance we shall 

 discuss shortly. 



The smallness of the area occupied by the southern detachment 

 suggests that, before it had had time to spread, something effectually 

 checked and interrupted the advance, and that consequently it had 

 vei-y long distances to travel from its retreat ; this, in turn, implies 

 that the habitat during the period of least distribution was not South- 

 western, but South-eastern Europe. 



Now, it can ])e demonstrated from the geological record by 

 references to the flora of the submerged peat in the sea between 

 Denmark and Scandinavia, that birches and alders spread to Scandi- 

 navia (and therefore presumably to the British Islands) during the 

 Inter-glacial period just prior to the development of the Baltic system 

 of glaciers. Presumably, our insect followed the lines of advancing 



