1916.] 135 



itself, and not clearly allied to any other known family. Further, 

 its range is purely European, as it only occurs in the British Isles, 

 France (from which it just overflows into the valley of Piedmont in 

 Italy), Switzerland, G-ermany, Austria-Hungary, Eussia (as far east 

 as Moscow), Finland, Roumania, Scandinavia (as far north as 66°), 

 and it barely enters Belgium and Holland on the east. Thus we 

 see that it includes in its stations, habitats varying from those 

 enjoying a mild and oceanic climate, to those possessing the vigorous 

 Continental one of Central Eussia. But, let it be noted, it seems to 

 object to passing the Arctic circle and into the warmer regions of 

 Europe. Its limits, whethcn* as a mere coincidence or not, seem to be 

 the winter isotherm of 5° C, and the summer one of 7° C. The species, 

 then, is wholly European, and shows no sign of ever having wandered 

 beyond our boundaries. Coupled with this is the fact that if one 

 judges from the remoteness of its nearest relatives, it must be a very 

 old one. Its history and geography then, if worked out clearly, should 

 betray to some extent the past history of the distribution of the 

 European floi*a and fauna. The present range, except for the 

 break in Britain, is just what one would expect of a birch feeder 

 which had originated in Europe and spread there. Still, one must 

 note that its distribution does not coincide with that of the food 

 plants, birch and alder, or of any one species of birch or alder, for 

 they exist throughout the Holarctic area, occurring even in the 

 Mediterranean Islands. Inability then to stand a high average 

 annual temperature, or a veiy low one, seem to be the sole limits to 

 any extension of range ; and if this be the case now, presumably 

 the same factors had the same power formerly. Hence, granting that 

 the limits of its colonising ability were attained in late Pliocene 

 times, of necessity on the approach of the glacial period, in spite of 

 the probable continued existence of the dwarf birch (Bettila nana) in 

 Britain throughout that time, one cannot conceive of survival here, 

 at any rate in its present habitats. It would, therefore, be driven 

 forth southward to more favourable climes, whence when the climate 

 ameliorated, it would issue and regain lost ground. Evidently, 

 this re-advance would be across the lower reaches of the dry bed of 

 the North Sea. But this temptingly simple position is clearly un- 

 tenable in this crude form, as it offers no explanation of the broad 

 gap in the British distribution, such as occurs in Mid and North 

 England and in South and South Central Scotland — that is, unless we 

 grant that this gap has been caused by subsequent geological or other 

 events. Geologically, the only event which v.ould satisfactorily 



