170 Proceedings of the Royal Physical Society. 



make their crossing, or if they have crossed, it has been by 

 means of a bridge. I am able, in many instances, to trace the 

 exact path by which squirrels advanced through this country, 

 and that, in some instances, as long as fifty years ago. The 

 crossing from Stirlingshire into Perthshire by the Bridge of 

 Frew, over the Forth, as shown before, and the entrance of 

 Sutherland by the bridges over the Kyle, instance the above 

 remarks. 



The extensive planting of wood which took place in the 

 south of Scotland at Tynninghame, where large areas of 

 ground were planted in 1707 — towards the end of last century 

 and beginning of the present century — was no doubt a most 

 important factor in aiding the increase of the squirrel ; and 

 we have evidence in the foregoing pages that, in many locali- 

 ties, to the new feeding grounds thus opened up, the squirrel 

 is indebted for its increase and prosperity. 



Large tracts of country were also planted in Perthshire by 

 the Duke of Athole and Lord Breadalbane, which would 

 afford abundant feeding grounds for the species. We have 

 seen already that the exception to the general rule of the 

 northerly waves being the strongest, is exemplified at Dun- 

 keld Centre, high mountains and scarcity of wood, at that 

 early date, combined, checking their advance to the northward, 

 and forcing them to roll southward, until the force of the 

 pressure being exhausted, they recovered, and again advanced 

 north-eastward through Strathmore and Forfar. 



Similar planting took place on the Spey, and favoured their 

 northward advance, when at last they revived in the old 

 forests of Eothiemurchus. 



Finally a comparison of the boundaries of the several areas 

 populated from the Centres of Dalkeith and Minto, Dunkeld 

 and Beaufort, and other minor tracts, with the map in Skene's 

 " Celtic Scotland " " showing mountain chains," will result in 

 very approximate results ; showing that to a considerable 

 extent the advances of the species have been checked and 

 delayed, regulated or influenced, more or less, by the various 

 ranges of hills, though a continued pressure has in many 

 places, as already explained, caused an overflow, and the sur- 

 mounting of unusual difficulties. It may be well also in this 



