The Scottish Naturalist, 153 



south-west. Between England and Ireland was a large lake 

 reaching from opposite Wales to beyond the north of Ireland. 



Assuming that Crambus furcatellus is (as is most probably the 

 case) a grass or moss eater, we may regard it as one of the earliest 

 of our mountain Lepidoptera to arrive. In the first place, because 

 of its probable food-plant, which, as already said, would be one 

 of the first to be established ; and in the second place, because 

 it is the only one of the species known to inhabit Wales. When 

 the plants and animals began their northward march the arctic- 

 alpine species would, as already remarked, follow closest on the 

 retiring ice-sheet, and close behind them would come the more 

 numerous species that inhabit the lower and less arctic localities. 

 As these seem to be stronger, they would occupy all the ground 

 behind, where, moreover, the climate would be becoming some- 

 what unfavourable to the mountain and arctic species. There 

 would therefore be no possibility, on the part of the latter, of 

 turning back, and they would be driven either up the hills or to 

 the north. Some of the Crambus furcatellus^ therefore, we may 

 imagine, were pressed up the Welsh hills, where the then still 

 existing local glaciers would afford a climate suitable to them and 

 adverse to their pursuers ; others would be driven northwards, 

 and find resting-places on the hills of the north-west of England 

 and of Scotland, on which, doubtless, the species had once a wide 

 range. Another view of the possible history of the arrival and 

 spread of Crambus furcatellus may be taken — namely, that it 

 managed to get across the sea to the south of England, and 

 thence gradually spread northwards, taking Wales on its way. 



Perhaps the next — if not as early an — arrival would be Erebia 

 Epiphron, which we know is a grass-feeder. It does not occur 

 in Wales, and perhaps never did, but entering England on the 

 east has left colonies on the north English and Scottish hills. 

 From Scotland it probably reached Ireland by the north of the 

 great lake, which, as we have seen, occupied a large part of the 

 Irish Sea, and which to such a weak flier as the Erebia would 

 prove an obstacle to direct migration from England. Ireland, or 

 at least the north and west of it, not having been in all proba- 

 bility fully colonised, would not present such obstacles to a 

 southward march as would the more accessible sister country. 



The insects most likely to be next in succession would be 

 Zelleria saxifrages and Swammerdamia nanivora. 



The Zelleria feeds on various species of saxifrages, especially 

 (in this country) on Saxifraga aizoides^ but also on S. oppositi- 



