
LIMITS TO THE RANGE OF PLANT-SPECIES. 211 
In Dumfriesshire there are but few localities in which these 
Alpine plants occur. If one were to imagine the peat hags to be 
an ocean, then the localities of these Alpine plants would be like 
islands in an archipelago. Within these islands the range is 
limited by the occurrence of special habitats. One form oceurs 
on bare dry rocks or screes of stones, another on steep slopes of 
short turf, some only on wet rocks, others in rock crannies or the 
mud of spring-heads, In the rare places in which a rich 
accumulation of good loam has taken place, Epilobiwm angusti- 
folium, Linn., will be found, and perhaps Trollius; and so on. 
Hence the absence of peat soil and the existence of special 
habitats are the limiting causes for those forms in Dumfriesshire. 
The absence of any species cannot be taken to prove anything. 
The ordinary idea of the origin of these Alpine plants is that 
they are still, so to speak, on the fringe of the glacial ice. They 
are supposed to have followed the ice sheet northwards to the 
Arctic regions, and also up any convenient mountain which kept 
the proper climate. They were driven north and upwards by 
invading swarms of European species. But this process took a 
considerable time, and the retreat was almost certainly very 
gradual. Hence we should be grateful that during all that long- 
continued period a perpetual series of appropriate habitats was 
prepared for our Scottish Alpine plants. . The curious Hieracia 
which we find on isolated rocky places or corries in Dumfriesshire 
must always have had a rock or corrie of the proper kind to fly to 
when they were driven, step by step, from lower altitudes during 
at least 30,000 years.1 ‘Their absence, even if appropriate 
habitats exist at present, cannot be taken to prove anything. 
The existence of the right insect also appears to limit the range 
of some of these forms. Many special bumble-bee and butterfly 
flowers of the Alps do not exist in Scotland. The reason is 
probably that it is in the highest degree unlikely that a Bombus 
would be foolish enough to fly over several miles of peat-moss to 
visit a little Alpine “island” on the chance of finding honey, 
hence no Aguilegia alpina, Linn., and few of the characteristic 
Leguminos and Gentians. This point must not be pushed too 
far. Most flowers have a most-favoured-nation clause for one 

1Keane, Ethnology, 1896, p. 58. The discussion here of the glacial 
period is the best, or at least the most recent, known to me. 
