200 TRANSACTIONS—PERSHSHIRE SOCIETY OF NATURAL SCIENCE. 
in the British Isles. To see them again we must go to south-west 
France and the Pyrenees. Their presence here forms a strange link 
between this out-of-the-way corner of Britain and the distant moun¬ 
tains of Spain, and has given to naturalists plenty of scope for theories 
as to how they found their way here. It has been suggested that 
they are the legacy of Spanish invasions and of Spanish traffic with 
Ireland in early and mediaeval times—that we see in them relics of 
that Spanish intercourse of which there are still traces in the quaint 
old town of Galway, in the architecture of its great houses now falling 
into ruins and with little to show of their former glory ; in the bright 
colours of the national costume which still lingers here; in the names, 
and even in the features of the people. “Naturalists, however” (to 
quote from the delightful Open-Air Studies of an Irish botanist), 
“ are now agreed that a former extension of the coast-line of Europe 
allowed the plants to migrate overland from Portugal to Ireland, and 
that, aided by a mild and equable climate, they have here remained 
to tell us of byegone ages and of vanished lands.” 
XX .—Plant Associations of the Pay Basin. 
By Robert Smith, B.Sc. 
(Read 13th January, 1898.) 
It is a simple observation that the plants conspicuous in the 
landscape are comparatively few in number. These few are the 
so-called “ social ” species, whose individuals are so plentiful as 
to dominate over all the other plant inhabitants of the same area. 
Thus, on the moor the heather is the dominant social species, occupy¬ 
ing by far the greater part of the ground. With the chief species 
are associated some which are kept down by it, and others which 
are dependent upon it for food, shade, or shelter. Such a community, 
made up of chief species, subordinate species, and dependent species, 
constitutes a Plant Association in the sense used in this paper. 
There are many social species in our district which may dominate 
in a Plant Association, and all stand out from their neighbours by 
certain well-defined characters. 
I. —The most conspicuous are the Trees, whose tall growth 
excludes them from much competition with smaller forms. 
II. Shrubs are similarly, although to a less degree, raised above 
the humbler herbs, and constitute important societies. Of these, 
Calluna has by far the greatest extension. Other Ericacece^ although 
usually subordinate to it, may here and there find a more suitable 
spot where they rise to the rank of dominant species. On the high 
