xxii INTRODUCTION 



The foregoing table wiU convey a general idea of the distribution 

 of the more prominent members of these groups. With one or two 

 exceptions, which are stated, the plants are confined to those 

 county-divisions which border on the Atlantic. It is to be noted 

 that the Atlantic seaboard of Ireland naturally divides itself into 

 five geological districts, characterised alternately by a marked 

 prevalence or absence of limestone. Many of the plants of the 

 groups under consideration are distinctly calcicole or calcifuge 

 (lime-loving or lime-avoiding), and it will be seen from the table 

 that their distribution is profoundly affected by the petrological 

 conditions which prevail. An attempt is made to show the head- 

 quarters of each species by the use of a heavier or lighter line 

 according to its abundance or rarity in each division. The divi- 

 sions are enumerated in the order in which they lie along the coast. 

 East Donegal is entered twice, as it forms the coast-line of two 

 widely separated areas. 



Though the Cantabrian and American plant-groups are members 

 of widely separated floras, a stronger reason exists for considering 

 them together than the fact of their similar distribution in Ireland. 

 There can be little doubt that both arrived in the island, one from the 

 south, the other from the north, over land-surfaces long since de- 

 stroyed, and that with the alpine group they represent the very oldest 

 components of our existing flora. What the period was when these 

 land-connexions existed has not yet been determined with any degree 

 of certainty ; but that it was pre-Glacial appears an almost neces- 

 sary admission, despite the difficulty of accounting for the survival 

 of the southern plants throughout the " Great Ice Age." 



The Central Plain. — The great Limestone Plain, which next 

 demands consideration, stretches uninterruptedly from Dublin Bay 

 on the east coast to Galway Bay on the Atlantic. So little has 

 been written on its characteristics, physical or botanical, that a 

 brief description appears desirable here. The Central Plain of 

 Ireland is the largest tract of Carboniferous limestone in Western 

 Europe. It may be considered as a wide flat syncline, its axis 

 parallel to the main NE. and SW. folding of the island. On the 

 north, the plain terminates against the flanks of the limestone 

 highlands of Cavan, Leitrim, and Sligo. In the south, its con- 



