10 36 Proceedings of the Royal Irish Academy. 



group will correspond pretty closely with the native flora ; while in the 

 latter we shall find almost all the species which are under suspicion of 

 introduction. The main sources of modern introduction are well summarized 

 by Dunn (pp. xiii-xiv) and Adams (loc. cit.). 



In populous or highly tilled districts it is generally a very difficult 

 problem to determine which plants are indigenous, and which have been 

 brought in, directly or indirectly, by human operations. The problem is 

 simplified in an isolated and primitive area such as Clare Island, where the 

 conditions are tolerably uniform, and where the large proportion of ground 

 still remaining in a state approaching its primeval condition permits, at 

 least partially, of a reconstruction of the original surface, and of the vegetation 

 which it supported. 



The doubtful section of any flora consists largely of annual plants, whose 

 life-history renders them especially suited for life in the cultivated lands. 

 They are abundant seeders ; the resting-stage during which they are dispersed 

 comes frequently ; and being short-lived, they can pass through a generation 

 between ploughings. The home of a large number of such plants is in the 

 Mediterranean region, whence they have worked their way northwards 

 across the cultivated lands. The dry soils of the south-east of England 

 possess a greater number of such species, both as natives and as introduc- 

 tions, than the north of England, or Scotland, where they appear mainly 

 as introductions ; and some are native in eastern Ireland, which die out 

 or become dependent on man, as the light soils of the Leinster sea-board 

 graduate into the peaty soils of Connaught. As Dunn remarks (p. ix) : 

 " The total range of any plant comprises all the countries in which it occurs, 

 and, in cases where artificial dissemination has enlarged the original area, the 

 total range may sometimes be satisfactorily divided into concentric zones 

 corresponding to the increasing dependence of the plant on man as it recedes 

 from its native centre." 



In remote areas in western Ireland many of the doubtful items are 

 eliminated. Such genera as Papaver, Fumaria, Silene, Lychnis, Trifolium, 

 Yalerianella are rare or absent — at least as regards annual species; while 

 the annuals on the peninsula of Howth (4J- square miles) on the east side 

 of Ireland include about 130 species, or 24 per cent, of its flora of 545 

 species, the annuals of Clare Island (6£ square miles), the flora of which 

 comprises 389 species, comprise only 18 per cent. On Clare Island almost 

 every plant which is found only on cultivated ground may be ruled out as 

 either introduced by man, or dependent on him for its continued existence. 

 This is because the island, in its primitive condition, contained no open ground 

 such as gravelly wastes, sand-dunes, &c, on which these species might have 



