26 Proceedings of the Roy al Iri^h Acadenvj. 



certain divisions. All species ranked as probably or certainlj iatro- 

 dnced in Ireland Trere kept apart, as their range cotdd throw bnt little 

 ligbt on natural plant groups. It may be remarked that the maps 

 Ivronglit out very clearly the discontinuity of range which marks the 

 alien flora. Speaking generally, tbe native lowland plants are cbarac- 

 terized by a continuous range between tbeir limits, while the alien 

 plants frequently exhibit a broken and discontinuous range (figs. 13, 

 14). Eemarkable exceptions to botb rules exist, and will be men- 

 tioned later on. 



In arrangiag tbe maps under types of ^distribution, one difficulty 

 was quick to make itself felt. This was, tbat in the natural flora 

 every gradation exists between any two types of distribution whicli 

 we may select. The difficulty was met by using "Watson's plan — the 

 only possible one — of modified or intermediate types as already referred 

 to ; but considering the unsatisfactory nature of such fine distinctions 

 in a flora not yet fully worked out, the creation (by publication) of 

 such intermediate types is for the present withbeld, and lists will be 

 given chiefly of those plants whose range is sufficiently characteristic 

 to allow of their beiag referred without qualification to one definite 

 type of distribution. 



The grouping of the maps established in the first place two classes: 

 (a) plants which show no aggregation in any portion of the country; 

 and (b) plants which show an aggregation or diminution in some 

 portion of the country. 



Class A consists of (1) universal species, i.e. species on record for all 

 the forty botanical divisions, and showing no marked increase or decrease 

 ia frequency in any direction; (2) species of probably universal distribu- 

 tion, the occasional gaps on the maps being with little doubt the result 

 of incomplete knowledge. To sections (1) and (2) some 360 species may 

 be referred, or about one-third of the Irish flora. (3) Following these we 

 have a range of species of diminishing frequency but wide distribution, 

 the list extending from the border of the "probably universal " species 

 flown to plants which have only a few widely scattered stations in 

 Ireland, and which might be separately classed as of local type. 

 Tollowing the nomenclature of "Watson, who gave the name of British 

 type to all species evenly spread throughout Great Britain (though not 

 necessarily c&«f/«Mot«7y distributed}, we may define the three groups of 

 the above Class A as of "Irish" type so far as the Irish flora is 

 concerned; but the use of this term, except with the qualification 

 appended, might mislead ; and I prefer to employ, in the j»resent 

 paper, the term " General type of distribution," for all species whose 



