Clare Island Survey — Phanerogamia, 10 57 



Clement Eeid considers that wind and water migration, plus the "accumulated 

 accidents of some thousands of years," are sufficient to account for the intro- 

 duction of the whole present flora of the British Isles across the existing 

 seas. 1 



These instances will suffice to show how much opinion is divided on 

 this subject, and how students of the problem have arrived at very different 

 conclusions. 



If the flora of Clare Island came across a barrier of sea, we might hope 

 to find, if we analyze it, that plants especially fitted for crossing such a 

 barrier are well represented, and, vice versa, that species to which such a barrier 

 would be a serious obstacle are conspicuous chiefly by their absence. This 

 will lead us to attempt a classification of the local flora according to the means 

 by which the constituent species are dispersed. 



But it must be conceded at once that no more than a hint as to the origin 

 of the flora is to be hoped for from so simple an analysis. As "Wallace long 

 ago pointed out, plants have existed long enough in most cases to have been 

 carried to all suitable localities, and the determining factor of their distribution 

 is to be found in their powers of adaptation to their new conditions. Guppy, 

 commenting on this, observes that time has long since discounted the means 

 of dispersal, and that as regards explaining plant-distribution, the study of 

 the means of dispersal is often superfluous. The process of dispersal is ever 

 going on ; but its results are mainly determined by the reception the plant 

 receives in its new country. 



Thus it is seen that the present distribution of plants in our islands is but 

 in a slight degree the result of the relative facility for dispersal which the 

 constituent species possess. We notice, for instance, that plants with flying 

 seeds, or, whose seeds are being continually scattered by birds, are no more 

 widely nor abundantly spread than are those which possess neither these 

 nor other special facilities for dispersal. Neither does the existence of a 

 land-bridge involve the conclusion that all the plants at one end of it will 

 eventually reach the other. Even where competition is reduced, suitability 

 of habitat is an all-important consideration. And where, as is usually the 

 case, competition comes keenly into -play, suitability of habitat becomes a 

 much more stringent term, for a plant can then spread and form a lodgment 

 on new ground only where the conditions are not only fitted for its growth, 

 but better fitted for its growth than for the growth of the other plants of 

 similar habitat which are holding the ground on its arrival, or arrive along 



1 Clement Eeid : The Origin of the British Flora, p. 31, 1899, and paper read at British 

 Association meeting, 1911. 



2 H. B. Guppy : The Distribution of Aquatic Plants and Animals. Scottish Geographical 

 Magazine, ix, pp. 28-33. 1S93. 



K.I.A. PEOC, VOL. XXXI. H 10 



