10 88 Proceedings of the Royal Irish Academy. 



the farm-land; and although one or two of them are fitted for wind-dispersal, 

 the probability is that all were brought by the birds — probably by means of 

 seeds adhering to their feet or plumage, as the Black-headed Gull is only 

 very sparingly a vegetable feeder. This case may seem to show the ease 

 with which plants may be introduced by the movements of birds. But the 

 conditions when studied are seen to be very special. In the first place, 

 the great concentration on a very small area of ground must be considered. 

 Assuming that only a hundred pairs of breeding birds formed this colony, the 

 feeding of the young involved some hundreds of thousands of journeys direct 

 from the farm-land, where grubs and worms were obtainable, to an area only 

 a few perches in extent. Again, the guano was probably as beneficial to the 

 growth of the weed-flora as it was mimical to the bog-vegetation, so that the 

 seeds arrived on ground unoccupied by a pre-existing competitive flora. A 

 little consideration will show how different the case of this bird-colony is from 

 a case such as that afforded by Clare Island, where we have to rely on the 

 casual visits of the members of a scanty avifauna. But, on the other hand, 

 that scanty avifauna has had a very long period in which to do its work. 



Probability of a Land-bridge. 



It has been seen that examination and analysis of the vegetation of Clare 

 Island show that only an insignificant fragment of it could have arrived by 

 water, and that wind-dispersal could not reasonably be held responsible for 

 the balance. If the plants arrived across a channel of sea, birds must be 

 held accountable for the introduction of the bulk of the indigenous flora. 

 Birds, no doubt, have played an important part ; and we may without hesitation 

 put down a number of the hard-seeded plants to their credit. But there still 

 remains a large portion of the flora ; and local conditions, such as the cliffy 

 character of the shore and the prevalence of westerly winds, tend to discount 

 the efficacy in this case of other accidental or occasional dispersal. It has 

 been seen also (pp. 5-9) that analysis of the flora of this and the adjoining- 

 islands seems to show a general invasion by the mainland plants, for the 

 island flora exhibits no predominance of species specially fitted for over-sea 

 dispersal, every section of the mainland flora being represented as far as the 

 habitats existing on the islands allow. 



Neither does the list of common mainland plants absent from the group 

 of islands show a predominance of species unfitted for over-sea dispersal. 

 This list (given on p. 10) is seen to contain indifferently representatives of 

 all the modes of dispersal — plants whose seeds float unharmed in sea-water 

 [Sium angustifoliuvi, JBidens cemua, Alnus ghdinosa, Sparganiimi simplex, 



