10 56 Proceedings of the Boyal Irish Academy. 



work putting together the merely possible means of distribution." ' If, on 

 the other hand, we cannot believe that the island populations have crossed 

 wide expanses of sea, we must invoke vast changes in the distribution of sea 

 and land, usurping, to quote Darwin again, the right " to make continents 

 as easily as a cook does pancakes," 3 and " sinking imaginary continents in a 

 quite reckless manner." 3 



In the present paper it is permissible to refer to those aspects of the 

 subject only which have a bearing on the question of the Clare Island flora ; 

 but even from this limited standpoint it will be seen that much evidence on 

 both sides can be quoted, and that certainty is not easily arrived at, though 

 the barrier to be crossed in this particular instance consists of only a few 

 miles of sea. 



Even a narrow channel is, without doubt, a serious obstacle to plant 

 dispersal. " Decidement," says De Candoile, 4 " les transports au travers d'un 

 bras de mer, quelque petit qu'il soit, par des causes naturelles, sont 

 infiniment rares." " Au travers d'un bras de mer comme la Manche, et a plus 

 forte raison, au travers d'un Ocean, ces causes de transport ont ete' ou nulles, 

 ou sans effet, depuis que l'homme observe." 



This is the opinion of many students of the question. Save by 

 dissecting captured birds, seeds have hardly ever been taken at sea, or 

 observed arriving on land at a considerable distance from their point of 

 departure under conditions where colonization was possible. 5 But much 

 observation and experiment are still required, and in the meantime opinion is 

 not unanimous. 



Sernander, 6 for instance, in Scandinavia, holds, against Hult, Blytt, and 

 Gunnar Andersson, that stretches of sea do not present serious barriers to 

 plant-migration. Blytt, 7 on the other hand, strongly emphasizes the slowness 

 and overland nature of plant migration. He affirms that seed-conveyance to 

 small distances is the rule, and long migration the exception ; and he holds 

 with Wallace 8 that discontinuous distribution signifies age, not long-distance 

 dispersal. 



Forbes, in Ids classic essay, 9 is strongly of opinion that the flora of Great 

 Britain and Ireland migrated thither over bygone land-surfaces ; while 



1 Life and Letters of C. Darwin, ii., p. 82. 2 Hid., ii., p. 74. 3 Ibid., iii., p. 230. 



* A. dk Candolle: Geographic Botanique Raisonnee, ii, pp. 708, 801-802. 1855. 



5 The casting up of tropical American seeds upon the coasts of Ireland or Norway, for instance, 

 could never have effected colonization. 



6 RuTOEit Sernandek: Den Skandinaviska Vegetationens Spridningsbiologie, p. 456. 1901. 



7 A Blvtt : Immigration of the Norwegian Flora, p. 31, &c. 



8 A. R. Wallace: Island Life, p. 69. 



9 Loc. tit., p. 399. 



