ScHARFF — On the Origin of the European Fauna. 433 



advocates of accidental introduction — a subject of much importance, of 

 which. I have now to treat. 



Accidental Means of Dispersal. 



This includes, of course, also introductions by man, and, following 

 Darwin, it should more properly be called " occasional means of 

 distribution." Darwin (21 a) has shown that seeds of plants may be 

 easily transported to islands by wind, or by floating logs of wood, or 

 even by birds. He has also referred to the fact that locusts and other 

 insects, and eggs of fish and snails are sometimes blown to great 

 distances from the land. He has given many other instances of the 

 manner in which animals and plants might have reached islands. 

 Mr. Wallace, and other naturalists, have likewise collected examples 

 of these occasional introductions. Moreover, all that is known of the 

 means of dispersal of land and freshwater Mollusca has recently been 

 brought together by Mr. Kew (50) in a painstaking and excellent 

 work. It is astonishing how many cases of accidental introductions 

 are known to this author, but nevertheless he remarks (p. 97) : "It 

 must be admitted that neither freshwater nor land shells are really 

 well furnished with means of dispersal ; the transportal of a species 

 of either group over a large expanse of ocean, or to great distances on 

 land, with subsequent establishment, must be an extremely rare and 

 exceptional occurrence, and one which happens only once or twice in 

 many hundreds of years." 



All these views, however, do not particularly refer to the fauna or 

 flora of Ireland, and the only hint that at least a portion of the flora 

 owed its existence in that country to an accidental introduction, was 

 given by Prof. Hennessy (39). He suggested that, as there were 

 times of prolonged and intimate intercourse between the people of the 

 northern coast of Spain and those of Ireland, the conditions for bring- 

 ing the seeds of various plants from one country to the other probably 

 existed, and that to this fact is due the similarity in the flora of the 

 two countries. 



I have already admitted (p. 429) that some of the Irish Mammals 

 may have reached Ireland by means of an accidental introduction 

 through the agency of man. That many of the non-resident birds, and 

 even, perhaps, some residents, are brought to this country by an 

 occasional means of dispersal is undoubted. The same we may assume 

 to be the case with a few shells, worms, wood-lice, spiders, and 

 centipedes, and to a greater extent, perhaps, with insects. But 



