6 Proceedings of the Royal Irish Academ;/. 



Yet the diversity and compaiative lichuess of the fauna and flora of some of 

 the oceanic islands, and the depth of water intervening between them and the 

 mainland, had to be accounted for in some other manner. Neither "Wallace 

 nor Darwin was inclined to admit extensive geogi-aphical changes within 

 the period of existing species. The distribution of plants and animals by 

 " accidental," or what Darwin called " occasional," means of dispersal seemed 

 to furnish them with a clue to the world-wide dissemination of certain 

 species. With his wonted indomitable industry and pei-severance, Darwin 

 collected every possible fact that might V>e of service to him in demonstrating 

 the dispei-sal of species by accidental means. He endeavoured to show by 

 the most ingenious experiments how certain plants and animals might be 

 floated by marine currents across many miles of open sea from one country 

 to another. He demonsti-ated that many kinds of seeds may retain their 

 vitality for several days in the crop of a freshly-killeil bird, which might drift 

 away to a distant island during that period. A pellet of earth adhering to 

 the leg of a Eed-lesged Partridge contained seeds from which eighty-two 

 plants germinated after having lieen kept in a (hied condition for three years. 

 He proved experimentally that yming frcsliwater molluscs are apt to attach 

 themselves finnly to the feet of ducks, and that they might thus ea.sily be 

 transported from one pond to another.' 



Darwin's experiments have found many imitators ; and vahiable observa- 

 tions tending to show that at any rate .some of the more minute animals 

 and plants are liable to be conveye<l by occasional means of dispersal, have 

 been made. Thus the winter-eggs of Cla<locera (water-fleas) have been found 

 adhering to the feathers of wild ducks and other water-fowl. It has l>een 

 demonstrated that alga- are sometimes earned by water-beetles from pool 

 to pool, and that water-bugs, during their flights, occasionally transfer 

 larval water-mites from one ditch to another.' 



The most practical e.xperiments of all are those unintentional trans- 

 mi.ssions of seeds, insect.^, and otiier terrestrial invertebrates, and even of 

 vertebrates such as the mouse and rat, which we witness every day from one 

 country to another, from island to island, across vast expanses of territory, 

 and from one climatic extreme to another. 



Beyond these we know very little of what actually happens in nature 

 without man's intervention. Many people have speculated on the possible 

 means of accidental transport. Yet their theories do not afford us any 

 real guidance as to the manner in which animals and plants are actually 



' Darwin. C, " Origin of Species," pp. 316-362. 



' Zacbanu, O., ■■ Tier- und Pflaozeowelt d. SiiMwaiwn," pp. 306-308. 



