136 OCEANIC ISLANDS. [CH. Ill 



the feet of wild ducks and found it to contain after 

 cultivation numerous small creatures such as Nematoda, 

 Rotifers, eggs of Cladocera, statoblasts of Plwmatella &c. 

 Not only are the eggs of most of these creatures ex- 

 tremely patient of desiccation, but the animals them- 

 selves would readily survive a short journey. A series of 

 journeys would scatter them far and wide over the globe. 

 Such small aquatic organisms are known to be widely 

 dispersed. 



We find that the facts of distribution are quite in 

 accord with these principles. Birds and winged creatures 

 generally, such as bats, are on the whole the most widely 

 distributed orders of animals. Amphibia and earthworms^ 

 rarely or never occur on both sides pf a stretch of sea 

 unless there be good evidence to show that a land 

 connection once existed. Oceanic islands which have 

 been formed de novo in mid ocean and are not detached 

 portions of pre-existing continents are almost invariably 

 free from such animals as are incapable of traversing the 

 sea. If sufficiently distant from any continent oceanic 

 islands are generally without mammals, reptiles and 

 amphibia, but have both birds and insects and certain 

 other invertebrates which are transported to them by 

 involuntary migration. 



Influence of geological terrain upon faunas. 



Not only the existence of forests or deserts or open 

 pampas are influential in favouring or checking the 

 advance of animals to fresh localities ; it is even held that 



