284 INTRODUCTION TO EVOLUTION 



would be inexplicable, since most of the islands prove to be entirely suit- 

 able to mammalian life once mammals are introduced by man or some 

 other agency. We have seen that continental islands, e.g., Borneo, have 

 abundant mammahan faunas which reached them over land connections. 



Means of Dispersal to Oceanic Islands 



If, then, we discount land connections as means by which animals reach 

 oceanic islahHs, what other means are availaBTe? 



Transportation by wind may well be the most important means of popu- 

 lating oceanic islands. The efficacy of this means will be at once apparent 

 in the cases of plants which produce microscopic seeds and spores. Dust 

 from the explosive volcanic eruption which destroyed a large part of the 

 island of Krakatoa in 1883 encircled the globe, remaining suspended in 

 the atmosphere for many months. Hence there is no difficulty in accounting 

 for dispersal of seeds and spores which rival dust particles in minuteness. 



Even larger seeds may be carried by winds and air currents. Thus 

 Click (1939) in connection with trapping insects in airplanes at high alti- 

 tudes caught plant seeds at altitudes as great as 5000 feet. He concluded 

 that transportation by prevailing winds in the upper air currents forms the 

 most important means by which seeds reach oceanic islands. There seems 

 no reason why this same method of transport may not be effective for 

 animals and animal eggs which resemble seeds in diminutiveness, providing 

 "only that the animals or eggs can withstand conditions encountered in the 

 upper atmosphere. Few people realize the vast multitude of minute land 

 snails living all around us. Many of these snails are less than a millimeter 

 in diameter and weigh less than one milligram. It should occasion no sur- 

 prise that most oceanic islands possess such tiny snails and that in some 

 cases snails found on these islands are hundreds, or even thousands, of 

 miles distant from their nearest relatives. 



The same situation applies to small insects. One investigator, Elton, has 

 observed that aphids and flies are blown across the 800 miles between 

 Europe and Spitzbergen (Zimmerman, 1948). In the investigation men- 

 tioned previously, Click trapped thousands of specimens at heights up to 

 14,000 feet. Included were flightless larvae and nymphs, wingless adults, 

 mites, and spiders. Significantly, the forms taken at greatest heights were 

 weak fliers, while the stronger fliers, having heavier bodies, were found at 

 lower altitudes. Thus for the most part transportation through the higher 

 atmosphere was being accomplished with relatively little cooperation on the 

 part of the insect. 



