HISTORICAL INTRODUCTION 



Meanwhile evidence was accumulating that these particles might be 

 carried by wind to distances vastly greater than had been imagined by the 

 ancients. In dust deposited after transport for hundreds of kilometres 

 by sirocco and trade winds, Ehrenberg (1849, 1872, 1872^?) found large 

 quantities of protozoa and plant spores, and gradually he became con- 

 vinced that viable micro-organisms could survive transport through the 

 atmosphere. When the Beagle w^as near the Cape Verde Islands, Darwin 

 (1846) found the atmosphere haz}' with dust from North Africa. In 

 samples of this dust Ehrenberg found sixty-seven kinds of organisms — 

 including freshwater infusoria and cryptogamic spores (Plate 2) — and 

 Darwin at once grasped the importance of the phenomenon in the 

 geographical distribution of organisms. 



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