GEOGRAPHICAL DISTRIBUTION 145 



that the rate of fall of various seeds in air is such that they 

 would have to be carried to improbable heights by the 

 wind in order to travel for very great distances before 

 falling to the ground, 



But no amount of negative evidence is conclusive in the 

 face of even one firmly established bit of positive evidence, 

 and the positive evidence is not only more conclusive but 

 more voluminous than the negative. Seeds of the pitcher 

 plant, Nepenthes ampullaria, are known to have been 

 transported from Ceylon to the Seychelles, a distance of 

 1500 miles, and Engler calculated that, out of a total of 

 about 675 species in Hawaii, 140 ferns and other spore- 

 bearing plants, and 14 angiosperms were quite certainly 

 transported thither by wind. In fact, a large percentage 

 of the vegetation of isolated oceanic islands is of plants 

 whose seeds could hardly have been transported there in 

 a viable condition except by winds. l 



As Warming has noted, there is not an oceanic island 

 destitute of plant life, though many of them are separated 

 from the present mainland by hundreds and even thousands 

 of miles of salt water, and have never, in all probability, 

 been connected with any continent; all their vegetation, 

 therefore, must have been transported thither by some 

 agency. In 1901 there fell in Switzerland large quantities 

 of dust which is said to have undoubtedly come from 

 Africa. If this were possible it is certainly not improb- 

 able that light seeds of various species might be trans- 

 ported very long distances in a similar manner. The 



1 Even small animals, and especially insects, are known to be trans- 

 ported to considerable distances by the wind. During his voyage on 

 the exploring ship Beagle (1832-1836), Darwin observed spiders, buoyed 

 up by their webs, being wafted over the vessel by the wind as far as 

 60 miles from land. 



