CHAP. V DISPERSAL OF ANIMALS AND PLANTS 81 



year a million seeds were brought by the wind to the 

 British Isles from the Continent, this would be only ten 

 to a square mile, and the observation of a life-time might 

 never detect one ; yet a hundredth part of this number 

 would serve in a few centuries to stock an island like 

 Britain with a great variety of continental plants. 



When, however, we consider the enormous quantity of 

 seeds produced by plants, that great numbers of these are 

 more or less adapted to be carried by the wind, and that 

 winds of great violence and long duration occur in most 

 parts of the world, we are as sure that seeds must be 

 carried to great distances as if we had seen them so carried. 

 Such storms carry leaves, hay, dust, and many small objects 

 to a great height in the air, while many insects have been 

 conveyed by them for hundreds of miles out to sea and 

 far beyond what their unaided powers of flight could have 

 effected. 



Birds as Agents in the Dispersal of Plants. — Birds are 

 undoubtedly imj)ortant agents in the dispersal of plants 

 over wide spaces of ocean, either by swallowing fruits and 

 rejecting the seeds in a state fit for germination, or by the 

 seeds becoming attached to the iDlumage of ground- 

 nesting birds, or to the feet of aquatic birds embedded in 

 small quantities of mud or earth. Illustrations of these 

 various modes of transport Avill be found in Chapter XII. 

 when discussing the origin of the flora of the Azores and 

 Bermuda. 



Ocean-currents as Agents in Tlant-dispersal. — Ocean-cur- 

 rents are undoubtedly more important agents in conveying 

 seeds of plants than they are in the case of any other 

 organisms, and a considerable body of facts and experi- 

 ments have been collected proving that seeds may some- 

 times be carried in this way many thousand miles and 

 afterwards germinate. Mr. Darwin made a series of in- 

 teresting experiments on this subject, some of which will 

 be given in the chapter above referred to. 



Bisperzal along Mountain Chains. — These various modes 

 of transport are, as will be shown when discussing special 

 cases, amply sufficient to account for the vegetation found 

 on oceanic islands, which almost always bears a close 



G 



