CHAP, v.] DISPERSAL OF ANIMALS AND PLANTS. 79 



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 un- 

 doubtedly important 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 plumage 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 will be found 

 in Chapter XII. when discussing the origin of the flora of the 

 Azores and Bermuda. 



Ocean-currents as agents in Plant-dispersal. — Ocean-currents 

 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 experiments have been 

 collected proving that seeds may sometimes be carried in 

 this way many thousand miles and afterwards germinate. 

 Mr. Darwin made a series of interesting experiments on 

 this subject, some of which will be given in the chapter above 

 referred to. 



Dispersal 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 relation to that of 

 the nearest continent ; but there are other phenomena pre- 

 sented by the dispersal of species and genera of plants over 

 very wide areas, especially when they occur in widely separated 

 portions of the northern and southern hemispheres, that are 

 not easily explained by such causes alone. It is here that 

 transmission along mountain chains has probably been effective; 

 and the exact mode in which this has occurred is discussed in 

 Chapter XXIII., where a considerable body of facts is given, 

 showing that extensive migrations may be effected by a suc- 

 cession of moderate steps, owing to the frequent exposure of 

 fresh surfaces of soil or debris on mountain sides and summits, 

 offering stations on which foreign plants can temporarily 

 establish themselves. 



