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, w T hich 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. 
