CH. Ill] BIRDS OF OCEANIC ISLANDS. 151 



on oceanic islands ; indeed they are more often entirely 

 absent than present even in small numbers ; hence it 

 must be inferred that they have peculiar difficulties in 

 crossing a considerable expanse of ocean. When they are 

 met with upon oceanic islands, as for example in the 

 Galapagos, they have undergone considerable modification, 

 thus emphasising from another point of view the diffi- 

 culties of their transference. On the other hand birds 

 are of all animals the most liberally endowed with the 

 capacity of crossing long stretches of ocean ; correlated 

 with this the birds of oceanic islands are not always 

 greatly modified as compared with their nearest allies 

 upon the mainland ; this is particularly well shown in- 

 such birds as are in the habit of taking long flights ; the 

 Waders for instance of oceanic islands are but rarely of 

 different species from those of the adjacent continents. 

 On the other hand the finches and such like " small birds," 

 frequently arboreal, depending entirely or largely upon 

 vegetable food, and not in the least marine in their 

 proclivities are commonly much specialised upon oceanic 

 islands ; often indeed to such a degree that their affinities 

 are hard to interpret, as is particularly well shown in 

 the case of some of the Sandwich Islands' birds (for which 

 see below). 



Influence of human interference upon Migration. 



In considering the facts and problems of distribution 

 it is very important to eliminate the changes in faunas 



