358 DARWINISM chap. 



ing nightjars, buntings, white-throats, willow-wrens, cuckoos, 

 house -sparrows, robins, wheatears, and blackbirds. These 

 had probably crossed from Somersetshire, and had they been 

 caught by a storm the larger portion of them must have been 

 blown out to sea.^ 



These facts enable us to account sufficiently well for the 

 birds of oceanic islands, the number and variety of which are 

 seen to be proportionate to their facilities for reaching the 

 island and maintaining themselves in it. Thus, though more 

 birds yearly reach Bermuda than the Azores, the number of 

 residents in the latter islands is much larger, due to the 

 greater extent of the islands, their number, and their more 

 varied surface. In the Galapagos the land-birds are still more 

 numerous, due in part to their larger area and greater proxi- 

 mity to the continent, but chiefly to the absence of storms, 

 so that the birds which originally reached the islands have 

 remained long isolated and have developed into many closely 

 allied species adapted to the sj^ecial conditions. All the 

 species of the Galapagos but one are peculiar to the islands, 

 while the Azores possess only one peculiar species, and 

 Bermuda none — a fact which is clearly due to the continual 

 immigration of fresh indi\dduals keeping up the j^urity of 

 the breed by intercrossing. In the Sandwich Islands, which 

 are extremely isolated, being more than 2000 miles from 

 any continent or large island, we have a condition of things 

 similar to what prevails in the Galapagos, the land -birds, 

 eighteen in number, being all peculiar, and belonging, except 

 one, to peculiar genera. These birds have probably all 

 descended from three or four original types which reached 

 the islands at some remote period, probably by means of 

 intervening islets that have since disappeared. In St. Helena 

 we have a degree of permanent isolation which has pre- 

 vented any land-birds from reaching the island ; for although 

 its distance from the continent, 1100 miles, is not so great 

 as in the case of the Sandwich Islands, it is situated in an 

 ocean almost entirely destitute of small islands, while its 

 position within the tropics renders it free from violent storms. 

 Neither is there, on the nearest part of the coast of Africa, 

 a perpetual stream of migrating birds like that which 

 ^ Report of the Brit. Assoc. Committee on Migration of Birds during 1886. 



