272 



ISLAND LIFE. 



[part it. 



distant Bermudas almost every year, and extending its range as 

 far as Paraguay, is the only species of land-bird which remains 

 completely unchanged in the Galapagos ; and we may therefore 

 conclude that some stragglers of the migrating host reach the 

 islands sufficiently often to keep up the purity of the breed. 

 Next, we have the almost cosmopolite short-eared owl (Asio 

 hrachyotus), which ranges from China to Ireland, and from Green- 

 land to the Straits of Magellan, and of this the Galapagos bird 

 is probably only one of the numerous varieties. The little 

 wood warbler {Dendroeca aitreola) is closely allied to a species 

 which ranges over the whole of North America and as far south 

 as New Grenada. It has also been occasionally met with in 

 Bermuda, an indication that it has considerable powers of flight 

 and endurance. The more distinct species — as the mocking- 

 thrushes (Mimus), the tyrant fly-catchers (Pyrocephalus and 

 Myiarchus), and the ground dove (Zenaida), are all allied to non- 

 migratory species peculiar to tropical America, and of a more 

 restricted range ; while the distinct genera are allied to South 

 American groups of finches and sugar-birds which have usually 

 restricted ranges, and whose habits are such as not to render 

 them likely to be carried out to sea. The remote ancestral 

 forms of these birds which, owing to some exceptional causes, 

 reached the Galapagos, have thus remained uninfluenced by 

 later migrations, and have, in consequence, been developed into 

 a variety of distinct types adapted to the peculiar conditions of 

 existence under which they have been placed. Sometimes the 

 different species thus formed are confined to one or two of the 

 islands only, as the two species of Certhidea, which are divided 

 between the islands but do not appear ever to occur together. 

 Mimus parvidus is confined to Albemarle Island, and M. trifascia- 

 tus to Charles Island ; Cactornis pallida to Indefatigable Island, 

 and C. ahingdoni to Abingdon Island. 



Now all these phenomena are strictly consistent with the 

 theory of the peopling of the islands by accidental migrations, 

 if we only allow them to have existed for a sufficiently long 

 period ; and the fact that volcanic action has ceased on many of 

 the islands, as well as their great extent, would certainly 

 indicate a considerable antiquity. 



