60 ZOOLOGICAL GEOGEAPHY. [part hi. 



mammals are peculiar, indicates that the separation is not a very- 

 recent one. At the same time, as all the species are very closely 

 ' allied to those of the opposite coasts when not identical, we may 

 be sure that the subsidence which isolated them is not geologi- 

 cally remote. 



Socorro, the largest of the Revillagigedo Islands, is altogether 

 different from the Tres Marias. It is situated a little further 

 south (19 S. Latitude), and about 300 miles from the coast, in 

 deep water. It is about 2,000 feet high, very rugged and bare, 

 and wholly volcanic. No mammalia were observed, and no 

 reptiles but a small lizard, a new species of a genus {Utci) 

 cliaracteristic of the deserts of N.-Western Mexico. The only- 

 observed land-shell (Orthalicus undatus) also inhabits N.-W. 

 Mexico. Only 14 species of birds were obtained, of which 9 

 were land-birds ; but of these 4 were new species, one a peculiar 

 variety, and another (Parula insularis) a species first found in 

 the Tres Marias. With the exception of this bird and a Buteo, 

 all the land-birds belonged to different genera from any found on 

 the Tres Marias, though all were Mexican forms. The peculiar 

 species belonged to the genera Harporhynchus (Turdidse) ; Trog- 

 lodytes (Troglodytidae) ; Pijjilo (Fringillidse) ; Zenaidura (Colum- 

 bidai) ; and a variety of Conui'us holochrous (Psittacidce). 



The absence of mammals and snakes, the large proportion of 

 peculiar species, the wholly volcanic nature of these islands, and 

 their situation in deep water 300 miles from land, — all indicate 

 that they have not formed part of the continent, but have been 

 raised in the ocean ; and the close relation of their peculiar 

 species to those living in N.-Western Mexico, renders it pro- 

 bable that their antiquity is not geologically great. 



The Cocos Islands, about 300 miles S.-W. of the Isthmus of 

 Panama, are known to possess one peculiar bird, a cuckoo of the 

 Coccyzus type, which is considered by some ornithologists to con- 

 stitute a peculiar genus, Nesococcyx. 



IV. The West Indian Islands, or Antillean Sub-region. 



The West Indian islands are, in many respects, one of the 

 most interesting of zoological sub-regions. In position they 



