60 
ZOOLOGICAL GEOGRAPHY. 
[part III. 
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 Fevillagigedo 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 (Uta) 
characteristic 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 M arias, though all were Mexican forms. The peculiar 
species belonged to the genera Harporhynchus (Turdidse) ; Trog- 
lodytes (Troglodytidse) ; Pipilo (Fringillidse) ; Zenaidura (Colum- 
bidse) ; and a variety of Conurus holoehrous (Psittacidae). 
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 Irving 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 
