PDF - Wallace Online

CHAP. XIV.] THE NEOTROPICAL REGION. 59
by one or more arms of the sea, as above indicated, is further
rendered necessary by the character of the molluscan fauna of
the Pacific shores of tropical America, which is much more
closely allied to that of the Caribbean sea, and even of West
Africa, than to that of the Pacific islands. The families and
many of the genera are the same, and a certain proportion of
very closely allied or identical species, shows that the union of
the two oceans continued into late Tertiary times. When the
evidence of both land and sea animals support each other as
they do here, the conclusions arrived at are almost as certain as
if we had (as we no doubt some day shall have) geological proof
of these successive subsidences.
Islands of the Mexican Sub-region.—The only islands of
interest belonging to this sub-region, are Tres Marias and
Socorro, recently investigated by Col. Grayson for some of the
American Natural History societies.
Tres Marias consist of four small islands lying off the coast
of north-western Mexico, about 70 miles from San Bias. The
largest is about 15 miles long by 10 wide. They are of horizon-
tally stratified deposits, of moderate height and flat-topped, and
everywhere covered with luxuriant virgin forests. They appear
to lie within the 100 fathom line of soundings. Fifty-two species
of birds, of which 45 were land-birds, were collected on these
islands. They consisted of 19 Passeres ; 11 Picarise (7 being
humming-birds) ; 10 Accipitres ; 2 parrots, and 3 pigeons. AU
were Mexican species except 4, which were new, and presumably
peculiar to the islands, and one tolerably marked variety. The
new species belong to the following genera; Panda and
Granatelhts (Mniotiltidse) ; Icterus (Icteridse) ; and Amazilia
(Trochilidse). A small Psittacula diffei-s somewhat from the
same species on the mainland.
There are a few mammalia on the islands ; a rabbit (lejpu^)
supposed to be new ; a very small opossum {Didelphys), and a
racoon {Procyon). There are also several tree-snakes, a Boa, and
many lizards. The occurrence of so many mammalia and snakes
is a proof that these islands have been once joined to the main-
land ; but the fact that some of the species of both birds and