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 



