IN THE ' BIOLOGIA-CENTRALI AMERICANA.' 545 



restorations of the distribution of land and water during previous 

 geological epochs. Also a faunistic table. 



The Neotropical and ISTearctic faunas and floras do not meet 

 at the Isthmus of Panama, but in Mexico. The isthmus was 

 originally very much broader. 



The various groups of the fauna seem to fall into three 

 categories : — 



1. Those which are of undoubted northern provenance. Some of 



these stop with the plateau ; others descend thence into the 

 hot lands, and most of these continue into Central— even 

 far into South America. 



2. Those which are of Southern, Neotropical provenance. Many 



of them have overrun Central America and extended into 

 Mexico, where their current has, so to speak, been divided 

 to east and west by the wedge-like plateau. 



These two main categories interdigitate, with many com- 

 plications. Some have become derelicts in their new home, 

 whilst they have died out in their older home, e, g., Tapirs. 

 Others have hooked back, not the families, but genera 

 and species rathei-, e. g. Opossums and the Tree-Porcupine 

 Erethizon, 



3. There is a considerable number of forms, drawn from all 



classes, which seem to be endemic, rather archaic, developed 

 into what they are on the spot. They are the most inter- 

 esting and most difficult to interpret. 



3 A. Some seem to be real aborigines. 3 b. Others are neither 

 from North America proper nor fi-om South America. 

 They must have come from elsewhere. Some of these 

 puzzling groups seem to be a legacy from a more western 

 extension of land, say from Lower California to the 

 Galapagos and South America, analogous to the "Andines" 

 of botanists, which date back far into the Cretaceous 

 period. 



Others point unmistakably to Mediterranean lands and to 

 Africa. A " land-bridge " implies also coasts with all their 

 concomitant ph^'sical features, suitable land-conditions for ter- 

 restrials and freshwater-fish, shallow seas for corals and shells, etc. 

 Such " bridges" need not have ever existed in their entirety, being 

 rather like changing pontoon-bridges. Such restorations rest 

 upon circumstantial evidence ; much of it will, no doubt, be ruled 

 out of court, but there is a great deal of cumulative evidence 

 and much that is mutually supporting (both negative and direct) 

 presented by plants, Vertebiutes, and Invertebrates, terrestrial 

 and marine, so that the Afro-American connections are becoming 

 more than a good workable hypothesis. The chief question is now, 

 how long and into what geological groove did they last ? Did 

 they last long enough, say into the Oligocene, to be available for 

 comparatively recent groups ? 



Proc. Zool. Soc— 1916, No. XXXVIII. 38 



