256 A HISTORY OF FISHES 



and Antarctic Zones, encircling the North and South Poles of 

 the earth (Fig. 98). 



The Tropical Zone contains by far the greatest number and 

 diversity of genera and species, although as far as actual 

 numbers of individuals are concerned some of the more northerly 

 species like the Cod (Gadus) and the Herring [Clupea) probably 

 surpass any of those in tropical climes. Among the character- 

 istic oceanic forms inhabiting this region are the Tunnies, 

 Bonitoes, and Albacores {Scombridae) , "Sword-fishes" {Xiphii- 

 due, Istiophoridae) , Flying-fishes {Exocoetidae) , and so on, and as 

 all the coral reefs of the world are included within its limits, 

 such typical reef-dwelling forms as the Butterfly-fishes (Chaeto- 

 dontidae)^ Pomacentrids {Pomacentridae) , Wrasses {Labridae), 

 Parrot-fishes {Scaridae), File-fishes [Monacanthidae) , Trunk-fishes 

 {Ostraciontidae), etc., are largely confined to this zone. The 

 vast majority of the marine Perch-like fishes (Perciformes) — 

 Groupers, Grunts, Drums, Carangids, and the like — are in- 

 habitants of the Tropical Zone, which also includes many 

 diverse members of the Herring family (Clupeidae) as well as 

 numerous kinds of Flat-fishes {Heterosomata) . 



As far as the coastal fishes are concerned, two main divisions 

 or regions of the Tropical Zone may be recognised: Indo- 

 Pacific and Atlantic. The same genera may occur in both 

 regions, but are nearly always represented by distinct species. 

 A careful examination of the fishes living on either side of the 

 continent of America, and particularly of those in the neighbour- 

 hood of the narrow isthmus connecting North and South 

 America, reveals the fact that those of the Atlantic bear a 

 marked resemblance to those of the Pacific. Indeed, in the 

 Panama region many of the species can be arranged in pairs, 

 one being found on the Atlantic side and its nearest relative 

 on the Pacific, the two being frequently so alike that they can 

 only be distinguished with difficulty. How has this similarity 

 of the two faunas been brought about? The artificially con- 

 structed Panama Canal, with its series of locks, may be 

 dismissed as a connecting passage, and it is equally impossible 

 for any mixing to have taken place via the cold waters of the 

 Arctic and Antarctic Oceans. It is well known, however, that 

 in the geological period known as the Eocene the present isthmus 

 of Panama was submerged beneath the sea and the Atlantic 

 and Pacific Oceans were continuous (Fig. 97). The same types 

 of fishes were then found on both sides of the continent, but the 

 subsequent formation of the isthmus provided a definite physical 



