2»0 



Isthmus Barriers Separating Fish Faunas 



During the lifetime of most of the present species, the Isth- 

 mus has not been depressed. It was depressed in or before 

 Pliocene time, during the hfetime of most of the present genera. 

 We learn from other sources that few of the extant species of 

 fishes are older than the PHocene. Relatively few genera go 

 back to the Eocene, and most of the modern families appear 

 to begin in the Eocene or later Cretaceous. In general the 

 Miocene may be taken as the date of the origin of modern 

 genera. The channel formed across the Isthmus was relatiA'ely 

 shallow, excluding forms inhabiting rocky bottoms at consider- 

 able depths. It was wide enough to permit the infiltration from 



Fig. 1S3. — Channel Catfish, IdaJurns pinirtntus CRafinesquc). Illinois River. 



Family Siluridcp. 



the Caribbean Sea of numerous species, especially of shore 

 fishes of sandy bays, tide pools, and brackish estuaries. The 

 currents set chiefly to the Avestward, favoring the transfer of 

 Atlantic rather than Pacific types. 



Since the date of the closing of this channel the species left 

 on the tAvo sides have been altered in varying degrees by the 

 processes of natural selection and isolation. The cases of actual 

 specific identity are few, and the date of the establishment as 

 species, of the existing forms, is subsequent to the date of the 

 last depression of the Isthmus. 



We may be certain that none of the common genera ever 

 found their way around Cape Horn. ^lost of them disappear 

 to the southward, along the coasts of Brazil and Peru. 



While local oscillations, inA'olving changes in coast-lines, 

 have doubtless frequently taken place and are still going on, 

 the past and present distribution of fishes does not alone give 

 adequate data for their investigation. 



