GENEKAL REMAEKS ON DISTRIBUTION. 



The ichlliyological province to which Panama belongs extends to the north- 

 ward as far as the Gulf of California and Magdalena Bay. Of the 374 species recorded 

 from Panama, 204 are now known to occur in the Gulf of California, and further 

 exploration will certainly increase this list. The two regions differ principally in the 

 greater development at Panama of Siluroids and Scifenoids, the majority of which fail 

 to reach the northern limits of the province. 



To the south of Panama, the faunal relations are as yet poorly defined. The 

 coast of Ecuador is known to us lirincipally from Boulenger's (1898-9) brief account 

 of a collection from the Bay of Santa Helena, near Guayaquil. The marine species 

 there listed belong almost exclusively to the Panama fauna, and include many char- 

 acteristic forms. How much farther to the southward these extend their range is 

 unknown. The coast of Peru is largely unexplored, but the very incomplete lists 

 which we possess indicate an almost total absence of Panama species. When these 

 coasts shall be adequately investigated, there will probably be discovered a rather 

 sharp line of demarcation of faunas, corresjionding with the interval between the 

 areas of the South Equatorial and the Equatorial Counter Currents. 



Much has been written concerning the close parallelism between the fish- 

 faunas on opf)osite sides of the Isthmus of Panama, and the bearing of this upon the 

 question of a water-way formerly open between the two oceans. A full bibliogra]>hy 

 of the subject is given by Gregory (1895), together with a resume of the geological 

 and biological evidence for the former existence of such an interoceanic connection, 

 and a discussion of the probable date of its occurrence. 



From the biological side, the subject is treated in a most satisfactory way by 

 Faxon (1895), with whose views we find ourselves wholly in accord. The ichthy- 

 ological evidence is overwhelmingly in favor of the existence of a former open 

 communication between the two oceans, which must have become closed at a period 

 sufficiently remote from the present to have permitted the sj^ecific differentiation of 

 a very large majority of the forms involved. That this differentiation progressed at 

 widely varying rates in different instances becomes at once apparent. A small 

 minority of the species remain wholly unchanged, so far as we have been able to 

 determine that point. A larger number have become distinguished from their 

 representatives of the opposite coast by minute (but not "trivial") differences, 

 which are wholly constant. From such "representative forms," we pass by imper- 

 ceptible gradation to species much more widely separated, whose immediate relation 

 in the past we cannot confidently affirm. Of identical species, occurring in both 



( 27 ) January 26, 1904. 



