552 



SCIENCE. 



[N. S. Vol. XIV. No. 354. 



waatiag iu the Mediterranean. The only- 

 genera which could have crossed the 

 Isthmus are certain shallow- water or 

 brackish- water forms, sting-rays, torpe- 

 does, sardines, eels and mullets, widely- 

 diffused through the East Indies and found 

 also in the Mediterranean. The former 

 channel if one ever existed, had, therefore, 

 much the same value in distribution of 

 species, as the present Suez Canal. 



GEOLOGICAL EVIDENCE OF SUBMERSION 

 OF THE ISTHMUS OF SUEZ. 



Yet, from geological data, there is strong 

 evidence that the Isthmus of Suez was sub- 

 merged in relatively recent times. The 

 recognized geological maps of the Isthmus 

 show that a broad area of post-Pliocene or 

 Pliocene deposits constitutes the Isthmus 

 and separates the nummulitic hills of Suez 

 from their fellows about thirty miles to the 

 eastward. The northern part of the Isth- 

 mus is alluvium from the Nile, and its 

 western part is covered with drifting sands. 

 The E,ed Sea once extended farther north 

 than now and the Mediterranean farther to 

 the southeast. Assuming the maps to be 

 correct, the Isthmus must have been open 

 water in the late Pliocene or post-Pliocene 

 times. 



Admitting this as a fact, the difference 

 in the fish fauna shows that the waters over 

 the submerged area must have been so 

 shallow that rock-loving forms did not and 

 could not cross it. Moreover, the region 

 must have been overspread with silt-bear- 

 ing fresh waters from the Nile. To such 

 fishes as Chcetodon, Solocentrus, Thalassoma, 

 of the lied Sea, or to Crenilabrus, Boops and 

 Zeus, of the Mediterranean, such waters 

 would form a barrier as effective as the 

 sand-dunes of to-day. 



CONCLUSIONS AS TO THE ISTHMUS OF SDEZ. 



"We are led, therefore, to these con- 

 clusions : 



1. There is no evidence, derivable from 

 the fishes, of the submergence of the Isth- 

 mus of Suez. 



2. If the isthmus was submerged in Pli- 

 ocene or post- Pliocene times, the resultant 

 channel was shallow and muddy, so that 

 ordinary marine fishes or fishes of rock 

 bottoms, or of deep waters, did not 

 cross it. 



3. It formed an open water to brackish- 

 water fishes only. 



4. The types common to Japan and the 

 Mediterranean did not enter either region 

 from the other, by way of the Eed Sea. 



5. As most of these are found also in 

 India or Australia or both, their dispersion 

 was probably around the south coast of 

 Africa or by the Cape of Good Hope. 



THE CAPE OF GOOD HOPE AS A BARRIER 

 TO FISHES. 



The fishes of the Cape of Good Hope are 

 not well enough known for close compar- 

 ison with those of other regions. Enough 

 is known of the Cape fauna to show its 

 general relation to those of India and Aus- 

 tralia. The Cape of Good Hope lies in the 

 South Temperate zone. It offers no abso- 

 lutely impassable barrier to the tropical 

 fishes from either side. It bears a closer 

 relation to either the Eed Sea or the Med- 

 iterranean than they bear to each other. 

 It is, therefore, reasonable to conclude that 

 the transfer of tropical shore fishes of the 

 Old World between the Atlantic and Pa- 

 cific, in recent times, has taken place mainly 

 around the southern point of Africa. To 

 pelagic and deep-sea fishes the Cape of 

 Good Hope has offered no barrier whatever. 

 To ordinary, fishes it is an obstacle, but 

 not an impassable one. This the fauna 

 itself shows. It has, however, not been 

 passed by many tropical species, and by 

 these only as the result of thousands 

 of years of struggle and point-to-point 

 migration. 



