October 11, 1901.] 



SCIENCE. 



553 



RELATIONS OF JAPAN TO MEDITERRANEAN 

 EXPLAINABLE BY PRESENT CONDITIONS. 



We may conclude that the resemblance 

 of the Mediterranean fish fauna to that of 

 Japan or India is no more than might be 

 expected, the present contour of the conti- 

 nents being permanent for the period of 

 duration of the present genera and species. 

 The imagined removal of barriers on any 

 large scale would necessitate much closer 

 resemblances than those which actually 

 exist. 



THE ISTHMUS OF PANAMA AS A BARRIER 

 TO DISTRIBUTION. 



Conditions in some regards parallel with 

 those of the Isthmus of Suez exist in but 

 one other region — the Isthmus of Panama. 



IDENTITY OF GENERA ON TWO SHORES OF 

 THE ISTHMUS OF PANAMA. 



Here the first observers were very 

 strongly impressed by the resemblance of 

 forms. Nearly half the genera found on 

 the two sides of this isthmus are common 

 to both sides. Taking those of the Pacific 

 shore for first consideration, we find that 

 three fourths of the genera of the Panama 

 fauna occur in the West Indies as well. 



This identity is many times greater than 

 that existing at the Isthmus of Suez. 

 Moreover, while the Cape of Good Hope 

 offers no impassable barrier to distribution, 

 the same is not true of the southern part of 

 South America. The subarctic climate of 

 Cape Horn has doubtless formed a com- 

 plete check to the movements of tropical 

 fishes for a vast period of geologic time. 



UNLIKENESS OP SPECIES ON THE SHORES OF 

 THE ISTHMUS OF PANAMA. 



But curiously enough, this marked re- 

 semblance is confined chiefly to the genera 

 and does not extend to the species on the 

 two shores. 



Of 1,400 species of fishes recorded from 

 tropical America north of the Equator, 

 only about 70 are common to the two 

 coasts. The number of shore fishes com- 

 mon is still less. In this 70 are included a 

 certain number of cosmopolitan types which 

 might have reached either shore from the 

 Old World. 



A few others invade brackish or fresh 

 waters and may possibly have found their 

 way, in one way or another, across the 

 Isthmus of Nicaragua. Of fishes strictly 

 marine, strictly littoral, and not known 

 from Asia or Polynesia, scarcely any species 

 are left as common to the two sides. This 

 seems to show that no waterway has ex- 

 isted across the isthmus within the life- 

 time, whatever that may be, of the exist- 

 ing species. The close resemblance of 

 genera shows apparently with almost equal 

 certainty that such a waterway has ex- 

 isted, and within the period of existence of 

 the groups called genera. How long a 

 species of fish may endure unchanged no 

 one knows, but we know that in this regard 

 great diff'erences must exist in different 

 groups. Assuming that difierent species 

 crossed the Isthmus of Panama in Miocene 

 times, we should not be surprised to find 

 that a few remain to all appearances un- 

 changed ; that a much larger number have 

 become ' representative ' species, closely re- 

 lated forms retaining relations to the en- 

 vironment to those of the parent form, and, 

 finally, that a few species have been radi- 

 cally altered. 



This is exactly what has taken place at 

 the Isthmus of Panama with the marine 

 shore fishes. Curiously enough, the move- 

 ment of genera seems to have been chiefly 

 from the Atlantic to the Pacific. Certain 

 characteristic genera* of the Panama region 

 have not passed over to the Pacific. On the 



* Hoplopagrus, Xeniehthys, Xenistius, Xenocys, Micro- 

 desmus, Cerdale, Craiinus, Azevia, Microlepidotus, Or- 

 thostoechus, Isaciella, etc. 



