262 Isthmus Barriers Separating Fish Faunas 



the Pacific coast of the United States and Mexico. With 

 Hawaii Japan shares 90 genera, with New Zealand 62; 204 are 

 common to Japan and India, 148 to Japan and the Red Sea, 

 most of these being found m India also. Two hundred genera 

 are common to Japan and Australia. 



From this it is CA'ident that Japan and the ilediterranean 

 have much in common, but apparently not more than Japan 

 shares with other tropical regions. Japan naturally shows most 

 likeness to India, and next to this to the Red Sea. Proportion- 

 ately less is the resemblance to Australia, and the likeness to the 

 Mediterranean seems much the same as that to the AYest Indies 

 or to the Pacific coast of America. 



But, to make these comparisons just and effective, we should 

 consider not the fish fauna as a whole ; we should limit our dis- 

 cussion solely to the forms of ecjuatorial origin. From the 

 fauna of Japan we may eliminate all the genera of Alaskan- 

 Aleutian origin, as these could not be found in the other regions 

 under comparison. We should eliminate all pelagic and all 

 deep-sea forms, for the laws wdiicli govern the distribution of 

 these are very different from those controlling the shore fishes, 

 and most of the genera ha\'e reached a kind of equilibrium 

 over the world. 



Significance of Rare Forms. — We may note also, as a source 

 of confusion in our investigation, that numerous forms found 

 in Japan and elsewhere are very rarely taken, and their real 

 distribution is unknown. Some of these will be found to 

 have, in some unexpected cjuarter, their real center of disper- 

 sion. In fact, since these pages were AA'ritten, I have taken in 

 Hawaii representatives of three * genera which I had enumer- 

 ated as belonging chiefly to Japan and the West Indies. 

 Numerous other genera common to the two regions have since 

 been obtained by Dr. Gilbert. Such species may inhabit 

 oceanic plateaus, and find many halting places in their circuit 

 of the tropical oceans. AYe have already discovered that 

 Madeira, St. Helena, Ascension, and other volcanic islands con- 

 stitute such halting places. We shall find many more such, 

 Avhen the dee]ier shore regions are explored, the region between 

 market-fishing and the deep-sea dredgings of the Challenger and 



* Antigonia, Etclis, Eminclichthys. 



