260 Isthmus Barriers Separating Fish Faunas 



therefore undertake a somewhat fuller analysis of the evidence 

 bearing on this and similar problems with a view to the con- 

 clusions which may be legitimately drawn from the facts of fish 

 distribution. 



Differences between Japanese and Mediterranean Fish Faunas. 

 We may first, after admitting the alleged resemblances and 

 others, note that differences are equally marked. In each re- 

 gion are a certain number of genera which we may consider 

 as autochthonous. These genera are represented by many 

 species or by many individuals in the region of their supposed 

 origin, but are more scantily developed elsewhere. Such genera 

 in Mediterranean waters are Crenilabrus, Labrus, Spicara, Pagel- 

 lus, Mullus, Boops, Spondyliosoma, Oblata. None of these occurs in 

 Japan, nor have they any near relatives there. Japanese autoch- 

 thonous types, as Pseudoblennius, Vellitor, Duymcsria, Anoplus, 

 Histiopterus, Monocentrus, Oplegnathus, Plecoglossus, range south- 

 ward to the Indies or to Australia, but all of them are totally 

 unknown to the Mediterranean. The multifarious genera of 

 Gobies of Japan show very little resemblance to the Mediter- 

 ranean fishes of this family, while blennies, labroids, scaroids, 

 and scorpaenoids are equally diverse in their forms and alliances. 

 To the same extent that likeness in faunas is produced by con- 

 tinuity of means of dispersion is it true that unlikeness is due 

 to breaks in continuity. Such a break in continuity of coast- 

 line, in the present case, is the Isthmus of Suez, and the uulike- 

 ness in the faunas is about what we might conceive that such a 

 barrier should produce. 



Sources of Faunal Resemblances. There are two main 

 sources of faunal resemblances: first, the absence of any barriers 

 permitting the actual mingling of the species; second, the like- 

 ness of temperature and shore configuration on either side of 

 an imperfect barrier. Absolute barriers do not exist and ap- 

 parently never have existed in the sea. If the fish faunas of 

 different regions have mingled in recent times, the fact would 

 be shown by the presence of the same species in each region. 

 If the union were of a remote date, the species would be changed, 

 but the genera might remain identical. 



In case of close physical resemblances in different regions, as 

 in the East Indies and West Indies, like conditions would favor 



