258 A HISTORY OF FISHES 



in length, there are no locks on its course, and the only- 

 physical barrier to bar the progress of fishes is the high degree 

 of salinity of the Bitter Lakes which lie in its southern half. 

 These seem to be effective in preventing the dispersal of some 

 species, but do not form an insuperable barrier to others, and 

 a recent investigation of the canal fauna revealed that the 

 Bitter Lakes themselves possessed quite a rich fish-fauna, and, 

 curiously enough, some of the fishes were found to grow to a 

 larger size than the individuals living outside the canal in the 

 Red Sea or Mediterranean. The results obtained from this 

 investigation showed that about forty species of fish live 

 habitually in the canal itself, including species from the Medi- 

 terranean and Red Sea in almost exactly equal numbers. Of 

 these, one or two forms from the Mediterranean have succeeded 

 in reaching the Gulf of Suez at the north end of the Red Sea, 

 but have not gone far beyond the neighbourhood of the canal 

 entrance. On the other hand, no less than eight or nine Red 

 Sea fishes have appeared at Port Said, of which several appear 

 to have become thoroughly established in the Mediterranean. 

 For example, a little Sand Smelt from the Indo-Pacific 

 [Hepsetia) has been caught about two hundred miles to the 

 west of Alexandria, as well as on the coast of Palestine, and 

 a species of Siganid [Siganus nebulosus), a genus of fishes normally 

 confined to the Indo-Pacific region, has appeared near the 

 island of Cyprus. It may be concluded, therefore, that a very 

 gradual and far from complete mixing of the two faunas is 

 taking place through the Suez Canal, and that the journey 

 from Red Sea to Mediterranean seems to present fewer 

 difficulties than that in the reverse direction. This may be 

 explained by the presence of rapid tidal currents in the lower 

 part of the canal, which would tend to sweep floating fish- 

 eggs and larvae into the Bitter Lakes, and by the fact that in 

 the northern half of the canal there is a slow, constant streaming 

 to the north for ten months in the year. 



Turning to the Southern Hemisphere, it may be noticed that 

 south of the Tropical Zone the currents are not deflected by the 

 land masses to nearly the same extent as in the north, and the 

 zones of distribution are easier to define and the isotherms more 

 nearly parallel. As far as the subtropical region of the South 

 Temperate Zone is concerned, it is not unusual to find genera 

 common to this region and that of the North Temperate Zone, 

 the Cape fauna having many points in common with that of 

 the Mediterranean. In the accompanying map the dotted areas 



