146 THE WEALTH OF THE SEA 



travel far; and, remarkable as it may seem, they 

 generally stay within comparatively restricted areas. 



In general the geographical range of fishes ap- 

 pears to be limited, first, by barriers of some sort; 

 second, a fish may reach a certain locality and be 

 unable to maintain itself there; and, third, it may 

 maintain itself but undergo such pronounced 

 changes in the new environment that in time a species 

 distinct from the original type may be evolved. The 

 most common barriers hindering fish from becoming 

 more widely distributed consist of land, of higher 

 and lower temperatures, and of shallow and deep 

 water. It is, of course, well known that some species 

 cannot withstand a high temperature while others 

 require warm water. A fish from tropical waters 

 would meet death just as certainly as an Arctic fish, 

 if an exchange of waters at their normal tempera- 

 tures were to take place. Similarly deep water would 

 mean death to the shoal-water fish just as surely as 

 shoal water would be fatal to the deep-sea fish. 



Most of our common food fishes live in compara- 

 tively shallow water near shore ; many enter bays and 

 inlets; some enter estuaries of streams; and several 

 ascend fresh-water rivers. The swordfish, the tile- 

 fish, the red snappers, the halibut, and to a certain 

 extent the cod, however, are common and important 

 food fishes which generally remain some distance 

 offshore. 



Some of the common marine fishes are pelagic in 

 their habits; that is, they swim and feed in open 

 water at or near the surface. The menhaden, the 



