SWIMMING ANIMALS 89 



zones or in the vicinity of rocky coasts, as opposed to those 

 of more open water. Such differences can be noticed in 

 comparatively small areas ; but there are, as well, differ- 

 ences to be found over very large regions. Everyone 

 knows that if he visits tropical latitudes he will see fishes 

 that he never sees m more northern waters. A visitor to 

 such widely separated fish-markets as those of Grimsby in 

 England, Trieste in Italy, and Colombo in Ceylon, will at 

 once be convinced of this. Each market will display its 

 own characteristic fish, and it is doubtful whether any fish 

 would be found common to all three markets. Such 

 localization of faunas is easily understandable on land, 

 where such barriers as deserts, high mountain ranges or 

 large areas of water, limit the different animals to their own 

 special regions. But in the sea, with all the oceans of the 

 world connected, there are no such purely mechanical 

 barriers present to limit the dispersal of the different kinds 

 of fish and prevent them from presenting a perfectly uniform 

 distribution from ocean to ocean and sea to sea. How is it 

 then that one finds nevertheless that certain widely separ- 

 ated areas each possess their own characteristic population 

 of fishes ? It is evident that there must be some barrier, 

 and a general exploration of the chemical and physical 

 properties of sea water has shown that in all probability the 

 chief factor in the distribution of the fishes is the temperature 

 of the water itself. 



It is generally to be noticed that in our northern waters, 

 wherever the temperature of the water is less than ten 

 degrees Centigrade, our typical northern fishes will be 

 found, cod, halibut, haddock, herring, and many others. 

 On our south-western coasts, those of Devon and Cornwall, 

 we find that we are on the southern limit of the distribution 

 of these northern fishes ; and, at the same time, here occur 

 the northern limits of certain southern warm-water loving 



