ANIMAL LIFE IN THE SEA 161 



littoral animals of temperate latitudes must be eury- 

 thermal, while those of tropical and of polar seas may 

 be stenothermal. As the actual temperature in the 

 case of many stenothermal animals seems to matter less 

 than its constancy, the temperate zone must form 

 a barrier between the animals of the two other zones, 

 and more or less prevent their mixing. Further, as 

 sudden changes are always more dangerous than gradual 

 ones, the contact of a cold current with a warm one 

 seems often to lead to a great destruction of animal 

 life, though this perhaps affects pelagic animals more 

 than littoral ones. 



Variations in the saKnity of the water seem to have 

 similar effects, and Hjort states that of the seventeen 

 species of the cod family found in the North and 

 Norwegian Seas, each one seeks out a special spawning- 

 ground, having its characteristic depth, temperature, 

 and salinity, a fact which may be partly due to greater 

 susceptibility of the larvae than of the adults to minor 

 variations in pressure, temperature, and saUnity. 



Something must now be said as to the characters of 

 shore animals. In the chapter on the tundra we men- 

 tioned some of the common forms to be found in Arctic 

 regions, and to that account some more general state- 

 ments may be added. Of the marine mammals we 

 must regard all the seals and the walrus as httoral 

 forms, for though many show a perfect mastery of the 

 water, all come on shore for breeding purposes, and 

 thus are dependent on a substratum, at least at certain 

 seasons. In addition to the seals already mentioned, 

 we have the true fur-seals (Otaria), which occur in the 

 north, outside the Arctic area, and the many seals of 

 the Far South, such as WeddeU's seal (Leptonychotes 

 weddelli), Ross's seal (Ommatophdca rossi), the crab- 



1404 L 



