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 salinity 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 salinity. 
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 littoral 
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 Weddell’s seal (Leptonychotes 
weddelli), Ross’s seal (Ommatophoca rossi), the crab- 
1404 Ty 
