160 THE DISTRIBUTION OF 
against the movement of the waters, and they are found 
dead or stranded on the beach. 
The weaker abandon the attempt to struggle against 
the waters—the lumpsucker and the gobies among fish, 
the limpet among molluscs, the acorn shells and bar- 
nacles among crustacea—these do not attempt to resist 
the tides, but cling tight and allow the danger to pass. 
Another danger, quite as real, though not so obvious, 
which the shore forms must guard against is the con- 
stant change of temperature and of salinity in the sea 
water, which is characteristic of the shore. The 
temperature of the oceans varies much, and is in- 
fluenced by local causes, but the shore waters especi- 
allyare greatly affected by the variations in the tempera- 
ture of the land, that is by latitude. In tropical waters 
the range of temperature is small (about 10° F.), and 
the mean temperature is high (about 80° F.). In polar 
waters, while the range is similarly small, the mean 
temperature is low (about 28° F.). Between the two, 
in temperate regions, the range is considerable, while 
the mean is neither high nor low. Nowhere, of course, 
can the ranges of temperature in the sea be compared 
with those on land, owing to the high specific heat of 
water. We find, however, that very many land animals, 
for instance all the warm-blooded ones, have a heat- 
regulating mechanism, whereby they can adjust them- 
selves to changes of temperature. Such a mechanism 
is absent in all sea animals except those (e. g. whales, 
&c.), who are descended from terrestrial ancestors. 
According as they can or cannot tolerate variations 
in the temperature of sea water, sea animals may 
be divided into eurythermal and stenothermal forms. 
The former are tolerant of great variations, the latter 
are rapidly killed by them. Speaking generally, the 
