160 THE DISTEIBUTION 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 chng tight and aUow the danger to pass. 



Another danger, qiiite 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- 

 ally are 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 



