54 Causes of Emigrations 



ZONATION AND STRATIFICATION 



On ocean beaches all over the earth animals are arranged more 

 or less in zones (Allee, 1923, 1923a, 1923b; Crane, 1947; Evans, 

 1947; Hewatt, 1937; Kuhnholtz-Lordat, 1926, 1926a; Pearse, 1914; 

 Pearse, Humm, 6C Wharton, 1942; McDougall, 1943; Stephenson, 

 1942). Such tendencies toward stratification in the distribution of 

 animals in habitats is general, and at times such segregation has 

 apparent relation to differences in temperature, available water, and 

 other environmental conditions. Even among land vegetation there 

 is a tendency for plants and animals to segregate into strata, which 

 begin below the surface of the soil and extend to the tree tops. 



Along ocean beaches the tendency of animals to segregate into 

 zones has apparently led to the taking up of life on land by some 

 species. Along the Irish coast the distribution of salt-marsh plants 

 is correlated with the salinity of the water in which they grow 

 (McCrea, 1926) . Where mangroves flourish on the coast of Java 

 the littoral crustaceans are arranged in five rather definite zones 

 through the tidal areas (Verwey, 1930) . On the Scottish coast 

 each species of intertidal animal reaches its maximum size at a cer- 

 tain level (Stephen, 1930, 1930a). On the French coast Fischer 

 (1928) has attempted to discover the factors which determine the 

 upper limits of distribution of intertidal marine animals. He finds 

 that exposure of beaches to the open sea may cause waves to distort 

 the zonation which is elsewhere dependent largely on tides. The 

 alga Fucus platycarpus Farlow and the barnacle Balanus balanoides 

 (L.) have upper limits at mean neap-tide level. "Various other lit- 

 toral organisms have limits more or less close to high-tide mark or 

 to low-tide mark; almost none stop in the intermediate zone. It is 

 thus frequency rather than duration of immersion that acts upon 

 organisms. Ecological subdivisions may be established in the tidal 

 zone, corresponding to the successive upper limits of distribution of 

 various species." 



