Gravity and Molar Agents 81 



transformed from marine into terrestrial types (Flattely, 1920, 

 1921; Fulton, 1921), but most sessile intertidal animals tend to 

 be radially symmetrical, resisting rather than alert, and unpro- 

 gressive (Hayes, 1927; Pearse, 1914, 1922), so that few of them 

 through the ages have gone across beaches to land. The tide offers 

 a reward to animals which can establish themselves above high-tide 

 mark. This is the accumulations along the drift line — the flotsam 

 and jetsam of the sea, which furnish food and shelter. 



Molar agents in the form of currents, winds, and waves may 

 bring food to littoral animals, transport species to new localities 

 and thus permit them to obtain a foothold, and produce changes 

 in littoral waters which perhaps at times lead to adaptation which 

 help fit animals for terrestrial life. The faunas on wave-washed 

 beaches or on current-swept shores where there are few waves are 

 similar (Fischer, 1927) ; littoral water in motion has some similar 

 effects, whether it is in the form of waves or currents. The pound- 

 ing of waves during storms makes it impossible for some animals 

 to live on exposed shores, and the wetting of higher ground by 

 spray at such times makes it possible for marine animals to become 

 established temporarily on land surfaces or in little pools. Where 

 tides are strong they have rather uniform effects on the littoral 

 water in relation to such qualities as content of Oo and H^S, pH, 

 salinity, temperature, and turbidity (Miller, Ramage, & Lazier, 

 1928) , but where they are feeble or erratic this is not true (Moberg, 

 1927) . Winds help flying fishes to leave the ocean for short sails 



Fig. 13. A beach near Port Antonio in Jamaica. The 



rocks serve as homes for crabs and snails, some of 



which wander into the humid shade of the forest; 



the wet sand teems with amphipods, isopods, crabs 



insects, and many other animals; and the lagoon is 



the home of fishes, sea urchins, jellyfishes, and /oG^C,4/\ 



other swimming or floating life. (Photo by R. E. /o^N-^ '^T"^^ /\ ' 



Blackwelder.) /4^ ..O^"^^ ''^^ tA 



\5\ -4.;.- /^ 



