The origin of life was undoubtedly in the sea. 

 Physiological adjustments were necessary before or- 

 ganisms could occupy fresh water. Although some 

 organisms may have become air-breathers and in- 

 vaded land habitats directly from the sea, most evolu- 

 tion of terrestrial forms has doubtless come from fresh 

 water. Relatively few major groups of animals have 

 been successful in this invasion of land, the most 

 notable being oligochaete worms ; gastropod mollusks ; 

 many arthropods, especially the insects and spiders ; 

 reptiles : birds ; and mammals. 



ADJUSTMENTS TO THE TERRESTRIAL 

 HABITAT 



Living on land presents many problems. 

 Our present concern is to analyze the ways in which 

 animals have met these problems and to trace the suc- 

 cession of communities in the extreme terrestrial 

 habitats of rock, sand, and clay. 



Gravity 



8 



Local Habitats, 



Communities, and 



Succession: 



Rock, Sand, 

 and Clay 



In water, organisms counteract gravity by 

 means of various flotation and swimming mech- 

 anisms. Fluid buoyancy permits water-dwellers to 

 attain huge size ; consider the whale. A land animal, 

 on the other hand, must support its entire weight. 

 Some terrestrial animals gain a modicum of support 

 by burrowing into the soil : others drag their bodies 

 over the ground surface. But the supportive advan- 

 tage they thus gain is costly in other directions, for 

 they are slow moving and relatively helpless before 

 predators. The animals best adapted to terrestrial life 

 have evolved appendages in the form of legs or wings 

 that not only raise the body above the ground but are 

 also the means of more or less rapid locomotion and 

 adroit movements over the surface or through the air. 

 Terrestrial adaptation has involved the development 

 of a tough body covering to hold fluids and internal or- 

 gans in place ; a skeletal framework to give permanent 

 shape to the body and, as a system of levers, to fur- 

 nish means of locomotion ; and powerful muscles to 

 lift and move the heavy body. Gravity thus limits the 

 mass of land animals ; dinosaurs, mastodons, and 

 elephants approach the maximum practicable size. 



Moisture 



In sharp contrast to aquatic forms, terrestrial 

 animals are not constantly enveloped with a continu- 

 ous watery medium, with the limited exceptions of 

 protozoans, nematodes, and other small organisms 

 living in moist soil. 



96 



