Gravity and Molar Agents 79 



Gravity and Molar Agents 



Gravity is in some respects to be regarded as a factor which is 

 unfavorable to the migration of animals from water to land. The 

 specific gravity of many animals is little greater than that of water. 

 Motile aquatic animals therefore have little difficulty in remaining 

 suspended in the medium in which they live, but those that have 

 taken up life on land live in the atmosphere, and their bodies are 

 much heavier than it. 



Terrestrial animals burrow in soil, drag their heavy bodies over 

 the surface of the earth, or have more or less effective and special 

 means for support and locomotion. If they ascend elevations, they 

 may be subject to injury from rapid falls and violent collisions 

 with the surface of the earth. A land animal must, if it is to attain 

 any degree of dominance in terrestrial habitats, have skeletal and 

 locomotor structures which support it against the pull of gravity 

 and give it ability to move fast enough to compete with other species. 

 A jellyfish can never become established on land. Gravity in a 

 general way limits the size of land animals. Perhaps the ideal ter- 

 restrial types are such animals as active flying insects, swift lizards, 

 agile birds, and saltatorial mice. But some types of land animals, 

 like dinosaurs and elephants, have successfully overcome gravity 

 and attained gigantic size. To do this they consume great quantities 

 of food, which has its ultimate source in land vegetation and sun- 

 light. 



Gravitational forces are responsible for the tides which exert 

 such a profound influence on littoral plants and animals. The 

 rhythmical ebb and flow of the ocean along shore alternately ex- 

 poses animals on beaches twice each day to the influence of the 

 atmosphere and sea water. Intertidal animals therefore develop a 

 certain degree of resistance to the desiccating action of air and the 

 extreme and rapid variations in temperature which are more or less 

 characteristic of terrestrial habitats. 



Perhaps some littoral animals have through such influences been 



