130 



ANALYSIS OF THE ENVffiONMENT 



experimentation diflBcult, and it is hard to 

 determine the ecological e£Fects that can be 

 ascribed solely to its action. As yet, it is 

 impossible to free an animal from gravity, 

 although the pull can be equalized by ro- 

 tation on a wheel or kUnostat. A certain 

 amount of experimentation is possible, be- 

 cause, within limits, animals react to centrif- 

 ugal forces as they do to gravity. Since 

 gravity is always acting on the animal 

 while it is in the centrifuge, this method is 

 mainly efiEective for centrifugal forces some- 

 what stronger than gravity. Difficulties in 

 analysis are increased by the presence in 

 nature of many vertical gradients such as 

 those of light intensity, pressure, tempera- 

 ture, and related viscosity, or of oxygen or 

 carbon dioxide content. These vertical in- 

 tensity gradients are frequently steepei in 

 \vater than in air; in either medium, some 

 animals may react to environmental strati- 

 fications much as other organisms do to 

 gravity itself. 



THE EFFECT OF GRAVITY ON THE 

 ENVIRONMENT 



Isostasy 



Probably the most far-reaching ecological 

 effect of gravity is in relation to the prin- 

 ciple of isostasy. According to this principle, 

 the distribution of the continental platforms 

 and the deep ocean basins is, in the main, 

 a result of isostatic balance. Determinations 

 of the pull of gravity show that this force 

 is weaker over the continents than over the 

 ocean, furnishing a strong indication that 

 the materials underlying the oceans are 

 heavier than are those in or under the con- 

 tinental protuberances. 



If the earth originated largely according 

 to the postulations of the planetesimal hy- 

 pothesis, as it now seems reasonable to be- 

 Ueve, differential distribution of densities 

 in the earth's crust can be explained, as 

 it was by T. C. ChamberUn, in terms of 

 selective ingathering of planetesimal debris 

 combined with selective placement of the 

 products of weathering (Mather, 1939). 

 Under any conditions, isostasy strongly 

 suggests that the great continental plat- 

 forms are relatively fixed and stable in loca- 

 tion; hence gravity, working to establish 

 and maintain isostatic balance, has been of 

 fundamental injportance in setting up the 

 primary division of world habitats. 



The principle of isostasy is related to the 



evolution of habitats rather than to that ot 

 individuals; like certain other environmental 

 principles, isostasy is more closely con- 

 cerned wdth community ecology than with 

 the ecology of individuals. 



Earthquakes 



Earthquakes are of decidedly smaller 

 importance than is the general isostasy of 

 the planet. They are not rare. Including the 

 slight tremors, earthquakes may well run 

 to about 30,000 in a year. Those strong 

 enough to destroy towns come much less 

 often and averaged between one and four 

 a year during the nineteenth century. 



The majority of earthquakes take place 

 in two relatively narrow bands that Ue 

 along the Alpine-Italian-Caucasian-Himalay- 

 an great circle and along the giant horse- 

 shoe made by the Pacific coast hne. These 

 two belts meet in eastern Asia. Of 160,000 

 earthquakes, 53 per cent were recorded 

 from the Mediterranean-Himalayan circle 

 and 38 per cent from the Malayan-Jap- 

 anese-Andean horseshoe. Were the study 

 of these two regions equally careful, these 

 percentages would probably be different. In 

 any event, 68 per cent of earthquakes 

 strong enough to affect a tenth of the 

 earth's surface originated near the borders 

 of the Pacific Ocean. In contrast, earth- 

 quakes are rare in the great continental low- 

 lands of central Russia, Brazil, and in mid- 

 North America. The general rule is that 

 of two adjoining regions, the one with the 

 greater average slope is the more unstable. 



Many earthquakes occur near volcanoes, 

 particularly preceding eruption, or near 

 those that are nearly or quite extinct. Such 

 tremors cover a relatively small area of 100 

 to 200 square miles, have shallow foci that 

 are usually less than a mile in depth, and 

 show great intensity near the focus. Gen- 

 eral tectonic earthquakes have deeper foci, 

 often ranging from 6 to 20 or more miles 

 below the surface. Still deeper ones are 

 known that have an estimated depth of 

 from 125 to 375 miles or even more. The 

 really large earthquakes cover areas up to 

 two million square miles. 



Earthquakes commonly result from dis- 

 placement along a fault line. At times the 

 disturbance follows a warping of the earth's 

 crust with no visible fault. The amount of 

 slipping ranges from a fraction of an inch 

 to several feet and may be local or extend 

 for a few hundred miles. 



