336 The Geofecfonic Factor 



levelling forces operated, the surface of the earth would gradually 

 become peneplained even though high uplands might erode to 

 form low mountains as a transitory product of the erosion. Climates 

 would approach relative uniformity over the whole world. As more 

 extreme climates disappeared, so would the species and commu- 

 nities restricted to them. With climatic change going only in one 

 direction, the ranges of the remaining species would come together 

 but not divide, and no new geographic isolates would form. As 

 ecological diversity diminished, so would the opportunities for geo- 

 graphic isolation through colonization because climatic barriers be- 

 tween similar but distant areas would disappear. One by one, islands 

 would erode away, removing other opportunities for colonization. 

 As species became extinct, few new ones would evolve to take their 

 places. Presumably comparable processes would occur in the oceans. 

 The trend would be backwards, toward fewer and fewer species 

 occurring in fewer and fewer communities. 



What if crustal disturbances had never occurred since the initial 

 time when conditions favored the origin of pre-life organic com- 

 pounds? Without the mechanism provided by crustal movements 

 for dividing and reuniting areas, life might not have evolved at all. 

 If it had, only marine life would have evolved because erosion 

 would have worn away any original continents, and there would 

 have been no exposed land. 



Even in this continuous ocean, life would have been quite dif- 

 ferent. Without barriers arising periodically to divide species ranges 

 there would have been no increase in the number of species through 

 the mechanism of geographic isolation. A few species might have 

 arisen by the occurrence of polyploid series or curious genetic de- 

 vices, but based on the behavior of present polyploid series this 

 process could not have gone far. 



THE "GEOTECTONIC MACHINE " 



The dynamic changes of the earth's topography and climate have 

 acted as a veritable machine which has brought about a continual 

 increase in the number of different kinds of living things and a 

 subsequent congregation of these into biotic communities. As was 

 discussed in Chapter 2, many conflicting theories have been ad- 

 vanced to explain the cause and nature of the crustal unrest pro- 

 ducing these changes. 



Whether this geotectonic force is gravitational pressure follow- 

 ing contraction, convection currents beneath the crust, mechanics 

 of an expanding crust, or something else, the fact remains that it is 

 one of the basic forces influencing the direction of organic evolution. 



