The Evolution of the Universe 25 



differences in heat radiation from the ocean floor, the apparent dif- 

 ferences in thickness of the crust in the Rise area, and the sub- 

 marine topographic features of upthrust, skimping, and faulting. 



Few hypotheses have caused as much controversy as Wegener's 

 hypothesis (1924) of continental drift. Followers of this hypothesis 

 believe that South America, Africa, India, Australia, and Antarctica 

 once fitted together as one large continent, Gondwana land, which 

 has since broken up and drifted apart. Jardetsky ( 1954 ) postulated 

 that the difference in rotational speeds between equatorial and 

 polar regions would provide a mechanism for such continental 

 fragmentation and reported supporting experiments on the rupture 

 of thin plates under the influence of such simulated conditions. 



Wegener (1924) and Du Toit (1937) believed that the separation 

 of the African mass from the South American mass had occurred 

 within the last 40 or 50 milhon years, at least within the Cenozoic. 

 Studying polar wanderings, as deduced from rock paleomagnetism, 

 Runcorn (1959) and Irving (1959) arrived at figures that suggest 

 little drifting in the northern hemisphere since the Cretaceous ( about 

 70 million years ago) and an amount of drifting that would bridge 

 only a third of the Atlantic Ocean since the Cambrian, some 600 

 million years ago (Fig. 11). 



Origin of the Hydrosphere and Atmosphere 



Two other components of the earth are of vital importance to the 

 process of organic evolution: the oceans and the atmosphere. Urey 

 ( 1952 ) believed that the hot period of the earth's crust lasted only 

 a short time. He deduced that, as temperatures in the earth's 

 atmosphere rapidly cooled to their present state, the various gaseous 

 elements combined to form an atmosphere of water vapor, hydrogen, 

 ammonia, methane, and smaller quantities of other gases. A large 

 part of the water vapor condensed and filled the low areas to form 

 the oceans. Urey also asserted that the water comprising possibly 

 95 per cent of the oceans has escaped from the interior of the earth 

 during geologic time. Thus the early oceans would have been 

 small and would have supplied only a limited amount of rain to 

 erode areas of the cooled earth. As the volume of surface water 

 increased, so would the amounts of rain, ice, and snow, bringing 

 about great erosive forces constantly tending to level the highland 

 areas of the earth's crust. 



Other investigators believe that the primeval earth lost all its 



