TIT II NATURAL HISTORY OF Tllli SUA. 387 



gratUial evolution of llic surface fcatunts of the planet, conti- 

 nental land appears, on the whoht, to have become more 

 compact, m(;re circumscribed and higher, while the Ocean 

 Basins have become more shut off from each other and 

 deeper. Continental land has been far from permanent, 

 but there are many reasons for believing that the areas on 

 the surface of the j^lanet, within which the present continents 

 are situated, are areas within which continents have been 

 torn down and built up again since the dawn of geological 

 history, while similar revolutions have not taken place in 

 abysmal (jr pelagic areas of the Ocean Basins to anything 

 like the same extent, and not at all during any (jf the later 

 geological periods." 



Many naturalists besides the authors of the Challenger 

 Memoirs have called attention to the striking resemblances 

 between the Arctic and the Antarctic faunas and floras — both 

 littoral and pelagic. vSo long as there has been climate on 

 the globe, and geologists can point to no trace of it before 

 Mesozoic times, these areas have been separated by the 

 heat barrier of the equatorial zone. It is only fair to 

 suppose that they are derived from a common stock. In 

 an examination of this problem together with the origin of 

 the deep sea fauna, and in seeking an explanation of the 

 fact that the Arctic Ocean was probably a warm coral sea in 

 paleeozoic and even later geological times, Dr. Murray 

 brings forward a daring hypothesis, which will no doubt 

 meet with both criticism and ridicule. It has the merit of 

 simplicity, the further one that nobody can dc^ny it, and its 

 spirited author may well ask astronomers and others for a 

 better one. " The waters of these ancient oceans must have 

 had a temperature of 65' or 70' Fahrenheit at the poles," 

 and, "the same temperature and the same marine fauna 

 prevailed from equator to poles, the temperature not being 

 higher at the equator. . . . In early Mesozoic times cooling 

 at the poles and differentiation into zones of climate appear 

 to have commenced, and temj)erature conditions did not 

 afterwards admit of coral reefs in the polar area. But the 

 colder and hence denser water that in consequence descended 

 to the greater depths of the ocean carried with it a large 



