THE ORDOVICIAN PERIOD 521 



been identified in the far north (in North Devon, the west coast of 

 King William's Land, Boothia, etc.) by fossils akin to those of low 

 latitudes, indicates that the climatic conditions of North America 

 and Europe must have been less diversified than now. This 

 apparent lack of diversity of temperature through wide ranges of 

 latitude is one of the unexplained problems of geology. Its 

 solution is possibly to be found in a much higher average tem- 

 perature of the ocean, due to a deep circulation the reverse of that 

 which exists now. 1 If the body of the ocean-water was relatively 

 warm (instead of cold as now), it would have done much to 

 counteract the effect of slight insolation during the cooler part of 

 the year. 2 



LIFE 



Just as no great physical change took place in the passage 

 from the Cambrian to the Ordovician period, so there was no pro- 

 nounced break in the succession of life. The time from the begin- 

 ning of the Cambrian to the close of the Ordovician appears to have 

 been one long eon of progressive development and expansion of life, 

 and its division into two nominal periods is artificial rather than 

 natural. 



The fossil record of the Ordovician is fuller than that of the 

 Cambrian. This is due partly to an increase in fossilizable forms, 

 partly to an increase in numbers of individuals, and partly to 

 better conditions of preservation. 



The general aspect of life was cosmopolitan, though it was not 

 the same everywhere. It varied with the physical evolution of the 

 continent, and largely as the result of it. The variations assumed 

 three general phases: (1) adaptation to the immediate physical 

 environment, particularly the nature and depth of the sea-bottom 

 (edaphic adaptation)', (2) modification by auto-evolution within 

 restricted areas isolated by barriers (provincial evolution), and 

 (3) modification toward a universal type through intermigration 

 (cosmopolitan development) . 



1 Chamberlin, Proc. Am. Phil. Soc., Vol. XLV, 1906, pp. 1-11. 



2 Chamberlin and Salisbury, Earth History, Vol. Ill, pp. 437-445 



