AND THEIR DESCENDANTS 



signs of becoming a continent; Europe's countries, or 

 some of them, assumed a distinct existence. With the 

 advent of mountains came streams and rivers; and the 

 streams, fed by the abundant rainfalls, rushed down to 

 the seas in torrents that performed the work of erosion 

 with a rapidity perhaps unequalled by even the greatest 

 rivers of our present-day knowledge though at first 

 the land areas were not large enough to give rise to 

 streams as long as great rivers like the Amazon or 

 Mississippi. We cannot say exactly what the areas and 

 localities of the water and the land were ; but it is safe 

 to assume that at the beginning of the Silurian period 

 beds of sediment brought down by the rivers and the rain 

 were accumulating about the borders of the land, and as 

 far out as the waves and currents were able to convey the 

 earth materials. The climate was still equable and was 

 much the same over great areas of the world's surface, for 

 the forests of warm temperate latitude are, in part, the 

 same as those in Arctic regions. Certain parts of the land 

 appear to have been desert. 



Life began to change a great deal in Silurian times. 

 The extensive withdrawal of the sea from great stretches 

 of submerged surface reduced the area of shallow water 

 available for the forms of life that had so richly peopled 

 it during the Ordovician period. Then there came an 

 age during which the sea invaded some of the regions of 

 the earth's crust, and again withdrew, leaving behind it 

 great stretches of water which gradually grew more 

 intensely salt. All these things had naturally a great 

 effect on the development of the plants and animals of 

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