THE AGE OF MAMMALS 



and new lands arising. We shall not again consider this 

 idea in all its bearings, or ask whether there is any 

 simpler explanation to be found in the never-ceasing 

 explosive tremors of the crust; but we shall only say 

 that the last of these great changes set in at the end of 

 the Chalk age. After that era we arrive at the period 

 among the rocks which, with all its subdivisions, Eocene, 

 Miocene, Pliocene, Quaternary, is classed as Cainozoic or 

 Modern. 



Let us sum up the changes broadly. The Tertiary 

 period, which now begins, has been called the Age of Lakes : 

 but this merely means that there were great lake deposits, 

 and it is true to say that as contrasted with a period of 

 great waters, the Tertiary is to be considered as the 

 period of land. That does not mean that there were in 

 all the hundreds of thousands of years which it embraces 

 no advances and retreats of the sea, no submergings and 

 uprisings of the land. There certainly were. But the 

 land was dominant, and it is the land animals and the 

 land vegetation that are the most important and progress 

 most. After the earth movements which occurred at the 

 end of the Mesozoic or Secondary period there appears to 

 have followed a period of quiet. There was a consider- 

 able area of land standing high above the waters; and 

 there began one of the minor but considerable encroach- 

 ments of the sea in North America. It is probable that 

 the Pacific and the Atlantic joined between North and 

 South America. At the end of this first period the sea 

 withdrew again, and what is called the Miocene period 

 began with a lowering of the temperature of the waters 

 R 257 



