THE COMING OF THE BIRDS 89 



the waters of the North Sea. Sea-serpents wandered 

 up the mouth of the Thames, and rhinoceroses and 

 hippopotami splashed in the rivers of Yorkshire. 



But the earth was growing colder. I am not 

 satisfied that any good explanation has yet been 

 given of this progressive cooling of our globe, and 

 I will not speculate on it. Ice Ages we fairly under- 

 stand, as an effect of the rise of the land ; but general 

 cooling and the formation of permanent ice-caps 

 at the Poles have not yet been explained. We will 

 simply state the fact. The northern hemisphere 

 became more and more temperate. The warm- 

 loving plants and animals were slowly driven south. 

 The land was rising in very many parts of the earth. 

 The great masses of the Rocky Mountains and Andes, 

 the Alps and Atlas and Himalaya, were dtiring all 

 this time slowly rising towards the snow-line. A new 

 Ice Age was approaching. This, of course, explains 

 much of the cooling of the earth, but there is a 

 steady lowering of temperature which it does not 

 explain. After an Ice Age the earth never returns 

 quite to the degree of warmth which it had had 

 before. 



However, the Ice Age at last set in. Seven million 

 square miles of northern Europe and America were 

 covered with permanent ice and snow. Glaciers, 

 immensely larger than those which now flow slowly 

 down the flanks of the higher Alps, flowed from the 

 hills of Scotland, Cumberland, and Wales. I will say 



