48 EVOLUTION 



five ice-sheets. As to the cause of this extraordinary 

 phenomenon we are still quite unsettled. All the 

 theories I have mentioned in the course of this chapter 

 have their partizans displacement of the earth's axis, 

 purification of the atmosphere, uplifting of the land, 

 change of the ocean currents, etc. and the question 

 must be left open. It must be borne in mind that a fall 

 of a few degrees in temperature is said by many recent 

 students to be sufficient to account for the phenomenon. 

 As to the date and duration of the ice-age there is an 

 even greater difference of opinion. Some geologists 

 bring the close of it down to 20,000 years ago, while 

 others make it begin nearly a million years ago. 

 Professor Keane says that (on Croll's principles) the 

 whole ice-age (with intervals) must have lasted from 

 700,000 to 800,000 years, and that it closed definitively 

 80,000 years ago. Dr. A. R. Wallace (on the same 

 principles, somewhat modified) thinks it began 240,000 

 years ago, and lasted 160,000 years. We can only say 

 that the most weighty of recent estimates put the climax 

 of the last ice-sheet (a relatively small one) at between 

 20,000 and 60,000 years ago, and the beginning of the 

 glacial period is hopelessly uncertain. This brings us 

 down to quite recent times, according to the geological 

 scale, and here we may leave the physical story of 

 the earth and turn to the development of its living 

 inhabitants. 



