152 LIFE ON THE EARTH. 



posed that the mass of the Earth has grown alto- 

 gether colder, since the earliest times, by radiation 

 of its heat into and beyond the cold starry spaces 

 of the universe. It has been conjectured that the 

 Earth's axis has been displaced, so that parts once 

 under the more direct action of the sun have lost 

 much of his beneficent influence; and it has been 

 thought that the differences in question might arise 

 from a different distribution of land and water, which 

 is a known cause of considerable inequality of tem- 

 perature in the same latitudes. 



In considering this subject we may remark that 

 inequalities of temperature in the same parallel of 

 latitude may be observed when an area of water is 

 compared with a surface of land; and when land 

 varies in elevation, and the nature of its surface. Sir 

 Charles Lyell has founded on these facts the sup- 

 position that if a tract of land, equal to and of the 

 same form as our existing continents and islands and 

 rising to the same height, were collected round the 

 poles, while the equatorial zones were occupied by an 

 encircling sea, the whole earth would be cooler than 

 it is now. There is no doubt that this would be the 

 case, but it happens not to agree with the observed 

 facts regarding the glacial deposits, for these require 

 deep ocean over much of what is now land in circum- 

 polar zones. 



