142 LIFE ON THE EARTH. 



since the epoch when the Falls began their back- 

 ward progress from Lake Ontario. 



This epoch, however, is not of necessity the same 

 as that of the origin of the river action, which may 

 have gone on for some unknown time previously. 

 It does not then give us the desired information of 

 the length of the Postglacial period, or the date, as 

 De Luc might have expressed it, of the birth of our 

 continents. But it seems to point in the same direc- 

 tion as all the other natural chronometers, and to 

 compress within a few thousand years the later part 

 of the Pleistocene Period, when the main features of 

 the Land, the Rivers and Lakes, and Plains and 

 Mountains, had been finally redeemed from the power 

 of the sea, and peopled by the now existing races 

 of plants and animals. 



CHANGES OF CLIMATE. 



Few inferences have obtained a more general 

 assent among geologists than that which affirms the 

 change of climate during the progress of life on the 

 globe. The evidence on which reliance has been 

 placed has been sometimes adopted on light grounds, 

 sometimes rejected for fresh and better testimony; 

 but the conviction of almost every writer has been 

 deliberately recorded in favour of the prevalence of 

 much higher temperatures during early geological 



