THE ADVENT OF MAN. 235 



man or any of the contemporary mammals back to the Mio- 

 cene. In the Pliocene the continents had attained to their 

 present elevations, and climates were not dissimilar from those 

 prevail'-ig at present ; but still we have no certain indication 

 of the presence of man ; and if other modern mammals extend 

 back to this period their number is very small. In this age 

 also the greater part of the continents must have been covered 

 with a great thickness of soil and disintegrated rock favourable 

 to vegetation, and there seemed nothing to preclude the intro- 

 duction of man. But a new and at first sight most unfavourable 

 change was to intervene. Whether through internal changes 

 affecting the distribution of land and water, or through astro- 

 nomical vicissitudes, the northern hemisphere, and possibly the 

 whole world, entered on an era of refrigeration, the so-called 

 " Glacial Age " of the Post-Pliocene or Pleistocene period. 

 That in this period our continents as far south as the latitude 

 of 40° were overwhelmed with ice or ice-laden seas is rendered 

 evident by the fact that the whole surface up to several thou- 

 sands of feet above the sea-level has been bared of its accu- 

 mulated debris and polished and grooved by ice, and laden Avith 

 boulders and other glacial deposits, while in many places at 

 heights of even 1,000 or 1,200 feet these deposits contain sea- 

 shells of species now living in the colder parts of the ocean. 

 These phenomena do not exist in the tropical regions, except in 

 the vicinity of high mountains, but they recur in the southern 

 hemisphere. It is still uncertain whether the period of greatest 

 cold in the two hemispheres was at the same time or in 

 successive ages. Geologically, however, they are approximately 

 contemporaneous, both occurring between the end of the 

 Pliocene and the modern period ; but nevertheless they may 

 not have coincided in absolute date. 



Very different views have been held as to the precise condi- 

 tion of the continents in the Glacial Age, though all agree in 

 the prevalence of cold and the action of ice, and in the fact 

 of a great submergence at one or more stages of the period. 



